Structural, thermal and mechanical properties of aluminum nitride ceramics with CeO₂ as a sintering aid

Choi, H.S.,Im, H.N.,Kim, Y.M.,Chavan, A.,Song, S.J. Ceramurgica ; Elsevier Science Ltd 2016 CERAMICS INTERNATIONAL Vol.42 No.10

AlN ceramics have been prepared with CeO<SUB>2</SUB> as a sintering aid at a sintering temperature of 1900<SUP>o</SUP>C. The effect of CeO<SUB>2</SUB> contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO<SUB>2</SUB> exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO<SUB>2</SUB> content and was greater than that of Y<SUB>2</SUB>O<SUB>3</SUB>-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50wt% of CeO<SUB>2</SUB> had the highest relative density of 99.94%, thermal conductivity of 156Wm<SUP>-1</SUP>K<SUP>-1</SUP> and hardness of 72.46kg/mm<SUP>2</SUP>.

Mechanochemical synthesis and fast consolidation of a nanostructured CoTi-ZrO₂ composite by high-frequency induction heating

Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.11

CoTi and ZrO<SUB>2</SUB> nanopowders were mechanochemically synthesized from CoTiO<SUB>3</SUB> and Zr powders according to the reaction (2CoTiO<SUB>3</SUB>+3Zr→2CoTi+3ZrO<SUB>2</SUB>). The milled powders were then consolidated using high frequency induction heated sintering (HFIHS) within one minute under the applied pressure of 80MPa. The average hardness and fracture toughness of the nanostructured 2CoTi-3ZrO<SUB>2</SUB> composite sintered from high energy ball milled powder were 1205kg/mm<SUP>2</SUP> and 6.9MPam<SUP>½</SUP>, respectively. The mechanical properties of composite were compared with those of monolithic CoTi and ZrO<SUB>2</SUB>, respectively.

Simultaneous synthesis and consolidation of nanocrystalline Al-TiC composite by high-frequency induction heating

Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.13

Al-TiC nanopowders from the mixture of Ti and Al<SUB>4</SUB>C<SUB>3</SUB> were made by high energy ball milling. Highly dense nanostructured 4Al-3TiC composite was simultaneously synthesized and consolidated by high-frequency induction heating within one minute from the mechanically activated powders. The microstructure and mechanical properties (hardness and fracture toughness) were investigated using FE-SEM, XRD and Vickers hardness tester.

Effect of GDC interlayer thickness on durability of solid oxide fuel cell cathode

Khan, M.Z.,Mehran, M.T.,Song, R.H.,Lee, J.W.,Lee, S.B.,Lim, T.H.,Park, S.J. Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.6

Long-term performance degradation of solid oxide fuel cell (SOFC) cathode as a function of gadolinium doped ceria (GDC) interlayer thickness has been studied under accelerated operating conditions. For this purpose, SOFC half-cells with GDC interlayer thicknesses of 2.4, 3.4 and 6.0@?m were fabricated and tested for 1000h at 900<SUP>o</SUP>C under constant current density of 1A/cm<SUP>2</SUP>. The half-cells consisted of lanthanum strontium cobalt ferrite (LSCF)/GDC composite cathode, GDC interlayer, scandia-ceria stabilized zirconia electrolyte and platinum anode as a counter electrode. Area specific resistance (ASR) of the half-cells was continuously measured over time. Higher increase in ASR was observed for the half-cells with GDC interlayer thickness of 2.4 and 6.0@?m, which is attributed to higher strontium (Sr) diffusion towards electrolyte and to cathode/GDC interface delamination coupled with small Sr diffusion, respectively. However, half-cell with GDC interlayer thickness of 3.4@?m showed smaller degradation rate due to highly dense GDC interlayer which had less interfacial resistance and suppressed Sr diffusion towards electrolyte.

Application of sonochemical processing to LSC(La_0.6Sr_0.4CoO₃)/SDC(Sm₂O₃-doped CeO₂) composite cathodes for solid oxide fuel cells involving CeO₂-based electrolytes

Ko, M.H.,Hwang, J.H. Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.10

Nanocrystalline Sm<SUB>2</SUB>O<SUB>3</SUB>-Doped CeO<SUB>2</SUB> (SDC) powders were synthesized in a single- and two-phase material system by using sonochemical processing with high-frequency agitation. The synthesized SDC nanocrystalline powders were used to coat the mixed-conducting La<SUB>0.6</SUB>Sr<SUB>0.4</SUB>CoO<SUB>3</SUB> (LSC) cathode materials. The combined synthesis processing allows the artificial coating of the LSC materials with the ionic-conducting SDC electrolyte. The electrochemical polarizations of the SDC/LSC composites are characterized using a geometrically constricted contact between the ionic probe and the SDC/LSC composites. The lowest cathode polarization was obtained for the LSC (85wt%)/SDC (15wt%). The lowest electrode polarization is believed to result from the high density of the triple-phase boundaries when the constituent phases are interconnected in a 3-dimensional manner.

Comparative study on storing energy for (Ba,Zr)TiO₃ and CuO-(Ba,Zr)TiO₃ ceramics for piezoelectric energy harvesting applications

Shin, D.J.,Koh, J.H. Ceramurgica ; Elsevier Science Ltd 2017 Ceramics international Vol.43 No.suppl1

The environmental regulations for lead-based materials have resulted in the piezoelectric energy harvester based on lead-free piezoelectric ceramics attracting increased attention. In this work, two different types of piezoelectric materials, Ba(Zr<SUB>0.04</SUB>Ti<SUB>0.96</SUB>)O<SUB>3</SUB> and CuO-Ba(Zr<SUB>0.04</SUB>Ti<SUB>0.96</SUB>)O<SUB>3</SUB> ceramics, have been employed for energy harvesters applications to replace lead-based energy harvesters. The BZT and CuO-BZT ceramics were prepared by the conventional solid solution method. The piezoelectric properties of CuO-BZT ceramics showed higher piezoelectric values (d<SUB>33</SUB> = 302pC/N and g<SUB>33</SUB> = 20.64mVm/N) than those of BZT ceramics: (d<SUB>33</SUB> = 268pC/N and g<SUB>33</SUB> = 19.11mVm/N). The BZT and CuO-BZT ceramics were stacked in a multilayered structure to form triple layers. The stored energy in the capacitors (3μF) for multilayered BZT and CuO-BZT were 1.72 and 2.67mJ/cm<SUP>3</SUP>, respectively. The energy harvesters based on the lead-free ceramics BZT and CuO-BZT were analyzed and compared.

Effect of SiO₂ content on the microstructure, mechanical and dielectric properties of Si₃N₄ ceramics

Lee, S.J.,Baek, S. Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.8

This study investigated the effect of SiO<SUB>2</SUB> content in the Y<SUB>2</SUB>O<SUB>3</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB> additive system on the microstructure, mechanical and dielectric properties of silicon nitride (Si<SUB>3</SUB>N<SUB>4</SUB>) ceramics. The total sintering additive content was fixed at 8wt% and the amount of SiO<SUB>2</SUB> was varied from 0 to 7wt%. The crystalline phases of the samples were determined by X-ray diffraction analysis. Complete α-to-β transformation of the Si<SUB>3</SUB>N<SUB>4</SUB> occurred during sintering of all of the samples, which indicated that the phase transformation was unaffected by the SiO<SUB>2</SUB> content. However, the microstructures showed that the aspect ratio of the β-Si<SUB>3</SUB>N<SUB>4</SUB> grains decreased and the residual porosity increased with increasing SiO<SUB>2</SUB> content. Additionally, the flexural strength and the dielectric constant decreased with increasing SiO<SUB>2</SUB> content because of the residual porosity and the formation of the Si<SUB>2</SUB>N<SUB>2</SUB>O phase via a reaction of SiO<SUB>2</SUB> with Si<SUB>3</SUB>N<SUB>4</SUB>.

Effects of Sc₂O₃ sintering aid for the densification and mechanical properties of SiC-ZrB₂ composites

Muksin,Yoon, D.H.,Raju, K. Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.6

This study examined the effects of a Sc<SUB>2</SUB>O<SUB>3</SUB> sintering aid on the density, microstructure and mechanical properties of SiC-5vol% ZrB<SUB>2</SUB> composites prepared by hot-pressing. Microstructural studies showed that the addition of Sc<SUB>2</SUB>O<SUB>3</SUB> not only caused a decrease in the hot-pressing temperature from 1950 to 1750<SUP>o</SUP>C by liquid phase sintering, but also resulted in the formation of crystalline Sc<SUB>4</SUB>Zr<SUB>3</SUB>O<SUB>12</SUB> at the grain boundaries via a reaction with ZrO<SUB>2</SUB> on the surface of the ZrB<SUB>2</SUB> powder. The addition of Sc<SUB>2</SUB>O<SUB>3</SUB> produced a fine-grained microstructure with a 43% (430→615MPa) and 20% (3.6→4.3MPam<SUP>½</SUP>) increase in flexural strength and fracture toughness, respectively, compared to the SiC-ZrB<SUB>2</SUB> composite without Sc<SUB>2</SUB>O<SUB>3</SUB>.

ZnO-capped nanorod gas sensors

Mirzaei, A.,Park, S.,Kheel, H.,Sun, G.J.,Lee, S.,Lee, C. Ceramurgica ; Elsevier Science Ltd 2016 Ceramics international Vol.42 No.5

This paper reports on the synthesis of pristine α-Fe<SUB>2</SUB>O<SUB>3</SUB> nanorods and Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO core-shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core-shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5-200ppm) at different temperatures (150-250<SUP>o</SUP>C). The XRD and SEM revealed the excellent crystallinity of the Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO core-shell nanorods, as well as their uniformity in terms of shape and size. The Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO core-shell nanorod sensor showed a stronger response to ethanol than the pristine Fe<SUB>2</SUB>O<SUB>3</SUB> nanorod sensor. The response (i.e., the relative change in electrical resistance R<SUB>a</SUB>/R<SUB>g</SUB>) of the core-shell nanorod sensor was 22.75 for 100ppm ethanol at 200<SUP>o</SUP>C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO core-shell nanorods was 15.96s, which was shorter than that of the pristine nanorod sensor (22.73s). The core-shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO core-shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe<SUB>2</SUB>O<SUB>3</SUB>-ZnO interface.

Optical absorption and XPS studies of (Ba_1-xSr_x)(Ce_0.75Zr_0.10Y_0.15)O_3-δ electrolytes for protonic ceramic fuel cells

Jaiswal, S.K.,Hong, J.,Yoon, K.J.,Son, J.W.,Lee, H.W.,Lee, J.H. Ceramurgica ; Elsevier Science Ltd 2016 CERAMICS INTERNATIONAL Vol.42 No.8

Perovskite-type materials containing metal and cerium cations have been used as catalysts, membranes, electrodes in fuel cells, and hydrogen pumps. The effect of strontium doping on the perovskite-type structure has been evaluated and effectively maintains the cubic structure of (Ba<SUB>1-x</SUB>Sr<SUB>x</SUB>)(Ce<SUB>0.75</SUB>Zr<SUB>0.10</SUB>Y<SUB>0.15</SUB>)O<SUB>3-δ</SUB>. X-ray photoelectron spectroscopy (XPS) was performed on the oxidation states of barium, strontium, cerium, zirconium, yttrium and oxygen. With strontium doping, the oxidation state of cerium ions in 4+ increases and results in the decrease in oxygen vacancy concentration. These are attributed to the strong binding energy of (A/B)-O bond which in turn, enhances the structural stability of electrolyte system. The free volume (V<SUB>f</SUB>) is maximum for x=0.2 (V<SUB>f</SUB>=33.1A<SUP>3</SUP>) and then decreases with further strontium insertion. The optical absorption peaks are attributed to charge transfer transitions and energy band gap (increases from ~4.13 to ~4.35eV). However, the increase of band gap is only ~5% with partial replacement of barium by strontium. These characteristics are useful to make (Ba<SUB>1-x</SUB>Sr<SUB>x</SUB>)(Ce<SUB>0.75</SUB>Zr<SUB>0.10</SUB>Y<SUB>0.15</SUB>)O<SUB>3-δ</SUB> a promising candidate for futuristic and reliable fuel cell technology.