Bridgman Anvil Apparatus Research Articles

Shift of Glass Transition Temperature under High Pressure for Ge<sub>20</sub>Te<sub>80 </sub>Glass

Amorphous solids prepared from their melt state exhibit glass transition phenomena upon heating. Derivatives of volume like viscosity, specific heat and thermal expansion coefficient show rapid changes at the glass transition temperature (Tg). In general, application f high pressure increases the Tg (a positive dTg/dP). This positive dTg/dP has been well understood with the Free Volume and Entropy models. However, there are few exceptions where a negative dTg/dP has been observed. It has been proposed that the glasses which undergo negative thermal expansion can exhibit a negative dTg/dP. In this study, electrical resistivity of semiconducting Ge20Te80 glass at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at Tg. The pressure dependence of Tg (dTg/dP) shows a decreasing trend (-dTg/dP). Chalcogenide glasses like Se, As2Se3 and As30Se30Te40 show a positive dTg/dP in contradiction to the present observation of negative dTg/dP. A model proposed by deNeufville and Rockstad finds a linear relationship between Tg and the optical band gap (Eg) when they are grouped according to their connectivity (Zav).Application of high pressure decreases the interatomic distance which in turn decreases the separation between the valence and conduction bands (optical band gap). This reduction in optical band gap shifts the glass transition to lower values. It is also suggested that the sign of the pressure derivative of Tg can be negative (-dTg/dP) if the thermal expansion coefficient is negative. Inelastic neutron diffraction studies show a negative thermal expansion coefficient for most of the Te based chalcogenide glasses. Hence, Ge20Te80 glass is uniquethat its pressure dependence of Tgobeys both thermodynamic and the Tg-Eg-Zav models.

Pressure dependence of glass transition in As2Te3 glass.

Amorphous solids prepared from their melt state exhibit glass transition phenomenon upon heating. Viscosity, specific heat, and thermal expansion coefficient of the amorphous solids show rapid changes at the glass transition temperature (Tg). Generally, application of high pressure increases the Tg and this increase (a positive dT(g)/dP) has been understood adequately with free volume and entropy models which are purely thermodynamic in origin. In this study, the electrical resistivity of semiconducting As(2)Te(3) glass at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at Tg. The Tg estimated from the slope change in the resistivity-temperature plot shows a decreasing trend (negative dT(g)/dP). The dT(g)/dP was found to be -2.36 °C/kbar for a linear fit and -2.99 °C/kbar for a polynomial fit in the pressure range 1 bar to 9 kbar. Chalcogenide glasses like Se, As(2)Se(3), and As(30)Se(30)Te(40) show a positive dT(g)/dP which is very well understood in terms of the thermodynamic models. The negative dT(g)/dP (which is generally uncommon in liquids) observed for As(2)Te(3) glass is against the predictions of the thermodynamic models. The Adam-Gibbs model of viscosity suggests a direct relationship between the isothermal pressure derivative of viscosity and the relaxational expansion coefficient. When the sign of the thermal expansion coefficient is negative, dT(g)/dP = Δk/Δα will be less than zero, which can result in a negative dT(g)/dP. In general, chalcogenides rich in tellurium show a negative thermal expansion coefficient (NTE) in the supercooled and stable liquid states. Hence, the negative dT(g)/dP observed in this study can be understood on the basis of the Adams-Gibbs model. An electronic model proposed by deNeufville and Rockstad finds a linear relation between Tg and the optical band gap (Eg) for covalent semiconducting glasses when they are grouped according to their average coordination number. The electrical band gap (ΔE) of As(2)Te(3) glass decreases with pressure. The optical and electrical band gaps are related as Eg = 2ΔE; thus, a negative dT(g)/dP is expected when As(2)Te(3) glass is subjected to high pressures. In this sense, As(2)Te(3) is a unique glass where its variation of Tg with pressure can be understood by both electronic and thermodynamic models.

High pressure characterization and modelling of CaCO3 powder mix in the Bridgman anvil apparatus

For investigating high pressure sintering processes, numerical models can be used. This will demand material models which give realistic mechanical response throughout the whole parameter space of the actual process. As the pressures become higher, the material density approaches its full theoretical value and the elastic part of the material properties becomes increasingly important. In this investigation, Poisson's ratio was determined using ultrasonic pulse-echo measurements. A new elastic model and an improved plasticity model were implemented into a user-defined material subroutine in a finite element (FE) code. To experimentally investigate the load displacement response and pressure distribution in powder compacts during pressing, a pressure instrumented Bridgman anvil apparatus was used. Validation of the FE model was conducted against experimental data from pressing experiments using two different start densities. The results show that the simulation model is indeed capable of reproducing load–thickness curves and pressure profiles reasonable close to the experimental curves.

Experimental characterization of CaCO 3 powder for use in compressible gaskets up to ultra-high pressure

High Pressure High Temperature (HPHT) equipment is used for commercially produce synthetic diamond and other polycrystalline products. The common denominator for almost all high-pressure systems is to use capsules where a powder material encloses the core material. In this work, the properties of CaCO 3 powder up to ultra-high pressure have been studied using an instrumented Bridgman anvil apparatus. Bismuth phase transformations were used as fix point calibrant. Three different parameters were studied, density, moisture and diametral support dependence on the load–thickness and pressure build-up. The experiments are done such as they can be used as validation and calibration of constitutive models for finite element simulations of the HPHT-process. The results show that increasing the density of the powder compact increases the load needed to reach maximum pressure. In addition, the residual stresses in the compact seem to delay the phase transition on the down-ramp. Moisture content within 0.5%–1.2% does not significantly influence the compaction properties of the discs. Diametral support increases the phase transition load.

An attempt to produce ex situ TTS to understand their mechanical formation conditions – The case of an ultra high purity iron

High pressure torsion (HPT) tests can be used to transform the structure of materials. Several studies in the literature highlight that such tests are a means of investigating certain formation mechanisms of Tribological Transformed Structures (TTS) outside contacts, as they reproduce conditions that are close to those found inside them (quasi-hydrostatic pressures and shearing) under conditions that are easier to control than in a real contact. As there is no unique path leading to deformation with TTS in real contacts, it has been decided to simplify the history of deformation as the first step in investigating the occurrence of TTS. In this study, a Bridgman anvil apparatus, which allows shearing samples in torsion under pressure, is used to test ultra high purity iron (99.99999%) with an initial grain size of 100 μm, under a mean pressure of 0.5 GPa. The microstructural investigations led to establishing a map identifying the refined zones within the volume of the sample and linked to contact conditions. This work considers these zones with the finest microstructure (size grains of about 220 nm) as “TTS”. The experimental study is coupled with finite element analysis to investigate the specificity of the stress and strain cycles of the microstructure produced. Although further work is needed to validate the suggested comparison to friction induced transformed layers, the results showed that “TTS” are produced in a specific sample volume linked to sliding/adhesion contact conditions. The coupling between experiments and modeling shows that “TTS” formation can be associated with high hydrostatic stress components, high local plastic strains and high strain gradients.

Inhomogeneous microstructural evolution of pure iron during high-pressure torsion

A Bridgman anvil apparatus (unconstrained high-pressure torsion configuration) was used to shear samples of ultra high purity iron (99.99999%) in torsion under pressure (0.5 GPa). The evolution of the resulting microstructure was characterized by focused ion beam imaging (FIB), electron backscattered diffraction mapping (EBSD) and automated crystal orientation mapping in a transmission electron microscope (ACOM-TEM). The initial large grains (100 μm) were fragmented first; with the resulting deformation causing low angle dislocation boundaries followed by a gradual increase in their misorientation, finally leading to the formation of fine structural or microstructural elements (substructure) with high misorientations. These final structural elements (about 220 nm) could be considered as grains, if viewed from the conventional terms to describe grain (as that being entirely delimited by continuous HAB). One possible mechanism of formation could be continuous dynamic recrystallization. Furthermore deformation caused by the tests led to continuous misorientation in the grains whereas a significant continuous misorientation gradient in the grains was highlighted. The formation of predominant sub-micronic crystalline structure resulted in a significant increase in strength while good ductility was maintained. At a microscopic scale there was evidence that the refinement first occurred in the near surface layer of the sample and then propagated through the whole volume with a necessary condition of adhesion at the interfaces sample/anvils.

High-pressure compaction modelling of calcite (CaCO 3) powder compact

Numerical simulation of manufacturing processes with working conditions at high pressure (above 1 GPa) requires constitutive data of the powder for the whole range of pressure and density. Most of the test apparatuses commonly used to obtain such data are only working in the lower pressure regions. Because of the absence of high-pressure data, many parameters have to be guessed or extrapolated. A material used in high-pressure applications is Calcite (CaCO 3). The material can be used as an insulator in high-pressure capsules it is also a common material in the earth core. An apparatus often used to generate high pressure during compaction is the Bridgman anvil apparatus. In this work experimental tests with a Bridgman anvil set-up using Calcite powder discs with different thicknesses were done. A nonlinear elastic-plastic cap model was developed to model the behaviour of powder material from low pressure and loose state to high pressure and solid state. The constitutive model was implemented in a finite element code. The constitutive data were identified by optimization of experimental data. Validation was done by numerically reproduce the mechanical behaviour of uni-axially pressing Calcite to different pressure (up to 5 GPa) including unloading. The load–displacement curves, density distribution and the surface displacement were measured and compared to the finite element results. The results of the compaction simulations agree reasonably well with the experimental results.

Formation conditions of mechanically modified superficial structures for two steels

The aim of this work was to study the formation mechanisms of tribologically transformed structures (TTS). As these transformations need not be associated with phase modifications but usually with grain refinement or nanostructuring, the term mechanically modified superficial structures (M2S2) has been proposed to name any intermediary modification of material structures between the initial and nanostructured states. Severe plastic deformation processes allow producing transformations of material in volume and reproducing conditions that are close to those found under contact. Thus, a Bridgman Anvil apparatus which allows shearing samples in torsion under pressure is used to test two carbon steels (AISI 1005, AISI 1045). This experimental study is coupled with finite element analysis used to simulate the test and to study the specificity of the stress and strain cycles of the transformed layers obtained. For both materials, M2S2 layers are produced. The results show that the two materials follow the same trends. M2S2 formation can be associated with high hydrostatic pressure, high plastic strains, high strain rates and high strain gradients. This work is a first step for proposing mechanical criteria for the M2S2/TTS formation, but also for studying the influence of the initial alloy (microstructure, composition) on these transformations.

Preparation, thermal expansion, high pressure and high temperature behavior of Al2(WO4)3

The titled compound Al2(WO4)3 was synthesized by a conventional solid state reaction and characterized by powder XRD. It crystallizes in an orthorhombic (Pbcn, No. 60) lattice, with unit cell parameters as 12.582(2), 9.051(1), 9.128(2) A, and V = 1039.5(3) (A)3. The compound was found to show negative thermal expansion (NTE) behavior in the temperature range of 25 to 850°C. The average linear NTE coefficient (α1), in this temperature range, was −1.5 × 10−6 K−1. The effect of pressure at ambient temperature, was studied by a Bridgman Anvil (BA) apparatus, to reveal that there is no irreversible phase transition up to 8 GPa. The effect of high pressure and high temperature on this compound was studied by a Toroid Anvil (TA) apparatus. This compound has a limited stability under high pressure and temperature, as it undergoes a decomposition to AlWO4 and WO3−x with a partial oxygen loss. As an off-shoot of this work, certain new modifications of WO3−x under pressure and temperature were observed, viz., monoclinic, tetragonal and an orthorhombic modifications at 5 GPa/1400°C, 3 GPa/900°C and 1.8 GPa/1030°C, respectively. The detailed XRD studies of the products are presented here.

The generation of mechanically mixed layers (MMLs) during sliding contact and the effects of lubricant thereon

In unlubricated and boundary lubricated sliding, materials touch only at a restricted number (typically n≈10) of isolated, typically microscopically small ‘contact spots’ that occupy but a small fraction of the macroscopic interfacial area. It is here that the load is supported by the local hardness of the softer of the two materials, and that friction and wear are generated. The intermittent local shear strains at the contact spots in the course of sliding, and eventually through their statistical movements of the entire top layers of wear tracks, are very large. This behavior has been simulated, both for dry sliding and lubrication, by means of stacked foils of pure copper and silver sheared under high superimposed pressure in a Bridgman-anvil apparatus. Strain hardening curves were obtained and the samples, now equivalent to material at wear tracks and specifically ‘mechanically mixed layers’ (MMLs), were examined microscopically by means of a variety of techniques. From the workhardening curves the coefficient of friction as well as the hardness of the MMLs was inferred. The experiment is complicated by a strong shear strain anisotropy, in fact comparable to that found at actual contact spots, namely rising from near zero strain at the anvil-sample interfaces and at the axial center of the samples to a maximum at the mid-plane and the circumference. Microscopic analysis by means of focused ion beam microscopy (FIBM) and secondary ion mass spectrometry (SIMS) revealed that in the course of sliding, MMLs are formed through the proliferation of ‘tongues’ where local folding of the material occurs. An unexpected and potentially highly important discovery was the bodily migration of volume elements of one of the metals through the other, e.g. of lumps of silver through copper and vice versa, without leaving a trace. This phenomenon was enhanced through oil lubrication.

Pulverization of rubber under high compression and shear

Non-cryogenic pulverization of rubber material was obtained under high compression and shear using a modified Bridgman Anvil apparatus. The effects of operating variables, such as temperature, normal and shear forces, shear rate, and residence time were examined, and optimum conditions for obtaining desired particle size distribution with minimum agglomeration were identified. Based on our strain energy storage theory, a criterion of pulverization was obtained and computational analysis of deformation of rubber using a Mooney-type equation for stored energy was performed. The numerical values for strain energy distribution in a rubber disk, which indicate potential pulverization under different compression and shear forces, were obtained using the ANSYS computer program. This information was used as a guide to obtain optimum operating conditions and design parameters for optimum design of the solid state shear extrusion (SSSE) pulverization process [H. Arastoopour, Single Screw Extruder for Solid State Shear Extrusion Pulverization, U.S. Patent No. 8,101,468 (1998)].

Pressure-induced crystalline to amorphous transition in hydroxylapatite

Hydroxylapatite, prepared by the reaction of an aqueous suspension of Ca(OH)2 with H3PO4 at pH>7 and sintered at 700°C was subjected to various pressures in a Bridgman anvil apparatus, and the retrieved material was examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) methods. The studies showed that the crystalline hydroxylapatite transforms irreversibly to an amorphous phase 2 GPa. The transformation occurs through an intermediate phase and is completed at around 10 GPa pressure.

What role for contact spots and dislocations in friction and wear?

Sixteen years of research on the properties of ‘contact spots’ of ductile materials in ‘adhesive’ wear are surveyed. They are the locations (typically occupying less than 1% of the macroscopic area of contact) at which solids are in load-bearing contact during wear. Their properties determine the coefficient of friction, and friction and Joule heat are generated at them. It was found that the mechanical behavior of contact spots is governed by ordinary dislocation plasticity. They remain elastic if the local pressure falls short of the indentation hardness of the softer side. In that case (almost) no wear occurs. Elastic contact spots are promoted by light loads and fine polishing. Ordinarily, wear particles are detached through tangenital shearing-off where, statistically, the two sides at contact spots momentarily interlock. The Holm-Archard wear ‘law’ is a simple direct result of this mechanism. However, quite typically the bulk of sliding takes place within adsorbed moisture films which are ubiquitous in our daily surroundings. Outside of contact spots the adsorbed moisture behaves much like ordinary water. At the typical contact spot the moisture is squeezed down to but two monomolecular layers; and the relative motion between sliding solids overwhelmingly takes place between these. This causes the prevalence of friction coefficients about μ ≈ 0.3 in our surroundings. The complex behavior of the absorbed water can cause stick-slip. As water is desorbed at ⋍ 170°C, the determination of flash temperatures at contact spots is critical. A corresponding theory was developed and verified in connection with experiments on graphite lubrication which depends on the presence of adsorbed moisture. Further, plastic contact spots and solid lubrication have been simulated by means of a Bridgman anvil apparatus. The results show that the same work-hardening behavior applies at contact spots as known from the bulk. They also proved that at contact spots intimate mixing of the materials of the two sides gives rise to a finely mixed layer which can amorphize by much the same dislocation mechanism as is believed to cause melting. The amorphous material promptly recrystallizes at least in the CuAg system. It is proposed to try and inhibit such recrystallization by selected lubricants, so as to promote beneficial wear-resistant tribo-films.

Bridgman Anvil with a Sintered Diamond Core for Phase Transformation Studies at High Pressures and High Temperatures.

A new Bridgman anvil composed of tungsten carbide with a sintered diamond core is described. The truncated face of the anvil is 26mm in diameter, the central part of which consists of the sintered diamond core 15mm in diameter. Using the newly developed Bridgman anvils, pressures up to about 20.5GPa were successfully generated at room temperature. Phase transformation experiments were carried out using the Bridgman anvils with an internal heating system. 5-10mg sample was charged in a single high-pressure high-temperature run. The disproportionation boundary of Zn2SiO4 modified spinel to a mixture of ZnSiO3 ilmenite and ZnO rocksalt was determined as P(GPa)=12.3+1.3×10-3T(°C) by the Bridgman anvil apparatus.

Effect of high pressure on the electrical resistivity of glassy oxygenated selenium

The application of pressure is one of the well-known methods for investigating the properties of glassy semiconductors ([1] and references cited therein). It is well established that small quantities of impurities can change the electrical and optical properties of some chalcogenide glasses [2]. In the case of amorphous selenium, the d.c. conductivity increases by six to seven orders of magnitude when small amounts (10 to 200 p.15.m.) of oxygen are added. Simultaneously the thermal activation energy of conductivity is lowered from 1.2 to 1.0 eV [3, 4]. Twaddell et al. [4] found the corresponding changes of the infrared transmission spectra in the range 4 to 25/~m and concluded that the conductivity alteration depended not only on the total concentration of impurity but also on the definite impurity bonding configuration. On the other hand, many other investigators found the conductivity of pure bulk glassy selenium to lie in the range 10-~0 to 10-13~-I cm -1 [5-12]. In this letter, a systematic study of the pressure and temperature dependence of the electrical resistivity of glassy oxygenated selenium is reported. The sample undergoes a continuous pressure-induced semiconductor-to-metal transition. The value of the thermal activation energy obtained for the electronic conduction for the samples agrees very well with the Motrs model [13]. The sample remains amorphous even up to 14GPa pressure, whereas pure glassy selenium crystallizes at 13 GPa pressure with a discontinuous transition to metallic state. Bulk glassy selenium (99.9999% purity) was obtained from Nuclear Fuel Complex, Hyderabad. The oxygenated glass Se + 400 p.p.m. O was prepared (without any special purifying) by incorporating oxygen in the form of SeO2. The mixture of SeO2 and selenium was heated in cleaned and evacuated (10-6 torr) quartz ampoules under argon atmosphere. The molten alloy (at 800 K) was agitated thoroughly in a rotary furnace with a speed of 10 r.p.m, for 2 days. Then the homogeneous glass was obtained by quenching the melt in an ice-water mixture. The glassy nature was confirmed by X-ray diffractometry. High pressure measurements were carried out in a Bridgman anvil apparatus up to 14GPa pressure under quasihydrostatic pressure environment. The details of high pressure apparatus and its calibration were published elsewhere [14, 15]. The electrical resistivity measurements at atmospheric pressure were made in a four-probe configuration using the van der Pauw technique. In calculating the electrical resis

Effect of pressure on the electrical resistivity of bulk amorphous Al23Te77 alloy under various stages of crystallization

Effect of pressure on the electrical resistivity of bulk amorphous Al23Te77 alloy under various stages of crystallization

Investigation of pressure induced electronic transitions in lanthanam, uranium and thorium

Pressure-induced transitions in lanthanum, uranium and thorium were investigated by measuring the room temperature thermoelectric power ( S) of these elements in the pressure range zero to one hundred kbar in a Bridgman anvil apparatus. In lanthanum two isostructural electronic transitions that are precursors to the structural changes at 23 kbar and 62 kbar are observed. Subtle electronic transitions are observed in uranium and thorium at 43 kbar and 30 kbar respectively. A schematic phase diagram of lanthanum and its similarity to that of uranium is discussed.

Time dependence of electrical resistance at high pressure in some tellurium based amorphous alloys

Electrical resistance measurements are reported for amorphous In 20Te 80 and Cu 25Au 5Te 70 alloys up to a pressure of 80 Kbar using a Bridgman anvil apparatus and a four lead arrangement to measure resistances. The amorphous samples are produced by liquisol quenching. The resistance shows time dependent changes which are analysed in detail. The contention that there is a pressure-induced transformation from the amorphous to the crystalline phase is confirmed by X-ray diffraction of samples recovered after they were pressurised to 35 Kbar in a hydrostatic environment.

Pressure-induced phase transitions in InBi

The electrical resistivity of single-crystal InBi has been measured in a hydrostatic pressure cell to 15 kbar and in a Bridgman anvil apparatus to 90 kbar. The resistivity decreases linearly with pressure to 19 kbar, where an abrupt jump indicates a first-order crystallographic transition. A distinct change in slope occurs at 40-45 kbar and is possibly due to a second-order displacive transition.

Notes on Compressible Gasket and Bridgman-Anvil Type High Pressure Apparatus

The squeezing process of a disc-shaped gasket, which is compressed between Bridgman anvils, is analyzed by a simple model. It is made clear that plastic flow and pressure distribution in the disc is governed by its thickness. A normalized characteristic thickness, which can be determined for each disc material by thickness measurement on squeezed discs, is defined and correlated experimentally with shear strength of the material under pressure. According to the correlation, a simple method is developed to evaluate substances for solid pressure materials. A Bridgman anvil apparatus and sample assemblies for it are so designed as to provide the best performance in a range of from 100 kbar to 200 kbar. Noncatalytic syntheses of diamond, stishovite and cubic boron nitride are described as examples of utilizing the apparatus. Some features of anvil fracture are also described.