Materials Science Forum Vols. 830-831

Abstract: Transition metal diborides, especially zirconium and hafnium diboride are potential ceramic material for ultra high temperature applications above 1800°C. These borides are characterized by high melting point, formation of high melting point oxides, good oxidation resistance and excellent thermo-mechanical properties. In this present exploration, zirconium diboride (ZrB₂) has been selected for its moderate density (6.09 gm/cc) and better oxidation resistance compared to high density hafnium diboride (11.2 gm/cc). The developed ZrB₂ composite in the present study contains 10 wt. % SiC and 10 wt. % MoSi₂ as sintering additives. SiC and MoSi₂ were added to improve the thermal shock resistance and sinterability of the ultra high temperature ceramics (UHTCs). Vacuum hot pressing was carried out at 1800°C for a holding period of 30 minutes and applied pressure of 30 MPa. Attractive feature of this ZrB₂ composite is good machinability due to better electrical conductivity and complicated shapes can be realized easily through electro discharge machining (EDM) process. Detailed XRD phase analysis and microstructural investigation of the polished and fractured composites was carried out using SEM. Mechanical and thermal properties tests have been carried out for the optimized ZrB₂ composite material.

Abstract: BaTi₄O₉(BT4), has dielectric constant (εr)≈36 and unloaded Q-factor (Qf) >35,000GHz, but has temperature coefficient of frequency (τf~16-20ppm/°C). Ba₂Ti₉O₂₀(B2T9), very near to BT4 in the BaO-TiO₂ phase diagram, has εr~38, Qf~35,000GHz and τf<4ppm/°C. Though this phase is selected for most of the applications, there are many challenges like slow formation, decomposition, reaction with substrate etc. But, BT4 formed quickly and densified easily, but only high τf precluded it from many practical applications. Most of the literature on the modification of BT4 decreased the εr≤35. Here, the preparation and properties of ZnO/ZnTiO₃ added BaTi₄O₉ ceramics are reported. The ceramics have been prepared by the solid state method. Raw materials have been homogenized and calcined at 1125°C. Calcined powders are ball-milled, compacted and sintered in 1275−1350°C to get dense ceramics with ρ=4.38-4.44 g/cc (TD>97.5%). XRD analysis has confirmed the BaTi₄O₉ phase formation along with secondary phases. FeSEM microstructures show liquid phase assisted sintering leading to fused grains and exaggerated grain growth. The dielectric properties have been measured near 4GHz by standard rod resonator methods using cylindrical specimens. The ceramics have εr36.5−38, τf<11ppm/°C and Qf>17THz. It is possible to further improve the Qf of these ceramics by controlling the processing.

Abstract: The paper illustrates Carbon nanotubes reinforced pure Al (CNT/Al) composites and fly ash reinforced pure Al (FA/Al) composites produced by ball-milling and sintering. Microstructures of the fabricated composite were examined and the mechanical properties of the composites were tested and analysed. It was indicated that the CNTs and fly ash were uniformly dispersed into the Al matrix as ball-milling time increased with increase in hardness.

Abstract: Alumina powder dispersions in molten urea medium were prepared using ammonium poly (acrylate) dispersant. The dispersant concentration for achieving very good dispersion of the powder was optimized through rheological measurements. The optimum dispersant concentration for alumina powder dispersion in molten urea (1.25 wt.% of alumina) is higher than that required in aqueous medium (0.35 wt%). The dispersions with alumina loading as high as 55 vol.% having low viscosity could be achieved in molten urea medium at 135 °C. The viscosity and yield stress of the alumina powder dispersions in molten urea is low enough for casting.

Abstract: Mica containing glass-ceramic is prepared by crystallizing magnesium alumino silicate glass between 600°C and 1100°C without the use of any additives or nucleating agents. Effects of ceramization heat treatment conditions on phase constitution and microstructures are analyzed using XRD and HRSEM respectively. Developed glass-ceramics have been analyzed for its microhardness and machinability. Optimum heat treatment condition for achieving uniformly distributed fluorophlogopite phase essential for machinability, and at the same time, magnesium aluminum silicate phase for increasing the hardness, is presented.

Abstract: Friction stir processing (FSP) is a novel technique used to fabricate surface composites. This investigation is an attempt to made Al/Al₂O₃ and Al/SiC surface composite using FSP and compare the mechanical and metallurgical properties influenced by the types of reinforcement particles. Two plates were grooved with 0.8 mm in width, 5 mm in depth and 100 mm in length in the middle of the aluminium plate using wire EDM and compacted with Al₂O₃ and SiC powder. The FSP was carried out automatically on an indigenously built FSW machine at tool rotational speed of 1200 rpm, processing speed of 60 mm/min and axial force of 10 kN. The optical and Scanning Electron microstructures are precisely revealed the homogeneous distribution of Al₂O₃ and SiC particles in the stir zone of surface composite layer (SCL). The microhardness was measured across the cross section of SCL layers of Aluminium and Aluminium with SiC and Al₂O₃. The higher microhardness was obtained in Aluminium with SiC and Al₂O₃ composites fabricated by FSP. This is because of the higher hardness value of SiC particles than Al₂O₃ particles.

Abstract: Materials with relative permittivity or dielectric constant near to that of air (εr~1) are known as ‘ultra-low k’ materials. They find a number of applications in inter-connects of micro-electronic circuits, antennae, high-speed communication substrates etc. Among the inorganic materials, porous silica is the widely studied candidate. Porous silica can be of many types depending upon the extent of porosity and size and connectivity of pores. This paper presents the details of measurement of permittivities and the results of silica beads and silica aerogels. Silica beads, prepared by microwave heating of silica gels, are spherical beads of average 1mm size. Hydrophobic silica aerogels, prepared by ambient pressure drying of silica gels, are irregular chunks of 5-10 mm size. Both are potential bulk fill insulation materials and hence the permittivity can be measured as an aggregate filling a definite volume. The permittivities of these have been measured upto 1 MHz by 3-terminal method using a precision LCR meter and a powder-paste cell as per ASTM-D150-11. The εr values of silica aerogels and silica beads in 20 Hz-1 MHz range could be measured and are less than 1.6 at 1 MHz.

Abstract: Porous ceramics was successfully fabricated by a sacrificial porogen leaching method using in-situ synthesised aluminium based binders by reaction bonding at low sintering temperatures of 600-1000°C. Porous ceramics with porosity in the range of 27 to 65% were obtained by leaching technique. Interconnected bimodal pores were produced by both stacking of starting particles and leach out of salt. During sintering, the aluminium binder experienced metal to ceramic transformations and act as good binder.

Abstract: Magnetic composite comprising zinc ferrite and barium hexaferrite; BaFe₁₂O₁₉/ZnFe₂O₄ nanoparticles having super paramagnetic nature were synthesized by co-precipitation of iron, zinc and barium chloride salts using 7.5 M sodium hydroxide solution. The resulting precursors were heat treated (HT) at 800 and 1200°C for 4 h in nitrogen atmosphere. The hysteresis loops showed an increase in saturation magnetization from 1.040 to 52.938 emu/g with increasing heat treatment (HT) temperatures. The ‘as synthesized’ particles have size in the range of 20–22 nm with spherical and needle shapes. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plate shape with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on microwave absorption properties were estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite reaches-23.12 dB (more than 99% power attenuation) at 10.46 GHz which make it a potential material in the area of stealth technology.

Abstract: The present study describes the processing and characterization of hypoeutectic A319 functionally graded Aluminium metal matrix composite (FGMMC) reinforced with 10 weight percentage SiC_p particles of 23 μm size. FGMMC’s are processed by liquid stir casting method followed by vertical centrifugal casting. Metallographic analysis of FGMMC casting reveals the influence of the centrifugal force on the gradation of various phases in the matrix and an increasing gradient distribution of SiC reinforcements gradually from inner towards the outer periphery forming different zones. Tensile and the compressive tests show that the variation in properties are structure sensitive and is confirmed by the dry wear tests. The study clearly depicts the gradient nature in the structure and mechanical properties of the FGM castings produced by centrifugal casting method.