Cleaved NaCl Crystals Research Articles

Surface diffusivity of cleaved NaCl crystals as a function of humidity: Impedance spectroscopy measurements and implications for crack healing in rock salt

Rock salt offers an attractive host rock for geological storage applications, because of its naturally low permeability and the ability of excavation‐induced cracks to heal by fluid‐assisted diffusive mass transfer. However, while diffusive transport rates in bulk NaCl solution are rapid and well characterized, such data are not directly applicable to storage conditions where crack walls are coated with thin adsorbed water films. To reliably predict healing times in geological storage applications, data on mass transport rates in adsorbed films are needed. We determined the surface diffusivity in such films for conditions with absolute humidities (AH) ranging from 1 to 18 g/m3 (relative humidities (RH) of 4%–78%) by measuring the surface impedance of single NaCl crystals. We use the impedance results to calculate the effective surface diffusivity S = DδCusing the Nernst‐Einstein equation. TheS values obtained lie in the range 1 × 10−27 m3 s−1 at very dry conditions to 1 × 10−19 m3 s−1 for the deliquescence point at 296 K, which is in reasonable agreement with existing values for grain boundary diffusion under wet conditions. Estimates for the diffusivity D made assuming a film thickness δ of 50–90 nm and no major effects of thickness on the solubility C lie in the range of 1 × 10−14 to 8 × 10−12 m2 s−1 for the highest humidities studied (14–18 g/m3 AH, 60%–78% RH). For geological storage systems in rock salt, we predict S values between 1 × 10−22 – 8 × 10−18 m3 s−1. These imply crack healing rates 6 to 7 orders of magnitude lower than expected for brine‐filled cracks.

The influence of substrate temperature, substrate material and heat treatment on the micro structure of Ag/Si nanocomposite films prepared by r.f. co-sputtering

Nanocomposite films consisting of silver and silicon were manufactured by r.f. co-sputtering deposition. The influence of substrate temperatures varying from 20 to 200 °C and different substrate materials such as cleaved NaCl crystals, C-coated Cu grids and (100) Si single-crystal wafers was investigated. Subsequent heat treatment at different temperatures ranging from 500 to 800 °C and its effect on the microstructure of the deposited films was also investigated by using a combination of differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy, conventional transmission electron microscopy and high-resolution electron microscopy. The typical microstructure of the as-deposited film consisted of nanosized Ag particles embedded in an amorphous Si matrix. The crystallization temperature of the amorphous Si matrix was reduced from 550 to 490 °C with increasing Ag content in the films. The microstructure of the annealed Ag/Si film after crystallization consists of Si crystallites with sizes from several nanometres to hundreds of nanometres and Ag particles with average sizes from 12 nm (600 °C) to 28 nm (800 °C). Surface segregation of silver was also observed for the annealing temperatures of 600 °C and above. We suggest a possible explanation for the silver-induced lowering of the amorphous Si matrix crystallization temperature.

Epitaxial Growth of Al (100) Films under High Rate Deposition on Cleaved NaCl Crystals

High rate deposition of Al (99.999% in purity) from dual sources of W-baskets was carried out onto NaCI crystals which were cleaved in air and heated at temperatures of 523 to 643 K in an ordinary high vacuum (2 x 10 -3 Pa). From a transmission electron microscopic observation, it was found that when deposited in thickness of 80 to 200 nm at a substrate temperature of 623 K the films grew continuously with island structure in (100) orientation. Moreover, (100) flat single-crystal films thicker than 200 nm often developed without containing islands and they usually included numerous dislocations. Most dislocations were come out from the films during heating to a high temperature above 873 K in the electron microscope.

Preferentially Oriented Growth of Aluminum under High Rate Deposition on Cleaved NaCl Crystals

Simultaneous evaporation of Al (99.999% in purity) from dual sources of W baskets was carried out onto a surface of NaCl crystal cleaved in an ordinary high vacuum (10 -3 Pa), and the crystallinity of the deposited film was examined through a transmission electron microscope (TEM). The substrate temperature varied in the range from room temperature to 523 K during the preparation of Al films of thickness 100 nm. On the other hand, the deposition thickness was changed from 20 to 130 nm at a substrate temperature of 473 K. A continuous single-crystal film with the (111) plane of thickness more than 70 nm was obtained on a substrate heated to 473 K.

Calorimetric and other studies of intermetallic phase formation in Ag/Al bilayer thin films

Silver and aluminium bimetallic thin film couples have been studied using constant heating rate differential scanning calorimetry both on cleaned glass substrates and freshly cleaved NaCl crystals. The most striking feature was the occurrence of two maxima in the reaction rate during the formation of a single product phase, Ag2Al, suggesting a two-step growth process. The activation energy for this reaction was 0.98 eV in the first step and 0.93 eV in the second step. These values are in good agreement with values obtained by a different technique, i.e. Rutherford back-scattering. Transmission electron microscopy, thin film X-ray diffraction, the change in resistance of bilayer films with temperature and thermodynamic and kinetic analyses have been used to investigate the intermetallic phase formation. It was observed that substrate plays an important role in the kinetics of thin-film reaction.

Infrared spectroscopy of CO on NaCl(100) IV. Bandshape analysis

The infrared bandshape of monolayer CO on NaCl(100) at 4 K is slightly asymmetric and its bandwidth of 0.09 cm −1 is the narrowest observed for any surface molecule. From a variety of experiments we know that this band profile is determined by heterogeneous rather than homogeneous broadening. We offer an explanation for the experimentally observed bandshape by proposing that irregularities in the surface of the cleaved NaCl crystals give rise to many distinct two-dimensional domains of physisorbed CO. A first-order perturbation calculation, which generates the infrared cross-section as a function of domain size, has been used to estimate that on the order of 10 5 molecules reside in an average size domain. The accuracy of the calculation, whose Hamiltonian represents the intermolecular potential among CO molecules by electric multipole coupling, will be discussed. We use the same coupling Hamiltonian to explore the effect of submonolayer coverages, isotopic dilution, and surface heterogeneities on the infrared spectroscopy of CO on NaCl(100). Our calculations can account for some, but not all, of the observed spectroscopic data.

Effect of heat treatment on structure and resistivity of Bi-SiO cermet films

Since considerable changes take place during heat treatment of cermet films, an attempt was made to study the changes in structure and electrical properties accompanying the heat treatment. A detailed study was done for the films containing 80 and 60 at% bismuth. The Bi−SiO cermet films were prepared by the coevaporation technique on to well cleaned Corning 7059 glass substrates and freshly cleaved NaCl crystals, at room temperature (303 K) in a vacuum better than 1×10 −5 torr

The development of thermal etch pits on cleaved NaCl crystals in the presence of an electric field

The development of thermal etch pits on cleaved (100) surfaces of sodium chloride crystals was investigated in relation to an external DC electric field applied along the [100] direction. Measurements of pit size and density were made after isothermal and isochronic evaporation in fields with gradients ranging up to 130 kV m -1. Below a minimum (threshold) value of field gradient, the size and density of pits were unaffected by the presence of the field. In the presence of a threshold field gradient, evaporation resulted in the enhancement of nucleation of pits as judged by a marked increase in their density with no change in their size. At higher fields, enhanced pit nucleation and pit growth are evident. The field gradient threshold observed in relation to thermal pit development corresponded to previously reported threshold phenomena for enhanced evaporation from cleaved (100) surfaces of NaCl. Measurements of dislocation densities in regions below the evaporated surface showed a decrease in density with increasing field strength beyond the threshold value. These results are interpreted as suggesting a migration of charged dislocations in the presence of the DC field.

Decoration of NaCl with Au: Contamination effects

The traditional procedure for decorating cleaved NaCl crystals is found to be substantially affected by contaminants frequently found in high-vacuum systems. Transmission electron microscope photographs of Au decorant show effects caused by vapors from mechanical vacuum-pump oils, diffusion-pump oils, and other oils. No appreciable decoration of monatomic steps was observed in those cases where decoration was carried out in a sufficiently cleaned ion-pumped system.

Adsorption of water on sodium chloride: The effect of prior exposure to hydrogen chloride, carbon dioxide and water vapor

Water isotherms were obtained on cleaved NaCl crystals which had been treated with water vapor, HCl-H 2O vapor and CO 2-H 2O vapor. These treatments are known to produce surfaces containing hydroxide, chloride and bicarbonate. The water isotherms were in accord with proposed structures for hydrogen-bonded first-layer adsorption of one molecule of H 2O for each OH − site and one molecule of H 2O for two Cl − sites or two HCO 3 − sites. The adsorption of krypton reflected only the surface area changes produced by crystallite growth resulting from surface dissolution in the adsorbed water layer and subsequent recrystallization during outgassing. Vacuum heating of NaCl powder at 180°C and 250°C after exposure to water vapor gave evidence of the conversion of the hydroxide surface to chloride surface by bulk diffusion.

Loops and Spirals on Thermally Etched Surfaces of Cleaved NaCl Crystals

Freshly cleaved (100) surfaces of laboratory grown single crystals of NaCl were thermally etched by placing them in a muffle furnace on a platinum lid. In addition to the usual square etch pits, various closed loop patterns have been beautifully illustrated. It is conjectured that the circular loop patterns, having a central spiral, might have been formed as explained by Lang (1957), while the pattern similar to that originating at Frank-Read sources and the multiple loop pattern might have been formed at pairs of dislocations of opposite nature.