High Pressure Behavior of Mullite-Type Oxides: Phase Transitions, Amorphization, Negative Linear Compressibility and Microstructural Implications

Even though mullite occurs rarely in nature, it is perhaps one of the most important phases in both traditional and advanced ceramics. Existing and emerging applications of mullite and mullite-type materials include: high-temperature composites, aerospace materials, ballistic shielding for military applications and even non-linear optical materials. There are many uncertainties regarding the basic physical properties of mullite-type materials, particularly in terms of their high-pressure structural stability and mechanical behavior that are important to address for emerging applications of mullites as engineering materials. This work is the first reported comprehensive investigation of the high –pressure structural behavior of several different mullites and synthetic mullite-type oxides. The materials investigated are representatives of different structural and chemistry branches of the mullite family. The goal is to elucidate how the most fundamental building blocks of mullite oxides accommodate high pressure compression. Mullites and mullite-type oxides are investigated at extreme pressures using synchrotron x-ray diffraction and laser Raman spectroscopy. These experiments enable the extraction of the materials’ structure and its modifications in a function of increasing pressure: deformation of polyhedra, phase transitions, formation and breaking of bonds. The experimental techniques used here are ideally suited to provide a synergical interplay in the study of oxides under high-pressure conditions: Raman spectroscopy is a technique for investigating short range order phenomena while x-ray diffraction accesses structural changes occurring at the long range order. The following phenomena are discussed: phase transitions, equations of state, pressure-driven amorphization, and the very rare effect of negative linear compressibility. The unprecedented discovery of negative linear compressibility in mullite-type oxides presented here opens the door to military applications as incompressible optical materials

Titanium hydride up to 90 GPA: Synchrotron X-ray diffraction studies

The structure and behavior of titanium hydride was studied on compression to about a megabar. Angle-dispersive as well as energy-dispersive synchrotron X-ray diffraction studies were carried out in situ, in diamond anvil cells, in several compression and decompression sequences. A phase transition from a cubic to a tetragonal structure was observed. The pressure evolution of the diffraction patterns revealed that the cubic (Fm-3m) to tetragonal ((4/mmm) phase transition occurs at about 0.6 GPa. The transition was found to be reversible on decompression from 34 GPa but irreversible on decompression from 90 GPa. The influence of the pressure transmitting medium on the pressure-induced structural transformations of TiH2 was also examined. An equation of state fit of the evolution of unit cell volume as a function of pressure, up to 90 GPa yielded a zero pressure bulk modulus K0=143(7) GPa, and its pressure derivative K0=3.0(0.1) for the high-pressure tetragonal phase of TiH2

Equation of State of Gallium Oxide to 70 Gpa: Comparison of Quasihydrostatic and Nonhydrostatic Compression

Synchrotron x-ray diffraction and diamond-anvil cell techniques were used to characterize pressure induced structural modifications in gallium oxide. Gallium oxide was studied on compression up to 70 GPa and on the following decompression. The effect of the pressure-transmitting medium on the structural transformations was investigated in two sets of compression and decompression runs, one with nitrogen as a quasihydrostatic pressure-transmitting medium and the other in nonhydrostatic pressure conditions. The x-ray diffraction data showed gradual phase transition from a low-density, monoclinic β-Ga2O3 to a high-density, rhombohedral α-Ga2O3. With the use of nitrogen as a pressure transmitting medium, the β- to α-Ga2O3 transition begins at about 6.5–7 GPa and extends up to ∼40 GPa, confirming recent theoretical calculations. This pressure-driven transition is irreversible and the material decompressed from 70 GPa to ambient conditions was composed, in both sets of experimental runs, of α-Ga2O3 only. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero pressure bulk modulus K0=199(6) GPa, and its pressure derivative K0′=3.1(4) for theβ-Ga2O3 phase, and K0=220(9) GPa and K0′=5.9(6) for the α-Ga2O3 phase for the experiments performed in quasihydrostatic compression conditions. When for the same experiment K0′ is held at 4, then the bulk modulus values are 184(3) and 252(14) GPa for β-Ga2O3 and the α-Ga2O3, respectively. We compare the results of this work with our previous studies on the high-pressure behavior of nanocrystalline gallium oxid

Neutron diffraction of NaBD4: Phase transition, Rietveld Structure refinements, and equation of state

NaBH4 is a hydride with possible applications as a hydrogen storage material for future renewable energy technologies. It’s dehydrogenation properties are enhanced with the mixture of particular catalysts through ball-milling techniques during which local pressures may exceed several GPa’s. It is for this reason that understanding the behavior of pressure induced phase changes of its crystalline unit cell is an area of interest

Stability and Equation of State of a Nanocrystalline Ga-Ge Mullite in a Vitroceramic Composite: A Synchrotron X-ray Iffraction Study

Synchrotron x-ray diffraction and diamond anvil cell techniques were used to characterize the phase transformations and to evaluate the structural stability at elevated pressures of a developed nanocrystalline composite. The optically transparent material was built of a germanium oxide-based amorphous host matrix with homogeneously dispersed 13±3 nm Ga-Ge mullite-type nanocrystals, which had a structure similar to the conventional Al2O3-SiO2 mullite. The equation of state of the nanocrystals and the overall structural integrity of the nanocomposite were investigated in quasihydrostatic conditions on compression to 36 GPa and on the following decompression to ambient conditions. The overall pressure-induced changes of x-ray diffraction patterns evidenced that the structural integrity of the material is well preserved up to about 14–16 GPa. The nanocomposite decompressed from 36 GPa to ambient pressure showed a very limited reversibility of the pressure-driven changes. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus, K0, for the nanocrystalline phase of 229(15) GPa which makes this material potentially interesting for structural applications at elevated pressures

High pressure studies of titanium hydride up to 50 GPa with synchrotron x-ray diffraction

Titanium dihydride has the potential to play an important role in the efficiency of high density hydrogen storage. The structural instability of TiH2 at high pressures makes an accurate characterization of its structure a vital part of understanding its behavior. A sample of TiH2 was placed in a diamond anvil cell and studied from ambient pressure up to 53 GPa using in situ synchrotron x-ray diffraction at the Advanced Photon Source (APS) of Argonne National Laboratory (Sector 16, HPCAT). From data of the evolution of the structure with pressure, an equation of state was obtained to model the behavior of the unit cell of TiH2 between 0 and 51 GPa

Carcinosarcoma of the vulva: a rare case report

Carcinosarcomas are rare and clinically aggressive neoplasms with poor outcome. A very few cases has been reported in the literature. We present a case of a 54 yrs woman with bleeding per vagina for 1.5 months. On per vaginal examination, a lump of size 3×2 cm was found over the left anterior region of vulva. CECT abdomen revealed a well-defined heterogeneously enhancing lesion with calcific foci involving both labia minora. MRI pelvis reveals carcinoma left vulva with involvement of lower 3rd of vaginal wall. Histopathological examination showed features suggestive of carcinosarcoma of the vulva. This was further confirmed by immunohistochemistry which showed positivity for CK, SMA, caldesmon and negative for S100, CD34, desmin, CD31, ERG and MyoD1. Because it is a very rare tumor with severe prognosis, we presented this case as this case report can be a useful addition to the literature.

Oral squamous cell carcinoma with clear cell change: a rare case report

Clear-cell variant of oral squamous cell carcinoma is an extremely rare entity. Clear-cell change can be seen in any of the neoplasms, but as pure form variant, it is difficult to find in head-and-neck SCC. We hereby present a case report of 71-year-old male who presented with a growth with erythematous patches on the lateral pharyngeal wall for the past 3 months. Histopathologic examination showed nests, islands and sheets of malignant squamous cells with vesicular nuclei and abundant clear cytoplasm. Neoplastic cells constituting majority of nests exhibited clear cell changes. Special stain was performed to identify the nature of clear cells. periodic acid Schiff-diastase (PAS-D) showed positivity in the clear cells. Immunohistochemical study using antibody for pan-cytokeratin revealed diffuse positivity in the tumor cells

"Mother-weights" and lost fathers: parents in South Asian American literature

That parent-child relationships should play a significant role within South Asian American literature is perhaps no surprise, since this is crucial material for any writer. But the particular forms they so often take – a dysfunctional mother-daughter dynamic, leading to the search for maternal surrogates; and the figure of the prematurely deceased father – are more perplexing. Why do families adhere to these patterns in so many South Asian American texts and what does that tell us about this œuvre? More precisely, why are mothers subjected to a harsher critique than fathers and what purpose does this critique serve? How might we interpret the trope of the untimely paternal death? In this article I will seek to answer these questions – arguably key to an understanding of this growing body of writing – by considering works produced between the 1990s and the early twenty-first century by a range of South Asian American writers

High-Pressure Structural Integrity and Structural Transformations of Glass-derived Nanocomposites: A Review

A review of our recent and ongoing extensive high-pressure synchrotron X-ray diffraction and high-pressure optical spectroscopy studies of nanocrystalline composites is presented. These heterophased, nano-architectured composites consist of amorphous matrices with dispersed nanocrystals or quantum dots. We show how besides compositional variations, additional tuning of these glass-derived nanocomposites can be done by exploiting elevated pressure. We examine stability and pressure-driven phase transitions occurring in nanocrystals as well as structural changes occurring in the glass matrix. Finally, we discuss the influence of the glass matrix of a composite on the structural transformations occurring in the embedded nanocrystals