Zero and controllable thermal expansion in HfMgMo(3-x)WxO(12)

HfMgMo（3-x）WxO（12） with x = 0.5, 1.0, 1.5, 2.0, and 2.5 are developed with a simple solid state method. With increasing the content of W, solid solutions of Hf Mg Mo3-xWx O12 crystallize in an orthorhombic structure for x≤2.0 and a monoclinic structure for x2.0. A near-zero thermal expansion（ZTE） is realized for HfMgMo（2.5）W（0.5）O（12） and negative coefficients of thermal expansion（NCTE） are achieved for other compositions with different values. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and incorporation of a different amount of W, which cause variable distortion of the octahedra and softening of the MoO4 tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and x-ray diffraction.

Crystal structure and microwave dielectric properties of novel BiMg_(2)MO_(6)(M=P,V)ceramics with low sintering temperature

Novel low-firing BiMg_(2)PO_(6)(BMP)and BiMg_(2)VO_(6)(BMV)ceramics with excellent microwave properties were synthesized and analyzed.The Rietveld refinement indicated that BiMg_(2)MO_(6)(M=p,V)ceramics crystallized in orthorhombic structure.The analysis results of the scanning electron microscope proved a dense and uniform microstructure.Further research showed that there was a strong correlation among permittivity,quality factor,and bulk density at different temperatures.The correlation between the properties and the structure of BiMg_(2)MO_(6)(M=P,V)ceramics were characterized by the chemical bond theory.The outstanding dielectric properties of BiMg_(2)MO_(6)(M=P,V)ceramics(BMP:εr=12.503,Q×f=25,760 GHz,tf=-20×10^(-6)/℃;BMV:εr=13.472,Q×f=37,270 GHz,tf=78×10^(-6)/℃)make them expected to become the novel low temperature co-fired ceramics(LTCC).

An explicit and novel structure,lattice dynamics,and photoemission of La-doped nanocrystalline SrZrO_(3) perovskite

As no complete and comprehensive studies have been previously reported for La-doped nanocrystalline SrZrO_(3)(SZO),we researched herein a detailed investigation for pure and La-doped samples.A modified solid-state reaction process,including successive cycles of milling and sintering at high temperature,was followed to produce SZO and Sr_(0.9)La_(0.1)ZrO_(3)(SLZO)powdered ingots.Rietveld analysis of X-ray diffractometer data predicts that the two samples exhibit orthorhombic structure with an increase in crystallite size by~25%for doped sample.A great reduction in Raman modes intensity(~60%)and an annihilation of several vibration modes were detected using Raman spectroscopy.The degree of ordering on the B-site was recorded to be higher in La-doped sample.According to ultraviolet-visible(UV-Vis)absorption,a decrease in the optical gap width(E_(g))from 4.40 eV to 4.21 eV was achieved by La incorporation due to the presence of additional defect states such as oxygen and Sr vacancies at the band edge.The process of electron-hole recombination was studied using photoluminescence(PL)spectroscopy.Deconvolution of PL spectra yielded four emission bands:one green band,one blue band,and two violet bands.Highly intense violet emission atλ=393 nm approximately five times greater than that detected for pure SZO is realized as La^(3+)substitutes for Sr^(2+).Such property nominates SLZO for technological applications requiring highly intense violet emission,e.g.,light-emitting diodes.

A new class of high-entropy M_(3)B_(4) borides

A new class of high-entropy M3B4 borides of the Ta_(3)B_(4)-prototyped orthorhombic structure has been synthesized in the bulk form for the first time.Specimens with compositions of(V0.2Cr0.2Nb0.2Mo0.2Ta0.2)3B4 and(V0.2Cr0.2Nb0.2Ta0.2W0.2)_(3)B_(4) were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors.The sintered specimens were〜98.7%in relative densities with virtually no oxide contamination,albeit the presence of minor(4-5 vol%)secondary high-entropy M5B6 phases.Despite that Mo_(3)B_(4) or W_(3)B_(4) are not stable phase,20%of M03B4 and W3B4 can be stabilized into the high-entropy M3B4 borides.Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N.This work has further expanded the family of different structures of high-entropy ceramics reported to date.