Effect of pressure on mechanical and optical properties of ThO2 and PuO2

We have used DFT + U with spin–orbit coupling to understand the effect of pressure on the mechanical and optical properties of ThO₂ and PuO₂. Both the compounds are mechanically stable in a cubic and an orthorhombic structure. The cubic AnO₂ has higher elastic constants and Bader charges on each atom than the orthorhombic AnO₂. With an increase in pressure on cubic AnO₂, elastic modulus increases for both the structures. Bader charge on AnO₂ and magnetic moment of PuO₂ decrease with an increase in pressure for cubic structure. The static refractive index and static dielectric function are higher for PuO₂ in both the cubic and orthorhombic structures as compared to that in ThO₂. The mechanical and optical properties drawn comply with the experimental outcomes.     

Full densification of zirconium carbide ceramics sintered under high pressure at low temperature

Zirconium carbide (ZrC) ceramic is one of the most important and promising materials with a high melting point. However, its low diffusion coefficient affects its densification behavior, which dramatically limits its engineering application. For polycrystal ceramic materials, sintering by thermal diffusion is the most widely used method for consolidation. But in view of the difficult densification behavior of ZrC, it is necessary to develop new sintering methods together with another driving force. In the study, we reported an ultra-high-pressure sintering strategy to fully densify ZrC ceramic under 1.7 GPa at 1600℃. The sintered sample exhibited high density, fine grain size, excellent mechanical properties, and a large number of crystal defects, including dislocation networks and walls, which were similar to those in deformed metals. Its hardness increased to 2057.44 HV0.1 because of its unique microstructure.     

Elasticity,mechanical and thermal properties of submicron h-AlN: in-situ high pressure ultrasonic study

Compressional (V_P) and shear (V_S) wave velocities of high pressure (P) and temperature (T) sintered submicron hexagonal aluminum nitride (h-AlN) have been measured up to 15 GPa using ultrasonic interferometry. Using finite strain equation-of-state approaches, yielding K_S0 = 293.1 ± 2.9 GPa, G₀ = 141.9 ± 1.4 GPa and K′_S = 3.72 ± 0.04, G′ = 0.52 ± 0.02. It is found that the Vickers hardness of submicron h-AlN is as high as 11.7 GPa, and its fracture toughness is up to ～4.25 MPa m^1/2. Meanwhile, based on the elastic wave velocities and density data, the elastic-related properties of h-AlN are derived as Debye temperature Θ₀ = 1006 K, Grüneisen parameter γ = 1.72 and acoustic thermal conductivity λ_acoustic = 196 W/mK. These results may provide a comprehensive elasticity and elastic-related properties of submicron h-AlN under high pressure, which may be of great importance for its uses at extreme conditions.     

Elastic and piezoelectric properties of PbTiO3 at room temperature

We have calculated the phonon dispersion curves of lead titanate (PbTiO₃) at room temperature. A lattice dynamical formalism using the shell model is used. This microscopic model includes the short-range interactions of axially symmetric type and the long-range Coulomb interactions and taking into account of the electronic polarizability of constituent ions. Zone center phonon and a few phonons in the first Brillouin zone are used for fitting. In general, we obtain the best agreement between the calculated and observed frequencies. The values of relevant parameters are critically analyzed. A calculation of the elastic and piezoelectric properties is presented and compared with the available experimental data.     

Structural transformations of Muscovite at high temperature by X-ray and neutron diffraction

Structural transformations of Muscovite at temperature up to 1095 °C were determined using powder X-ray and neutron diffraction. Data were collected at room temperature from preliminary heated and quenched samples at 650 °C, 980 °C and 1095 °C. X-ray data were interpreted by either Rietveld method and neutron data, which complete the structural information by a better assignation of oxygen positions. With neutron data atom position was refined by fitting Pair Distribution Functions. It was found to be a progressive but continuous microstructural change, with the formation of an increasingly disorganized structure, but the layered organization of muscovite is maintained up to 1095 °C. Rietveld refinements from X-ray confirm the 6 to 5 coordination of Al atoms above 650 °C. It induces some structural changes as the orientation and mutual position of tetrahedrons in silicate layers. Pair Distribution Function refinements show the weakening of the long range structural organization, above 5 Å. At lower distance, a local order is maintained and the preferential alignments of both alumina unit pairs and silica tetrahedron were observed. This residual structural order of high-temperature muscovite is favorable to the achievement of textured ceramics.     

Self-poling and electromechanical response of crystallographically textured PMN-32PT prepared by templated grain growth

Crystallographic texturing of ferroelectrics is known to improve the piezoelectrics response due to the alignment of optimal grain orientations in polycrystalline materials. Using high-energy x-ray diffraction, a ferroelastic self-poling effect was observed in crystallographically textured 0.68 Pb(Mg_1/3Nb_2/3)O₃− 0.32PbTiO₃ ceramic. It is shown that the BaTiO₃ platelet templates used to induce crystallographic texture imposed a biaxial strain causing ferroelastic domains to re-orient parallel to the template plate normal. In-situ high-energy x-ray diffraction was then used to characterize the response mechanisms of the material with applied electric fields. The textured ceramic produced a (111) lattice strain of 0.13% in the remanent state, and a 0.16% (111) unipolar lattice strain at 2 kV/mm while the untextured ceramic had a higher (111) lattice strain of 0.18% in the remanent state and a smaller (111) unipolar lattice strain at 2 kV/mm of 0.096%. This contrast in the strain magnitudes can be linked to the self-poling effect. A strain mechanism incorporating the self-poling effect is proposed, furthering our understanding of how crystallographic texture impacts the piezoelectric properties and providing a pathway for engineering the self-poling effect to further enhance material response.     

Manufacturing and deformation behavior of alumina and zirconia helical springs at room temperature

Ceramic helical springs with identical dimensions were produced by hard machining from alumina, alumina toughened zirconia (ATZ), and tetragonal zirconia polycrystals (TZP) stabilized with different oxides. According to the results of the spring constant determination under deformation rates of 3 mm/min, the deformation behavior of all ceramic springs obeys to Hook's law. However, variation of the deformation rate, tests under constant load, and spring recovery behavior revealed differences in the deformation behavior of alumina, TZP, and ATZ springs. Alumina springs exhibited time-independent deformation in all tests. In contrast, anelastic deformation at room temperature was demonstrated in all springs containing TZP. This deformation is completely reversible over a period of several days. Anelastic behavior is particularly pronounced in Y-TZP springs, whereas Ce-TZP springs exhibit comparatively very low but still reliably detectable anelasticity. Oxygen vacancies in the TZP ceramic are considered the most likely explanation for the anelastic behavior of TZP springs at room temperature.     

有机硅密封剂高温力学性能研究

在温度25~250℃对3类硫化体系的5种有机硅密封剂的拉伸强度、180o剥离强度和剪切强度的研究表明:随着温度的升高,5种有机硅密封剂的拉伸强度、180o剥离强度和剪切强度均不断下降,其中拉伸强度和剪切强度降低的速率递减。有机硅密封剂的高温性能变化与硫化体系类型密切相关,脱氢硫化体系HM321密封剂在25~150℃拉伸强度、剪切强度和180o剥离强度均最高,但在25~250℃拉伸强度、剪切强度和180o剥离强度保持率均最低。脱氨硫化体系XY-602S有机硅胶粘剂在232~250℃的拉伸强度和剪切强度最高,25~250℃的拉伸强度保持率也最高。     

In situ crystallization and elastic properties of transparent MgO–Al2O3–SiO2 glass‐ceramic

Glass‐ceramics (GC) generally possess enhanced mechanical properties compared to their parent glasses. The knowledge of how crystallization evolves and affects the mechanical properties with increasing temperature is essential to optimize the design of the crystallization cycle. In this study, we crystallized a glass of the MgO–Al₂O₃–SiO₂ system with nucleating agents TiO₂ and ZrO₂. The crystallization cycle comprised a 48 hour nucleation treatment at the glass‐transition temperature followed by a 10 hour growth step at a higher temperature. During this cycle, the evolution of crystalline phases was followed by high‐temperature X‐ray diffraction (HTXRD), which revealed the presence of karooite (MgO·2TiO₂), spinel (MgO·Al₂O₃), rutile (TiO₂), sillimanite (Al₂O₃·SiO₂), and sapphirine (4MgO·5Al₂O₃·2SiO₂). The same heat treatment was applied for in situ measurement of elastic properties: elastic modulus, E, shear modulus, G, and Poisson's ratio, ν. The evolution of these parameters during the heating path from room temperature to the final crystallization temperature and during the nucleation and the crystallization plateaus is discussed. E and G evolve significantly in the first two hours of the growth step. At the end of the crystallization process, the elastic and shear moduli of the GC were approximately 20% larger than those of the parent glass.     

In-situ X-ray observation of phase transformation of rhombohedral boron nitride under static high pressure and high temperature

In-situ X-ray diffraction study for phase transformation of rhombohedral boron nitride (rBN) to a denser phase was performed under static high pressure (HP) and high temperature (HT) up to 9 GPa and 1600 °C. It was found that the layer stacking sequence of rBN structure began to change at less than 1 GPa, and the phase transformation to wurtzite structure (wBN) was observed at 6–7 GPa and room temperature. After conversion to wBN, further transformation to the zincblend type cubic structure (cubic BN) at 8 GPa and 1400 °C was observed, which is quenchable and the P-T conditions yielding cBN form were similar to that from hexagonal boron nitride. The observed behavior of the phase transformation of rBN by using in-situ X-ray diffraction study is well consistent with the results obtained from the quenching experiment from HP/HT by using belt type HP apparatus.

No structural change was observed at 600°C isothermal compression up t0 8GPa, while wBN formation was observed at room temperature compression at 7 GPa. This variation of the transformation behavior under HT isothermal compression may essentially be caused by the reduction of shear stress which affects the rotation and/or slip of hexagonal plane of rBN under HP.     

Facile and highly efficient wet synthesis of nanocrystalline BiFeO3particles by reverse co-precipitation method

In this study a nanocrystalline bismuth ferrite BiFeO₃ (BFO) powder was synthesized using a novel, facile and highly efficient reverse co-precipitation method. Proposed method is also highly cost-efficient and environmentally friendly. It was demonstrated that the optimal calcination temperature for the preparation of phase-pure BFO is 400 °C, while at higher temperatures formation of undesirable crystal phases occur. X-Ray diffraction studies were employed for the investigation of crystal structure and phase composition of obtained powders. Scanning electron microscopy (SEM) was used to evaluate the morphology of synthesized powders. For piezoelectric studies, precursors powders were pressed into 12.5 mm diameter pellets and calcined at the same temperatures. Obtained materials show very good piezoelectric properties, as a result of high defects homogeneity and very low crystallite size. Determined piezoelectric coefficient is comparable with praseodymium, samarium or europium doped BiFeO₃ ceramic.     

Compressive response of PMMA microcellular foams at low and high strain rates

Microcellular foams are widely applied in various applications in both civil and military applications for barriers and energy absorption materials. Poly(methyl methacrylate) microcellular foams were fabricated via supercritical foaming method. Field emission scanning electron microscopy, differential scanning calorimetry, and mechanical test machine were used to visualize the foam structure and test the quasi‐static compression properties. Moreover, Split Hopkinson Bar (SHPB) setups were adopted to explore the dynamic compression properties. The experimental results show that the microcellular foams have homogeneous cell size distribution and exhibit superior compressive behavior at both quasi‐static and high strain rates. The mechanical properties depend on both foam density and strain rate. Strain rate effects are clearly observed. At quasi‐static strain rate and 7500 S^?1 regime, cell wall bucking and folding are the main failure mechanism. However, at high strain rate regime, softening phenomenon is observed. By roughly calculating the energy absorbed and the temperature rise, the temperature of the foams will rise up to as high as 130 °C after conducting high strain rate compression, and it is postulated that the generated heat will destroy the cell structure of the foams. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46044.     

Photoelectrochemical study of nickel base alloys oxide films formed at high temperature and high pressure water

The oxide film formed on nickel base alloys at high temperature and high pressure water exhibits semi-conducting properties evidenced by photocurrent generation when exposed to monochromatic light. The use of macro- and micro-photoelectrochemical techniques (PEC and MPEC) aims to identify the different semiconductor phases and their distribution in the oxide film.Three different nickel base alloys were corroded in recirculation loop at 325 °C in pressurised water reactor primary coolant conditions for different exposition durations.PEC experiments on these materials enable to obtain macroscopic energy spectra showing three contributions. The first one, with a band gap around 2.2 eV, was attributed to the presence of nickel hydroxide and/or nickel ferrite. The second one, with a band gap around 3.5 eV, was attributed to Cr₂O₃. The last contribution, with a band gap in the range of 4.1-4.5 eV, was attributed to the spinel phase Ni_1−xFe_xCr₂O₄. In addition, macroscopic potential spectra recorded at different energies highlight n-type semi-conduction behaviours for both oxides, Cr₂O₃ and Ni_1−xFe_xCr₂O₄.Moreover, MPEC images recorded at different energies exhibit contrasted regions in photocurrent, describing the distribution of nickel hydroxide and/or nickel ferrite and Cr₂O₃ in the oxide film at a micron scale.It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale.     

Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

One‐ or two‐dimensional channel structures and properties of piezoelectric composites via freeze‐casting

By employing carefully tailored tert‐butyl alcohol (TBA)‐based freeze‐casting parameters, a large amount of porosity (>70 vol%) and one or two‐dimensional pore channels created were produced into alkali niobate‐based (NKN) ceramics. The relationship between processing factors and microstructures has here been studied, in terms of (i) porosities controlled by adjusting the solid loading in the initial slurry and (ii) strategically attempted freezing direction to make varied pore channels, in which two freezing directions from the bottom or side of mold can produce unidirectional elongated and radially centrosymmetric microstructures, respectively. In addition to that, NKN/epoxy composites with 3‐1 or 3‐2 type polymer channels in the NKN matrix have been fabricated by infiltration of the polymer into the porous NKN hosts. The effect of the channel directions on the mechanical and piezoelectric properties of the composites was investigated for varied volume fractions of the active ceramic phase, mechanical loading, and the poling direction, leading to very high‐piezoelectric g₃₃ coefficients at >60 mV·m/N in the composites with unique channel structures.     

Preparation and infrared to visible upconversion luminescence of Yb2O3:Ho3+ nanocrystalline powders

Yb₂O₃:Ho³⁺ nanocrystalline powders were synthesized through a solid state reaction method. X-ray diffraction analysis and field emission scanning electron microscopy were used to analyze the phase composition and morphology of the powders. Then under the 980 nm excitation of laser diode, the fluorescence of the crystals was studied via a fluorescence spectrometer. The green and red emissions centered on 551 and 668 nm were observed, and the green band dominated the emission spectrum. The effect of the concentration of Ho³⁺ on the upconversion luminescence intensity was discussed and the possible upconversion emission mechanism was explained. It indicates that like other metal oxide nanoparticles, Yb₂O₃ could also be a potential host material for doping to prepare the upconversion phosphor.     

X-ray diffraction of multiwalled carbon nanotube under high pressure: Structural durability on static compression

The structural durability of multiwalled carbon nanotubes under hydrostatic and non-hydrostatic compression was examined by in situ X-ray powder diffraction at room temperature. No interlayer interaction such as sp³ hybridization that could lead to hexagonal diamond in graphite was observed under compression up to 52 GPa, even though the nanotubes were similar in compressibility to graphite. This result could be attributed to the nested structure, which makes the interlayer stacking of carbon atoms take on an irregular arrangement. Despite the history of non-hydrostatic compression, electron microscopic observation revealed that the structure remained nested tubular. This reversibility suggests the nanotubes have strong durability on non-hydrostatic compression under extreme pressures.     

Tensile mechanical properties and constitutive model for HTPB propellant at low temperature and high strain rate

To investigate the mechanical properties and fracture mechanisms of hydroxyl‐terminated polybutadiene (HTPB) propellant at low temperature and high strain rate, uniaxial tensile tests were conducted over the range of temperatures 233 to 298 K and strain rates 0.4 to 14.14 s^?1 using an INSTRON testing machine, and scanning electron microscope (SEM) was employed to observe the tensile fracture surfaces. The experimental results indicate that the deformation properties of HTPB propellant are remarkably influenced by temperature and strain rate. The characteristics of stress–strain curves at low temperatures are different from that at room temperature, and the effects of temperature and strain rate on the mechanical properties are closely related to the changes of properties and the fracture mechanisms of HTPB propellant. The dominating fracture mechanism depends much on the temperature and changes from the dewetting and matrix tearing at room temperature to the particle brittle fracture at low temperature, and the effect of strain rate only alters the mechanism in a quantitative manner. Finally, a nonlinear viscoelastic constitutive model incorporating the damage evolution and the effects of temperature and strain rate was developed to describe the stress responses of this propellant under the test conditions. During this process, the Schapery‐type constitutive theories were applied and one damage variable was considered to establish the damage evolution function. The overlap between experimental results and predicted results are generally good, which confirms that the developed constitutive model is valid, however, further researches should be done due to some drawbacks in describing the deformation behaviors at very large strain. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42104.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

Effect of pressure on electric generation of PZT(30/70) and PZT(52/48) ceramics near phase transition pressure

Impact experiment of Pb(Zr_0.3Ti_0.7)O₃ and Pb(Zr_0.52Ti_0.48)O₃ ceramics were conducted by empolying shock reverberation techniques within 3-7 GPa and X-ray diffraction patterns of these materials have been measured at pressure up to 32 GPa with a diamond anvil cell and synchrotron radiation. To refine the crystal structure, Rietveld analysis was performed and bulk moduli were calculated using Birch-Murnaghan equation of state. We found a tetragonal phase transforming to a cubic phase in Pb(Zr_0.3Ti_0.7)O₃ and Pb(Zr_0.52Ti_0.48)O₃ ceramics at ∼7.4 GPa and ∼4 GPa respectively. For dynamic pressure experiment, a metal flyer accelerated by a gas gun facility impacts into PZT ceramics to investigate electric energy. As pressure increased, output voltage of Pb(Zr_0.3Ti_0.7)O₃ and Pb(Zr_0.52Ti_0.48)O₃ ceramics slightly increased below ∼7 GPa and ∼4 GPa. But the voltage increased near ∼7 GPa and ∼4 GPa. From the result, we could confirm that the phase transition influenced the considerable effect on the electrical power generation.