Experimental characterization of CaCO3 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 CaCO₃ 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.     

Experimental characterisation of CaCO3 powder mix for high-pressure compaction modelling

The mechanical properties of powders at high pressures are difficult to measure and therefore such data are rarely reported in open literature. Available test equipment mainly operates in the low-pressure region, 0-200 MPa. Calcite (CaCO₃) is a mineral suitable for high-pressure processes, e.g. sintering of diamond compacts. It is also a very common material in the earth core and therefore of interest for geoscientists. In order to model the processes in the high-pressure region (above 1 GPa), knowledge of the mechanical properties of the powder in the entire pressure range is needed. Experimental studies have been conducted to investigate the pressure-density relationship of a CaCO₃ powder and also to correlate the relative density to elastic and strength properties using experimental results. Further, a methodology has been introduced to provide a foundation for an elastic-plastic constitutive model. The mechanical behaviour of a CaCO₃ powder mix has been investigated using the Brazilian disc test, uniaxial compression testing and closed die experiments. The experiments showed increasing elastic modulus and strength with increasing density. An empirical expression of the dependence of the bulk modulus on density has also been introduced.     

Simulation of high velocity compaction of powder in a rubber mould with characterization of silicone rubber and titanium powder using a modified split Hopkinson set-up

The paper introduces a method for characterization of silicone rubber and titanium powder in high velocity compaction using the split Hopkinson set-up. The impact test data has been used to estimate parameters in constitutive models for rubber and powder. A finite element study has been performed with different geometrical design of the high velocity compaction of titanium powder against an aluminium mandrel using a rubber mould as pressing medium. One goal of this study is to investigate if and how the manufacturing method can be applied for making dental copings.A conclusion of the experimental work is that it is possible to characterize rubber material and powder material for high velocity compaction of metal powder by the use of a modified split Hopkinson pressure bar set-up. The numerical simulation shows qualitatively good agreement with the experience from practical tests. In conclusion, the work shows the possibility to numerically study the geometric design and to optimize the densification behaviour of a complex high velocity compaction process.     

Fabrication of Transparent γ-Al2O3 from Nanosize Particles

The compaction and heat-treatment behavior of nanosize γ-Al₂O₃ powder (average diameter = 20 nm) was studied. A diamond anvil high-pressure cell was used to compact the powder at pressures up to 3 GPa, both in air at room temperature and under liquid nitrogen, followed by pressureless heat treatment at 800°C. For all conditions studied, the fabricated compacts were optically transparent. X-ray diffraction confirmed retention of the γ-phase. The compacts were also characterized before and after heat treatment by microhardness measurements and by transmission electron microscopy. For both ambient and cryogenic compaction, sample hardness increased with pressure, and heat treatment resulted in about a 50% increase in hardness independent of the initial green-state value. Samples compacted in LN₂ were significantly harder (up to 9.6 GPa) than those compacted in air. TEM examination revealed a random-dense-packed particle structure and interconnected porosity; interstitial void dimensions, however, were always less than the average particle diameter (20 nm). Observed effects on the increase in hardness could not be explained by microstructural changes normally attributed to increased compaction pressure or heat treatment, most notably densification. Alternative explanations are proposed.     

Study of cold powder compaction by using the discrete element method

The discrete element method (DEM), based on a soft-sphere approach, is commonly used to simulate powder compaction. With these simulations a new macroscopic constitutive relation can be formulated. It is able to de-scribe accurately the constitutive material of powders during the cold compaction process. However, the force-law used in the classical DEM formulation does not reproduce correctly the stress evolution during the high density compaction of powder. To overcome this limitation at a relative density of about 0.85, the high density model is used. This contact model can reproduce incompressibility effects in granular media by implementing the local solid fraction into the DEM software, using Voronoi cells. The first DEM simulations using the open-source YADE software show a fairly good agreement with the multi-particle finite element simulations and experimental results.     

无助剂AlN陶瓷的高压烧结制备及热导率

以碳热还原法生产的AlN粉体为原料,用国产六面顶压机,在5.0GPa,1 300～1 800℃,在无烧结助剂的情况下,高压烧结制备了AlN陶瓷.用X射线衍射、扫描电镜对高压烧结AlN陶瓷微观结构进行了表征.结果表明:经1 300℃烧结50 min制备的AlN陶瓷的相对密度达94.8%.经1 400℃烧结50min制备的AlN陶瓷的断裂模式为穿晶断裂.经1 800℃烧结50min制备的AlN陶瓷由单相多晶等轴晶粒组成,该样品的热导率达115.0W/(m·K).高压烧结制备的AlN陶瓷的晶格常数比AlN粉体的略有减小.高压烧结温度的提高和烧结时间的延长有助于提高AlN陶瓷的热导率.     

Experimental investigation of the elastoplastic response of aluminum silicate spray dried powder during cold compaction

Mechanical experiments have been designed and performed to investigate the elasto-plastic behaviour of green bodies formed from an aluminum silicate spray dried powder used for tiles production. Experiments have been executed on samples obtained from cold compaction into a cylindrical mould and include: uniaxial strain, equi-biaxial flexure and high-pressure triaxial compression/extension tests. Two types of powders have been used to realize the green body samples, differing in the values of water content, which have been taken equal to those usually employed in the industrial forming of traditional ceramics. Yielding of the green body during compaction has been characterized in terms of yield surface shape, failure envelope, and evolution of cohesion and void ratio with the forming pressure, confirming the validity of previously proposed constitutive models for dense materials obtained through cold compaction of granulates.     

Fabrication of Transparent Silicon Nitride from Nanosize Particles

Compaction of ultrafine silicon nitride (Si₃N₄) powder at high pressures and various temperatures followed by pressureless sintering was investigated. The powder, consisting of nearly spherical particles (16 nm in diameter) of amorphous stoichiometric Si₃N₄, was pressed in a diamond anvil cell under pressures up to 5 GPa and temperatures ranging from liquid nitrogen to 500°C. Quality of compaction, evaluated by visual transparency and hardness of the produced compacts, depended on the amount of adsorbed gases on the surface of the particles and on the temperature of compaction. Visually transparent compacts were produced by pressing the starting powder without outgassing in liquid nitrogen under 5 GPa. The transparent compacts exhibited a hardness of 1200 kg/mm² after pressing in the diamond anvil cell at 500°C for 3 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 1400°C in a tube furnace under nitrogen, the hardness of these samples increased to over 2000 kg/mm² and the visual transparency was maintained. The results demonstrated that transparency was maintained. The results demonstrated that transparent compacts of nanosize amorphous Si₃N₄ particles could be sintered to high hardness at relatively low temperatures without using sintering aids or applying pressure during sintering.     

A validation procedure for numerical models of ceramic powder pressing

This paper describes an experimental procedure to validate numerical models used to simulate powder pressing. It consists mainly of two steps: closed die uniaxial pressing followed by isostatic pressing. The uniaxial pressing causes a non-homogeneous density distribution in the pressing direction as a consequence of friction between die walls and powder. In the isostatic pressing, less compacted regions have a larger volumetric strain, resulting in a non-trivial shape of the re-compacted part, which computes indirectly the previous density distribution. Experimental data from both steps are compared to the results from finite element models. The Drucker-Prager/Cap constitutive model was used to represent the compaction of alumina powder. Several simulations covering a range of parameters obtained from the literature were performed to calibrate the model, through an inverse analysis. The developed procedure sheds a light in the methods to calibrate and/or validate constitutive models used for powder pressing.     

Modeling of effective design of high pressure anvils used for large scale commercial production of gem quality large single crystal diamond

In order to overcome disadvantages of the substantial cost associated with the tungsten carbide (WC) anvil used for large scale commercial production of gem quality large single crystal diamonds, we have developed a modeling of effective design of WC anvil by finite element method (FEM). Our results indicate that the bevel length (l) and the bevel angle (α) of anvil have a significant effect on the performance of large volume cubic high pressure apparatus (LV-CHPA). Firstly, to gain the same cell pressure, the system oil pressure is decreases as l decreases or α increases. Secondly, the probability of failure crack in the anvil is increases as l decrease to 12 mm or α increase to 43°. Thirdly, the optimum value of l and α should be 13 mm and 41.5°, which can increase the performance of LV-CHPA farthest. The results have been proved by technical production. This would greatly help to improve the cubic anvil type high pressure techniques.     

Cold compaction study of Armstrong Process® Ti-6Al-4V powders

The Armstrong Process® developed by Cristal US, Inc./International Titanium Powder, is an innovative, low-cost technology for producing Ti and Ti alloy powders in a one-step, continuous process. In this work, Armstrong Ti-6Al-4V powders were characterized and the cold compaction behavior of the powders were investigated in detail. As-received as well as milled powders were uniaxially die-pressed at designated pressures up to 690 MPa to form disk samples with different aspect ratios. Samples with high aspect ratio exhibited non-uniform density along the pressing axis and the density distribution was consistent with the result predicted by finite element analysis. The model developed from the linear regression analysis on the experimental density data can be used to predict density of compacts with different aspect ratios. In the studied pressure range, an empirical powder compaction equation was applied to linearize the green density — pressure relationship. Cold compaction parameters were obtained for the as-received and milled Armstrong Ti-6Al-4V powders.     

High P-T phase transformations and metastability in the Zr0.5Hf0.5O2 solid-solution ceramic

High pressure-temperature (P-T) phases of the Zr_xHf_1−xO₂ (x = 0.5) solid-solution have been stabilised in a CO₂ laser heated diamond anvil cell. At room-temperature the monoclinic to orthorhombic-I structural transformation is initiated at 5-8 GPa. The X-ray diffraction (XRD) studies show these two phases coexist to above ∼15 GPa. A progressive increase in the orthorhombic-I phase abundance occurs, to culminate in full conversion at ∼20 GPa. At this lower threshold of ∼20 GPa transformation to the orthorhombic-II (cotunnite) structure can be initiated by heating in the range of 600-1200 °C. Substantial conversion to the cotunnite phase occurs in the same temperature range at 25-30 GPa. Raman signatures have been assigned to the two orthorhombic high-pressure phases, aided by the qualitative assessment of the complementary XRD data. Decompression experiments show that phase mixture composites of these high pressure structures, possibly with enhanced tribological properties, can be recovered to ambient conditions.     

Constitutive modeling of the behaviour of cermet compacts during reaction sintering

This study deals with the identification of a constitutive equation describing the mechanical behaviour of a nickel ferrite based cermet during sintering. This constitutive equation considers the material as a continuum and may enable one to predict the densification behaviour of a powder under different thermal treatments and the impact of compact geometry, external loading on strain and stress generation. A classical viscous equation of the Newtonian type that includes a term describing free sintering densification has been chosen. The method used for the identification of the parameters of this equation is the one proposed Gillia et al., which is based on dilatometry measurement. It includes a stairway thermal cycle for the determination of the free sintering term and intermittent loading for estimating the viscosity. This approach has been successfully applied to nickel ferrite cermet. The model has been found to be adequate to model the densification behaviour up to 1250 °C, but experimental and theoretical efforts must be accomplished to describe the behaviour above this temperature, when the material exhibits swelling.     

Pressure induced reactions amongst calcium aluminate hydrate phases

The compressibilities of two AFm phases (strätlingite and calcium hemicarboaluminate hydrate) and hydrogarnet were obtained up to 5 GPa by using synchrotron high-pressure X-ray powder diffraction with a diamond anvil cell. The AFm phases show abrupt volume contraction regardless of the molecular size of the pressure-transmitting media. This volume discontinuity could be associated to a structural transition or to the movement of the weakly bound interlayer water molecules in the AFm structure. The experimental results seem to indicate that the pressure-induced dehydration is the dominant mechanism especially with hygroscopic pressure medium. The Birch–Murnaghan equation of state was used to compute the bulk modulus of the minerals. Due to the discontinuity in the pressure–volume diagram, a two stage bulk modulus of each AFm phase was calculated. The abnormal volume compressibility for the AFm phases caused a significant change to their bulk modulus. The reliability of this experiment is verified by comparing the bulk modulus of hydrogarnet with previous studies.     

A small-angle X-ray scattering study of local variations within powder compacts

This work used two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate the compaction behaviour of pre-gelatinised starch (PGS) and microcrystalline cellulose (MCC), which are commonly used as pharmaceutical excipients. By analysing azimuthal variations in scattering intensity, reproducible relationships were found between the compaction pressure, relative density and changes in the shapes of 2D-SAXS patterns for each material. These results indicated differences in the compaction mechanisms between PGS and MCC.The relationships also provided a means for investigating local variations in compaction behaviour within specimens prepared using different materials and compaction conditions. Relative density results from 2D-SAXS were consistent with expectations based on the effects of friction during compaction and appeared similar to data from other methods. In addition, however, 2D-SAXS measurements revealed local variations in the effective direction in which compaction occurred, with significant radial components observed near the die walls. This appeared to be consistent with the transfer of some compaction pressure to friction on the die wall. These observations represent an important advance, since other experimental methods do not easily reveal the direction of force transmission within the powder compact.     

Synthesis of diamond with high nitrogen concentration from powder catalyst-C-additive NaN3 by HPHT

Diamond crystals with high nitrogen concentration, 1000-2000 ppm, have been successfully synthesized from the system of powder catalyst (Fe-Ni)-C-additive NaN₃ in a cubic anvil high-pressure and high-temperature apparatus. The synthetic diamond crystals are cubo-octahedral or octahedral shape with a green or dark green color. The FTIR spectra of the diamond synthesized indicate that its nitrogen concentration increases with an increase of the NaN₃ additive. Furthermore, such additive increase also leads to an increase in the minimum temperature and pressure for graphite/diamond conversion. We found iron nitride in the sample synthesized with high content of NaN₃. We believe its presence is an indication that Fe content in the Fe-Ni alloy is reduced and the characteristics of catalyst are changed, leading to the increase of the minimum temperature and pressure for graphite/diamond conversion.     

Uniaxial compaction behaviour and elasticity of cohesive powders

The compression and compaction behaviour of bentonite, limestone and microcrystalline cellulose (MCC) — three cohesive powders widely used in industry were studied. Uniaxial compression was performed in a cylindrical die, 40 mm in diameter and 70 mm high, for three selected cohesive powder samples. The initial density, instantaneous density and tablet density were determined. The influence of maximum pressure and deformation rate was examined. The secant modulus of elasticity E_sec was calculated as a function of deformation rate v, maximum pressure p and powder sample. After compaction experiments in hydraulic press at three pressures - p = 30, 45 and 60 MPa - and two different deformation rates, the strength of the produced tablets was examined in a material strength testing machine.From uniaxial compression tests performed on the universal testing machine for loading and unloading, the modulus of elasticity E was calculated on the basis of the first linear phase of unloading. The total elastic recovery of tablets was also obtained.     

Refinement of Nanoscale Grain Structure in Bulk Titania via a Transformation-Assisted Consolidation (TAC) Method

Bulk nanocrystalline TiO₂ samples (100% rutile) with a relative density as high as 97% and a grain size of <20 nm have been produced via high-pressure (up to 8 GPa)/low-temperature (∼0.3 T _m, where T _m is the melting temperature) sintering, using a toroidal-type high-pressure apparatus. Nanophase TiO₂ powder with a metastable anatase structure and an initial grain size of ∼38 nm was used as the starting material. During sintering, the anatase phase transformed to either the rutile or srilankite phase, depending on the pressure–temperature ( P – T ) combination. The starting temperature of the anatase-to-rutile phase transformation decreased from ∼550°C at ambient pressure to ∼150°C at 2.5 GPa. Grain growth was limited by the low sintering temperature and the multiple nucleation events in the parent phase. The grain size of the transformed rutile decreased as the sintering pressure increased, which can be explained by the combined effect of increasing the nucleation rate and decreasing the growth rate with high pressure. We have demonstrated that it is possible to produce a dense sintered compact with a grain size even smaller than that of the starting powder. The high-pressure srilankite phase was observed at P – T conditions as low as 4.75 GPa and 250°C, respectively; however, unlike the anatase-to-rutile phase transformation, the rutile-to-srilankite phase-transformation temperature increased as the pressure increased. Also, in contrast to the irreversible anatase-to-rutile phase transformation, the srilankite will reversibly transform to rutile under the appropriate circumstances. This observation provides an opportunity to further refine the TiO₂ grain structure by switching the sintering conditions (temperature and pressure) between the regions in which the rutile or srilankite phase are stable.     

High-pressure gas-liquid equilibrium of the system carbon dioxide-citral at 50 and 70 °C

Measurements of high-pressure gas-liquid equilibria of the binary system carbon dioxide-citral were carried out in the present work. The knowledge of the phase equilibrium behaviour of this system is relevant with regard to the design and optimization of the supercritical deterpenation process. The measurements were carried out at 50 and 70 °C, in the pressure range 7.8-15.6 MPa, by means of a two-chamber gas-phase recirculation apparatus of 340 cm³. Both the liquid and the gas phase composition were measured. The data at 50 °C measured in this work were compared with literature data, whereas no comparison was possible at 70 °C because of their lack. The experimental data measured in this work were successfully correlated by means of a thermodynamic model based on the Peng-Robinson equation of state.     

Reaction Propagation Rates in HMX at High Pressure

We have measured the reaction propagation rate (RPR), or deflagration rate, in octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) powder in a diamond anvil cell over the pressure range 0.7–35 GPa. Numerical simulations of the RPR of pressurized HMX were also performed for comparison to the experimental results obtained. The simulated RPR values closely approximate the observed rates at pressures up to 3 GPa, and serve as a bridge to lower‐pressure deflagration rates for HMX in the literature. However, at higher pressures the simulated RPR values deviate significantly from our experimental results. This suggests that further refinement to the computational model is required for the calculated RPR values to approach those observed at higher pressures.