Widok Tom 67 Nr 3 (2015)

www.ptcer.pl/mccm

TWO-STAGE SINTERING, DOPING, AND MICROSTRUCTURE REFINEMENT OF

POLYCRYSTALLINE ALUMINA

Katarína Bodišová

_{, Václav Pouchlý}

_{, Karel Maca}

_,

Dušan Galusek

_*

1_{Faculty of Chemical and Food Technology, Slovak University of Technology, Bratislava, Slovakia} 2_{Brno University of Technology, CEITEC BUT, Brno, Czech Republic}

3_{Joint Glass Centre of the IIC SAS, TnUAD and FChFT STU, Trenčín, Slovakia}

*e-mail: dusan.galusek@tnuni.sk

Microstructure refi nement in advanced ceramic materials is frequently related to an improvement of

me-chanical properties. Moreover, it often provides additional functionalities, such as transparency in the visible

and infrared wavelength range. Many densifi cation techniques, mostly pressure assisted processes such as

hot-pressing, hot-isostatic pressing, spark plasma sintering, or pressureless techniques, such as microwave

assisted sintering and two-step sintering, are therefore frequently used in order to achieve full densifi cation,

while simultaneously suppressing grain growth in the fi nal stage of sintering. Among them, two-step sintering

is of particular interest due to its simplicity, and possibility to achieve complete densifi cation at relatively low

temperature without application of pressure: its capacity has been demonstrated for a range of various

sys-tems. Few works dealing with two-step sintering of alumina report on certain refi nement of microstructure in

comparison to conventional sintering. However, entire grain growth elimination was not achieved. In our work

we demonstrate a combination of two approaches, i.e. two- step sintering, and doping with suitable oxide

ad-ditives, such as MgO, Y

O

or ZrO

, which results in suppression of the grain growth in the fi nal stage of

sin-tering. Positive eff ect of the used additives on the microstructure refi nement, especially the impact of yttria,

zirconia, and magnesia doping on densifi cation and grain growth during the two-step sintering, as well as the

change of activation energy of sintering through doping with various metal oxides has been demonstrated.

Fully dense doped aluminas with relative densities up to 99.7% were prepared. Neither doping of aluminas

sintered under conventional conditions nor the two-step sintering of pure alumina without doping alone results

in entire grain growth suppression in the fi nal stage of sintering. A combination of the two-step heating regime

with suitable doping (500 ppm MgO added in the form of a MgAl

O

nanopowder) leads to complete grain

growth suppression at relative densities above 99%.

The authors gratefully acknowledge the funding provided by the Slovak national grant agency VEGA under the Contract numbers 2/0058/14, and the grant APVV 0108-12. This publication was created in the frame of the project “Centre of

Abstracts of SINTERCER Workshop on

Nanomaterials Sintering with the Use of SPS and

HP-HT Techniques, and SINTERCER Seminar on

Infl uence of Advanced Techniques of Sintering

on Microstructure and Physico-mechanical

Properties of Nanomaterials,

Zakopane, September 17

th

_{, 2015}

excellence for ceramics, glass, and silicate materials” ITMS code 262 201 20056, based on the Operational Program Research and Development funded from the European Regional Development Fund.

Keywords: Two-stage sintering, Alumina, Doping, Microstructure, MgAl2O4 nanopowder

MAGNETOELECTRIC EFFECT IN COMPOSITES BASED ON SINGLE-CRYSTALLINE

PIEZOELECTRICS

Nikolai A. Sobolev

_{*, Julien V. Vidal}

_{, Andrey A. Timopheev}

_{, Andrei L. Kholkin}

_{, Ilya V. Kubasov}

_,

Svetlana P. Kobeleva

_{, Mikhail D. Malinkovich}

1_{Universidade de Aveiro, Departamento de Física & I3N, 3810-193 Aveiro, Portugal} 2_{National University of Science and Technology “MISIS”, 119049 Moscow, Russia}

3_{Universidade de Aveiro, Departamento de Engenharia de Materiais e Cerâmica & CICECO, 3810-193 Aveiro, Portugal}

*e-mail: sobolev@ua.pt

We present studies of the anisotropic direct magnetoelectric (ME) properties of tri- and bilayered 2-2 type composites fea-turing Metglas and single-crystalline piezoelectric plates of LiNbO3 (LNO), GaPO4 (GPO) and LiTaO3 (LTO) possessing

unidomain or bidomain structure with opposite polarization vectors (i.e. a head-to-head or tail-to-tail domain structure). The possibility of generating large direct ME voltage coeffi cients at electromechanical (EM) resonance in LNO based ME laminate systems is reported. The crystalline LNO and GPO are identifi ed as strong candidates to form their own fi eld of ME-based lead-free linear low-cost high-temperature magnetic-fi eld sensors.

Numerical and experimental studies of the anisotropy of the ME response in composites based on diff erently oriented piezocrystals has been carried out. The transversal ME eff ects were found to be strongly dependent on the direction of the applied in-plane magnetic bias fi elds as well as the orientation of the piezocrystals. The precise control of the latter should allow one to engineer almost any desired quasi-static and resonant ME anisotropic properties for any specifi c application.

The bidomain systems behave as bimorph piezoelectrics. Large ME eff ects were obtained in resonances correspon-ding to low-frequency bencorrespon-ding modes of vibration. The contour modes were suppressed in the bidomain systems, whe-reas the bending modes were enhanced. Bidomain-based laminates may be useful e.g. in the fabrication of ME vector magnetic fi eld sensors operating at low frequencies in the bending mode.

The work has been supported by FCT of Portugal through the projects UID/CTM/50025/2013, RECI/FIS-NAN/0183/2012 (FCOMP-01-0124-FEDER-027479), SFRH/BPD/74086/2010 and SFRH/BD/89097/2012, as well as by NUST „MI-SiS” through grant no. K3-2015-003.

Keywords: Magnetoelectric (ME) properties, LiNbO3 (LNO), GaPO4 (GPO), Laminate system, Bimorph piezoelectrics

INFLUENCE OF SYNTHESIS CONDITIONS ON FORMATION OF CORE-SHELL

TITANATE-FERRITE PARTICLES AND PROCESSING OF COMPOSITE CERAMICS

Bojana Mojić-Lanté

_{, Liliana Mitoseriu}

_{, Paula Vilarinho}

_{, Vladimir V. Srdić}

_*

1_{Department of Materials Engineering, Faculty of Technology, University of Novi Sad, Serbia} 2_{Department of Solid State & Theoretical Physics, Al. I. Cuza University, Iasi, Romania} 3_{Department of Materials and Ceramics Engineering, University of Aveiro, Portugal}

*e-mail: srdicvv@uns.ac.rs

Titanate-ferrite (SrTiO3-NiFe2O4 and BaTiO3-NiFe2O4) composite and core-shell particles have been prepared by

the combination of sol-gel and co-precipitation route. Modifi cations of the synthesis procedure (pH value, functionaliza-tion of core and shell particles) have been carried out in order to maximize the electrostatic forces between the phases and, thus, optimize the core-shell morphology of the synthesized powders. It has been found that the synthesis pH has considerable infl uence on the morphology of the as-synthesized particles and their phase composition. The prepared titanate-ferrite powders were processed into dense composite ceramics by conventional sintering in air and/or by spark

plasma sintering. One of the main goals was to fi nd the link between particle morphology and composite microstructure, optimize the sintering regime and conduct the functional characterization of the obtained ceramics with diff erent phase mass ratio. Regarding the sintering regime of the obtained powders, combination of low-temperature conventional and spark plasma sintering has given the best results in terms of achieving adequate composite density, phase preservation and homogeneous phase distribution. Functional characterization of the sintered ceramics with diff erent phase mass ratio confi rmed the expected dielectric, ferroelectric and ferromagnetic behaviour of the obtained titanate-ferrite composites. Keywords: Core-shell particle, Spark plasma sintering (SPS), Titanate-ferrite composite, Electric properties, Magnetic properties

PROCESSING OF HYDROXYAPATITE OBTAINED BY COMBUSTION

Maria Canillas

_{, Rebeca Rivero}

_{, Raul Garcia-Carrodeguas}

_{, Flora Barba}

_{, Miguel A. Rodriguez}

_*

1_{Instituto de Cerámica y Vidrio (CSIC), C/ Kelsen 5, Madrid, Spain}

2_{Instituto Nacional del Carbón (CSIC), C/ Francisco Pintado Fe 26, Oviedo, Spain} 3_{CERTBIO. U. Federal Campina Grande, Campina Grande, Brasil}

*e-mail: mar@icv.csic.es

The synthesis of hydroxyapatite by combustion and its following processing in order to obtain dense bodies with improved mechanical resistance have been studied in the present work. Hydroxyapatite was synthesized by combustion method in order to obtain nanostructured powders. These powders were densifi ed by thermal treatment with controlled sintering rate with the aim to avoid trapped porosity and minimizing grain growth. It was carried out in order to obtain an improve-ment of mechanical properties in the bulks products.

Combustion synthesis was carrying out in two diff erent media, aqueous and oxidizing media. Oxidizing media improves homogenization for synthesis and increase the energy of the reaction. Combustion products were characterized by XRD and specifi c surface area and particle size distribution were measured. After densifi cation, bulk materials were charac-terized by SEM and their mechanical properties evaluated by diametral compression and Weibull modulus statistic was applied.

Keywords: Hydroxyapatite, Nanopowder, Combustion synthesis, Microstructure, Mechanical properties

SPS SINTERING OF SILICON NITRIDE-CNT

Csaba Balázsi

Bay Zoltán Nonprofi t Ltd. for Applied Research Fehérvári út 130, H-1116 Budapest e-mail: csaba.balazsi@bayzoltan.hu

In the present work we examined the validity of a novel technique, namely spark plasma sintering (SPS) in consolidating and tailoring the microstructure of multiwall carbon nanotube (MWNT) reinforced silicon nitride-based ceramic compos-ites. SPS makes it possible to prepare fully densifi ed composites at comparatively lower temperature with substantial short holding. It also provides a means of precious modifi cation of the kinetics of densifi cation, reactions and grain growth that are involved in an entire sintering cycle. SPS has been applied with success to a wide range of ceramics (oxides, nitrides, carbides and composites). The SPS method is comparable to the conventional hot pressing process, where the precursor powders are loaded in a die and a uni-axial pressure is applied during the sintering. However, instead of using an external heating source, a current, which is typically a few thousands of ampers (and a few volts) can pass through the graphite die, the sample or both. Conduction along the die it represents basically resistance heating, i.e. the die also acts as a heating source. Conduction through the sample may generate breakdown, arcing, spark or plasma among powder parti-cles that induce a fast densifi cation process. By using the SPS method the densifi cation of samples without considerable grain growth process can be achieved within few minutes. In this article this new sintering method and the conventional hot isostatic pressing (HIP) was also used for composite elaboration. Morphological, structural, compositional observa-tions, as well as characterization of mechanical properties will be presented.

SPARK PLASMA SINTERING AND TEXTURING OF THERMOELECTRIC MATERIALS

FOR ENERGY CONVERSION

Jacques G. Noudem

1_{CRISMAT-CNRS UMR 6508, ENSICAEN 6 boulevard Maréchal Juin, 14050 Caen Cedex 04, France} 2_{LUSAC EA-4253, Université de Caen Basse-Normandie, rue Aragon, 50130 Cherbourg, France}

e-mail:jacques.noudem@ensicaen.fr

The pressing device in Spark Plasma Sintering (SPS) equipment was modifi ed with the aim of obtaining the n-type and

p-type oxide and inter-metallic thermoelectric bulk as Ca1-xSmxMnO3, Ca3Co4O9, Bi2Te2,7Se0,3, Bi0,5Sb1,5Te3, respectively. The new process is referred to as “Spark Plasma Texturing” (SPT). During SPT, the bulk material can freely deform. As a result, inter-grain preferential crystallographic orientation is created. Materials processed by conventional SPS are usually quasi-isotropic.

In this work, neutron diff raction measurements were used to investigate the bulk texture of the resulting materials. Their thermoelectric properties were assessed at low and high temperatures in the in-plane and out-of-plane confi gurations. In addition, anisotropy of mechanical and thermal properties was evidenced on thick specimens for use as thermoelements. Keywords: Anisotropy, Bulk texture, Spark plasma texturing (SPT), Thermoelectric materials, Thermoelectric properties

PREPARATION BY POLYMERIZED- GEL MEDIATED SYNTHESIS AND

CHARACTERIZATION OF La

Sr

Fe

Co

O

NANOCERAMICS FOR CATHODES OF

INTERMEDIATE TEMPERATURE SOLID OXIDE FUEL CELLS (IT-SOFC)

José Manuel Pérez-Falcón, Domingo Pérez Coll, Jesus Tartaj*

Electroceramics Department, Instituto de Cerámica y Vidrio (CSIC), Kelsen 5, 28049 Madrid, Spain *e-mail: jtartaj@icv.csic.es

Solid oxide fuel cells (SOFCs) are energy conversion devices that produce electricity with high effi ciency and negligi-ble pollution. Yttria stabilized zirconia with 8 mol% Y₂O₃ represents the state of the art electrolyte for high temperature (800–1000 ºC) SOFCs. However, many problems related to the correct use of the other components of the SOFC are to be present during service at the above operating conditions. Therefore, a lower operating temperature becomes neces-sary to reduce manufacturing costs and to increase their durability. For that purpose new materials are required demand for materials that can effi ciently operate at lower temperature, i.e. electrolytes with higher conductivity such as Gd-doped ceria or La(Sr)Ga(Mg)O₃, and also is crucial the development of more eff ective cathode materials with increased electro-catalytic activity in compare with the perovskite-type materials based on lanthanum manganites. La_1-xSrxFeO3 (LSF) has

demonstrated higher electrical and ionic conductivity than La_1-xSrxMnO3 (LSM). In this same way, La1-xSrxCoO3 (LSC) can

be considered as a promising cathode for intermediate temperature SOFCs with high values of conductivity. However, the good electrocatalytic performance of this cobaltbasedcathode is somehow limited by a thermal expansion coeffi -cient (TEC) mismatch with other components of the SOFC. TECs of both the electrolyte and electrode layers should be well matched to ensure long-term operational stability of the SOFCs. Alternative materials are ferrite/cobaltite cathodes. The electronic conductivity of these samples in air is characterized by the higher values at increasing Co contents and besides, the presence of Fe helps to minimize mechanical stresses as it can reduce the TEC mismatch. Here we show that a synthetic route based on the auto-combustion of an ethylene glycol-metal nitrate polymerized gel precursor can be effi ciently used to easily produce a range of La_0.6Sr_0.4Fe_1-yCoyO3-x (LSFC) nanoceramics at moderate temperatures. We

have therefore been able to determine on air-sintered samples how the electronic conductivity and thermal expansion coefi cients change with the Fe, Co composition. The results are important not only for the synthetic route itself but also because they establish practical cathode operational parameters on sintered samples for the selection of the most suita-ble composition, considering the TECs of the electrolyte utilized and the operating temperature of the cell.

Keywords: Cathode, IT-SOFC, La_0.6Sr_0.4Fe_1-yCoyO3-x Nanoceramics, Polymerized-gel mediated synthesis, Thermal

MICRO-MECHANICAL TESTING OF WC, Si

N

AND ZrB

CERAMICS -

NANOINDENTATION AND MICROPILLAR TESTING

Ján Dusza

Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, Košice, Slovakia e-mail: jdusza@imr.saske.sk

The orientation dependence of hardness, reduced modulus and compressive strength of diff erently orientated WC grains/ crystals in WC-Co, Si3N4 grains/crystals in the reaction bonded Si3N4 system and ZrB2 grains/crystals in ZrB2 polycrystal

was investigated. Depth-sensing indentation and micro-compression tests of micropillars prepared by focused ion beam from oriented facets of grains were studied. Electron backscatter diff raction (EBSD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were performed to determine the grain orientation and to study the surface morphology and the resulting deformation and damage mechanisms around indents and in micropillars.

The hardness and of the diff erently orientated grains showed signifi cant angle dependence from the basal towards the prismatic directions. A strong infl uence of the grains orientation on yield stress and rupture stress values was also found. The active slip systems for individual ceramics have been recognized.

Keywords: Nanoindentation, Micropillar Testing, Si3N4, ZrB2, WC

SUPERHARD NANOCRYSTALLINE COATINGS: PROCESSING AND

CHARACTERIZATION

František Lofaj*, Lenka Kvetková, Petra Hviščová

Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, Košice, Slovakia *e-mail: fl ofaj@saske.sk

Nanocomposite structure consisting of 10-20 nm crystallites of one phase embedded in thin amorphous matrix of another phase obtained by spinodal decomposition during vacuum deposition suggested by Veprek two decades ago assumes dramatic increase of hardness of the resulting coatings. The evolution of magnetron sputtering based deposition tech-niques, which are suitable for the formation of nanocomposite structures, occurred toward the increase of the level of ionization of the sputtered material from conventional DC and RF magnetron sputtering toward modulated pulse power (MPP) and high power impulse magnetron sputtering (HiPIMS). Recently, a novel technique, high target utilization sput-tering (HiTUS) appeared which is diff erent from magnetron sputsput-tering. It has an independent plasma source off ering high plasma density, higher target utilisation and better control of the deposition conditions. However, the potential of the latest technologies in hard nanocomposite coatings is still mostly unknown. Therefore, the objectives of the work is to

investigate the possibilities of HiPIMS and HiTUS technology in the control of the mechanical and tribological properties of W-C based coatings by means of the modifi cation of the processing parameters. The work includes investigations of the infl uence of standard parameters such as total power and substrate bias and acetylene content on the changes in hardness, elastic modulus, strength and friction behaviour of the coatings and the obtained results are compared with the properties of the conventional DC sputtered W-C coatings.

Keywords: High power impulse magnetron sputtering (HiPIMS), High target utilization sputtering (HiTUS), Nanocrystalli-ne Coating, Mechanical properties, W-C coating

TRIBOLOGICAL PROPERTIES OF SILICON NITRIDE BASED COMPOSITES WITH

GRAPHENE AND BORON NITRIDE NANOPARTICLES

Pavol Hvizdoš*, Ján Balko, Alexandra Kovalčíková, Martin Fides

Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04001 Košice, Slovakia *e-mail: phvizdos@saske.sk

Nanocomposites with silicon nitride matrix mixed with various amounts of graphene or hexagonal boron nitride nanopar-ticles, respectively, were prepared by hot pressing. Their microstructure and mechanical properties were studied. Wear and friction was tested at dry conditions in a range of loads by the standard self-mated ball-on-disc method. Microme-chanisms of wear damage as well as wear regimes were identifi ed and related to the microstructure, type and amount of nanofi ller, and to the loading conditions.

It has been found that both carbon and hBN fi ller phases make densifi cation more diffi cult, leading to higher porosity. In all materials frequent stacks of several platelets are present. Graphene platelets were well integrated in the microstruc-ture, improved indentation fracture toughness.

Monolithic silicon nitride had at higher temperatures reduced friction coeffi cient due to formation of tribofi lm, tri-bochemical reaction created oxide particles. Carbon phases did not participate in lubricating process. Signifi cant graphite transfer fi lm was not observed. However, improved fracture toughness by graphene did lead to better wear resistance at temperatures up to 300 °C.

Wear resistance at and above 500 °C signifi cantly decreased, especially for porous materials.

Financial support from projects VEGA 2/0075/13 and APVV-0108-12 is gratefully acknowledged. Keywords: Graphene, hBN, Microstructure, Mechanical properties, Si3N4

TITANIUM NITRIDE – TITANIUM DIBORIDE MATERIALS OBTAINED

BY THE SPS METHOD

Piotr Wyżga

_{*, Lucyna Jaworska}

_{, Jolanta Laszkiewicz-Łukasik}

_{, Jerzy Morgiel}

1_{The Institute of Advanced Manufacturing Technology, Centre for Materials Research and Sintering Technology, Wrocławska}

37a st., 30-011 Kraków, Poland

2_{Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta st., 30-059 Kraków, Poland}

*e-mail: piotr.wyzga@ios.krakow.pl

Titanium nitrides and borides are important for engineering applications due to their exceptional properties such as oxida-tion resistance, high melting point and high hardness. In addioxida-tion, they are characterized by good electrical conductivity,

therefore

reason of poor sinterability and thus it is diffi cult to obtain highly densifi ed sample using conventional methods.

In the presented work the two kinds of powdered micro- and nanosized titanium nitride - titanium diboride com-posite material were used for spark plasma sintering (SPS). The fi rst starting material was the mixture consisted of TiN (0,8-1,2 μm) powder with an addition of 30 wt.% TiB2 (2,5-3,5 μm) powder, prepared using planetary ball milling.

The second one had the same quantitative composition but it was TiN-TiB2 (30-40 nm) composite nanopowder obtained

in-situ by plasmachemical synthesis. The study aimed to compare the properties of composites sintered from micro- and nanopowders. Vickers hardness and Young’s modulus were the criteria of thesintering process optimization.

Composites sintered from micropowders at 1600 °C for 5 min had the best properties while the optimal sintering tem-perature and duration for the nanopowders was 2200 °C and 3 min, respectively. In the case of nanopowders, duration of the sintering process had strong infl uence on the phase composition of materials.

The results of the phase composition analysis carried out by X-ray diff raction method. Microstructure observations were realized using TEM and SEM techniques. Materials obtained from nano- and micropowders after the SPS sintering process showed the favorable compressive stress state in the surface layer.