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1.
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. 相似文献
2.
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. 相似文献
3.
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. 相似文献
4.
AbstractSiC 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. 相似文献
5.
The superconducting transition temperatures (T c's) for La 3S 4 and La 3Se 4 vary in a non-linear fashion under hydrostatic compression to 22 kbar. Both T c and the crystallographic transformation temperature rise with pressure for La 3S 4 and show a maximum near 10 kbar for La 3Se 4. Non-transforming isostructural La 3Te 4 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. 相似文献
6.
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. 相似文献
7.
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)。 相似文献
8.
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. 相似文献
9.
27Al MAS NMR spectra of synthetic calcium aluminoferrites, Ca 2Al xFe 2−xO 5 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. 相似文献
10.
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° 2 θ 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. 相似文献
11.
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. 相似文献
12.
AbstractThe variation of the electrical resistivity of amorphous simple metal alloy Mg 70Zn 30 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 相似文献
13.
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. 相似文献
14.
Measurements of the superconducting transition temperature T c for a transforming polycrystalline sample of V 3Si 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 . 相似文献
15.
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. 相似文献
16.
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. 相似文献
17.
Influence of alkalinity (OH -/SiO 2) 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 Al 2O 3·H 2O and NaAlO 2 are respectively used as aluminium sources. At an alkalinity higher than 0.39, zeolite beta can not be obtained when Al 2O 3·H 2O 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. 相似文献
18.
We report on the near-infrared (NIR) luminescent properties of Bi 2O 3-GeO 2 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 Bi 2O 3 concentration instead of assisting to construct Bi centers. 相似文献
19.
CeRhIn 5 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. 相似文献
20.
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 Rt 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, Rt, dropped by 30% to 50%. When a steel coverplate was used, total interface defeat occurred at 1.5 km/s. 相似文献
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