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The processing, microstructure, and material properties of monolithic Si3N4 were investigated by using two amorphous Si3N4 nanopowders doped with (1) 6 wt% Y2O3 and (2) 6 wt% Y2O3 + 8 wt% Al2O3. The orthorhombic Si2N2O-based phase was found in the two as-sintered bulks. Carbothermal reduction treatment (CRT) was thus applied at 1400 °C for 10 h with 3 wt% and 6 wt% carbon black added to the precursor powders of the Si-Y-O-N and Si-Y-Al-O-N systems, respectively, resulting in an increased nitrogen-to-oxygen (N:O) ratio and elimination of Si2N2O within the sintered bulks. The possible mechanisms of nucleation and grain growth of Si3N4 are discussed during CRT and the sintering process. The best wear resistance was achieved in ultrafine Si3N4 doped with Y2O3, which has good hardness, indentation toughness and a protective tribo-film. [ABSTRACT FROM AUTHOR]

Abstract: Nanocomposites of Cr3C2/Al2O3 were prepared by metal-organic chemical vapor deposition (MOCVD) method in a fluidized bed and sintered under hot-pressed condition at 1500°C for1h. The TEM microstructure showed that the Al2O3 matrix contained both intragranular and intergranular phase of Cr3C2. The hot-pressed Cr3C2/Al2O3 nanocomposites possessed better mechanical performance as regard to bending strength and fracture toughness in comparison to the monolithic Al2O3. The phenomenal change of the fracture mode from intergranular fracture of monolithic Al2O3 to transgranular fracture of nanocomposites is attributed to the enhancement of the residual stress and formation of a solid solution, eventually improving the strength of the Cr3C2/Al2O3 nanocomposite. Furthermore, it has been deciphered that the toughening mechanisms of the nanocomposites contribute to the crack deflection and the residual stress. [Copyright &y& Elsevier]

We report the enhanced mechanical properties of WC-reinforced Al 2 O 3 nano composites fabricated by spark plasma sintering (SPS). The addition of WC accelerates the sintering process and helps in improving the mechanical properties of Al 2 O 3 matrix by hindering its grain growth. The resulting microstructure shows uniformity with a grain size of 300–500 nm. Due to the refined microstructure of composites, the hardness and fracture toughness values are found to increase with the decrease of the Al 2 O 3 grain size. In addition, toughening mechanism of this composite also has been discussed in this study. [ABSTRACT FROM AUTHOR]

Abstract: We report the synthesis of a new class of Al2O3–WC nanocomposites for the first time by using metal–organic chemical vapor deposition process in a spouted bed followed by spark plasma sintering technique. The microstructure and mechanical properties of these prepared nanocomposites have been analyzed for various sintering parameters. From microstructure observation, it is found that the nanosized WC particles are dispersed within alumina matrix grains and intergrains. The microstructure of transgranular and step-wise fracture surface are found in these nanocomposites. The basic mechanical properties like density, hardness, and toughness also have been analyzed and the results are interpreted by correlating with that of corresponding microstructures. [Copyright &y& Elsevier]

Abstract: We proposed a novel approach to investigate the three-dimensional microstructures and sintering behaviors of Si3N4-based ceramic nanocomposites by electrochemical impedance spectroscopy. Si3N4/TiC and Si3N4/TiN with various weight percentages of conductive phases were prepared by spark plasma sintering (SPS) at different temperatures and dwell times. The incorporation of TiC and TiN into β-Si3N4 provides pulse current paths inside the ceramics due to their higher conductivity. These paths enable the localized Joule heating and mass transport, facilitating the densification and grain growth of ceramic compact. The electrochemical study of such nanocomposites has revealed three-dimensional information of the evolution of their microstructures, and the capacitive and resistive characteristics of the nanocomposites reflect the densification, grain growth, and element distribution in the compact. The impedance model presented in this work suggests isolated distribution of TiN in Si3N4 while Si3N4/TiC of the same amount of additives at the same sintering conditions formed conductive network. This impedance analysis further explained the differences in densification mechanism of SPS in Si3N4/TiN and Si3N4/TiC. [Copyright &y& Elsevier]

Abstract: The fine grains of Al2O3–Cr2O3/Cr-carbide nanocomposites were prepared by employing recently developed spark plasma sintering (SPS) technique. The initial materials were fabricated by a metal organic chemical vapor deposition (MOCVD) process, in which Cr(CO)6 was used as a precursor and Al2O3 powders as matrix in a spouted chamber. The basic mechanical properties like hardness, fracture strength and toughness, and the nanoindentation characterization of nanocomposites such as Elastics modulus (E), elastic work (W e ) and plastic work (W p ) were analyzed. The microstructure of dislocation, transgranular and step-wise fracture surface were observed in the nanocomposites. The nanocomposites show fracture toughness of (4.8MPam1/2) and facture strength (780MPa), which is higher than monolithic alumina. The strengthening mechanism from the secondary phase and solid solution are also discussed in the present work. Nanoindentation characterization further illustrates the strengthening of nanocomposites. [Copyright &y& Elsevier]

Abstract: The densification behaviors of Al2O3–Cr2O3/Cr3C2 nanocomposites prepared by a Spark Plasma Sintering (SPS) were investigated in this work. The initial powders used for sintering were Al2O3–Cr2O3, which were prepared by metal organic chemical vapor deposition (MOCVD) in a spout bed. Different colors of the compacts such as green, purple and black were observed after densification process at different SPS temperatures from 1200°C to 1350°C. These changes of color were relevant to the existence of secondary phase of green Cr2O3, pink solid solution of Cr2O3–Al2O3 and black Cr3C2, which were formed under the different SPS temperature. The secondary phase of Cr2O3 retarded the processing of densification for spark plasma sintering at 1200°C. The Cr2O3 reacted with Al2O3 to form solid solution of Cr2O3–Al2O3 and with carbon to form Cr3C2 as sintering temperature increased to 1350°C. The characteristics of high heating rate, shorter sintering time for SPS and the formation of secondary phase of Cr3C2 effectively reduced the substrate''s grain growth, making Al2O3–Cr2O3/Cr3C2 nanocomposites with small grain size. [Copyright &y& Elsevier]

Abstract: Fully dense β-Si3N4 based nanoceramics were consolidated by spark plasma sintering (SPS). A commercially available β-Si3N4 nano-powder was used as starting material. The sintering bulks were fabricated with a heating rate of 90°C/min by varying SPS temperature from 1550°C to 1700°C, and the linear shrinkage was used to evaluate the sintering behavior of the nano-powder. The microstructures of the developed Si3N4 based ceramics were achieved, and the grain length, grain width, and aspect ratio for these sintering bulks were found to increase gradually with elevating sintering temperature. The hardness and fracture toughness are associated with the microstructural characteristics. The crack propagation and fracture behavior for these β-Si3N4 based nanoceramics have been observed for the specimens sintered at different sintering temperatures. Crack deflection is found to be one of the toughening mechanisms for these β-Si3N4 based nanoceramics. [ABSTRACT FROM AUTHOR]

Abstract: We report our novel result regarding the observation of unusual but clean ferromagnetic (FM) signature at room temperature (RT) in high pure (99.999%) ZnO bulk ceramics processed by slow step sintering schedule (SSSS). All the sintered samples showed ferromagnetic signature at room temperature. However, sample sintered at 850°C showed more prominent M–H loop and saturation magnetization with respect to samples sintered at 500°C and 1300°C. Observation of FM behavior in sintered ceramics may be attributed to the presence of defects like cation (Zn)/anion (O) vacancies and interstitials created during the processing of sample. It is anticipated that SSSS promotes enhanced physical bulk as well as surface densification and at the same time confined the outgassing of free oxygen and Zn from the bulk which are acting as suitable defect sites inside the sintered grains and grain boundary junctions. Our micro-Raman and X-ray photoelectron studies revealed the existence of these defects which are believed to be the origin of the unconventional ferromagnetism (FM) of ZnO bulk ceramic at room temperature. [ABSTRACT FROM AUTHOR]

Abstract: TiN/Si3N4 nanocomposites were fabricated from β-Si3N4 and TiN nano powders by powder processing routes. The specimens were consolidated at 1600°C for 3min by spark plasma sintering, and nearly full densification was obtained in the resulting sintered bulk composites. The TiN phase grows rapidly and a typical twin structure of TiN can be observed in the composites. The grain size and distribution for TiN may affect the electrical resistivity, leading to a pulse current through the sintering compact. The resulting microstructure for the composite containing 10wt% TiN reveals that grain coarsening behavior for β-Si3N4 based grains is accelerated, and the composite has the highest toughness of 4.9MPam1/2 among all these composites. The other composites show nanosized β-Si3N4 based grains with a fracture toughness of nearly 4.2MPam1/2. [ABSTRACT FROM AUTHOR]

Abstract: Binary phase TiC/Ti3SiC2 composites have been synthesized by reactive hot-pressed sintering (HPS) with the aim of developing a new hard product. Raw powders of TiC, Ti and Si with compositions 2TiC/1Ti/1Si (2:1:1) and 2TiC/1Ti/1.1Si (2:1:1.1) have been used as the starting materials for the synthesis. The phase content and microstructure of synthesized composites have been analyzed using X-ray diffractometer (XRD) and scanning electron microscope (SEM). The mechanical properties such as four-point flexural strength and fracture toughness have been investigated for different processing parameters. The phase content (Ti3SiC2:TiC) of the composite synthesized from (2:1:1) powder vary from (81:19) to (71:29) with the hot pressing temperature increased from 1500 to 1700°C. This sample exhibits maximum flexural strength of 627MPa and fracture toughness of 6.84MPam1/2. The maximum apparent density is found to be 4.65g/cm3 for this sample at optimum hot pressing temperature of 1500°C. The composite synthesized from (2:1:1.1) composition shows improvement in the mechanical properties compared to 2:1:1 composition. The relationship between the phase content and mechanical properties has been investigated. [Copyright &y& Elsevier]

Abstract: Porous aluminum oxide (Al2O3) preforms were formed by sintering in air at 1200°C for 2h. A356, 6061, and 1050 aluminum alloys were infiltrated into the preforms by squeeze casting in order to fabricate Al2O3/A356, Al2O3/6061, and Al2O3/1050 composites, respectively, with different volumes of aluminum alloy content. The content of aluminum alloy in the composites was 10–40% by volume. The resistivity of Al2O3/A356, Al2O3/6061, and Al2O3/1050 composites decreased dramatically from 6.41×1012 to 9.77×10−4, 7.28×10−4, and 6.24×10−4 Ωm, respectively, the four-points bending strength increased from 397 to 443, 435.1, 407.2MPa, respectively, and the deviations were smaller than 2%. From SEM microstructural analysis and TEM bright field images, the pore volume fraction and the relative density of the composites were the most important factors that affected the physical and mechanical properties. The ceramic phase and alloy phase in Al2O3/aluminum alloy composites were found to be homogenized and uniformly distributed using electrical and mechanical properties analysis, microstructure analysis, and image analysis. [Copyright &y& Elsevier]

Abstract: Sintering behaviors and microstructural characteristics in solid solutions of Ba[Mg1/3(Nb x/4Ta(4−x)/4)2/3]O3 (BMN x T4−x , x =0, 1, 2, 3 and 4) were investigated by X-ray diffraction, SEM and TEM. Microwave dielectric properties, such as the relative permittivity (ɛ r), quality factor (Q) value and temperature coefficient of resonator frequency (τ f), were also measured. The excellent microwave dielectric property of Ba(Mg1/3Ta2/3)O3 (BMT) sample imply the necessity to sinter at higher temperature (1650°C) and to use longer soaking times (9h), but not for Ba(Mg1/3Ta2/3)O3 (BMN). The 1:2 B-site ordering was maintained at all Nb substitution contents and the 1:2 B-site ordering existed in the grains with antiphase domain boundaries (APBs). The Ba[Mg1/3(Nb1/4Ta3/4)2/3]O3 specimen exhibited excellent microwave dielectric properties, ɛ r =25.534, Qf =140666GHz, and τ f =4.8(ppm/°C). The excellent microwave dielectric property is due to the improvement of sintering property by appropriate Nb atoms substitution in the BMT matrix and the maintaining of 1:2 ordering in the BMN x T4−x series. [Copyright &y& Elsevier]

Abstract: We have developed a simple low-cost novel synthesis route to synthesize nanosized La0.7Sr0.3MnO3 (LSMO) without any requirement of a calcinations step at high temperature. The study of these nanoparticles indicates excellent properties (magnetization and resistance at 0T) similar to colossal magnetoresistance (CMR) materials sintered at 1600°C for 20h. The size and topography of the oxides are dependent on the calcinating temperature of the precursors. The resulting particle size is in the range of 20–100nm as determined by transmission electron microscopy. The magnetization of LSMO nanophase is strikingly dependent on the particle size. The transition temperature from the paramagnetic to ferromagnetic state of the sample is quite sharp and occurs at 372K for 600°C calcined sample sintered at 1200°C for 4h. Magnetoresistance of magnitude −5.7% at the field up to 1T was also observed at room temperature. [Copyright &y& Elsevier]