Etching of diamond octahedrons at high temperatures and pressure with controlled oxygen partial pressure

Etching of a natural diamond octahedron was carried out at temperatures of 800 to 1400° C and at pressures of 15 and 40 kbar under controlled oxygen partial pressure in the range 10⁻¹⁷ to 10⁴ atm by use of oxygen buffers. Well-defined etch pits of equiangular triangule outline were formed. When the results were plotted based on logP _{O 2} versus 1/T, reversal of the pit orientation clearly occurred on a boundary curve expressed by an equation, logP _{O 2}=−9.0×10⁴/T+63, whereP _{O 2} (atm) andT (K) are oxygen partial pressure and temperature, respectively. Etch pits with the same orientation as an octahedral face were produced in a low temperature and highP _{O 2} region, and those with the opposite orientation, i.e. the same as for natural “trigon”, were produced in the other region.     

Maximizing bubble segregation at high liquid fluxes

This study is concerned with a common class of problem involving two phase separation of a dispersed gas flow from a continuous liquid flow under extreme processing conditions. Relatively fine spherical bubbles of order 500 μm were generated in the presence of a surfactant under a high shear rate within a rectangular, multi-channeled, cuboidal downcomer. Liquid fluxes, as high as 176 cm/s through each channel of the downcomer, sheared bubbles from a sintered surface mounted flush to the channel wall before disengaging the downcomer flow into a vertical vessel. Both high feed fluxes, up to 15 cm/s, and high gas fluxes, up to 5.5 cm/s, ensured a high gas holdup beneath the downcomer and the hindered rising of the bubbles. Enhanced bubble–liquid segregation was achieved using an arrangement of parallel inclined channels incorporated below the main vertical chamber. This novel device, referred to as the Reflux Flotation Cell, prevented the entrainment of bubbles to the underflow, and significantly reduced the liquid flux to overflow, even in the absence of a conventional froth zone. Extreme upward bubble surface fluxes of up to 600 s⁻¹ were achieved, while counter-current downward liquid fluxes reached 14.4 cm/s, arguably four times the bubble terminal rise velocity. Hence successful phase separation was achieved while operating well beyond the so-called flooding condition arising from extreme levels of gas and feed fluxes. This hydrodynamic arrangement should find application in increasing surfactant extraction rates in foam fractionation and ion flotation, gas absorption, and even particulate flotation.     

Laboratory tests at elevated temperatures for the prediction of the rates of pressure rise in hydrazine tanks at normal storage temperatures

Various types of laboratory equipment have been used to measure the rates of gas evolution from liquid phase decomposition of hydrazine and from surface decomposition at elevated temperatures. In general the rates were found to be independent of time after the first few days on test and the surface rates obeyed the Arrhenius relationship with respect to temperature, hence enabling extrapolations to be made to normal storage temperatures. Good agreement was obtained between the measured rate of pressure rise in a 32 litre, titanium alloy tank at 70°C and that predicted on the basis of samples tested in laboratory equipment at the same temperature. A procedure by which rates of pressure rise can be converted to rates of hydrazine decomposition, taking into account the solubilities of nitrogen and ammonia, is given.     

Interfacial reactions of zirconium and zirconium-alloy liquid metals with beryllia at elevated temperatures

Argonne National Laboratory and Integrated Thermal Sciences, Inc. are developing crucible materials for melting reactive metals. A major part of this effort involves identifying reusable materials because they would have little or no interaction with the molten metals at elevated temperatures. Sessile drop-type experiments have been performed using pure zirconium and stainless steel-zirconium alloys (e.g., HT9-15Zr) on beryllia (BeO) substrates. The system was heated in high-purity argon to about 2000°C, held for 5 minutes, and cooled to room temperature. An external video camera monitored the interfacial interaction and wetting behavior. The zirconium melted and wetted the BeO at 1600°C, far below its melting point (1855°C). Post-test examinations show beryllium and oxygen dissolving in the zirconium metal. In addition, zirconium infiltrated the BeO substrate. No third phase reaction product was present at the zirconium-beryllia interface either at the top of the substrate or in the infiltrated region. HT9-15Zr also reacted with BeO; the alloy infiltrated partially into the BeO and formed a reaction-like layer attached to the ceramic substrate at the interface with the solidified metal. The rest of the liquid metal alloy did not wet the reaction product band. The results indicate that BeO is a poor crucible for the present application, but the observed wetting and infiltration phenomena are relevant to understanding the behavior of the liquid metal-ceramic interfaces.     

Experiments on expanded liquid metals at high temperatures

The electrical resistivity of liquid tungsten was measured using electric pulse heating of the wires inside capillary tubes. Under fast heating (10 µs) or slow heating (50 µs), the wire expands and fills the inner cavity of the capillary. On the oscillogram traces of the voltage drop across the wire, one can see the phases solid, liquid, fast expansion, and then the moment when the cavity is filled with the metal. Using the voltage drop, current, and volume of the capillary cavity, one can calculate the electrical resistivity,, of the expanded metal. Tungsten densities from 7.5 to 1 g · cm^–3(3 x 10²² to 0.5 x 10²² atoms · cm^–3) were investigated at temperatures from 10 x 10³ to 14 x 10³ K. For these densities, the electrical resistivity increased from 0.5 to 5m·cm.     

Sound speed in liquid lead at high temperatures

A dynamic technique, the isobaric expansion experiment (IEX), is used to reach high-temperature and pressure states in liquid lead. A unique technique is described for making sound-speed measurements once a final equilibrium end state is obtained. Data over an extended density range are presented. The sound speed in liquid lead over this range appears to vary linearly with density and has no dependence on temperature within our experimental precision (±7 %).Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Modelling of deformations of high strength concrete at elevated temperatures

A constitutive model for the analysis of deformations of concrete subject to transient temperature and pressures is proposed. In these severe conditions concrete structures experience spalling phenomenon, which is the violent or non-violent breaking off of layers or pieces of concrete from the surface of a structural element when it is exposed to high and rapidly rising temperatures. This process can lead to a loss of load-bearing capacity, trough a loss of section and a loss of protection to steel reinforcement. Many different form of spalling exist, but probably the most dangerous is explosive spalling, because it is sudden and capable to result in a general collapse of the structure.The constitutive model includes thermo-chemical and mechanical damage for taking into account the deterioration of the material due to mechanical loads, high temperatures and chemical changes and it is introduced into a general coupled mathematical model of hygro-thermo-chemomechanical behaviour of concrete structures.In this constitutive model the so called free thermal strains, which are the concrete strains during first heating, are decomposed in three main contributions: thermal dilatation strains (treated in a manner usual in thermomechanics), shrinkage strains (modelled by means of the effective stress principle) and thermo-chemical strains (which take into account for the thermo-chemical decomposition of the concrete and which are related to thermo-chemical damage). Thermo-mechanical strains occurring during first heating of concrete under load, known as LITS (Load Induced Thermal Strains), are also included in the framework of thermodynamics of porous media. The proposed model is applied to an illustrative example that demonstrates its capabilities.     

Hypervelocity impact tests on coated thermoplastic films at cryogenic and elevated temperatures

The hypervelocity impact facility at Space Research Institute (SRI), Auburn University has recently completed a series of tests on coated thermoplastic films at cryogenic (40 K) and elevated temperatures (420 K). With a 1-in gap, two films were mounted in a frame that applied biaxial tension to each film. The materials were impacted with 40–100 μm soda lime spheres utilizing a plasma drag gun to accelerate the particles to velocities between 5 and 12 km/s. The facility diagnostics allow for the determination each particle's, size, velocity and impact location along with micrographs of the nature of the impact damage. This summary of the test includes a general overview of the nature of damage on the films along with representative impact micrographs of the impact sites.     

Prediction of spalling in fibre-reinforced high strength concrete at elevated temperatures

This experimental study investigates two spalling test methods, for small scale specimens made of fibre-reinforced high strength concrete exposed to elevated temperatures, namely a blowtorch test and a furnace test. The study also aims at developing a relationship between relative maximum pore pressure and spalling, in order to determine a threshold relative maximum pressure for predicting spalling in heated concrete. The test results showed that the blowtorch test was a more effective and economical test method for carrying out spalling tests in small, unloaded and unrestrained specimens. Since such small specimens almost always do not spall during furnace tests, a blowtorch spalling test can help to provide relevant and useful data for mitigation of spalling in real scale structural elements. Also, with a blowtorch spalling test, a clear relationship between relative maximum pore pressures and spalling in heated concrete was observed, and a threshold relative maximum pore pressure above which spalling is likely to occur was suggested.     

Molecular hydrogen pressure in macrocollectors during hydrogen charging of steel at elevated temperatures

When a metal is being charged with hydrogen by exposing it to omnidirectional pressure of a hydrogen-bearing medium, the molecular hydrogen pressure in macrocollectors reaches the partial hydrogen pressure in the medium. In the case of metals exposed to unidirectional pressure of a hydrogen-bearing medium, the hydrogen pressure in collectors situated near the absorbing surface reaches the level of the partial hydrogen pressure in the medium and approaches zero in collectors situated near the desorbing surface. An inert (in respect to hydrogen) gas present in a collector under pressure does not inhibit the accumulation of hydrogen in this collector.     

Dual-phase magnesia-zirconia ceramics with strength retention at elevated temperatures

Two-phase polycrystalline ceramics containing MgO and ZrO₂ were fabricated by pressureless sintering powder compacts in air to near theoretical density. MnO was added as a densification aid in most compositions. For samples fabricated with 20 vol% ZrO₂ and 80 vol% MgO (which actually contained 23 vol% ZrO₂(ss) after sintering because some of the MgO dissolved in zirconia), densities in excess of 98% theoretical were achieved at temperatures as low as about 1250° C. However, most of the samples were typically sintered at 1420±10° C. The grain sizes of the two phases, ZrO₂(ss) and MgO(ss), were of the order of 1.4m. Thermal etching of the specimens showed the presence of very uniform sized domains (approximately 240 nm in size) in zirconia grains. Some samples were also fabricated in which 8 mol% CaO was added in order to stabilize the high-temperature cubic polymorph of zirconia to room temperature. The grain sizes of the two phases in this composition were also of the order of 1.4m. No domains were observed in zirconia grains in CaO-doped samples. Fracture strength was measured as a function of volume fraction of zirconia. Strength values in excess of 500 MPa have been measured on samples fabricated with 40 vol% zirconia (the amount of zirconia (ss) is 43 vol%). Samples of similar composition but with CaO doping exhibited strength of the order of 300 MPa despite an essentially identical grain size and density. Fracture toughness of samples containing CaO was 3.0 MPa m^1/2 while that of the samples without CaO was 5.2 MPam^1/2. No monoclinic phase was observed on either the fracture or the ground surfaces of CaO-doped and undoped samples. Fracture strength and toughness, measured as a function of temperature up to 1000° C, were found to be nearly independent of temperature. The temperature independence of the strength suggests that strengthening and toughening in this material does not occur by transformation toughening.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

Abstract

A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al₁₃Cr₂ intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al₁₃Cr₂ primary intermetallic particles. The solidification of these powders is discussed in terms of solidification front velocity controlled by external heat flow and by the initial undercooling. Particles less than 10 μm in diameter undercool significantly before solidification. Segregation free microstructures occur in the fine <1 μm) particles, where the solidification front velocity exceeds the absolute stability velocity.

MST/1247a     

Deformation of PC/ABS alloys at elevated temperatures and high strain rates

The objective of this paper is to experimentally study the deformation behavior of the alloys of polycarbonate (PC) and acrylonitrile–butadiene–styrene (ABS) at elevated temperatures and high strain rates. Four kinds of PC/ABS alloys with the ratio of PC to ABS being 80:20, 60:40, 50:50 and 40:60 and three different strain rates 8.0 × 10² s⁻¹, 2.7 × 10³ s⁻¹ and 1.0 × 10⁴ s⁻¹ are considered. The Split Hopkinson Pressure Bar (SHPB) experiments are carried out at 293 K and 343 K, respectively. The curves of engineering stress and engineering strain and true stress and true strain are obtained for the PC/ABS alloys at different temperatures and different strain rates, respectively. The effects of temperature, strain rate and the fraction of ABS on the deformation behavior of PC/ABS alloys are discussed in details, and then a temperature and strain rate-dependent phenomenological constitutive model for PC/ABS alloys is developed.