Thermodynamics of the oxidation of ZrB2-TiB2, ZrB2-SiC and ZrB2-B4C ceramics

The thermodynamics of the oxidation of three-high temperature ZrB2-based ceramics (ZrB2-TiB2, ZrB2-SiC and ZrB2-B4C) has been studied in order to find the stability domain of zirconium diboride, in terms of temperature, partial pressure of oxygen and composition, in which it is protected against oxidation. In the case of the ZrB2-TiB2 binary system, a plot of log pO2vs. 1/T in the temperature range of 500-2000 K and another plot of pO2 (x1014) vs.xTiB2 for T=2000 K are made taking into account the two-extreme possibilities of no solubility and 100% solid solubility between ZrB2 and TiB2, respectively. A plot of log pCOvs. log pO2 is made for 1773 K for the systems ZrB2-SiC and ZrB2-B4C. It was found that the ZrB2-TiB2 ceramics does not have sufficient oxidation resistance in the temperature range of 500-2000 K. ZrB2 of ZrB2-SiC ceramics can be protected under 1 atmosphere oxygen or in air if the liquid borosilicate (with the chosen composition, 70% B2O3-30% SiO2), which is an intermediate product, provides a kinetic barrier to the continuation of oxidation by forming an impervious layer on the exposed surfaces. In contrast, the ZrB2-B4C ceramics does not produce the borosilicate upon oxidation. In view of the volatility of pure liquid B2O3, it is recommended that the ZrB2-B4C ceramics can be used at a lower temperature, perhaps below 1373 K, when the vapour pressure of B2O3 is significantly small

Synthesis and consolidation of ZrC based ceramics: a review

Zirconium carbide is an extremely hard ultra high temperature ceramics (UHTC), used in nose cone, leading edge of rocket and supersonic vehicle, jet engines, fuel components of high temperature nuclear reactors, cutting tools, etc. Because of strong kinetic limitations of sintering like high covalent bonding, oxygen impurities on particle surface and low diffusion rate, both high temperature and external pressure are essential for consolidation of ZrC. Many researchers attempted various methods for sintering ZrC based systems. Addition of second phase, sintering additives and particle size and shapes can play significant roles in densification without deteriorating the high temperature desired properties. All the techniques for synthesis and consolidation of ZrC and its composites are reviewed

Electrocardiogram of Clinically Healthy Mithun (Bos frontalis): Variation among Strains

A study was conducted to establish the normal electrocardiogram in four different genetic strains of mithun (Bos frontalis). Electrocardiography, cardiac electrical axis, heart rate, rectal temperature and respiration rate were recorded in a total of 32 adult male mithun of four strains (n = 8 each). It was found that the respiration and heart rates were higher (P < .05) in Manipur than other three strains. Amplitude (P < .05) and duration of P wave and QRS complex differed (P < .01) among the strains. Mizoram strain had the highest amplitude and duration of P wave and QRS complex. On the other hand, higher (P < .05) amplitude and duration of T wave were recorded in Arunachalee and Mizoram strains. The mean electrical axis of QRS complex that were recorded for Arunachalee and Manipur strains were similar to that reported for other bovine species; whereas the electrical axis of QRS for Nagamese and Mizoram strains were more close to feline and caprine species, respectively. In conclusion, electrocardiogram of mithun revealed that the amplitude and duration of P wave, QRS complex and T wave were different among four different genetic strains of mithun and the electrical axis of QRS complex for Nagamese and Mizoram mithuns are dissimilar to bovine species

Sintering characteristics of Si3N4 with additives from Y2O3-La2O3 binary system

Si3N4 was sintered with sintering aids from La2O3-Y2O3 binary system varying their compositions from 10La2O3 90Y2O3 to 90La2O310Y2O3. Three eutectic compositions were found in the said system which governs densification of Si3N4, 60La2O340Y2O3 was found to be the best sintering composition and it can yield YLaO3 phase at the grain boundaries

Temperature and load dependent mechanical properties of pressureless sintered carbon nanotube/alumina nanocomposites

Multiwalled carbon nanotube (MWCNT)/alumina (Al2O3) nanocomposites having different CNT contents have been fabricated by wet-mixing and pressureless sintering in Argon at 1700 degrees C. Depending on indentation load, the highest improvement of 15-34% in hardness was achieved in 0.3 vol.% MWCNT/Al2O3 nanocomposite. Indentation size effect (Mayer's exponent = 1.753) on hardness was the most prominent in 1.2 vol.% MWCNT/Al2O3 nanocomposite due to presence of clustered CNTs, non-uniform interface and porous microstructure. similar to 34% increase in fracture toughness was achieved in 0.3 vol.% MWCNT/Al2O3 nanocomposite than pure Al2O3 (similar to 3.84 MPa m(0.5)). High MWCNT loaded nanocomposites had reduced R-curve sensitivity because of increased matrix grain refining effect and reduced matrix compressive residual stress. High temperature flexural strength results indicated that strength retention of nanocomposites up to 1100 degrees C in ambient was much better compared to pure Al2O3. To predict structure-property relationship in nanocomposites, detailed microstructural and fractographic studies were also performed. (C) 2011 Elsevier B.V. All rights reserved

Sintering behavior and properties of Si3N4 sintered with nitrogen-rich liquid in the system Y2O3-AlN-SiO2

Pressureless sintering of Si3N4 with selected liquid compositions from the system Y2O3-AlN-SiO2 has been reported. Compositions were selected so that the grain-boundary liquid produced YAG(3Y2O3.5Al2O3) after crystallization. Studies on dynamic sintering indicated that the first liquid formation occurred between 1278 and 1305C and the maximum sintering rate was observed between 1590 and 1610C depending on the compositions. Si2N2O, YAG, or YAG along with YSiO2N were the crystalline phases obtained on devitrification as the composition moved toward the AlN corner. The grain-boundary liquid phase could be reduced from 20 to 9% on crystallization. Highest flexural strength and fracture toughness obtained were 475 MPa (four-point, 45 to 20mm span) and 6. 8 MPa.square root m, respectively. The effect of crystallization on high-temperature strength and toughness was beneficial in all cases. The creep rate of the sintered product varied from 10.0 x10 to the -7 power to 0.00004/h between 1200 and 1450 C and at 200 to 350 MPa stresses

Effect of sintering temperature and nanotube concentration on microstructure and properties of carbon nanotube/alumina nanocomposites

Multiwalled carbon nanotube (MWCNT)/alumina (Al2O3) nanocomposites were prepared by simple wet mixing of MWCNT and Al2O3 powder followed by pressureless sintering in static Argon. X-ray diffraction (XRD) line broadening analyses using Williamson-Hall (W-H) technique revealed a polynomial dependence of crystallite size (D-V) on sintering temperature (T-sin) and volume percent CNT added (V-CNT) as D-V approximate to [0.9(T-sin)-770]+[-1.1(T-sin)+1489](V-CNT)+[0.4(T-sin)-547](V-CNT) (2). While the highest increase (similar to 14%) in Vickers hardness (HV) over pure Al2O3 was offered by 0.15 vol% MWCNT/Al2O3 specimen, the highest fracture toughness (K-IC similar to 5 MPa m(0.5)) and flexural strength (sigma(FS)similar to 260 MPa) were obtained for nanocomposite containing 0.3 vol% MWCNT which were similar to 26% and similar to 16% higher, respectively, than those of pure Al2O3. The 0.15 vol% MWCNT/Al2O3 specimen also offered maximum increase (similar to 22%) in thermal conductivity over unreinforced Al2O3 (similar to 39 W/m K). Electrical percolation in nanocomposites was observed between 0.6 and 1.2 vol% CNT loading. (c) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Microstructure and physicomechanical properties of pressureless sintered multiwalled carbon nanotube/alumina nanocomposites

Multiwalled carbon nanotube (MWCNT)/alumina (Al(2)O(3)) nanocomposites containing CNT from 0.15 vol.% to 2.4 vol.% have been successfully fabricated by simple wet mixing of as-received commercial precursors followed by pressureless sintering. Extent of densification of nanocomposites sintered at low temperature (e.g. 1500 degrees C) was <90%, but increased up to similar to 99% when sintered at 1700 degrees C and offered superior performance compared to pure Al(2)O(3). Nanocomposites containing 0.3 vol.% MWCNT and sintered at 1700 degrees C for 2 h in Argon led to similar to 23% and similar to 34% improvement in hardness and fracture toughness, respectively, than monolithic Al(2)O(3). In addition, the highest improvement (similar to 20%) in bending strength was obtained for 0.15 vol.% MWCNT/Al(2)O(3) nanocomposite compared to pure Al(2)O(3). Weibull analysis indicated reliability of nanocomposites increased up to 0.3 vol.% MWCNT, whereas, beyond that loading consistency was the same as obtained for pure Al(2)O(3). Detailed microstructure and fractographic analysis were performed to assess structure-property relationship of present nanocomposites. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Wear of some advanced ceramics under a sharp indenter in unidirectional sliding

The wear of reaction-bonded SiC (RSSiC), dense silicon nitride (DSN), tetragonal zirconia ZrO2(t), and alumina-titanium nitride (Al2O3-TiN) composites have been studied under a sliding diamond indenter having an included angle of 90-degrees and a tip radius of 6 mum at a sliding speed of 8 mm/s. The relative wear resistance as measured from the track diameter was in the order DSN > Al2O3-TiN > RSSiC > ZrO2(t). The track width showed a straight-line relationship with the square root of load, implying that lateral cracking was responsible for the wear. The wear rates decreased with load and varied from 1 x 10(-5) to 2 x 10(-5) mm/(m.g) for the above ceramics. The wear rate increased sharply as the velocity increased over 1 m/min at a load of 300 g and above due to frictional heat generated because of the work of sliding

Statistical analysis of mechanical properties of pressureless sintered multiwalled carbon nanotube/alumina nanocomposites

Mechanical properties of pressureless sintered 0.15-1.2 vol. multiwalled carbon nanotube reinforced alumina matrix nanocomposites have been analyzed using the 2-parameter Weibull statistics. Electron microscopy and phase analysis of nanocomposites sintered at 1700°C for 2 h in Argon revealed existence of interpenetrating network of nanotubes in alumina, formation of thin interface resembling stoichiometric aluminum monoxycarbide and matrix grain refinement by nanotubes. Statistical analyses indicated that with increasing Vickers hardness testing load (4.9-19.6 N) and flexural strength measurement temperature (room temperature to 1100°C), Weibull modulus of nanocomposites increased significantly suggesting improved consistency at higher load and temperature. The highest Weibull moduli were obtained for nanocomposites containing either 0.15 or 0.3 vol. nanotube which were �40 and �15 higher than single phase alumina for hardness and strength, respectively, supporting the specimen size effect on reliability of present brittle ceramic matrix nanocomposites. Superior mechanical reliability of nanocomposites over pure alumina was primarily attributed to the presence of structurally intact nanotubes forming effective interface region to ensure proper load sharing, matrix grain refinement, and especially, at higher testing load and temperature, overall averaging effect of flaws to yield higher Weibull moduli. © 2012 Elsevier B.V. All rights reserved