High pressure behaviors of nanoporous anatase TiO2

High pressure behaviors of nanoporous anatase TiO₂ were studied using in situ Raman spectroscopy up to 37 GPa. The nanoporous anatase phase starts to transform into the baddeleyite phase with poor crystallinity at ～15.2 GPa, and the baddeleyite phase coexists with anatase phase up to 18.4 GPa. The baddeleyite form transforms into an amorphous phase above 20.5 GPa. Upon decompression, the amorphous phase recovers to the baddeleyite phase and then transforms to the α-PbO₂ phase. The phase transition from the baddeleyite phase to the amorphous form is reversible. The poor crystalline baddeleyite phase acts as an intermediate state in the amorphization process. The phase transitions of the nanoporous anatase TiO₂ are obviously different from the pressure-induced amorphization in the anatase TiO₂ nanoparticles. These results indicate that the porous microstructure plays important roles in the high pressure phase transitions of the nanoporous anatase TiO₂.     

Measurement of Young’s modulus of oxides at high temperature related to the oxidation study

Abstract

α-Al₂O₃, Cr₂O₃, and α-Fe₂O₃ specimens were prepared by a sintering process. A 400 – 1000-Hz sine wave was applied to the specimen at 290 – 1273 K. The applied and respond waves were monitored by using force and acceleration sensors. The intensity ratio and phase shift between the applied and respond waves were analysed, and the anti-resonance frequency was obtained. Young’s moduli of α-Al₂O₃, Cr₂O₃, and α-Fe₂O₃ are estimated to be 386, 286, and 220 GPa at 298 K, respectively. The temperature dependence values of these oxides are estimated to be 54.3, 46.9, and 42.0 MPa K^-1, respectively. The temperature dependence of Young’s modulus can be classified on the basis of the crystal structure of solids. The estimation of Young’s modulus at 1273K is possible with an error range of 10 – 30 GPa for a crystalline solid if the crystal structure of the solid is known. It is found that the temperature dependence of Young’s modulus depends on the density of the oxides, and an experiment in which well-characterized crystalline solids are used must be conducted to minimize the error range.     

Absorption and circular-dichroism spectra of LaBGeO5 crystal doped with Pr and Ho ions

Absorption and circular-dichroism spectra of LaBGeO₅ crystal doped with Pr³⁺ and Ho³⁺ ions were investigated at 8 K in the range 300-900 nm. Electronic transitions of these ions, which substitute La³⁺ ions in positions with a local symmetry C₁ are observed in the spectra. All transitions are active in both absorption and circular-dichroism spectra. Values of the dipole strengths D_om, rotation strengths R_om and anisotropy factors g are calculated for some well-separated bands.     

Behavior of the refractive index of lithium disilicate glass ceramic processed at high pressure and high temperature

The aim of this work was to investigate the effect of high pressure and high temperature on the refractive index of lithium disilicate glass ceramic with the stoichiometric composition Li₂O·2SiO₂ (LS₂). A first group of monolithic LS₂ glass samples were processed at 2.5 GPa, 4 GPa and 7.7 GPa at room temperature and a second group was submitted to high pressure and, simultaneously, to heat treatments for nucleation and growth of the crystalline phases. For comparison, samples submitted to the same heat treatments at 1 atm were also investigated. The refractive index of the samples was obtained by spectral ellipsometry and the results were clearly dependent on the particular pressure and temperature conditions. The crystallization of the samples was investigated by X-ray diffraction. For the samples processed under high temperature at 1 atm and at 2.5 GPa a fraction of the originally amorphous glass was transformed to a monoclinic phase of lithium disilicate. For the samples processed under high temperature and at 4 GPa, a large fraction of the originally amorphous glass transformed to an orthorhombic phase while, at 7.7 GPa, it was observed the formation of lithium metasilicate.     

TEM Observation of the Zrc/Zro2 Interface Formed by Oxidation of Zrc Single Crystals

Microstructure at the interface of ZrC and ZrO₂ formed by oxidation of a single crystal of ZrC with 100 faces at 600°C at an oxygen pressure of 2 kPa was observed by high-resolution transmission electron microscopy and scanning electron microscopy. The ZrO₂ scale was shown to consist of two subscales, zones 1 and 2. The interfacial area in zone 1 was composed of regular lattice fringes corresponding to the 111, 200, and 220 lattices of cubic ZrO₂ (c-ZrO₂) crystallites 2 to 10 nm in size and an amorphous phase. The growth of crystallites occurred in zone 2, which visualized the interface of zones 1 and 2. Black thin films reminiscent of zone 1 were successfully separated, which were composed of amorphous carbon in which c-ZrO₂ particles 2 to 20 nm in size were included.     

HREM surface profile images of Bi2Sr2CaCu2O8

Surface profile images of Bi₂Sr₂CaCu₂O₈ have been obtained using high-resolution electron microscopy. The cleaved (001) surface of the crystals terminates with a single Bi-O atomic layer. The modulated structure developed in this surface atomic layer was observed directly. The (hk0) surfaces were found to decompose in air into an amorphous coating layer. This coating layer was unlikely recrystallized into the original structure under electron beam irradiation. The amorphous layer on the (hk0) surface formed in pure Ar atmosphere was relatively thin and could be recrystallized into some secondary phases in which a Bi loss was observed. The original (001) surface might also be covered by an amorphous-like layer. This disordered layer could be recrystallized under electron beam irradiation into BiSr₂Ca₂Cu₃O₉, BiSr₂CuO₅, etc. which intergrow with the parent crystal perfectly on the (001) planes.The author thanks the EPSRC for financial support.     

Structural and mechanical properties of titanium and titanium diboride monolayers and Ti/TiB2 multilayers

Nano-multilayers represent a new class of engineering materials that are made up of alternating nanometer scale layers of two different components. In the present work a titanium (Ti) monolayer was combined with titanium diboride (TiB₂) to form a Ti/TiB₂ nano-multilayer. Designed experimental parameters enabled an evaluation of the effects of direct current bias voltage (U_b) and bilayer thickness (Λ) during multilayer deposition on the mechanical properties of reactively sputtered Ti/TiB₂ multilayer films. Their nanostructures and mechanical properties were characterized and analyzed using X-ray photoelectron spectroscopy (XPS), low-angle and high-angle X-ray diffraction (XRD), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. Under the optimal bias voltage of U_b = − 60 V, it was found that Λ (varied from 1.1 to 9.8 nm) was the most important factor which dominated the nanostructure and hardness. The hardness values obtained varied from 12 GPa for Ti and 15 GPa for TiB₂ monolayers, up to 33 GPa for the hardest Ti/TiB₂ multilayer at Λ = 1.9 nm. The observed hardness enhancement correlated to the layer thickness, followed a relation similar to the Hall-Petch strengthening dependence, with a generalized power of ∼ 0.6. In addition, the structural barriers between two materials (hcp Ti/amorphous TiB₂) and stress relaxation at interfaces within multilayer films resulted in a reduction of crack propagation and high-hardness.     

High pressure Raman spectroscopic study of structural phase transition in samarium oxide

High pressure Raman spectroscopic study of Sm₂O₃ poly crystal was performed up to 21.0 GPa and room temperature using a diamond anvil cell. Pressure induced phase transition was observed at 2.6 GPa in the pressure increasing process. This phase transition corresponds to the monoclinic B type phase → the hexagonal A type transformation. The A type phase was stable up to 21.0 GPa. In the pressure release process, the A type phase was stable above 1.8 GPa, and was completely reverted to the B type phase at 1.1 GPa. The phase transition was confirmed to be reversible with a hysteresis of approximately 1.0 GPa.     

Microstructure and mechanical properties of TiAlN/SiO2 nanomultilayers synthesized by reactive magnetron sputtering

TiAlN/SiO₂ nanomultilayers with different SiO₂ layer thickness were synthesized by reactive magnetron sputtering. The microstructure and mechanical properties were investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and nano-indentation. The results indicated that, under the template effect of B1-NaCl structural TiAlN layers, amorphous SiO₂ was forced to crystallize and grew epitaxially with TiAlN layers when SiO₂ layer thickness was below 0.6 nm, resulting in the enhancement of hardness and elastic modulus. The maximum hardness and elastic modulus could respectively reach 37 GPa and 393 GPa when SiO₂ layer thickness was 0.6 nm. As SiO₂ layer thickness further increased, SiO₂ transformed back into amorphous state and broken the coherent growth of nanomultilayers, leading to the decrease of hardness and elastic modulus.     

Effect of Temperature and X-Ray Illumination on the Oxygen Ordering in La2CuO4.1 Superconductor

Structural properties of La₂CuO_4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a periodicity 2, along the c axis, due to a 3D long-range oxygen ordering, have been observed. The temperature-dependent study has allowed us to distinguish two order–disorder phase transitions, at 350 and 375 K for two different ordered phases, respectively. After rapid quenching from 380 to 100 K we were able to induce disorder-to-order-like transition because of a 2D rearrangement of the excess oxygen atoms in the temperature range of 130–180 K. The oxygen ordering could also be produced by X-ray beam illumination; a clear signature of X-ray-photo-induced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K.     

Synthesis of MoS2/C nanocomposites by hydrothermal route used as Li-ion intercalation electrode materials

MoS₂/C nanocomposites were synthesized by a facile hydrothermal route employing sulfocarbamide, sodium molybdate and D-grouse as starting materials. XRD analysis showed that the MoS₂ was a two-dimensional nanosheet crystal and C was retained as amorphous after their calcinations at 800 °C. TEM images showed that MoS₂ was uniformly dispersed in the amorphous carbon. The MoS₂/C composites exhibited high reversible capacity and excellent cyclic performance when used as Li-intercalation electrodes. The improvements in electrochemical performance are attributed to the incorporation of amorphous carbon, which can suppress the aggregation and pulverization of active materials, and keep the active materials electrically connected.     

Spectroscopic investigation on phase transitions for Ge2Sb2Te5 in a wide photon energy and high temperature region

We investigated the electronic properties of phase-change material Ge₂Sb₂Te₅ (GST) films using spectroscopic ellipsometry in a wide photon energy and high temperature region. Apart from the charge carrier response, the totality of optical conductivity spectra for three phases of GST films, i.e., amorphous (AM), face-centered-cubic (FCC), and hexagonal (HEX), is quite similar, composed of two interband transitions in visible and UV regions. From optical analysis in a wide photon energy region up to 8.7 eV, we found that the intensity as well as the position of the interband transition in the visible region changes significantly as the phase of GST films turns from the amorphous to the crystalline phase, which is consistent with previous theoretical studies. In high temperature measurements above room temperature for the three phases of GST films, we found that the change of optical response for the AM phase of GST film occurs abruptly through two successive phase transitions near 150 °C and 270 °C, while the optical spectra of the FCC phase shows a change only near 270 °C. In contrast to the two above-mentioned cases, a slight change in optical spectra is observed for the HEX phase with the increasing temperature. From the measured optical spectra, we derived the temperature dependence of optical bandgap for the three phases, which are closely correlated to the change of the transport property for the GST films.     

Mechanochemical synthesis and characterization of Ni<Subscript>25</Subscript>Te<Subscript>75</Subscript> nanocrystalline alloy

Ni₂₅Te₇₅ nanocrystalline alloy containing trigonal NiTe₂ and Te nanocrystals was prepared through mechanochemical processing of pure elemental tellurium and nickel powders in argon atmosphere. The Ni₂₅Te₇₅ samples processed from 3 h to 30 h milling times were characterized by X-ray powder diffraction, transmission electron microscopy, magnetization and Raman spectroscopy. Trigonal NiTe₂ crystals with average size of 16 nm can be obtained after only 3 h of processing time. For longer milling times, the trigonal NiTe₂ phase becomes majority (about 70% with 30% for nanometric Te and no pure Ni was detected) and its average crystallite size slightly increases to 20 nm. Transmission electron microscopy images and electron diffraction patterns confirm the nanometric size of the crystalline domains in the agglomerated particles. The magnetic properties of the Ni₂₅Te₇₅ powders are dependent on synthesis time, suggesting a paramagnetic behavior mainly associated with the NiTe₂ nanophase. Raman spectra showed peaks that can be associated with unreacted Te and tellurium oxides modes, but it also showed several modes that can be attributed to trigonal NiTe₂ nanophase. The high-pressure experiments showed no phase transitions for NiTe₂ up to 17 GPa and Te phase transitions from form I to forms II and III occurred simultaneously at 4.5 GPa, remaining up to 12 GPa; after that, only reflections of Te-III and the NiTe₂ were observed. All the phase transitions observed with pressure are reversible after decompression. The bulk modulus determined from the least-squares fit of first-order Murnaghan equation of states is 110 GPa for the NiTe₂ nanophase and 28 GPa for Te-I.     

First-Principle Calculations of Hardness and Melting Point of Mo2C

This paper has constructed two kinds of atomic and electronic models for hexagonal β-Mo₂C and orthorhombic α-Mo₂C. The optimized lattice parameters, elastic constant matrixes and overlap population for Mo₂C crystal cells have been obtained to realize the characterization of the hardness and melting point of the two structures by the first-principles plane wave pseudo potential method based on the density functional theory. The results reveal that the calculated lattice parameters of the Mo₂C crystal cells agree with the experimental and other calculated data. The calculated melting point/hardness are 2715 K/11.38 GPa for β-Mo₂C and 2699 K/10.57~12.67 GPa for α-Mo₂C, respectively. The calculated results from the density of states (DOS) demonstrate that the hybridization effect between Mo-3d and C-2p states in α-Mo2C crystal cell is much stronger than that in β-Mo₂C one.     

Investigation on electrical transport properties of nanocrystalline WO<Subscript>3</Subscript> under high pressure

The electrical transport properties of nanocrystalline tungsten trioxides (WO₃) under high pressures have been investigated by various electrical measurements up to 36.5 GPa. The discontinuous changes in direct-current resistivity under high pressures result from two electronic phase transitions at 4.3 and 10.5 GPa and two structural phase transitions at 24.8 and 31.6 GPa. Hall-effect measurement shows that the nanocrystalline WO₃ is n-type semiconductor within the whole investigated pressure range. The carrier concentration decreases monotonously with increasing pressure, but mobility increases first and then decreases at 10.4 GPa. Through alternate-current impedance measurement, it can be found that the variation of the ratio of grain boundary resistance to grain resistance synchronizes with that of the mobility under high pressures, indicating that the grain boundary plays more important role in the carrier transport process of nanocrystalline WO₃. The discontinuous changes of resistance and relaxation frequency of grain and grain boundary also provide the evidence for electronic phase transitions.     

Pressure-induced structural phase transformations in silicon nanowires

High-pressure structural behavior of silicon nanowires is investigated up to approximately 22 GPa using angle dispersive X-ray diffraction measurements. Silicon nanowires transform from the cubic to the beta-tin phase at 7.5-10.5 GPa, to the Imma phase at approximately 14 GPa, and to the primitive hexagonal structure at approximately 16.2 GPa. On complete release of pressure, it transforms to the metastable R8 phase. The observed sequence of phase transitions is the same as that of bulk silicon. Though the X-ray diffraction experiments do not reveal any size effect, the pressure dependence of Raman modes shows that the behavior of nanowires is in between that of the bulk crystal and porous Si.     

Magneto-optical activity of f–f transitions in elpasolite Rb2NaDyF6

Absorption and magnetic circular dichroism (MCD) spectra of f–f transitions ⁶H_15/2 → ⁶F_3/2, ⁶F_5/2, ⁶(F_7/2 + H_5/2) have been measured in cubic crystal Rb₂NaDyF₆ with Dy³⁺ ions in centrosymmetrical O_h positions. Magneto-optical activities (MOA) of the transitions (the ratio of zero moments of the MCD and absorption bands) have been obtained from the corresponding spectra. Origins of the transitions MOA have been analyzed and theoretical estimations of the MOA values have been made. It turned out, that MOA of the transitions in the centrosymmetrical crystal Rb₂NaDyF₆ (being allowed by odd vibrations) are noticeably larger than those in non-centrosymmetrical compounds, where f–f transitions are allowed by static odd components of crystal field.     

Solid-solid transition temperatures in SrCl2 from differential thermal analyses to 0.6 GPa

The reversible phase transition between the high-temperature, cubic C1 and the low-temperature, orthorhombic C23 polymorphs of SrCl₂ has been investigated by differential thermal analysis under hydrostatic pressure to 0.63 GPa. The C23→C1 transition temperature varies linearly with pressure at the rate of 0.42₄ μK(Pa)^?1 from the highest pressures down to ca. 0.34 GPa, and also linearly with slope 1.73 μK(Pa)^?1 at pressures ?0.26 GPa; the reverse, C1→C23 transition is not observed or deduced ?0.21 GPa. The observed curvature for the C1–C23 phase boundary over the range ?0.26–0.34 GPa, 1000–1050 K can be attributed to intersection with the “diffuse” transition in C1; the latter transition, however, could not be observed unambiguously. Linear extrapolation to 0.1 MPa places the C23→C1 transition near 553 K, which implies that C23 - not C1 - is the stable low-temperature polymorph. The recently-investigated transitions in PbF₂ closely parallel these in in SrCl₂.     

Deposition and characterization of pulsed direct current magnetron sputtered Al95.5Cr2.5Si2 (N1 − xOx) thin films

Aluminum rich oxynitride thin films were prepared using pulsed direct current (DC) magnetron sputtering from an Al_95.5Cr_2.5Si₂ (at.%) target. Two series of films were deposited at 400 °C and 650 °C by changing the O₂/(O₂ + N₂) ratio in the reactive gas from 0% (pure nitrides) to 100% (pure oxides). The films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and nanoindentation. The results showed the existence of three different regions of microstructure and properties with respect to the oxygen concentration. For the samples deposited at 650 °C in the nitrogen rich region (O₂/(O₂ + N₂) ≤ 0.08), the formation of the h-AlN (002) and Al-N bond were confirmed by XRD and XPS measurements. The hardness of the films was around 30 GPa. In the intermediate region (0.08 ≤ O₂/(O₂ + N₂) ≤ 0.24), the presence of an amorphous structure and the shifting of the binding energies to lower values corresponding to non-stoichiometric compounds were observed and the hardness decreased to 12 GPa. The lowering of mechanical properties was attributed to the transition of the clean target to the reacted target under non-steady state deposition conditions. In the oxygen rich region (0.24 ≤ (O₂/(O₂ + N₂) ≤ 1), the existence of α-Al₂O₃-(113), α-Al₂O₃-(116) and Al-O bonds confirmed the domination of this phase in this region of deposition and the hardness increased again to 30-35 GPa. Films deposited at 400 °C showed the same behavior except in the oxygen rich region, where hardness remains low at about 12-14 GPa.     

Preparation of free–standing transparent titania nanotube array membranes

在由乙二醇、水、氟化铵组成的电解液中添加钼酸钠调节阳极附近的离子浓度, 制备出厚度大约为10微米的透明二氧化钛纳米管阵列薄膜. 所得二氧化钛是无定型结构, 在120℃水热处理可以将其转化成锐钛矿结构, 并保持薄膜的结构完整性. 该薄膜的透射率与其表面结构和晶体结构有关. 这种透明二氧化钛纳米管阵列薄膜可望应用于染料敏化太阳能电池.