Polycrystalline NaCl Research Articles

New automated shear cell with diamond anvils for in situ studies of materials using X-ray diffraction

A new design of an automated shear cell with diamond anvils for X-ray in situ studies has been developed and manufactured. As compared with a previous design, the dimensions and weight of the new cell have been reduced almost by one half on retention of the pressure range up to 100 GPa. To demonstrate the efficiency of the apparatus, the investigations of the possibility to use polycrystalline NaCl as a sensor of pressure at diffraction studies of samples in nonhydrostatic compression conditions have been carried out.

The accuracy of diffraction patterns taken in a diamond-anvil cell using an X-ray diffractometer according to the Dobrovol’skii-Shvedov method

The accuracy of the diffraction patterns taken on a DRON-3 diffractometer by the Dobrovol’skii-Shvedov method in a diamond-anvil cell has been analyzed. The generally accepted reference materials (polycrystalline silicon and NaCl) have been used as samples. The long-term stability has been checked on a steel sample.

Orientation imaging microscopy of polycrystalline sodium chloride

A novel preparation technique is described that makes possible grain size analysis of polycrystalline NaCl using orientation imaging microscopy via electron backscatter diffraction (EBSD). The preparation methodology is specifically developed to overcome difficulties in preparing microporous NaCl for microscopy. The grain size and crystallographic texture of polycrystalline NaCl samples, prepared via solution pressure and sintered in the range of 650–780 °C, were able to be measured successfully with EBSD. The limitations of the preparation technique for EBSD analysis of NaCl are also discussed.

Catalytic characterization of pure SnO 2 and GeO 2 in methanol steam reforming

Structural changes of a variety of different SnO, SnO 2 and GeO 2 catalysts upon reduction in hydrogen were correlated with associated catalytic changes in methanol steam reforming. Studied systems include SnO, SnO 2 and GeO 2 thin film model catalysts prepared by vapour phase deposition and growth on polycrystalline NaCl surfaces and, for comparison, the corresponding pure oxide powder catalysts. Reduction of both the SnO 2 thin film and powder at around 673 K in 1 bar hydrogen leads to a substantial reduction of the bulk structure and yields a mixture of SnO 2 and metallic β-Sn. On the powder catalyst this transformation is fully reversible upon oxidation in 1 bar O 2 at 673 K. Strongly reduced thin films, however, can only be re-transformed to SnO 2 if the reduction temperature did not exceed 573 K. For GeO 2, the situation is more complex due to its polymorphism. Whereas the tetragonal phase is structurally stable during reduction, oxidation or catalytic reaction, a small part of the hexagonal phase is always transformed into the tetragonal at 673 K independent of the gas phase used. SnO 2 is highly active and CO 2 selective in methanol steam reforming, but the initial high activity drops considerably upon reduction between 373 and 573 K and almost complete catalyst deactivation is observed after reduction at 673 K, which is associated with the parallel formation of β-Sn. In close correlation to the structural results, the catalytic activity and selectivity can be restored upon an oxidative catalyst regeneration at 673 K. Tetragonal GeO 2 exhibits only a small activity and no pronounced selectivity to either CO or CO 2, at least after reduction. In its fully oxidized state release of surface/lattice oxygen results in a non-catalytic formation of CO 2 by oxidation of CO originating from catalytic dehydrogenation.

11511 真空蒸着法による合金薄膜の創製とTEM観察(表面工学,一般講演,学術講演)

This study was the manufacturing method of Al or Ni-Ti thin films by vapor deposition on polycrystalline NaCl. In the case which NaC1 was melted and solidified on a ceramic plate, it is effective to make a spherical dimple on the ceramic plate. The thin films were cut along grain boundary and in rectangles because of cracks which were made by difference of coefficient of thermal expansion between NaCl and ceramic. Al and Ni-Ti thin films were observed by transmission electron microscope (TEM) and their atoms arranged regularly. The Surface of Ni-Ti thin film was coated with golden color oxide film which made by remained water in the ceramic plate.

Grain boundary populations in wet and dry NaCl

The presence of water causes significant weakening of polycrystalline NaCl because fluid assisted recrystallisation and diffusion creep occurs along grain boundaries. In metals and ceramics, special low energy boundaries are often not wetted by fluids and this affects properties. The aim of the present paper is to use EBSD to determine whether special boundaries exist in NaCl and whether these are wetted. Wet and dry polycrystalline NaCl were examined, deformed by dislocation creep and statically and dynamically recrystallised. Coincident site lattice (CSL) boundaries in recovered material occurred with frequencies expected from the texture, whereas boundary populations of recrystallised material were significantly different from those expected from the texture, and may have been influenced by special boundaries, approximately parallel to {100} planes. Coincident site lattice boundaries became discontinuous with increasing strain in dry NaCl because of subgrain rotation. Both CSL and special boundaries migrated like general boundaries at low temperatures and are therefore likely to have been wetted.

Brillouin spectrometer interfaced with synchrotron radiation for simultaneous x-ray density and acoustic velocity measurements

We describe a new Brillouin spectrometer that has been installed on a synchrotron x-ray beamline for simultaneous measurements of sound velocities (by Brillouin scattering) and density (by x-ray diffraction). The spectrometer was installed at the 13-BM-D station (GSECARS) of the Advanced Photon Source. This unique facility has been tested in studies of transparent single crystal and polycrystalline materials at high pressure and temperature. The equation of state, acoustic velocities, and, hence, elastic moduli of materials as a function of pressure and temperature can now be determined without resort to a secondary pressure standard, such as the ruby fluorescence scale, or the equation of state of standard materials such as Au, Pt, or MgO, thus offering the potential to determine an absolute pressure scale. This article describes the design of the combined Brillouin-x-ray system and the first experimental results obtained. As a general-user facility, the system was designed to require minimal setup time and alignment of sensitive optics, run-time control and adjustments of optics from outside the experimental station, compatibility with powder and single crystal x-ray diffraction measurements, and no interference with other experimental techniques used on the beamline. To satisfy these requirements we adopted a novel optical design for the Brillouin system with a vertical scattering plane. Examples of measurements of acoustic velocities and elastic moduli on single crystal NaCl (B1) and polycrystalline NaCl (B2) at high pressure and single crystal velocities and elastic moduli of MgO at high temperature (to 600°C) and at high pressure are presented.

Kinetic Mechanism of ClONO2 Uptake on Polycrystalline Film of NaCl

Kinetic studies and the mechanism determination of ClONO2 uptake on polycrystalline NaCl were carried out using a coated-insert flow tube reactor combined with high-resolution, low-energy electron-impact mass spectrometer under the following conditions: p = 1-2 Torr, linear flow velocity v = 3.5-75 m s(-1), T = 293 and 387 K, [ClONO2] = (0.5-25) x 10(12) molecules cm(-3). The salt was deposited as a film from nonsaturated aqueous solution on the sliding rod. The temporal dependences of the uptake coefficient and the partial uptake coefficients leading to a formation of the prime Cl2 and HOCl products were measured for different ClONO2 concentrations. These dependences are established to be described by gamma = gamma0 exp(-t/tau) + gamma(s), gamma(0,s)(-1) = a(0,s) + b(0,s)[ClONO2]. In the framework of the proposed kinetic model, the data are explained and the main elementary kinetic parameters of the uptake are evaluated. The model is based on a combination of Langmuir adsorption, formation of surface complexes on initial active sites, Z(ch), followed by their unimolecular decomposition. Decomposition is proposed to proceed concurrently in two channels, one of which is a released surface site that conserves the properties of the initial site. In the other channel, the initial Z(ch) transforms into Z(ph) followed by steady-state uptake and reproduction of final Z(ph). The model gives an analytical expression for experimental parameters gamma0, gamma(s), and tau in terms of elementary rate constants and the reactant volume concentration. The final objective of the proposed model is the extrapolation of gamma0, gamma(s), and tau parameters to real tropospheric conditions.

Investigation of Subgrain Rotation Recrystallization in Dry Polycrystalline NaCl

NaCl is plastically anisotropic and forms a well developed substructure during deformation at 0.3-0.5Tm. EBSD was used to assess subgrain misorientations up to 0.5 true strain in dry NaCl. Equiaxed subgrains were ubiquitous but misorientations along segments of subgrain boundaries differed. Three types of subgrain boundary were identified: boundaries that surrounded equiaxed subgrains, boundaries that partly surrounded mantle subgrains, and extended subgrain boundaries, longer than the equiaxed subgrains. All of these subgrain features were recognised at low strains, <0.15. Misorientations of the majority of equiaxed subgrains were generally <2° at 0.5 strain, although segments could reach higher misorientations along kink-like boundaries. Mantle subgrains along grain boundaries tended to develop higher misorientations than in core subgrains. Long subgrain boundaries reached very high misorientations along segments of their length by 0.5 strain. Small new grains formed at triple points and more rarely within grains. Microstructures in NaCl are similar to those found in aluminium. Therefore, the dominant mechanism of high angle subgrain development at 0.5 strain and at 0.4Tm is probably an orientation splitting mechanism rather than equiaxed subgrain rotation.

Electroplasticity in metals and ceramics

The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75 T M and on the superplasticity of fine-grained oxides (MgO, Al 2O 3 and ZrO 2) at T>0.5 T M are considered. Regarding NaCl, at T≤0.5 T M an electric field E≥10 kV cm −1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm −1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5 T M, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5 T M, an internal electric field E≤0.3 kV cm −1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.

Plastic deformation of fine-grained Al 2O 3 in the presence of an electric field

Although polycrystalline ceramics are generally considered to be brittle, they can exhibit significant ductility at high homologous temperatures for certain conditions, namely a fine grain size (usually {le} 5 {micro}m) and non-stoichiometry. Prior studies by the present authors have shown that the concurrent application of an electric field of {approximately} 1 kV/cm during the tensile deformation of polycrystalline NaCl and 3Y-TZP (yttria-stabilized tetragonal zirconia polycrystals) gave a reduction in the flow stress and an increase in elongation. The present investigation was undertaken to determine the influence of an electric field on the plastic deformation of a fine-grained, polycrystalline Al{sub 2}O{sub 3}, which is normally considered to be highly stoichiometric. The objectives of the study were to determine: (a) the influence of a continuous electric field on the tensile stress-strain curve, (b) the degree of reversibility of any effect of the field on the flow stress and (c) the dependence of the flow stress on the field strength.

The rate-controlling mechanism(s) during plastic deformation of polycrystalline NaCl at 0.28–0.75 TM

The plastic deformation kinetics of polycrystalline 99.9% NaCl were determined in compression at 23–532°C (0.28–0.75TM) and a strain rate e = 8.3 × 10−4 s−1. The rate-controlling mechanism at 0.28–0.65 TM (σ/μ 3 × 10−4) were in accord with the Weertman-Dorn creep equation. At T > 0.5 TM the decrease in strain hardening with strain and temperature was attributed to cross slip, leading to a brittle-to-ductile transition at 0.5 TM. Dislocation climb was deduced to become more important at higher temperatures. The stress-strain curves were described reasonably well by the Bergstrom-Roberts dislocation multiplication model.

Some effects of an electric field on the plastic deformation of metals and ceramics

The external parameters generally considered in the plastic deformation of metals and ceramics are the temperature, pressure or stress and time. Usually neglected are the effects of electric and magnetic fields. However, such fields can often have a significant influence, especially when applied concurrently with the more common parameters. Some examples of the effects of an electric field on the plastic deformation of metals and ceramics found by the author and his coworkers are presented. Included are the following: (a) the influence of electropulsing on the flow stress of metals at 78–300 K, (b) the effect of an external electric field (surface charge) on the superplastic deformation of the 74754 Al alloy, (c) the influence of an electric field on the flow stress and ductility of polycrystalline NaCl at 0.28–0.75 TM and (d) the effect of an electric field on the super-plastic deformation of 3Y-TZP. Mechanisms responsible for the observed effects are considered.

Influence of an electric field on the plastic deformation of polycrystalline NaCl at elevated temperatures

The effect of a d.c. electric field of 1 kV/cm on the plastic deformation of polycrystalline 99.9% NaCl was determined in compression at 23–532°C (0.28–0.75 T M) and strain rate ε=8.3×10 −4 s −1. Upon removing the effect of Joule heating it was found that the reduction in the flow stress produced by the field increased with temperature from 0.28 to 0.4 T M, reached a maximum at 0.4 T M and then decreased, becoming nil at 0.75 T M. It is proposed that the reduction in flow stress resulted from an enhancement of cross-slip due to an increase in stacking fault energy. The field had no detectable effect on the plastic deformation kinetics at constant structure in the temperature range 0.28–0.6 T M (cutting of forest dislocations), nor did it appear to have a significant influence on dislocation glide and climb at 0.75 T M.

Enhancement of the ductility of polycrystalline NaCl by an electric field

The influence of an electric field E ≈ IkV/cm on the plastic deformation in tension of polycrystalline (do = 30μm) 99.9% NaCl was determined at 532°C (0.75Tm) using two electrode configurations. The following is a summary of the results and the conclusion derived therefrom: 1.1. The field produced a decrease in the flow stress and an appreciable increase in the ductility.2.2. The magnitude of the stress exponent n from strain rate cycling tests was 9.5, independent of strain and electric field. This suggests that the rate-controlling mechanism did not change with the field.3.3. Dynamic grain growth occurred during the tests without a field; grain refinement (dynamic recrystallization) occurred with the field.4.4. Turning the field “on” and “off” during the course of a test gave an increase in strain hardening when the field was turned “on” and a decrease when it was turned “off ”. The difference decreased with strain. ▪5.5. The results suggest that the field led to the development of a dislocation structure which was conductive to the nucleation of new grains. Moreover, the field retarded the growth of the new grains[10].

Influence of an electric field on the superplastic deformation of 3Y-TZP

In previous work by the authors it was found that an electric field of {approximately}1 kV/cm significantly reduced the plastic flow and fracture stresses of polycrystalline NaCl at room temperature. Subsequent studies determined that the electric field also lowered the brittle-to-ductile transition temperature and reduced the flow stress and increased the elongation at elevated temperatures. The objective of the present investigation was to determine whether an electric field has an influence on the superplastic (SP) deformation of yttria-stabilized, tetragonal ZrO{sub 2} (Y-TZP). Attention was directed to the effect of the field on the magnitude of the flow stress, the total elongation, grain growth and cavitation.

Electroplastic Effect in Cast Polycrystalline NaCl with Various Orientations of the Electric Field

The influence of various orientations and configurations of an electric field on the flow and fracture stresses in compression of cast polycrystalline NaCl was determined at room temperature. An electric field of }0.1 MV/m reduced the flow and fracture stresses by }50%, relatively independent of the orientation of the field. This field is one to two orders of magnitude lower than those previously reported to enhance {110} (110) dislocation motion in NaCl single crystals. It is concluded that in the present tests on polycrystals the electric field enhanced cross slip. Several mechanisms by which this might occur are proposed.

Effect of an electric field on the plastic deformation and fracture of polycrystalline NaCl

The influence of an electric field E ≤ 0.1 MV/m on the plastic flow and fracture of cast polycrystalline NaCl was determined in compression at room temperature. Without an electric field the major slip system was deduced to be 110 〈 110 〉, with impurity-cation-vacancy dipoles being the short-range obstacles to dislocation motion. The initial strain hardening rate was similar in magnitude to that for Stage II in single crystals; its decrease with strain was attributed to cross slip. Plastic flow preceded fracture, which was intergranular without an electric field. The yield, flow and fracture stresses and the strain hardening coefficient decreased with electric field, an approximately 50% decrease occurring at E = 0.1 MV/m. This field is one-to-two orders of magnitude lower than had previously been found to produce significant effects in NaCl single crystals. The fracture mode with the field was mixed cleavage and intergranular. It is concluded that the major effect of the electric field was to enhance cross slip, thereby reducing the rate of strain hardening and in turn the flow and fracture stresses.

A High-Pressure Test of Birch's Law

The compressional wave velocities of polycrystalline NaCl and KCl have been measured to over 17 gigapascals, with the use of Brillouin scattering and the diamond anvil cell. This pressure corresponds to 40% compression for NaCl and 60% compression for KCl (including the volume change across the B1-B2 transition). The data obey Birch's Law, which predicts that the velocity of each material is linear with density, except across the B1-B2 phase transition in KCl. This deviation from Birch's Law can be rationalized in terms of an interatomic potential model wherein the vibrational frequencies of the nearest neighbor bonds decrease when going to the eight-coordinated B2 structure from the six-coordinated B1 structure.

Comparative characteristics of the luminescence of crystalline and polycrystalline NaCl

The luminescence intensities of polycrystalline and crystalline NaCl specimens under a pressure of 28.5 GPa were determined experimentally. They differ approximately by a factor of 50. The excess radiation intensity associated with the polycrystalline specimens is explained by structural defects present initially in them.