The influence of Zr/Ti content on the morphotropic phase boundary in the PZT-PZN system

A quantitative structural investigation was carried out on (1-y)PbZrxTi1-xO3-yPbZn(1/3)Nb(2/3)O(3) where y=0.1 and 0.2 ((1-y)PZT-yPZN). High resolution XRD data have been used for quantitative phase analysis. The nominal compositions were prepared by a two-step low temperature calcining solid-state method. The sintered samples show an average grain size of 1-2 mu m. It is demonstrated that the increase in the concentration of PZN leads to the shift of the morphotropic phase boundary (MPB) of PZT towards the PbZrO3 end member. In the present work, an effort has been made to quantitatively determine the MPB phase contents and to regain the coexistence of tetragonal and monoclinic phases by varying the value of x(i.e. Zr/Ti ratio). The width of the MPB becomes considerably larger for y=0.10 and 0.20 as compared to pure PZT. This is attributed to the considerably lower grain size of our samples resulting from the adopted preparation method. (C) 2010 Elsevier B.V. All rights reserved

Apparatus to measure high-temperature thermal conductivity and thermoelectric power of small specimens

A thermal conductivity apparatus based on the parallel heat-flow technique has been fabricated to measure the thermal conductivity of small specimens. The steady-state method is used to measure samples of 1x1x10 $mm^{3}$ dimension in the temperature range of room temperature-700K. The details of instrument fabrication, the method of calibration,and typical measurements on test samples are described. The apparatus can also be used to measure the Seebeck coefficient in the same temperature range. As an example we report the thermal properties of $CrSi_{2}$, which is a potential candidate for high-temperature thermoelectric applications

Improved ductility and oxidation resistance in Nb and Al co-substituted MoSi2MoSi_2

Polycrystalline samples of $(Mo_{1-x}Nb_x)(Si_{1-y}Al_y)_2$ with x = 0.0-0.03 and y = 0.0, 0.04 were prepared by arc melting followed by densification by hot pressing. The mechanical and oxidation behaviors of high density polycrystalline pellets were evaluated. Thermogravimetric analysis shows lower oxidation in $(Mo_{1-x}Nb_x)(Si_{0.96}Al_{0.04})_2$ samples compared to pure $MoSi_2$. X-ray diffraction analysis of the oxide scale revealed the presence of $\alpha-Al_2O_3$ peaks. Mechanical characterizations carried out on (Mo_{0.99}Nb_{0.01})(Si_{0.96}Al__{0.04})_2 composition show a marked improvement compared to that of unsubstituted $MoSi_2$. Micro-hardness measurements reveal a lowering of the room temperature hardness and stiffness compared to pure $MoSi_2$. Plastic deformation under compression is observed at room temperature with a yield stress of 500 MPa. The $K_{1c}$ value increased from $3.33 (\pm3) MPa m^{0.5}$ to $4.05 (\pm0.3) MPa m^{0.5}$ with the fracture mode being predominantly transgranular

Role of milling parameters and impurity on the thermoelectric properties of mechanically alloyed chromium silicide

Mechanical alloying of Cr–Si powders (1:2 molar ratio) was carried out under different milling conditions using stainless steel milling media. In addition to formation of nanocrystalline $CrSi_2$ phase, depending on the milling duration, speed and use of dispersant, X-ray diffraction revealed presence of CrSi phase and EDAX showed iron contamination in varying amounts. The amount of contamination and the secondary phase are found to be linearly proportional to the input impact energy during milling. The nanocrystalline powders are seen to be thermally stable in air up to 900 K. SEM of the hot pressed powders reveal an equiaxed microstructure with grain size depending on the milling duration. Electrical resistivity \rho, Seebeck coefficient (S) and thermal conductivity (K) were measured up to 700 K. It is seen that the thermoelectric figure of merit (ZT) is high when the amount of secondary phases is minimum. The maximum ZT observed in the samples is $\sim 0.2$ at 600K which is comparable to other high temperature thermoelectric materials in the temperature range studied

Defect engineering of VO2 thin films synthesized by Chemical Vapor Deposition

Vanadium dioxide (VO2) has been a much sought after candidate for many electronics and photonics based applications. These applications demand CMOS compatibility and simple but scalable synthesis procedure. Herein we report the deposition of VO2 on CMOS compatible Si substrates via easily scalable Metal Organic Chemical Vapor Deposition (MOCVD) at different substrate temperatures between 520 and 550 °C. The morphology of the films deposited at different growth temperatures varied drastically despite retaining VO2 in the M1 phase. This was verified by XRD and Raman experiments. The thin films deposited at 535 °C showed sharp grain boundaries with a grain size of about 200 nm. Similarly, the electrical characteristics of the films deposited at 535 °C showed superior transition compared to the films deposited at other temperatures. However the films deposited at 550 °C retained the superiority in the transition strength, the transition width and the hysteresis of the transition increased at that temperatures. We understand that the reason for this observed behavior is the fraction of surface defects. It is seen from XPS measurements that vanadium was available in V3+, V4+ and V5+ oxidation states and the fractions of these individual components varied in all the thin films deposited at different temperatures. We believe that the defects fraction can give a handle to control the nature and quality of the transition in VO2

Hygroscopicity and bulk thermal expansion in Y2W3O12Y_2W_3O_{12}

Negative thermal expansion material, $Y_2W_3O_{12}$ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with $a = 10.098(1) \AA$ , $b = 13.315(3) \AA$, $c = 9.691(4) \AA$. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3–6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of $>100 \mu m$, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained $(2-5 \mu m)$ ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100–300 K and an average linear thermal expansion co-efficient of $-9.65 \times 10^{-6}/K$ is observed. The effect of water on the thermal expansion behavior of this system is discussed