o ^-Ui o a o t-' UJ
STABILITY OF TERNARY
HYDRIDES AND
SOME APPLICATIONS
PROEFSCHRIFT
TER V E R K R I J G I N G VAN DE GRAAD VAN DOCTOR IN D E TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL DELFT, OP GEZAG VAN D E RECTOR MAGNIFICUS, PROF. DR. IR. H. VAN BEKKUM, VOOR EEN COMMISSIE AANGE-WEZEN DOOR HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE VERDEDIGEN OP DINSDAG
4 MEI 1976 TE 16.00 U U R DOOR
HARMANNUS HINDERIKUS VAN MAL
N A T U U R K U N D I G INGENIEUR GEBOREN TE G R O N I N G E N''71^9 ""'
BIBLIOTHEEK TU Delft P 1799 4461 539584Natuurkundig Laboratorium van de N.V. Philips' Gloeilampenfabrieken te Eindhoven. De directie van dit laboratorium ben ik veel dank verschuldigd voor de gelegenheid die zij mij geboden heeft dit proefschrift te bewerken.
Ik wil graag iedereen die op enigerlei wijze heeft meegewerkt bij het onder-zoek en de realisering van de toepassingen niijn dank betuigen, in het bijzonder A. W. M. Sleegers, H. A. van Esveld en J. A. de Wit. Ook wil ik noemen P. Hokkeling die de materialen heeft vervaardigd en P. G. M. Vos die de reactie van waterstof met LaNij op de film heeft vastgelegd.
Veel van mijn coliega's ben ik dank verschuldigd voor de stimulerende ge-dachtenwisselingen, die hebben bijgedragen tot het resultaat van dit werk. Gaarne wil ik noemen A. R. Miedema, die me sterk heeft geinspireerd, K. H. J. Buschow, C. M. Hargreaves, J. H. N. van Vucht, F. A. Kuijpers en J. S. van Wieringen.
1. HYDROGEN COMPOUNDS, HYDRIDES 1
1.1. Introduction 1 1.2. Properties of hydrides 1
1.3. Characteristics of metal-hydrogen systems 3 1.4. Survey of hydrides (binary and ternary) 10
1.5. Scope of the present work 15 2. EXPERIMENTAL PROCEDURES AND RESULTS FOR THE
LaNij-H^ SYSTEM 18 2.1. Preparation of the compounds 18
2.2. The first reaction with hydrogen, activation 19
2.3. LaNij-hydride powder 24 2.3.1. Particle size 24 2.3.2. Specific surface area 25
2.3.3. Density of LaNis and LaNij-hydride powder 25 2.3.4. A complication related to the large expansion of the lattice 26
2.4. Measuring procedure for sorption isotherms 27 2.5. Isotherms for the LaNij-Hj system at different temperatures . . 28
2.6. Sorption hysteresis 29 2.7. Hydrogen absorption in non-stoichiometric LaNij 31
2.7.1. Phase diagram; homogeneity region of LaNij 31 2.7.2. Hydrogen absorption in LaNi;„ 4.8 < x < 5.5 33
2.8. The reaction of deuterium with LaNi; 35 3. EXPERIMENTAL RESULTS ON VARIOUS
METAL-HYDRO-GEN SYSTEMS • 37 3.1. The formation of hydrides by the LaNis.s^cCos;^ compounds
(x = O t o j c = 1) 38 3.2. The formation of hydrides by the LaNi^M compounds (M = Pd,
Ni, Ag, Cu ,Co, Fe and Cr) 39 3.3. The formation of hydrides by the Ro.2Lao.8Ni5 compounds (R =
Er, Y, Gd, Nd, Th and Zr) 41 3.4. The formation of hydrides by intermetallic compounds of thorium 42
4. HEATS OF FORMATION; DISCUSSION OF RESULTS . . . . 47
4.1. The atomic model for binary alloys 48 4.2. The extended atomic model for ternary hydrides 52
4.3. The formation of hydrides by the LaNi4M compounds . . . . 53 4.4. The formation of hydrides by the Ro.aLao.gNij compounds . . 55 4.5. The formation of hydrides by the RNij and RC05 compounds 56
thorium 58 4.7. Data from the literature 60
4.8. Conclusion 63 5. APPLICATIONS OF METAL-HYDROGEN SYSTEMS . . . . 64
5.1. Introduction 64 5.1.1. Hydrogen getters 64
5.1.2. Hydrogen pressure regulator with high absorption rate . . 65
5.1.3. A laboratory gas-circulation pump 65
5.1.4. Hydrogen fuel storage 65 5.1.5. Hydrogen storage in an electrochemical cell 67
5.2. A LaNij-hydride thermal compressor for a hydrogen refrigerator 67
5.2.1. Working principle of the compressor 67
5.2.2. Thermodynamics; efficiency 69 5.2.3. Configuration 71 5.2.4. Hydrogen refrigerator 73 5.3. A cold-accumulator 74 5.3.1. Design 76 5.3.2. Refrigeration data 77 5.3.3. Recondensation procedure 78 5.4. Heat pumps with metal hydrides 80
5.4.1. Introduction 80 5.4.2. General considerations on heat pumps 82
5.4.3. Thermally driven heat pump with metal hydrides . . . . 82
5.4.4. Two-stage heat pump 84 5.4.5. Multi-stage heat pump 84 5.4.6. Practical aspects 86
References 87 Summary 89 Samenvatting 91
I. HYDROGEN COMPOUNDS, HYDRIDES
1.1. Introduction
Hydrogen reacts with many elements to form hydrides. These reactions are of great technological importance and as a consequence have been the subject of many studies. For the physicist the study of these reactions is interesting because hydrogen is the smallest and the most simple atom. Furthermore, it offers the possibility of realizing very large relative variations of the atomic mass (deuterium, tritium), of great importance in both diffusion and neutron-diffraction studies. A large number of hydrides have been investigated, moti-vated by theoretical and practical interest.
In the past most of the work on hydrides has been concentrated on the binary compounds of hydrogen. These binary compounds of hydrogen are called hydri-des (in the literature also binary hydrihydri-des), and are divided by Remy into four classes: (1) gaseous or volatile hydrides, (2) polymeric hydrides (solid hydrides, which are neither salt-like nor metallic in character), (3) salt-like hydrides, (4) metallic hydrides.
Recently, however, interest has shifted towards the ternary compounds of hydrogen. These ternary compounds of hydrogen are called alloy hydrides (in the literature also ternary hydrides). In our study too, these alloy hydrides have been the main subject. We have been specifically interested in the reaction of hydrogen with intermetallic compounds of transition elements. These transition elements themselves, in so far as they can combine with hydrogen at all, generally form metallic hydrides. Some properties of these metallic hydrides are important in our description of the formation of ternary hydrogen com-pounds.
1.2. Properties of hydrides
A basic property of hydrides is the storage of hydrogen gas to a density which often exceeds the density of liquid hydrogen. The best example of such a hydride is water in which the partial hydrogen density is 110 kg Hj/m^, compared to a density of 71 kg Hj/m^ for liquid hydrogen at 20 K. Water, OH2, undoubtedly the most important hydride, was first recognised as product of the combination of hydrogen and oxygen by Cavendish, in 1781, who had also discovered hydro-gen in 1766 ' ) . The large storage capacity of hydrohydro-gen in hydrides is an attractive property for technical applications.
Some early applications include an electrochemical telegraph (Francisco Salvo ^) of Barcelona, 1804), a ''hydrogen clock" (Pasquale Andervalt ^), 1835) and the use of systems in the field for inflating balloons for meteorological ob-servations. In the electrochemical telegraph, bubbles of hydrogen and oxygen
were produced by decomposing water by an electric current. The hydrogen clocks were driven by the evolution of hydrogen generated by dropping zinc pellets into sulphuric acid H2SO4. For the filling of balloons the hydrogen was generated from calcium hydride CaHj, or lithium hydride LiH, by reaction with water. The hydrogen stored in H2SO4 and LiH has a concentration of 36 and 100 kg H2/m^, respectively, at room temperature. The above examples illustrate how hydrogen may be stored at a high concentration and in a form from which it may be conveniently liberated for use at atmospheric pressure and at room temperature, albeit by a non-reversible chemical reaction.
A very important and useful property of many technical applications is, how-ever, the reversibility of this chemical reaction. A large number of metals, alloys and intermetallic compounds have been found to react reversibly with hydrogen. For such a hydride at a given temperature the absorption (formation of the hydrogen compound) and desorption (formation of hydrogen and metallic substance) take place at nearly the same pressure (sometimes hysteresis is ob-served). The temperature at which the absorption and desorption take place at atmospheric pressure may be very different for the different hydrides. Very stable hydrides require a high temperature (up to 1000 °C) for the reversible reaction to take place at 1 atmosphere. Less stable hydrides have been found for which the reversible reaction takes place at atmospheric pressure at nearly room temperature. In recent years there has been a rapid growth in interest in the reversible reaction of hydrogen with intermetallic compounds at room tem-perature and nearly atmospheric pressure. This growing interest may stem from conjectures concerning the future role {energy transport) of hydrogen in our energy conservation — the so-called hydrogen economy.
So far we have considered several important properties of hydrides such as storage capacity, reversibility of the reaction and stability. Other properties of importance are: rate of absorption and desorption (kinetics of the reaction), constancy of the equilibrium pressure during the reaction at constant tem-perature, sensitivity of the reaction products to impurities.
The present work is mainly concerned with the thermal stability, the storage capacity and the observation of the equilibrium pressures at constant tem-perature of hydrides formed by intermetallic compounds, hydrides which can be formed in reversible reactions. Little attention has been paid to the other properties listed.
Since virtually all applications exploit the fact that hydrogen can be stored in a very compact form, it is useful to compare the partial hydrogen density in some hydrogen compounds, see table I.
Table I includes values for the molecular weight M, the density of the hydri-des o, the number //„ of hydrogen atoms per unit volume, the weight percentage of hydrogen wt% and the partial hydrogen density o„. LaNi; has a density of 8300 kg/m', the hydride LaNisH^ in the powdered form has a bulk density of
TABLE I
Comparison of the hydrogen contents in various hydrogen compounds compound H2O H2SO4 hq. CH4 h q H 2 T1H2 ZrH2 YH2 LaHj LaHj LaNisHg TiFeHi.95 M kg/kmole 18 98.1 16.0 2 49 9 93.2 90 9 140.9 141.9 438 5 105.7 Q kg/m^ 1000 1841 425 71 3800^) 5610") 3958") 5120") 5350") 6225 =) 5470 ") ^ H 10-28 m - 3 6.7 2.2 6.3 4.2 9.2 7.3 57 4.4 6 5 5 3 6 2 wt% 11 2 25 100 4 2 1 2 2 1.4 21 1.4 1.85 Qn kg/m^ 111 36 105 71 153 122 95 73 108 88 101
") Taken from Kleincrt *) ") Taken from Stdlinski ' ) ') Taken from ref 6 and ref. 7 '') Taken from Reilly and Wiswall ^)
3200 kg/m^ We then have # „ = 2 7 • 1 0 " atoms H/m^ and ^H = 45 kg H^jm^ This represents a potential hydrogen pressure at 0 °C of 780 atmospheres
For T1H2 the partial hydrogen density reaches a value which is more than twice that of liquid hydrogen at 20 K and atmospheric pressure. However, the temperature for the dissociation of the stable titanium hydride at a pressure of 1 atmosphere is about 700 °C Such a high dissociation temperature is not al-ways permissible or attractive for technical applications. The intermetallic com-pounds LaNij and TiFe form hydrides that combine a high hydrogen density with a dissociation pressure near one atmosphere at room temperature. 1.3. Characteristics of metal-hydrogen systems
The reaction of hydrogen with a metal M in which a stable hydride MH^ IS formed can be described as follows
M H , + i ( > ' - x ) H 2 * ± M H , , (1.1) where y > x.
The transition to MH^ is the absorption process, the reverse reaction is the desorption process. Since the reaction is exothermic the heat of reaction must
(aim)
" 0 1 2 3 4 - 5 6 — Hydrogen concentration (at H/mole RNig)
Fig. 1.1. Desorption isotherms (hydrogen equilibrium pressure versus concentration) at 40 °C for the compounds Lao,8Ro.2Ni5, where R represents La, Nd or Er. The Lao,8Ero.2Ni5 compound exhibits two hydride phases, one with 3.5 at. H per formula unit and the other with 5.7.
be supplied during the desorption process by external heating. The absorption and desorption process of metal-hydrogen systems is most conveniently studied by means of pressure-composition isotherms. These isotherms generally have the following features (see fig. 1.1).
At low hydrogen concentrations there is a strong composition dependence of the hydrogen pressure. This region (a-phase) refers to the original metal phase which is able to dissolve generally a small fraction of hydrogen gas without the occurrence of a second phase. An exception is Ba which takes up
10 at. % H at 300 °C and more than 50 at.% at 950 °C without a change in structure ' ) and without a discontinuous change in the lattice parameters. The next stage involves a region where the hydrogen pressure is concentration inde-pendent and where the saturated solid solution in the a-phase is in equilibrium with a hydrogen-deficient hydride phase (/J-phase). Further increase of the hydrogen concentration, after the a-phase has been completely converted into the /3-phase, is again accompanied by an increase in the equilibrium pressure, meaning that hydrogen dissolves in the hydride phase. In some cases a second hydride /3' with a higher hydrogen content than /? exists. In these cases one observes a second region where the pressure is independent of the concentration due to the equilibrium /3 + H2 -"-^ ^'. The /3-phases are to be regarded as binary compounds, for example LaH2, or ternary intermetallic compounds, for example LaNisHg, in which one of the components is hydrogen.
The concentration region determined by the horizontal parts of the isotherms can be regarded as a miscibility gap between two well defined phases. It originates from the fact that the partial heat of solution of hydrogen, starting from the
40 20 JO 5 2
[
. ^J
Er / "if /I
:i Nd _ > /La J
(-•• H concentration
Fig 1 2 Schematic plot of pressure versus concentration isotherms of a metal hydrogen system that forms a single hydride phase The hydrogen concentration of the saturated sohd solution and of the hydride phase are temperature dependent The miscibility gap disappears at the critical temperature The heat of formation A// is not independent of temperature T i , T2, T3 are increasing temperatures
hydrogen free compound becomes more negative with increasing hydrogen concentration The process of solution becomes more exothermic so that it is energetically more favourable for the hydrogen atoms to be concentrated locally rather than be distributed evenly throughout the metal ^°) At higher tem-peratures the influence of the entropy counteracts this tendency to form two separate phases so that the miscibility gap may disappear This is represented schematically in fig 12 where a critical pressure and temperature are indicated above which the homogeneous hydrogen alloy will be stable at any composition up to that of the saturated hydride The isotherms then have no region where the pressure is independent of the concentration As noted by Alefeld ' ' i^^ the situation is topologically comparable with that in the temperature-density diagrams of the normal gas-hquid transition of a one-component system
An example of a simple phase-diagram is given in fig 1 3, which shows isotherms for the Pd-H system One observes an increase in hydrogen con-centration in the solid solution and a decrease of hydrogen concon-centration in the hydride phase for increasing temperatures The critical temperatuie is r , = 295 °C with P^ = 20 atm at the composition PdHo 27 Above 295 °C there is a continuous change from the a-phase to the /3-phase More complicated systems are for example the Ti-H system or Zr-H system In the Zr-H system, see fig 14, the isotherms below 550 °C show only one plateau pressure, between this temperature and ca. 750 °C each isotherm has two plateaus at
^«2 (atm)
, 1
04 06 H/Pd
Fig I 3 Pressure versus concentration isotherms for the reaction of palladium with hydrogen as an example of a metal hydrogen system that forms a single hydride phase Only at tem-peratures below 295 "C do X-ray measurements show an abrupt change in the lattice param-eters, corresponding to the formation of a separate hydride phase
different pressures The equilibrium pressure in the two-phase region a + Hj -^ /3 (Pd-H system) or ;3 + Hj -«-»• /3' (see for example Zr-H system), usually known as the plateau pressure but also called transition pressure, can be regarded as a measure of the stability of the hydride with respect to the solid solution MH;, or of fl' with respect to /9 This plateau pressure, which
WOO
Fig 1 4 The reaction of zirconium with hydrogen is an example of a more complicated metal hydrogen system which forms more than one hydride phase Below 550 ' C there is a single hydride phase (d-phase) with the composition ZrH i 35 Above that temperature, because of the lowering of the alpha-to beta transition temperature of elemental zirconium ' ^), two hydride phases exist (/?-phase and 6-phase)
varies strongly with temperature, can be described approximately in a limited temperature range by
In PH2 = -A5/y? + AHIRT, (1.2) where AS and AH are respectively the changes in entropy and enthalpy per
mole H2 involved in the reaction (1.1). When AS and AH are taken to be temperature-independent in the limited temperature range considered, a plot of \np versus 1/r yields the values for AS and AH. The negative entropy effect is predominantly determined by the difference between the entropy of hydrogen in the gaseous state and in the hydride (solid state). Since the latter energy contribution is relatively small —AS will not be much different from the en-tropy contained in the gas, which is about 31 cal/deg mole H2 at 1 atm and at room temperature. From experimental data on the temperature dependence of the equilibrium pressure for metal-hydrogen phases we find values for AS close to this figure, viz. A5 = —30 ± 6 cal/deg mole Hj i^'^*-!'). Hence, for a hydride having a plateau pressure near 1 atmosphere at room temperature (300 K) and for which AG3oo,p=i = RT\np^^ = 0 , one obtains for the enthalpy effect (heat of formation) the value AH = J AS = —9 i 2 kcal/mole H2. Each factor of ten in the equilibrium pressure at room temperature cor-responds to a change in AH of RT\n 10 cal/mole H2 which is 1.4 kcal/mole H2. The enthalpy effect is therefore a direct measure of the relative stability of metal-hydrogen phases. Very stable hydrides have a large negative reaction enthalpy and dissociate (at atmospheric pressure) only at very high tempera-tures.
The most stable metal hydrides known are yttrium hydride and cerium hydride which dissociate at about 1 atmosphere only above 1100 °C *), see table II. Hydrides with a small negative reaction enthalpy can be formed at room temperature only at sufficiently high pressures. An example is GdNi5H2.9 for which a hydrogen pressure of 120 atmospheres is necessary ^*). Only those hydrides having an enthalpy of formation more negative than about —9 kcal/ mole H2 are stable at room temperature and 1 atm hydrogen pressure. From table II it can be seen that only those metallic hydrides are stable at room temperature which contain the elements Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sc, Y, La and other lanthanides, Ti, Zr, Hf, V, Nb, Ta, Th, U or Pu. In chapter 4 we make use of these data (table II) in our model describing the enthalpy change when hydrogen reacts reversibly with intermetallic compounds to form stable hydrides. We anticipate somewhat here and mention that the intermetallic compounds that form hydrides contain at least one of the above-mentioned elements.
Under the assumption that AS is a constant, and the same for different hydrides and also assuming that in the small temperature range considered,
TABLE II
Binary hydrides and their enthalpy of formation, in kcal/mole Hj. The numbers indicated are the experimental values for the enthalpy of formation. The values between brackets are estimated values due to Miedema, see ref. 52, and sec. 4.1 of this thesis LiH - 4 2 N a H - 2 8 KH - 2 8 R b H —26 CsH - 2 4 CeH2 - 4 9 EuH2 M g H j - 1 8 CaH2 - 4 2 S r H j 45 BaH2 42 PrH2 - 5 0 YbH2 ScHj - 4 8 YH2 - 5 4 LaH2 - 5 0 N d H j - 5 0 TiH2 - 3 0 ZrH 2 - 3 9 H f H j - 3 2 ThH2 - 3 5 SmH2 - 5 3 VH ( - 1 4 ) N b H ( - 1 8 ) TaH2 ( - 1 4 ) UH3 - 2 0 GdH2 - 4 8 CrH ( - 4 ) M o H
(+4)
WH ( + 1 0 ) PuH2 - 3 7 TbH2 M n H ( - 4 ) TcH ( + 1 2 ) ReH ( + 2 2 ) DyH2 F e H( + 8)
R u H ( + 1 6 ) O s H ( + 2 0 ) H 0 H 2 C o H NiH ( + 8 ) ( + 4 ) R h H PdH ( + 9 ) ( - 8 ) IrH P t H ( + 1 6 ) ( + 4 ) ErH2 - 5 4AH may be taken as a constant for each hydride, the temperature dependence of the plateau pressures of various hydrides is shown in fig. 1.5. The lines are described by formula (1.2), each with a different value of AH. All hydrides are represented (under the above conditions) by a bundle of lines converging to the point \np^^^ = —AS/R (which has a positive value for a negative en-tropy effect).
Although this presentation of metal-hydrogen systems is very much simplified (entropy changes involved are taken equal for all hydrides and independent of temperature) it proves useful as a guide in finding hydrides suitable for par-ticular technical applications (ch. 5).
The heat of formation of a hydride, which is a measure of the stability, is generally taken with respect to the pure metal phase. If, however, the region of solid solution is not very small, the plateau pressure in the two-phase region is not an exact measure of the stability of the hydride with respect to the pure
ion(K-')
Fig 1.5. Schematic representation of the dependence of the plateau pressure on the inverse absolute temperature (In p = —AS/R + IH/RT) of various metal-hydrogen systems. The lines converge to the point In p = ~ A 5 / / ? , where the value AS = —30 cal/deg mole H2 is taken for the entropy change. It must be emphasized that this picture is only schematic, since AS IS generally not independent of temperature and differs for different metal-hydrogen systems. However, this picture will prove useful in further considerations (ch 5)
7---r,
1^2
M at %H lAH,
Fig. 1.6. Schematic curves of Gibbs'free energy G of the binary M - H system at temperature T^, as a function of the hydrogen concentration (at. % H). Indicated are the region of solid solution and the hydride phase. The tangent to the curves determines the coexisting compositions of the solid solution and the hydride and also the chemical potential /ij = i RT^ In P2. A small region of solid solution (dotted curve) leads to the chemical potential /«i = i RTy I n p i for hydrogen, which is different from
^2-metal phase, but rather with respect to the coexisting solid solution. This is indicated in fig. 1.6 where the chemical potentials for hydrogen ju^ = ^ RT^ In p^ (with respect to the pure metal phase) and /<2 = i RTi In pi (with respect to the solid solution) are different.
When the temperature increases from T^ to J j the chemical potential for hydrogen is found to ht [j.^ = \ RT2 In p^,, where /^j again is the equilibrium hydrogen pressure in the two-phase region From isotherms measured at dif-ferent temperatures the heat of formation generally can be determined by making a plot of In/? versus 1/7", at a given hydrogen concentration in the hydride {AH = b{AG/T)/d{l/T)). In the case of a large solubility region, however, the heat of formation is not constant, because the compositions of the coexisting phases are changing with temperature. The plot of In;? versus 1/7" IS then not expected to result in a straight line
For all the metal hydrides considered in this work we have to do with relatively small regions of solid solution, see fig. 1.7, and a relatively small deviation from stoichiometry of the hydride with changes of temperature. The plateau pressures in the two-phase regions have therefore been taken as a measure of the stability of these hydrides.
G
W at %H I/2H2
Fig 1 7 Schematic curves of Gibbs' free energy G of the binary M-H system, as a function of hydrogen concentration Indicated are a small region of solid solution and a narrow region for the hydride phase When the temperature changes, the compositions of the coexisting phases stay nearly constant This results in a heat of formation which is nearly independent of temperature
1.4. Survey of hydrides (binary and ternary)
The binary compounds of hydrogen formed by boron, by gallium, and by all the elements of the 4th to 7th Mam Groups of the Periodic System are gaseous or volatile. This includes, for example CH4, NH3, H2O and HCl. The halogen hydrides, when brought into water form a positive univalent ion of
hydrogen (the ternary compound of hydrogen H2SO4 also mentioned in sec 1 2 IS another example) The hydrides AIH3, SnH4 and the germanium hydrides are typical molecular metal hydrides They have a high degree of volatility and a low melting point and are thermally unstable All the above mentioned hydrides and also other molecular-type hydrides such as those which are formed with the elements of the 3rd Main Group of the Periodic System will not be considered further in this thesis (see Remy, ref 1)
The binary compounds of hydrogen formed with most of the elements of the 1st and 2nd Main Group of the Periodic System are salt-like hydrides Hydrogen IS the electronegative component in these hydrides, and behaves m them like a halogen (cf lithium hydride) These salt-like hydrides are electrical insulators, exhibit high heats of formation and high melting points ^) The hydrides of LI, Na, K, Rb and Cs are up to 75% denser than the original alkali metal ' ' ) The hydrides of Ca, Sr and Ba are 20 to 25 % denser than the original alkaline earth metal " ) The salt-like hydrides will also not be considered further
The binary compounds of the transition metals, if they combine with hydro-gen at all, form metallic hydrides These hydrides do not always have composi-tions fixed by stoichiometric ratios The metals forming such hydrides often first incorporate hydrogen purely in solid solution, without change of crystal structure, although there is some expansion of the crystal lattice With many metals, however, a structural transformation sets in when the amount of hydro-gen taken up exceeds a certain concentration (sec 1 3) Since this is associated with a discontinuous change in properties, it is considered that a compound is formed These metallic hydrides are usually very hard and brittle Binary com-pounds of hydrogen have been reviewed almost exhaustively up to 1964, in-cluding some refs to 1966, in the book by Mueller, Blackledge and Libowitz ' ' ) . A review on palladium hydride, the most thoroughly investigated of all metal hydrides, up to 1966 can be found in the book by Lewis ''') Rare-earth metal hydrides recently have been reviewed by Bos and Gayer ^8) xhe solid state properties of metallic and salt-like hydrides have been surveyed by Libowitz " )
These binary compounds of hydrogen with transition metals were not the subject of our work However, for the explanation of the behaviour of inter-metallic compounds with hydrogen, which is our subject, we will make use of the knowledge of the existence and stability of these binary compounds and more specifically use the enthalpy of formation in the stability calculations (ch 4)
When hydrogen reacts with an alloy, a distinction should be made whether the alloy is a solid solution or an ordered intermetallic compound A hydride formed from a binary disordered alloy can be considered as a solid solution of two binary hydrides; for such cases there is only a small difference from the hydride of a single metal A hydride formed from an ordered compound, however, for example LaNisHg formed from LaNij, generally has no relation
at all to the binary hydrides of the constituents La and Ni. Since this hydride is formed by absorption of hydrogen at room temperature (where diffusion of La and Ni atoms is negligible), the crystal structure of the hydride can be derived from that of the original compound by relatively small changes in the La and Ni positions. As a matter of fact the lattice has to expand to accommodate the hydrogen.
For compounds that absorb hydrogen at high temperature (for instance above 300 °C) the diffusion of the metal atoms may become possible as well. Pebler and Gulbransen '*) studied at 600 °C and 1 atm hydrogen pressure the reversible reaction
3 Zr2Cu + 4.75 Hj <± 5 ZrH,.9 + ZrCus, (1.3) where Zr and Cu atoms must evidently diffuse with respect to each other.
Reilly and Wiswall ^°) studied at about 500 °C the reaction
2 Mg2Cu + 3 H2 ^ 3 MgH2 + MgCu2, (1.4) where, again, diffusion of Cu or Mg atoms must take place. In general, however,
relatively low temperatures are involved and such diffusion processes cannot take place. The system then shows binary instead of ternary behaviour.
In connection with nuclear reactors much effort has been directed since 1960 into research for materials that could store hydrogen to a very high density. The operating conditions of power reactors often require metal hydrides of high thermal stability as moderators. Most of the details of these investigations are in the classified literature and usually only the existence of the hydrides is published. A survey of hydrides, including hydrides of intermetallic compounds was made by Beck (quoted by Aitken, ref. 21), see table III.
In this table, only the number of hydrogen atoms absorbed per metal atom in the compound is indicated. No information is given concerning the stability of the hydrides. In 1966, Pebler and Gulbransen '*) reported on a study of the intermetallic compounds of zirconium with chromium, molybdenum, iron and cobalt. These authors mention a decreasing tendency to absorb hydrogen in the sequence ZrCrj, ZrMo2, ZrFe2 and ZrCo2. The stability of the hydrides of these compounds were in all cases found to be lower than that of pure Zr. In his study in 1963 of the reaction of hydrogen with thorium-aluminium com-pounds, van Vucht ^^) mentions that the stability of Th8Al4H4 is lower than the stability of ThH2; substituting Ce for Th in ThgAU increases the stability of the hydride. Reilly and Wiswall 23.20) studied the reaction of hydrogen at 300 °C with MgjNi, Mg2Cu, TijCo, Ti2Ni, TijCu, ZrCo, ZrjCo and ZrjCu. The pressure vs composition isotherms were given for MgjNi, MgCu and ZrCo; some results are also presented for the TiNi-H reaction at 300, 400 and 640 °C.
TABLE III
Ternary hydride compounds having hydrogen-to-metal ratios greater than 0.10 (ref. 21). The number of hydrogen atoms per metal atom of the compound is indicated by H/M compound TijAu TijPt TijAl TijCu Ti2Ni Ti2Ga Ti2Pd Ti2Mn ( + 1 0 a t % O ) Ti2Pt Ti2Fe ( + 1 0 a t . % 0 ) Ti2Cr ( + 1 0 a t . % 0 ) TiCr2 **) Y - T i C u VjSn N b j S n PrSb PrGa2 CaAl2 maximum observed H / M 1.17 0.50 0.94 1.18 0.97 0.85 0.49 0.47 0.35 0.27 0.39 0.22 0.21 0.06 0.25 0.22 0.21 0.11 0.15 compound Zr2Al3 Zr3Al2 ZrjAl Zr2Cr ( + 1 0 a t . % 0 ) ZrNi *) ZrV2 *) ZrCr2 *) ZrMn2 *) ZrMo2 Hf2Pt Hf2Co *) Hf2Mn *) HfCo *) HfV2 T h , N i 3 *) Th2Al ThAl T h C o ThMn2 Th6Mn23 *) maximum observed H / M 0.21 0.52 0.90 1.00 1.44 1.38 1.16 0.69 0.11 0.44 1.54 (1.50?) 1.49 1.06 (2.6?) 1.27 0.25 1.70 1.19 0.89
*) Compounds marked with an asterisk absorb more hydrogen than would be possible by gas-metal reaction with the constituent elements.
**) We recently prepared TiCr2Ho.8 at 24 atm and room temperature.
In a study on the reaction of hydrogen with RC05 and RNi, compounds (R stands for rare-earth metal) van Vucht, Kuijpers and Bruning *) found a tendency for the stabilities of these hydrides to decrease with increasing atomic number of the rare-earth metal. The stability of the RC05 hydrides is higher than that of the corresponding RNi; hydrides. The same authors also investiga-ted the change in stability when La was partially replaced by Ce (0-50 %) in
LaNij and found the stability of the hydride to decrease Kuijpers ' ' ) studied the RC05-H systems more extensively and obtained values for the reaction enthalpy from the temperature dependence of the equilibrium-pressures in the two-phase region All these hydrides are far less stable than the original rare-earth hydrides
Reilly and Wiswall ^° ^*) report on the stability of TiFe hydride and on TiFe hydride in which 10% of the Fe is replaced by Ni, causing an increase in the stability of the hydride The same authors ^'^) investigated the hydnding behaviour of the compounds of Mg with Ce, La and Y The dissociation pres-sures were found to be about 10°„ higher than that of MgHj at a given tem-perature Buschow and van Mai ^') established that LaNij has a large homo-geneity region (sec 2 7) at high temperatures and found a considerable change in the stability of the hydrides of these compounds within this region The stability increases with increasing deviations from stoichiometry in the nickel-poor direction and decreases in the nickel-rich direction (ch 3) Van Mai, Buschow and Kuijpers ^^) investigated the change in stability for the LaNij-hydrogen compound when Ni was replaced by Co They found a gradual increase in the stability with increasing cobalt content and a decrease in the absorption capacity (ch 3) Van Mai, Buschow and Miedema ^'') studied the change in hydrogen sorption properties caused by partly replacing La or Ni in LaNij by other metals Desorption isotherms at 40 °C have been measured for LaNi4M, where M represents Pd, Co, Fe, Cr, Ag or Cu, and for Lao sRo 2N15, where R represents Nd, Gd, Y and Er, also Th and Zr (ch 3) The stability for LaN^M-hydrogen compounds increases in the following order M = Pd, Ni, Ag, Cu, Co, Fe and Cr
The stability for Lao sRo 2Ni5-hydrogen compounds decreases in the fol-lowing order R = La, Nd, Zr, Gd, Y, Er and Th It is argued ^^) that the more stable the original RN15 compound, the weaker the tendency to form a RN15 hydride {Rule of reiersed stability) In the same paper a semi-empirical model IS given from which it is possible to draw quantitative conclusions as to the heats of formation of the ternary compounds (ch 4) A literature survey of metallic ternary and quaternary hydrides has been given recently by Newkirk ^ ^) Data on hydrogen absorption by AB5 compounds (A represents a rare earth and B a transition metal) were considered by Anderson et al ^*) Table IV lists the known ternary compounds of hydrogen (except for those already given in table III) Table IV also includes the hydrides of YC05 and ThCoj studied by Takeshita et al ^'), and furthermore the hydrides of RC03 compounds where R represents Dy, Ho and Er, investigated by the same authors ^^)
From this survey of all known ternary hydrides of intermetallic compounds, we may conclude that one feature they all have in common is that at least one of the metals in the compound itself forms a stable binary h>dride This immediately suggests a comparison of the thermodynamic stability of these
TABLE IV
Ternary hydride compounds, either not included in table III, or reinvestigated
c o m p o u n d Z r v N i i o H i v Z r N i H a o Z r 2 N i H 4 8 *) Z r C r 2 H 3 e Z r V 2 H 4 5 r e f e r e n c e 30 30,31 3 1 , 3 0 , 3 2 14,32 14,32 Z r M o j H o 9***)14,32 Z r F e 2 H o 2 Z r C o 2 H o , Z r 2 C u H i 8 Z r 2 C u H 3 0 Z r C o H j 9 Z r 2 C o H 5 4 T12C0H5 T12N1H3 3 T i N i H T i F e H j 14,32 14,32 14,32 14,32,23 23 23 2 3 2 3 , 3 7 2 3 , 3 7 7 c o m p o u n d M g 2 N i H 4 M g 2 C u H 3 *) M g , , C e 2 H 3 i M g i , L a 2 H 3 6 M g 2 4 Y 5 H 4 9 T h Y b 2 H 6 **) T h T i j H g **) T h Z r 2 H 7 **) T h 5 M n 2 3 H 2 4 T h C o H 4 2 T h N i 2 H 2 1 Y b P d H 2 1 L a C u j H j 2 P r C u 5 H 2 6 N d C u j H s 0 L a N i H 4 , L a P t H 2 8 r e f e r e n c e 23 23,20 24 24 24 28 59 59 28 34 28 28 24 24 24 16 16 c o m p o u n d L a C o 5 H 4 3 P r C 0 5 H 3 8 N d C o 5 H 2 7 C e C o 5 H 3 S m C o 5 H 3 0 G d C o j H j 8 T h C o 5 H 3 L a N i j H e P r N i ^ H e 4 N d N i s H e 2 S m N i s H i 6 Y C O 5 H 3 8 D y C 0 3 H 5 H 0 C 0 3 H 5 E r C 0 3 H 5 r e f e r e n c e 15 6,15 6,15 15 3 6 , 6 , 3 5 , 1 5 6,15 15,29 6,7,25,35,15 6 6 6 29,16 29 29 2 9
*) Compounds marked with an asterisk show a dissociation of the intermetallic phase during the reaction with hydrogen at high temperatures (reaction (I 3) and (I 4)) **) The binary compounds ThYb2 ThTi2 and ThZr2 do not exist at room temperature,
hydrides are prepared at high temperature
***) We recently prepared ZrMo2Hj §3 at 20 atm and at room temperature
ternary compounds of hydrogen with that of the corresponding binary hydrides Another feature is that in all cases the ternary hydride is found to be far less stable than the corresponding binary hydride. These observed facts will be explained in ch 4
1.5. Scope of the present worli
A large part of the work presented in this thesis is devoted to the LaNij-hydrogen system This compound has certain unique properties The main object of the study was to understand the large difference in stability of the LaNij hydride compared with the hydride of elemental lanthanum (nickel is relatively inert towards hydrogen) Other hydrides have been studied in so far as they have a bearing on LaNij or helped to elucidate the fundamental aspects of hydrogen sorption
systems and a description of the apparatus used for the activation and the measurements is presented in ch. 2. Also reported in that chapter are the results obtained on the reaction of hydrogen with LaNis and the experimental problems concerning the influence of a deviation from stoichiometry on the stability. Other matters investigated include activation of the samples, hysteresis, lattice expansion, and the reaction of deuterium with LaNis.
Chapter 3 gives the results of a systematic study of the stability of hydrides formed with the intermetallic compound LaNij in which a considerable fraction of La or Ni is replaced by other metals. A study was also made of the formation of hydrides from all the compounds in the binary systems of thorium with either nickel, cobalt or iron, representing 8 different structure types. These thorium compounds were chosen because their values of the enthalpy of formation are known experimentally (determined by Skelton, Magnani and Smith ^8). These enthalpies of formation are required in our semi-empirical model ^'') which gives a method of calculating the enthalpy of formation of ternary hydrides from the enthalpies of formation of the corresponding binary hydrides and binary intermetallic compounds; this is set out in ch. 4. The model for ternary hydrides is an extension of an atomic model for binary alloys and binary hydrides recently introduced by Miedema et al.^®'*°'*')- The ap-plication of the model requires the assumption that hydrogen in metalhydrides can be regarded as a metal *°). The energy effects that determine the stability of binary alloys are assumed to be dominated by the interactions between nearest-neighbour atoms. As a consequence it seems possible to express the enthalpy of formation of a ternary hydride like LaNisH^ in terms of the enthalpy of formation of the corresponding binary hydrides (here lanthanum and nickel hydrides) and of the original binary compound (here LaNij), in the following way:
A//(LaNi5H6) = A//(LaH3) + A7/(Ni5H3) — A//(LaNi5). (1.5) It is necessary to make the restriction that only one of the metals in the binary compound (AB„) is assumed to form a relatively stable hydride and that this metal (A) is the minority element in the compound AB„ (ch. 4). As is known (sec. 1.3, table II) lanthanum forms a very stable hydride and is the minority element (nickel is relatively inert to hydrogen). In a general formulation one may write
A//(AB„H2.) = A//(AH J + A//(B„H„,) - A//(AB„). (1.6) The knowledge of the enthalpy of formation of the thorium compounds there-fore facilitates the testing of expression (1.6). Since the thorium compounds represent 8 different crystal structures it is also possible to show the relatively small influence of the crystal structure on the energy effects involved.
almost 1 atmosphere at room temperature the enthalpy of formation of a hydride must be approximately AH = —9 kcal/mole Hj. One would therefore expect that according to expression (1.6), this condition is not a property unique to LaNis. A number of other compounds (ternary or quaternary) could also have this property and this could open new perspectives of practical interest.
In chapter 5 a short survey is given of some technical applications of these hydrides. A number of applications, involving LaNij hydride, will be examined in more detail. A description will be given of a LaNij-hydride thermal absorption compressor for hydrogen '') used in a hydrogen refrigerator. The compression work is derived from nearly isothermal desorption at about 160 °C and a nearly isothermal absorption at 15 °C. A description is also given of the so-called "cold-accumulator". In this device a bath of liquid hydrogen is coupled to a vessel containing LaNij. The purpose is to provide a portable, passive device giving cooling at about 20 K for a number of hours. The evaporated hydrogen is absorbed by the LaNij at an almost constant pressure and hence at constant temperature. For recondensation of the hydrogen the system is coupled to a cryogenerator. Finally a multi-stage thermally driven heat pump with metal hydrides is described. From a simple graphical presentation, given the temperature levels of the heat pump, the characteristics of the metal-hydro-gen systems required can be obtained.
2. EXPERIMENTAL PROCEDURES AND RESULTS FOR THE LaNij-H^ SYSTEM
In general the following experimental procedure was followed The sample, a single-phase alloy pellet prepared by arc melting, was first annealed and then exposed to hydrogen at high pressure to form a hydride After complete activa-tion, effected by a number of desorption-absorption cycles, desorption iso-therms were measured, as were absorption isoiso-therms in some cases also In all cases a desorption isotherm was measured at 40 °C in order to be able to com-pare the different metal-hydrogen systems In this chapter we consider the preparation of the samples, the activation process (the first reaction with hydro-gen) and the measuring procedure for the pressure versus concentration iso-therms This includes descriptions of the apparatus and the measuring systems used
Almost all compounds investigated disintegrated into a powder of small par-ticles when reacting with hydrogen for the first time This first reaction of LaNij with hydrogen has been recorded on film (sec 2 2) We determined the particle size after completion of the first reaction with hydrogen and after twenty ab-sorption-desorption cycles (sec 2 3) The bulk density of the LaNij-hydride powder and the specific surface area were also measured (sec 2 3) The expan-sion of the metal lattice accompanying the formation of a hydride is large (25% for LaNis) Since vibration during absorption-desorption cycles tends to increase the packing factor of the powder in the dehydrogenated state, the container may be exposed to large mechanical forces and become distorted (sec 2 3)
Experimental results for LaNij include our measurements of isotherms at 100, 120 and 140 °C, as well as measurements on the variations in the stability of hydrides formed by non-stoichiometric LaNi; compounds In addition we studied the hysteresis between absorption and desorption isotherms for the LaNij compounds The reaction of LaNij with deuterium at 40 °C is also considered
2.L Preparation of the compounds
Two methods of preparation were used The small alloy pellets (approxima-tely 10 g) were prepared by melting the components in an arc furnace under an argon atmosphere Larger quantities (more than 100 g) were prepared m an induction furnace using fireclay (chamotte) (AI2O3 with MgO and S1O2) crucibles, also under an argon atmosphere
In order to prepare the alloy pellets, the metal mixture in the correct weight ratio (correct to within 0 2%), using starting materials of 99 9% purity, was introduced into the arc furnace, outgassed for many minutes, after which some argon gas (20 mm Hg) was admitted (The argon gas is necessary to produce
the arc and it reduces the evaporation) The metal mixture was then melted in an arc between a movable, cooled tungsten rod and the sample on a water-cooled copper base plate The sample was allowed to solidify, was turned over and then remelted This was repeated several times to obtain a better homo-genization In order to anneal these alloy pellets they were placed in AI2O3 crucibles introduced into silica tubes which were sealed after thorough evacua-tion and outgassing For the annealing at rather high temperatures (7"> 1100°C) purified argon gas at a low pressure (a few mm Hg) was admitted The samples were then kept at high temperature (1100 1200 "C for LaNij) for several days and subsequently quenched in water Homogeneous alloy pellets up to 20 g could be prepared in this way
For the preparation of larger quantities an induction furnace was used The procedure will be described for LaN^Cu, the melting points of La, Cu and Ni are 920, 1083 and 1455 °C, respectively The nickel and copper in the cor-rect weight ratio are put into a fireclay crucible positioned in a radio-frequency coil inside a vacuum furnace The lanthanum is kept in a special holder at the top of the furnace (above the crucible) where there is little heating, to preclude loss by evaporation The furnace is evacuated and the metal mixture is out-gassed during the heating period When the nickel-copper mixture melts, purified argon is introduced into the furnace at a pressure of 20 mm Hg and small pieces of lanthanum are dropped into the melt at short intervals of time The melt is then quickly poured out into a graphite mould which is at room temperature and has a relatively large heat capacity in order to effect rapid cooling A more complete description is given by van Oss, van Esveld and van Mai '*'^) including details of the apparatus and the crucible In this way quantities of up to 300 g could be prepared In a similar way but in a different furnace (different crucibles and radio frequency coils) quantities up to 800 g or even 3 kg were prepared The annealing procedure for these large quantities was identical to that for smaller samples
A small fraction of all the compounds prepared was used for X-ray diffraction examinations and another small fraction was kept for metallographic examina-tion in order to verify that no second phase was present A quantity of about 7 g of each sample or batch was used in studies of the reaction with hydrogen and for this purpose was introduced into a high-pressure sample holder
2.2. The iirst reaction with hydrogen, activation
At room temperature, without any special treatment, the first reaction of a metallic substance with hydrogen does not usually take place spontaneously Most times it is necessary to granulate the samples to small particles (in an inert atmosphere), to heat to high temperatures (300-500 °C) in a high-vacuum system to facilitate outgassing and finally bring the samples into contact with
Hydrogen 0-ring b) Vacuum Controlled gas pressure Heating element Thermocouple Glass capsule Sample
Fig 2 I a High-pressure sample holder (length = 8 5 cm, diameter = 2 5 cm) used for the reaction of intermetallic compounds with hydrogen and for absorption and desorption measurements The sample is introduced into the reactor volume K, by removing part M As the sample disintegrates into a hne powder it falls into the annular space (width of 0 5 mm) b For the measurement of sorption isotherms at higher temperatures the sample holder is mounted inside a hermetically sealed container in which the gas pressure can be controlled The sample holder is provided with a thermocouple and a heating element and is used in the temperature range 100-180 °C
c An arrangement to activate samples in glass capsules in cases where the sample is used for different types of measurements (superconductivity, resonance, etc ) When value B is open the system can be evacuated via valve A the pressure inside and outside the glass capsule remaining the same Also hvdrogen can be admitted through A
After complete activation of the sample the pressure in the system is reduced and, at a pressure of a few atmospheres valve B is closed The bottom part of the container is removed any excess hydrogen can still be released from the capsule through valve B (by bubbling under water) The glass capsule is then held partly in liquid nitrogen while it is sealed with a flame, to be ready for measurements
hydrogen of 99 99 % purity Examples of this procedure are the activation of Mg, Mg ( + 5 % NI), V and TiFe ^° "^ ^)
However, for LaNij and related compounds, the activation procedure was found to be rather simple When the alloy pellets are contacted with hydrogen gas at a pressure between 20-50 atmospheres they react spontaneously at room temperature to form a hydride During this first reaction with hydrogen the sample disintegrates into a powder of small particles with a large, uncontam-inated surface
In detail, the procedure for activation and simultaneous measurement of the amount of hydrogen absorbed is as follows After a sample of precisely known weight (within 0 2%) has been introduced into a high-pressure sample holder (fig 2 1), the sample holder is tested for vacuum tightness by introducing helium gas at 20-30 atmospheres and using a helium leak detector This sample holder is then connected (see fig 2 2) via an o-ring coupling to a calibrated volume which includes a pressure indicator (usually 0-60 atm) and the dead space of the two valves B and C To determine the volume of the sample holder, valve A is opened, valve B closed and the sample holder together with the calibrated volume are evacuated to 10"' mm Hg (via valve C) The valves A (of the sample holder) and C are then closed and hydrogen at 40-50 atm is admitted from the high-pressure cylinder to the system via valve B After closing
Hz 0-ring coupling \) lA Reactor
i
Reducing valve ^ \ 2 ' Xa.
S
X High-pressure H2 cylinder.,^x::
VacuumFig 2 2 Three parallel systems used to activate samples contained in the sample holders The volume between the valves A B and C (including the pressure gauge) is a calibrated volume The system can be evacuated through C, hydrogen is supplied via B The high-pressure cylinder contains 99 9% pure hydrogen gas at high-pressures between 60-150 atm
B, valve A is opened and the volume of the sample holder is calculated from the initial pressure drop now observed. The calibrated volume was usually about 20 cm' and the volume of the sample holder 6 cm'. The error of measurement was less than 1
"o-When the sample is placed in contact with hydrogen it usually takes some time before any noticeable effects can be observed. This period might be any-thing between a few seconds to a few days and appears to be dependent on the stability of the compound and on the history of the samples. In general we observed that the more brittle alloys would activate more rapidly. It has also been found that fresh samples could be activated faster than identical samples that had been kept in air for some time. For samples that had been annealed, a longer time for activation was generally required than for as-cast samples of the same composition. We assume that as a first step in normal hydrogen ab-sorption, the hydrogen molecules dissociate at the surface and that subsequently the hydrogen atoms diffuse rapidly into the metal. An oxide layer will prevent this dissociation taking place and hence at the beginning only a very small fraction of the surface area will be active. Hydrogen diffuses through cracks in the oxide layer, the lattice expands upon hydrogen absorption and since most compounds are brittle, the activation process is self-accelerating. The sample
Fig. 2.3. Time-lapse pictures taken during the acti\ation process The first reaction of a fresh sample with hydrogen is accompanied by a break-up into small particles, thus forming a large, clean and highly reactive surface.
Fig. 2.4. Transparent reaction vessel used for observation of the activation process. The sample is positioned on a copper grid suspended from copper rods attached to the top plate. The ceramic microphone used to record the sounds accompanying activation is situated about 4 cm above the sample.
breaks up into small particles, thus forming a large, clean and highly reactive surface. The disintegration into a powder has been recorded on film and some characteristic pictures are shown in fig. 2.3.
A special transparent reaction vessel, fig. 2.4, was used for visual observation of this first reaction of a LaNij-alloy button with hydrogen. Film recordings were made of this first reaction of hydrogen with LaNij and with compounds based on LaNij containing Si, Cu and Cr respectively **-*^), The sounds generated during these activation processes were detected by a microphone mounted inside the reaction vessel and recorded (fig. 2.5). It has never been possible to predict the moment of the first crack, but it was found to be pre-ceeded by a noticeable rise in temperature: a thermocouple, therefore, was a reliable indicator of the moment the camera should be started.
2.3. LaNij-hydride powder (particle size, specific surface area, density) 2.3.1. Particle size
Our experience is that nearly all compounds measured become finely divided powdered hydrides when reacting with hydrogen. This is a complication in the use of hydrides for technological applications. The question arises whether there is some final size for the particles or whether they continue to break up into still smaller particles during absorption-desorption cycles. To examine this experimentally LaNij was activated under a pressure of 40 atm after which the hydrogen was desorbed. The powder obtained was examined under a micro-scope and 200 particles were classified according to size, see fig. 2.6. A similar
40afm Hydrogen Pellet LaNis 500 he Transparent reaction vessel
Fig. 2.5. Schematic diagram of the system used to observe the activation process and simul-taneously record the sounds produced during this first reaction of a sample with hydrogen.
24 32 40 *• d (ijm)
LaNig powder Particle sizes after 20 cycles
[L
JZL.16 24 32 40
dftjm)
Fig 2 6 Size distribution of LaNis particles (a) for 200 particles of a sample after one absorption-desorption cycle (6) for 200 particles of a similar sample after twenty cycles
sample was activated and subjected to twenty absorption-desorption cycles and then also examined under the microscope, this result is also shown in fig 2 6 For the first sample, peaks in the distribution are found at 4 (xm and at about 20 [Jim For the second sample the peak at 20 ptm has disappeared It appears that 4 (i,m is a sort of terminal size A sample which was cycled over a hundred times did not show appreciable differences from the second sample
2 3 2 Specific surface area
The specific surface area of a well-activated sample was determined by the gas-adsorption method (using krypton) and found to be about 0 25 m^/g If, in a simplified picture, all particles are assumed to be cubes of identical size one may calculate the particle size from the specific surface area and the density of LaNij (8 3 g/cm', sec 2 3) A particle size of about 4 [im is deduced, which agrees with the above microscopic observation
The occurrence of fine particles implies that in technical applications it will be necessary to use filters with very small pores (smaller than 2 y.m) The total surface area of these filters has to be large in many applications to prevent too large a flow resistance (sec 5) It seems, however, unlikely that very fine dust can be prevented from penetrating the total system For magnetic hydride powders a solution to this problem is of course the use of magnetic filters 2 3 3 Density oj LaNi, and LaNis-hydride powder
After the weight of a number of LaNij samples had been accurately deter-mined, the volume of these samples was determined by the displacement of water in a pyknometer The density was found to be 8 3 ± 0 1 g/cm', which is
in agreement with the value of 8.28 g/cm' calculated from X-ray data *). The effective density of the LaNij-hydride powder was determined in the following way. About 300 g of well activated powder was introduced in a measuring cylinder and the volume determined. The cylinder was then vibrated for a few minutes and the volume re-determined. The apparent or effective density varied between 3.0 and 3.6 g/cm'. After prolonged vibration a density of 4 g/cm' could be obtained. These experiments were performed after the powder had been transferred from the high-pressure reactor to the measuring cylinder. Since the equilibrium pressure at room temperature of LaNij hydride is above
1 atmosphere it is assumed that most of the hydrogen had been desorbed be-fore the start of these measurements. From an experiment in the transparent reaction vessel the bulk density of LaNij-hydride powder saturated with hydro-gen was found to be 3.3 ± 0.1 g/cm'; which means that the hydride powder has a density which is only 40% of that of the original pellet of LaNis. 2.3.4. A complication related to the large expansion of the lattice
From X-ray measurements van Vucht et al.*) found that the volume ex-pansion of the lattice of LaNij is 25%. This is an important factor in applica-tions and should not be ignored in studies on the safety characteristics of LaNij hydride. If hydrogen from newly activated LaNij-hydride powder is
Fig. 2.7. Photograph of a thin-walled stainless steel container deformed by the mechanical forces of the expanding powder particles during hydnding. The packing factor of the powder in the dehydrogenated state increases when the container is vibrated m vertical position during absorption-desorption cycles.
desorbed, the particles shrink by 25 % in volume and can fall into new positions inside the container. This was observed in the transparent reaction vessel. The next absorption, however, hardly changed the powder level, although all par-ticles had to expand again by 25%. The following desorption also had no influence on the powder level although all particles now shrink by 25%.
However, when the same experiments were repeated in a thin-walled stain-less steel cylinder, which was vibrated during desorption-absorption cycles, the expansion of the powder caused serious damage to the container. The result is shown in fig. 2.7.
2.4. Measuring procedure for sorption isotherms
For the determination of the pressure-composition isotherms the sample holder, containing an active (fine powder) sample, is connected to a measuring system, fig. 2.8. The measuring system consists of a manifold of 6 mm stainless
\)Pressure gauge Vacuum-''^ F \
i
X
— yInverted •• glass cylinder ' r • \ ) G C HI. ^ " Hydrogen R Reservoir •ii \I
A-' r
c
i n l .
1
:5 ^JM^ -1Fig. 2.8. Measuring system for pressure-composition isotherms. The volume of reservoir R, including the dead space of the valves A, D, F and G and the pressure gauge, is an accurately known volume. The sample holder is kept at constant temperature by a thermostatically con-trolled water bath between 20-100 °C. For higher temperatures the system shown in fig. 2.1b is used.
Steel tubing with connections to a reservoir R with pressure gauge, a vacuum pump, a high-pressure hydrogen cylinder and via a stainless steel capillary to an inverted glass cylinder in a water bath. The hydrogen can be collected in this inverted cylinder, with its calibrated scale, the displacement of the water level being a direct measure of the total quantity of hydrogen released. It is thus an integrating method. The overall accuracy is 1 %. However, this method can be used only when the hydrogen gas released is at a pressure above 1 atm. For pressures below 1 atm the hydrogen is withdrawn from the reservoir by a vacuum pump.
by withdrawing hydrogen from the sample holder in small decrements (via A) into the reservoir R of known volume (which could be either vented to the inverted glass cylinder (via D) or evacuated (via F) and measuring each sub-sequent equilibrium pressure. (The reservoir of known volume includes the dead space of the valves and pressure gauge.) Similarly, absorption isotherms are obtained by passing hydrogen into the sample holder from the reservoir of known volume in small increments, measuring each subsequent equilibrium pressure. (The reservoir is filled with hydrogen via valve G from a high-pressure hydrogen cylinder.) From/JKT" readings the change in composition of the metal hydride is calculated. During these measurements, the sample holder is kept in a stirred water bath, temperature being controlled to within 0.1 °C at 40 °C. For temperatures above 100 °C the system indicated in fig. 2.1b is used. For high plateau pressures a small reservoir of known volume is sufficient, whereas for low plateau pressures a large reservoir is required to reduce the number of points of measurement.
2.5. Isotherms for the LaNij-H^ system at different temperatures
Some desorption isotherms measured on LaNij at 40, 60, 100, 120 and 140 °C are shown in fig. 2.9. Experiments covering the temperature region 21-81 °C were already reported by van Vucht et al.*). We observed lower plateau pres-sures than these authors: this means that the stability of the hydride formed by our compound is larger, due probably to composition differences in the LaNi; samples. A plot of the logarithm of the pressure p versus the recip-rocal absolute temperature 7" (fig. 2.10), yields the value A / / = —7.4 kcal/ mole H2 for the enthalpy change and the value AS = —26 cal/deg mole Hj
50 6 « 3 to
t
'0 1 2 3 ^ 5 6* Hydrogen concentration (at H/mole LaNig)
Fig. 2.9. Pressure-composition isotherms at 40. 60, 100, 120 and 140 C for the reaction of hydrogen with LaNij.
24 26 28 30 3.2 34 '• lU / I I A /
Fig 2 10 A logarithmic plot of the plateau pressure versus the inverse absolute temperature for the composition LaNi5H2
Via the relation AG = —T AS + AH for the entropy change. The value AH = —7.4 kcal/mole H2 also points to a slightly more stable hydride than the sample measured by van Vucht et al. who found AH = —7.2 kcal/mole H2. 2.6. Sorption hysteresis
The first measurements on LaNij hydride *) were concerned only with the desorption isotherms, deviations from reversibility were not observed. In our results, however, it was immediately found that the pressure in the two-phase
PH2
(atm)
, 1
0 1 2 3 4 - 5 5 7
— ^ Hydrogen concentration (at l-f/mole LaNig)
Fig 2 11 Absorption and desorption isotherms for LaNij at 20, 40 and 80 °C The sorption hysteresis increases tovvards higher temperatures (data from ref 35, with a temperature cor-rection).
region is noticeably higher for absorption than for desorption This sorption hysteresis increases as the temperature is increased, see fig 2 11 (for a plot of the logarithm of the pressure versus \jT, see fig 2 12) Longer waiting times to attain more complete equilibrium did not change the observations It seems unlikely that surface effects are responsible for the hysteresis, since in that case a decrease of the pressure differences would be expected at higher tempera-tures In a further investigation, hysteresis was also found in SmCoj and in a number of related compounds ' ' )
Another interesting hysteresis experiment was the following A sample holder containing about 300 g of LaNij having an absorption capacity for hydrogen of about 50 litres (at 1 atm and 20 °C) with a relatively small dead space of about 20 cm', was provided with an electrical pressure gauge to monitor the pressure This sample holder with LaNij hydride was heated through steps of about ten degrees from room temperature to 100 °C and then cooled again via the same temperature steps back to room temperature (It was a closed system, no hydrogen being withdrawn or introduced into the system during the measurements) To arrive on a desorption isotherm the sample was first activated including several absorption-desorption cycles and then about 40 % of the hydrogen was desorbed from the saturated sample Starting from this point on the desorption isotherm at room temperature and heating to 100 °C, pressures at the different temperature steps were found corresponding to those of desorption isotherms On cooling, the pressures found corresponded again
lo' (atm) 5
f
2 ;o' 5 2 I 25 30 35 *• IOOO/T(K'')Fig 2 12 Logarithmic plots of the plateau pressures for absorption and desorption isotherms of two different LaNi; samples versus the inverse absolute temperature for the compositions LaNi5H2 The lines I and 2 correspond to absorption and desorption pressures of the one sample The lines 3 and 4 correspond to absorption and desorption pressures of the other sample (see also fig 2 11)
to those of the desorption isotherms. This is demonstrated by the straight line 2 of fig. 2.12. Next, the sample at room temperature was allowed to absorb a quantity of hydrogen, increasing the hydrogen concentration from 60% to about 65 % to arrive on an absorption isotherm. Starting from here at room temperature, on changing the temperature in steps (as previously described), all pressures were found to agree with the corresponding absorption isotherms, demonstrated by the straight line 1 of fig. 2.12. The result is very similar to the result of another LaNi; sample, where the lines 3 and 4 correspond to absorption and desorption pressures at different temperatures for the hydrogen concentration LaNi5H2 (fig. 2.11). One observes a nearly constant Apjp for the hysteresis effect. The hydrides of the two different LaNi; samples of fig. 2.12 are found to have different stabilities: this question will be discussed in sec. 2.7. We conclude that there is a definite hysteresis between the absorption and desorption isotherms, characterized by a nearly constant Apjp for different temperatures. It has been argued " ) that the hysteresis is related to the volume expansion of the unit cell and therefore also proportional to H/M (the number of hydrogen atoms in the compound). It has of course consequences in the use of LaNis hydride in technical applications (sec. 5).
2.7. Hydrogen absorption in non-stoichiometric LaNij
It has been found that for different LaNis samples the absorption of hydrogen may occur at somewhat different pressure levels, thus involving slightly different enthalpies of formation. A plausible reason could be the slight differences in the La-Ni ratio of the LaNis compounds. In order to understand the sorption behaviour of LaNis we investigated the phase relationships between the various compounds in the nickel-rich part of the La-Ni system ^^) and the sorption behaviour of the different compounds. In the range 50-100 at. % Ni there are six compounds: LaNi, La2Ni3*, LaNij, LaNis, LaaNi, and LaNis- The crystal structures, lattice constants and observed hydrides are summarized in table V. 2.7.1. Phase diagram; homogeneity region o/LaNis
From the results of X-ray diffraction, thermal analysis and metallography the phase diagram shown in fig. 2.13 was constructed. A relatively large homo-geneity region was observed for the compound LaNis. The concentrations limiting this region were determined by metallography. Samples of varying composition, homogenized at 1200 °C, were annealed for several days at 1200, 1100 or 1000 °C and then quenched rapidly in water. On the Ni-rich side the samples with a composition outside the homogeneity region showed a fine Ni precipitate on the grain boundaries as well as inside the grains. The nickel-deficient samples outside the homogeneity region, after quenching, showed a microstructure in which small amounts of relatively Ni-deficient liquid had
TABLE V
Crystallographic data for La-Ni compounds in the Ni-rich part of the binary system (after Buschow and van Mai, ref 25) and the hydrides observed compound LaNi L a N i , LaNi2 LaNi3 L a j N h LaNis lattice symmetry orthorhombic orthorhombic cubic rhombohedral hexagonal hexagonal structure type CrB unknown *) MgCu2 PuNi3 Ce2Ni, CaCus lattice constants (A) a= 3 9 1 ^) = 10 80 c = 4 3 9 0 = 5 114 b= 7 891 c = 9 7 1 5 a= 1.3S7 a= 5 083 c = 25 09 a= 5 058 c = 24 71 fl= 5017 c = 3 987 observed hydride LaNiHa 0 L a N i . H , L a N i j H , LaNi3H4 2 L a 2 N i , H , , 3 LaNisHg s
*) The stoichiometry of the compound LaNi^ has recently been determined by van Vucht and Buschow **), as being LajNis The compound La2Ni3 represents a new structure type The lattice parameters are a = 5.113, b = 9.727 and c — 7.898 A
1500 Temperature M I , woo 50 50 70 80 90 100 >• at Vo NI
Fig 2 13 Phase diagram of the Ni-rich part of the La Ni system It shows a relatively large homogeneity region at high temperatures for the compound LaNij (ref 25)
5.03 a (A) ' t 5.02 5.01 S.00 4.99 i..a 5.0 5.2 5.4 •• Composition x
Fig. 2.14. Lattice constants as a function of the composition for the compounds LaNi^^, annealed at 1200 °C and quenched (ref. 25).
accumulated at the grain boundaries. The following limits were found for LaNi;c in the homogeneity region:
X = 4.85-5.40 at 1200 °C X = 4.90-5.10 at 1100 °C X = 4.95-5.05 at 1000 °C.
The lattice parameters for the different LaNi^^ samples, quenched from 1200 °C, determined by X-ray diffraction, are shown in fig. 2.14. For the samples in which x exceeds the value 5.4, the occurrence of reflections of elementary nickel made the determination of the unit cell dimension in the hexagonal c-direction rather inaccurate. For LaNis we found a = 5.017, c = 3.987 A and V = 86.89 (A'). These results are similar to those of van Vucht et al.«) who found a = 5.0172, c = 3.9816 A, F = 86.8 (A') and Anderson et al.^*): a = 5.020, c = 3.981 A.
2.7.2. Hydrogen absorption in LaNi;„ 4.8 < x < 5.5
Desorption isotherms at 40 °C were determined for the LaNi;^ compounds with x values ranging from 4.80 to 5.50. The results are presented in fig. 2.15. The equilibrium pressures in the two-phase region increase from 2.75 atm for LaNi4.9o to 9.2 atm for LaNis.40. For stoichiometric LaNis the plateau pres-sure is 3.7 atm at 40 °C. The plateau prespres-sures for LaNi4.8o and LaNi4 gs are virtually the same as for LaNi4.9o. Also, the plateau pressures for LaNi5.4s and LaNis.4o were found to be nearly the same as for
LaNis.so-LaNi,
u.fji
