High pressure semiconductor to metal transition in CrS and CrSe

The effect of pressure on the electrical resistivities of sintered chromium sulphide and chromium selenide have been determined from four-probe measurements up to 70 kbar. A transition to the metallic state was found in CrS at about 24 kbar, in CrSe at 4 kbar. The pressure-induced semi-conductor-to-metal transition in CrS is correlated with a monoclinic-to-hexagonal structural transition.     

Phase transitions in an ionic conductor CuTeBr crystal

The dilatometric and pressure studies were carried out for CuTeBr crystal. The P–T phase diagram was constructed within the range of 3kbar. A triple point was found at 70 °C and 0.3 kbar, beyond which a new phase appears.     

Critical Field and Shubnikov-de Haas Oscillations of κ-(BEDT-TTF)2Cu(NCS)2under Pressure

We have used a novel experimental method to study the crossover of an anisotropic superconductor from a possible Pauli limited superconducting state to a vortex limited superconducting state by applying pressure. The new apparatus combined a tuned tank circuit with a nonmetallic diamond anvil cell to measure the change in critical field with angle in -(BEDT-TTF)₂Cu(NCS)₂ at pressures up to 1.75 kbar and at temperatures down to 70 mK. The critical fields (in the perpendicular or parallel orientation to the conducting planes) have been found to decrease by more than 90% within less than 2 kbar of pressure. In the parallel orientation, at 1.75 kbar, we have seen a clear change from the ambient pressure behavior of the critical field with temperature at low temperatures. Up to P=1.75 kbar, the H_c2 phase diagram is in good agreement with the theoretical prediction for weakly coupled layered superconductors. We have also succeeded in measuring oscillations in the resistivity of the normal state at higher magnetic field which could be used to find the effective quasi-particle mass. The -orbit Shubnikov-de Haas frequency was found to increase at a rate of 44T/kbar. Our experiment opens the possibility for further investigations of the effective mass with pressure, especially because the setup is suitable for pulsed fields as well.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

The effect of pressure on the superconducting and crystallographic transitions of La3S4, La3Se4 and La3Te4

The superconducting transition temperatures (T_c's) for La₃S₄ and La₃Se₄ vary in a non-linear fashion under hydrostatic compression to 22 kbar. Both T_c and the crystallographic transformation temperature rise with pressure for La₃S₄ and show a maximum near 10 kbar for La₃Se₄. Non-transforming isostructural La₃Te₄ exhibits a linear pressure dependence of T_c. However, the telluride undergoes a pressure induced phase transformation at ～70 kbar. In this case the sign of the pressure dependence is different from the sulfide and selenide. Variation of T_c with the crystallographic transformation temperature shows a new and unexpected behavior.     

Geopolymerization reaction,microstructure and simulation of metakaolin-based geopolymers at extended Si/Al ratios

Metakaolin-based geopolymers were synthesized at Si/Al ratios of 1:1, 1.5:1, 2:1, 3:1, 4:1, and 5:1 by using silica fume as silica corrector to alter Si ratios. The microstructure and strength of these geopolymers were characterized through XRD, SEM, NMR and compressive strength measurements. The dissolving rates of Al and Si species in geopolymerization were measured, and freeze-dried N-A-S-H gel was characterized by FTIR spectra. Modelling and simulation were employed to calculate the binding energy of one Si atom and the total energy of geopolymers formed at various Si/Al ratios. At Si/Al ratio of 2:1, high concentrations of Si and Al species are dissolved from precursors, high contents of Si-O-T linkages are formed and the geopolymer is of high compressive strength. The mechanical strength of geopolymers at various Si/Al ratios is dependent on the formation of N-A-S-H gel, rather than the zeolitic nuclei or silicate derivatives. This study might provide fundamentals for the geopolymerization of mine tailings, which usually possess high Si/Al ratios.     

甘氨酸-硝酸盐燃烧法制备Beta-Al_2O_3固体电解质研究

甘氨酸(GNP)为还原剂、硝酸盐为氧化剂,利用甘氨酸-硝酸盐燃烧法制备beta-Al_2O_3前驱粉料。利用热分析(TG/DSC)、X射线衍射分析(XRD)、扫描电镜(SEM)、核磁共振(NMR)和交流阻抗谱(EIS)等测试技术对beta-Al_2O_3的合成工艺进行研究。结果表明:该法合成beta-Al_2O_3前驱粉料的温度为1150℃,比固相反应法低了150℃,平均粒径约为42.0nm,具有较好的成型和烧结性能。将素坯在1620℃保温烧结,得到的烧结体的结构中Al(Ⅳ)和Al(Ⅵ)分别位于δ=45和δ=-6附近,相对密度为97.6%;350℃时的离子电导率为0.046S·cm~(-1)。     

Solid-phase low temperature steam-assisted synthesis of thermal stable alumina nanowires

A simple steam-assisted solid phase synthesis method was developed for synthesis of boehmite nanowires in the presence of TEAOH surfactant. The boehmite nanowires had uniform diameters (12-16 nm) and length up to 1-2 microm. The morphology of the nanostructured wires was well preserved after being converted to pure gamma-Al2O3 by thermal treatment at 600 degrees C for 5 h. The nanowires of Al2O3 exhibited excellent thermal stability by retarding the phase transformation and maintaining the wires-like nanostructure after being aged up to 1300 degrees C by preventing sintering between particles at high temperatures. The surface areas of Al2O3 nanowires could be maintained as high as 68 m2/g at 1300 degrees C while the surface areas of Al2O3 micropowder shrank to 0.89 m2/g after same thermal treatment. Both in-situ XRD and 27Al NMR results indicated that the crystal structure of gamma-Al2O3 nanowires was not transformed to alpha-Al2O3 at 1300 degrees C whereas micropowder Al2O3 was fully converted to alpha-Al2O3 at 1100 degrees C.     

Quantitative Aspects of 27Al MAS NMR of Calcium Aluminoferrites

²⁷Al MAS NMR spectra of synthetic calcium aluminoferrites, Ca₂Al_xFe_2−xO₅ with x = 0.93, 1, 1.33, reveal only a few percent of the expected intensity for the ²⁷Al central transition, indicating that the calcium aluminoferrite phase in Portland cements can barely be observed by ²⁷Al MAS NMR. This result supports the use of ²⁷Al MAS NMR for quantitative analysis of the tricalcium aluminate phase in Portland cements.     

Characterization of geopolymers from compositionally and physically different Class F fly ashes

The alkali-activation technology of coal fly ash is one of several potential solutions to minimize the harmful disposal of fly ash. This study reports high-resolution characterization of the alkali-activated reaction products for two representative Korean Class F fly ashes, which are significantly different in compositional and physical characteristics. The analysis confirms that differences in the network modifying element content, the amorphous phase content, and the particle size lead to large differences in compressive strength. Chabazite-Na and Al-rich chabazite-Na are identified as major crystalline phases in the high strength samples, supporting the favoring formation of ABC-6 family of zeolitic precursors for the higher mechanical strength while the C–S–H formation from the high CaO content (or crystalline CaO) is not a major source of the strength. The XRD analysis shows that the presence of amorphous humps located at 27–29° 2θ is not a sufficient indicator of geopolymeric gel formation. In the ²⁹Si MAS NMR, some portion of −108 ppm Q₄(0Al) peak is not related to quartz, implying that this portion of Si atoms actively participate in geopolymerization. The ²⁷Al MAS NMR spectra exhibit more conversion of Al(V) and Al(VI) aluminum atoms into Al(IV) units in the higher strength sample, which can be an indication of more geopolymeric reaction.     

A structural investigation of the Cape York meteorite by transmission electron microscopy

Examination of the kamacite phase of the Cape York meteorite by TEM revealed the following principal microstructural features: subgrains, Neumann bands, slip traces, dislocation networks, dislocation loops and precipitate particles. Quantitative data on many of these features have been collected. The significance of a pre-terrestrial shock event in explaining the origin of several of these features is discussed and microstructural evidence is presented showing that the maximum shock pressure encountered by the meteorite was in the range 70 to 120 kbar. This result is at variance with that claimed by other workers.     

Effect of pressure on electrical resistivity of Mg70Zn30 metallic glass

Abstract

The variation of the electrical resistivity of amorphous simple metal alloy Mg₇₀Zn₃₀ as a function of non-hydrostatic pressure has been studied. Measurements have been made in the range 0–10 kbar. The resistivity is found to increase with increasing pressure.

MST/982     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.     

Pressure dependence of Tc and phase stability for V3Si

Measurements of the superconducting transition temperature T_c for a transforming polycrystalline sample of V₃Si have been made as a function of hydrostatic pressure up to ≈29 kbar. T_c increases over the entire pressure range with no evidence of the maximum at ≈24 kbar predicted by Chu and Testardi. A distinct break in the slope dT_c/dP does occur near 16 kbar which is coincident with the discontinuity in the pressure dependence of the lattice parameter reported by Blaugher $\text{et al}$.     

NMR Study of Chain Motion in Atactic Polypropylene at High Pressure

Deuteron solid-state NMR techniques at high pressure are used to study the chain dynamics in the amorphous polymer atactic polypropylene. The arrest of the structural relaxation above the glass-transition temperature T_gis investigated using one- and two-dimensional deuteron NMR spectra. The slow reorientation of the main chain segments is identified with the -process observed in mechanical relaxation experiments. On approaching the glass transition, the time scale of the collective motion of the main chain becomes longer very rapidly at decreasing temperatures. Along isobars, at pressure values up to 5 kbar, the temperature dependence of the logarithmic average correlation time is very well described by a Vogel–Fulcher function. The motion of the main chain is strongly dependent on the pressure, while its character is determined mainly by the distance to T_g. The introduction of the equation of state allows the investigation of the dynamic behavior on isothermal and isochoric paths on approaching T_g. It is found that along an isotherm the mobility as a function of the density is also of the Vogel–Fulcher form.     

Influence of hydrostatic pressure on the mechanical behavior and properties of unidirectional, laminated, graphite-fiber/epoxy-matrix thick composites

This paper reviews and gives new insight into earlier work by the author and his co-workers on the experimental investigation of the influence of superimposed hydrostatic pressure on the mechanical behavior and properties of the epoxy used for the matrix and unidirectionally laminated, graphite-fiber/ epoxy-matrix thick composites. The direction of the fibers was, respectively, 0°, 45° and 90° for the compressive test samples and 0°, 45° -45° and 90° for the shear samples.

Hydrostatic pressure induces very significant, often dramatic changes in the compressive and shear stress/ strain behavior of composites, and consequently in the elastic, yielding, deformation and fracture properties. The range of pressures covered for the compressive experiments was 1 bar to 4 kbar, and for the shear tests 1 bar to 6 kbar. The shear modulus (G) of the epoxy increased bilinearly with pressure, with the break, or the discontinuity point, occurring at 2 kbar. The compressive elastic modulus (E) and the shear modulus (G) of the composites increase in the same manner as for the epoxy. The break, which is located at 2 kbar, represents a pressure at which physical changes in the molecular motion of the matrix epoxy occur. That is, segmental motion of molecules between the cross-links is frozen in by 2 kbar pressure. This pressure is known as the secondary glass transition pressure of the epoxy at room temperature. Alternatively, the sub-zero secondary glass transition temperature of the epoxy is shifted to ambient temperature by 2 kbar pressure. The increase in the moduli may also be given a mechanical interpretation. The elastic or shear modulus of an isotropic, elastic material due to small compressive or shear deformations, respectively, superimposed on a finite volume deformation, which is caused by hydrostatic pressure, increases with pressure. Such an increase in E or G has been predicted using finite deformation theory of elasticity.

The normally brittle epoxy develops yielding when the superimposed hydrostatic pressure exceeds 2 kbar. The shear yield stress (1% off-set) of the epoxy increases linearly with pressure above 2 kbar. This kind of yielding behavior can be predicted by a pressure-dependent yield criterion. The compressive yield strength of the 45° and 90° composites increases bilinearly with pressure, and the shear yield strength of the 0°, 45° and 90° composites also increases bilinearly with pressure. This bilinear behavior is also due to the secondary glass transition pressure of the matrix epoxy, being located at 2 kbar. The fracture strength of the composites also increases with pressure linearly and the greatest increase occurs in the 45° composite in compression and in the −45° composite in shear. The fracture modes of the composites undergo changes with increasing hydrostatic pressure. For instance, the 0° composite undergoes a brittle-ductile transition under shear stress, while no such transition appears to set in under compressive stress. The fracture mode of the 45° composite changes from matrix failure at lower pressures to fiber failure at high pressures under shear stress.     

Influence of alkalinity on particle size distribution and crystalline structure in synthesis of zeolite beta

Influence of alkalinity (OH^-/SiO₂) on particle size distribution and crystalline structure in synthesis of zeolite beta at a short crystallization period (60 h) has been studied. The results indicate that the highest crystallinity of synthetic zeolite beta at alkalinity of 0.24, 0.35 occurs when Al₂O₃·H₂O and NaAlO₂ are respectively used as aluminium sources. At an alkalinity higher than 0.39, zeolite beta can not be obtained when Al₂O₃·H₂O is used as aluminium source. The difference of particle sizes between zeolite beta synthesized from gels of different alkalinity is 0.7–0.8 μm. The widest particle size distribution of zeolite beta synthesized occurs when the alkalinity is 0.23–0.24. Along with the increase of the alkalinity, zeolite beta synthesized changes as follows: vibration peak of 1071 cm⁻¹ by IR moves to a higher position, the framework Si/Al ratio determined by ²⁹Si MAS NMR first increases gradually and then decreases, but the change of OhAl/TdAl ratio determined by ²⁷Al MAS NMR is contrary to that of the framework of the Si/Al ratio. In addition, the Si(O⁻) site existing in the TEA-β is confirmed by NMR, and the mechanism of alkalinity influencing crystallization is discussed.     

Study on multiple near-infrared luminescent centers and effects of aluminum ions in Bi2O3-GeO2 glass system

We report on the near-infrared (NIR) luminescent properties of Bi₂O₃-GeO₂ glass. Apparent differences of NIR luminescence, decay properties and thermal stability manifest the co-existence of more than one active center in this system. The effects of Al ions introduced both unintentionally and intentionally were investigated. ²⁷Al NMR analysis proposes 4-fold Al coordination as dominant structure which can also provide the environment for active centers. We suggest that Al ions play a crucial role to restrain concentration quenching at higher Bi₂O₃ concentration instead of assisting to construct Bi centers.     

Specific Heat of CeRhIn5: Pressure-Driven Transition from Antiferromagnetism to Heavy-Fermion Superconductivity

CeRhIn₅ is known to show an unusual transition at a critical pressure of ～15 kbar. Specific-heat data show a gradual change in the zero-field “magnetic” specific-heat anomaly from one typical of antiferromagnetic ordering at ambient pressure to one more characteristic of a Kondo singlet ground state at 21 kbar. However, at 15 kbar there is a discontinuous change from an antiferromagnetic ground state to a superconducting ground state, and evidence of a weak thermodynamic first-order transition. Above the critical pressure, the low-energy excitations are characteristic of superconductivity with line nodes in the energy gap, and, at intermediate pressures, of extended gaplessness.     

Penetration of semi-infinite AD995 alumina targets by tungsten long rod penetrators from 1.5 to 3.5 km/s

The performance of confined AD995 Alumina against L/D 20 tungsten long rod penetrators was characterized through reverse ballistic testing. The semi-infinite ceramic target was cylindrical with a diameter approximately 30 times the rod diameter. The target configuration included a titanium confinement tube and a thick, aluminum coverplate. The impact conditions ranged from 1.5 to 3.5 km/s with three or four tests performed at each of six nominal impact velocities. Multiple radiographs obtained during the penetration process allowed measurement of the penetration velocity into the ceramic and the consumption velocity, or erosion rate, of the penetrator. The final depth of penetration was also measured.

Primary penetration approaches 75% of the hydrodynamic limit. Secondary penetration is very small, even at 3.5 km/s. The effective R_t value decreased from 90 kbar to 70 kbar with increasing impact velocity over the range of velocities tested.

In tests in which the ratio of target diameter to penetrator diameter was reduced to 15, R_t, dropped by 30% to 50%. When a steel coverplate was used, total interface defeat occurred at 1.5 km/s.