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This paper describes the development of acousto-optic tunable filter (AOTF) devices that are fabricated using in-house grown, high-quality single crystals of mercurous bromide (Hg2Br2). The Hg2Br2 crystals were grown using physical vapor transport (PVT). The quality of the Hg2Br2 single crystals are determined using high-resolution x-ray diffraction (HRXRD). The full width at half maximum (FWHM) of the (220) reflection was found to be 0.029 degrees, which is a significant improvement in the growth of Hg2Br2 bulk single crystals. The diffraction efficiencies of the fabricated AOTF device in the long wavelength infrared (LWIR) found to be 26% which is the highest reported. The device was tested for point detection and for imaging capabilities in the LWIR range. The study shows promising results using Hg2Br2 in LWIR AOTF-based technologies.

Thermal expansions of single crystals of mercurous halides grown by the physical vapor transport method have been investigated. The unit cell lattice parameters were measured by using x-ray diffraction, whereas, the bulk thermal expansion was investigated using the dilatometer instrument as a function of temperature. It was found by both techniques that mercurous halides exhibit positive linear thermal expansion along the a-axis and negative linear thermal expansion along the c-axis. However, the volume expansion in both methods was found to be positive. The coefficient of thermal expansion along the a-axis was significantly larger than the one along the c-axis giving a net positive volume expansion.

Structural and compositional inhomogeneities in the free-standing epitaxial films of rare earth iron garnet films are characterized using high-resolution X-ray diffraction and energy dispersive X-ray fluorescence. These analyses show a variation in the lattice parameters as well as in chemical compositions, displaying inconsistency from the top and the bottom of the film, suggesting an inhomogeneous distribution of rare earth content. With these variations, intricate changes often occur in the magneto-optics properties. While with conventional X-ray diffraction techniques, the films appeared to be free of gross defects, our high-resolution rocking curves reveal variations in defect density and multiple lattice parameters. These results suggest that epitaxial films presumed to be homogeneous based on conventional X-ray diffractometry, may not be so. Therefore, films found to exhibit exotic physical properties should be carefully examined for the presence of structural and compositional anomalies to optimize the magneto-optic response. If such defects are present, the experimental results may be compromised due to sample imperfections, rather than intrinsic phenomena.

Single crystals of mercurous halide were grown by physical vapor transport method (PVT). The orientation and the crystalline quality of the grown crystals were determined using high resolution x-ray diffraction (HRXRD) technique. The full width at half maximum (FWHM) of the grown mercurous bromide crystals was measured to be 0.13 degrees for (004) reflection, which is the best that has been achieved so far for PVT grown mercurous halide single crystals. The extended defects of the crystals were also analyzed using high resolution x-ray diffraction topography. Preliminary studies were carried out to evaluate the performance of the crystals on acousto-optic modulator (AOM) and gamma-ray detector applications. The results indicate the grown mercurous halide crystals are excellent materials for acousto-optic modulator device fabrication. The diffraction efficiencies of the fabricated AOM device with 1152 and 1523 nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The results also indicate the grown crystals are a promising material for gamma-ray detector application with a very high energy resolution of 1.86% FWHM.

Mercurous halides; mercurous iodide (Hg2I2), mercurous bromide (Hg2Br2) and mercurous chloride (Hg2Cl2) are high figure of merit materials for fabricating Acousto-optic (AO) devices that operate in the visible and infrared regions. Single crystals of mercurous halides were grown by physical vapor transport (PVT) method. Thermal expansion as well as the effect of annealing on the material are discussed. Mercurous halides show positive thermal expansion along "a" axis whereas no significant thermal expansion along "c" axis. The coefficients of thermal expansion of Hg2Cl2, Hg2Br2 and Hg2I2 are 6.72 × 10-5 °C-1, 6.44× 10-5 °C-1 and 6.08 × 10-5 °C-1, respectively. The optical band gap of Hg2Cl2 was calculated using the transmission spectra as 2.9 eV.

In this paper, artificial flocs, similar to the ones that are found in a recently deposited cohesive seabed, were prepared using Ca-montmorillonite and kaolinite (two of the most abundant clay minerals), guar gum (a polysaccharide closely resembling natural marine organic matter), and artificial sea water, in order to investigate the effect of the composition on their rheological response. Shear rate effects on viscosity of these suspensions were also systematically addressed. These effects typically have a strong influence on the overall rheological response of materials incorporating any polymeric substance. It was found that the addition of guar gum, even as little as 5% relative to the clay content, increases the viscosity of the floc suspensions significantly. The higher the guar-gum and clay content, the greater is the viscosity of the floc suspensions, within the guar-gum loading range investigated in this study. The floc suspensions with higher solid content were found to exhibit a slight shear thinning behavior, apparently due to the breakdown of the floc structure under high shear stress. This behavior is more pronounced in montmorillonite floc suspensions than in the kaolinite suspensions due to the differences in the clay mineral surface charge properties and surface-to-mass ratios. We explore different rheological models in their applicability to describing the observed viscous response and conclude that a cross-type formulation may be most appropriate.

The structural properties of molecular beam epitaxially (MBE)-grown Hg1−x Cd x Te epilayers on CdZnTe (211) substrate have been investigated using high-resolution x-ray topography and rocking curves. High-resolution x-ray diffraction 2θ–θ scans of (422) reflections were utilized in calculating the out-of-plane lattice parameters of the HgCdTe layer and the CdZnTe substrate. The lattice strain of the HgCdTe layer was evaluated using the in-plane measurements of the (311) reflection. Etching seemed to improve the surface of the substrate by removing any damage caused by polishing or any post-processing. In spite of some localized line dislocations, a remarkable quality of the MBE-grown HgCdTe layer was observed. The full width at half maximum values of the HgCdTe layer and the CdZnTe substrate were determined as 43 arc-s and 16.2 arc-s, respectively.

Strength characteristics of the flocculated aggregates (flocs) of clay minerals in an aqueous marine environment are important in many modeling applications, such as penetration of heavy objects in the cohesive seafloor, hydrodynamic transport of coastal and seafloor sediments, dredging, and remote sensing among many. Measuring floc strength accurately has been a difficult task due to the fragile, amorphous, and transient nature of the flocs. Here, we describe a micromechanical technique that was used to measure the tensile strength of soft flocculated aggregates produced from simulated marine clay sediments. In this study, artificial flocs similar to the ones that are found on and near the seabed, were prepared using Na-montmorillonite, guar gum, and sea salt. Using the micromechanical technique the average tensile strength of the flocs was found to be 667 ± 189 Pa. The range of tensile strength values is consistent with the nature of the flocs and the measurement technique.

An acousto-optic devices were designed and fabricated using high quality single crystals of mercurous halide (Hg2X2) that were grown by physical vapor transport method (PVT). The orientation and the crystalline quality of the grown crystals were determined using high resolution x-ray diffraction (HRXRD) technique. The full width at half maximum (FWHM) of the grown mercurous bromide (Hg2Br2) crystals was measured to be 0.13 degrees for (004) reflection, which is the best that has been achieved so far for PVT grown mercurous halide single crystals. The extended defects of the crystals were also analyzed using high resolution x-ray diffraction topography. Preliminary studies were carried out to evaluate the performance of the crystals on acousto-optic modulator (AOM) and acousto-optic tunable filter (AOTF) applications. The results indicate the grown mercurous halide crystals are excellent materials for acousto-optic modulator as well as acousto-optic tunable filter device fabrications. The diffraction efficiencies of the fabricated AOM device with 1152 and 1523nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The diffraction efficiencies of the fabricated AOTF device with 10600 nm wavelength laser found to be 26%.

The roles of crystalline and osmotic swelling in the overall swelling of a volume of clay are poorly understood. In this paper, the swelling behaviour is simulated using the discrete-element method (DEM). The interparticle forces include mechanical, attractive (van der Waals), and repulsive (double-layer and interlayer) forces. Except for the interlayer repulsion, rational force–displacement relations had been established in previous publications. A systematic molecular dynamic study has recently been completed to establish the interlayer force–displacement relation. These relations are synthesised into a DEM study in the present paper. Numerical experiments are conducted by first forming dry powder assemblies and then saturating them under constant volume. The changes in internal fabric and the swelling pressures developing on the walls are evaluated. It is found that particles expand and fill up the interparticle pore spaces. The predicted relation of void ratio against swelling pressure is found to be reasonably close to that experimentally measured by others in the past. The results also indicate that the crystalline swelling influences swelling pressure values only when the void ratio is smaller than a certain value.

Swelling of clays due to water saturation causes structural damage in civil engineering structures. At the microstructural level, swelling is caused by interlayer and interparticle (double-layer) forces. The present article is focused on the interlayer force with the specific objective of developing a force-displacement relation. On the basis of the variation of the pressure tensor with distance, the interlayer repulsive force is evaluated. The interlayer force-displacement relation is thus established. It is found that (a) the interlayer force for small particle separations can be hundreds of times the double-layer force at the corresponding separations, and (b) the interlayer force decays very quickly with increasing particle separation. The results indicate that the fundamental mechanism controlling swelling changes as the specimen9s density changes, implying that mitigation strategies should depend on the density of the soil. The interlayer force-displacement relation is needed in assembly analyses such as the discrete element analysis. An ongoing study indicates that the interlayer force does not contribute to the overall swelling pressure when the void ratio is larger than a certain value.

Swelling clays, also known as expansive clays, are encountered extensively all over the world. These clays are problematic for geotechnical engineering applications because of distress caused to structures and infrastructure as a result of swelling and swelling pressure. These clays are also often used in geoenvironmental engineering applications because of their effective barrier properties as liner materials, e.g., in landfills, ponds, and cutoff trenches. In this study, we report the effect of dielectric constant of fluid on the permeability and consolidation characteristics of Na-montmorillonite swelling clay to investigate the role of clay-fluid molecular interactions on the macroscale properties of the clay. A new “porous rigid wall, flexible wall” permeability device specifically designed for swelling clays that allows for accurate measurement of permeability, swelling pressure and consolidation characteristics, and evaluation of microstructure of swelling clays with fluids with different polaritie...

The relationship between soil suction and degree of water saturation (also known as the soil-water characteristic curve or SWCC) for a given soil depends on a number of variables such as the soil type, mineral type, fluid type, pore size distribution, and anisotropy. In addition, the SWCC is found to exhibit a form of hysteresis, in that it is different in wetting and drying. Design of structures founded on unsaturated soils requires an in-depth microscopic understanding of the variables controlling SWCC and the underlying hysteresis. Most of the past basic studies have focused on granular materials, with solid particles represented by spheres. The present study is focused on clayey soils involving platelike solids. The analysis method of molecular dynamics (MD) is used to understand the nature of clay-water capillary meniscus. On the basis of the observations from the MD analysis and the Young-Laplace equation, a model is proposed for predicting SWCC and the associated hysteresis. The microscopic paramet...

Expansive clays such as montmorillonite cause severe distress to infrastructure due to swelling. The swelling of montmorillonite clay is also the basis for its use in many commercial applications such as drilling muds in petroleum engineering, as landfill liners in environmental engineering and in making polymer clay nanocomposites. The focus of this work is to carry out a systematic experimental and numerical study to understand and model behavior of Na-montmorillonite at molecular and particulate level to find mechanism of swelling in the Na-montmorillonite interlayer. Experimental results show breakdown of particles with an increase in swelling of the clay. This phenomenon was numerically studied by developing a modified Discrete Element Method (DEM) model that incorporates the latest developments in both clay and computer science, and can simulate particle subdivision. DEM results show the role of particle subdivision on swelling and swelling pressure. In understanding the true mechanism of swelling, it is essential to incorporate the interactions between clay molecular structure and the interlayer water molecules. For bridging the length scales, we have also evaluated the stress deformation response of the clay molecular structure using Molecular Dynamic (MD) simulations. Simulation results show that the deformation in the clay molecular structure due to external stress is mostly due to deformation of the water molecules in the clay interlayer. A new experimental technique which enables us to capture the molecular changes in the clay molecular structure upon hydration is also developed. This work provides a foundation for multiscale modeling of swelling clays.

Capillary forces between discrete colloid particles are of interest in many areas of science and engineering. This study used molecular dynamics (MD) to examine capillary phenomena that develop in a clay–water–air system, which requires an understanding of capillary forces, contact angles, and meniscus curvatures. The capillary formed between two parallel pyrophyllite clay particles and pure water was studied using MD for various system parameters which included particle separation, particle thickness, and strength of the clay–water van der Waals force. For a given set of potential energy parameters, the contact angle was independent of particle separation (≈ 550), the radius of curvature depended on particle separation, and particle thickness had a negligible influence on contact angles and meniscus radii. The van der Waals energy parameters, however, had a profound influence on contact angles and meniscus radii. In all cases, capillary forces on clay particles calculated by MD agreed with values calculated using the classical Young–Laplace equation. When the separation distance is 40 A, the force was calculated to be 9.87 × 10− 10 N by Young–Laplace equation and 8.37 × 10− 10 N by MD.

While swelling of certain clay types is an old geotechnical engineering problem, the fundamentals governing swelling still remain poorly understood. It is known that interlayer (crystalline) and double-layer (inter-particle) forces are responsible for the overall swelling. One could study the combined behavior using a particle-based modeling technique such as the discrete element method. A suitable constitutive law (in this case, the relationship between inter-particle force-displacement) is needed for this purpose. There is inadequate information available for developing a forcedisplacement relation for the interlayer force. Furthermore, the transition from interlayer to double-layer swelling has not been studied in the past. In the present study, analyses are performed using a molecular-level analysis technique known as the molecular dynamics. The molecular dynamics is used to study the interlayer force-displacement relation for selected particle separations. The study is limited to a montmorillonite-water-NaCl system. Based on the results of the present study, and those from past molecular studies, the force-displacement relation needed for discrete element analysis is developed. The details of the discrete element study is reported in a companion paper.

The mechanisms underlying swelling of clays during water saturation still remain poorly understood. Swelling is generally understood to result from interlayer forces (in the range of small particle separations) and double-layer repulsive forces (in the range of large particle separations). Recently, a systematic molecular dynamic study was conducted (see the companion paper entitled “Behavior of Swelling Clays: A Molecular Dynamic Study by Amarasinghe and Anandarajah) (a) to develop an understanding of the interlayer and double-layer forces and (b) to develop a forcedisplacement relation covering the full range of small to large particle separations. In the present paper, the force-displacement relation developed from the results of the molecular dynamic study, as well as the previously developed relations for the double-layer repulsive force and van der Waals attractive force, are used in a discrete element simulation to study the swelling behavior of clays in a comprehensive manner.

The changes in the H-O-H stretching vibration in the interlayer water and Si-O stretching vibration of a Na-montmorillonite (MMT) structure in the FTIR (Fourier transform infrared) spectra provide insight into the effect of fluids of different dielectric constants on the clay structure. Mechanisms by which the different fluids of varying polarities enter into the clay interlayer and the rates at which these molecules interact with the clay structure and the interlayer water are studied at the molecular level using six different fluids with dielectric constants ranging from 110 to 2.4. The shift in H-O-H bending vibrations of interlayer water and changes in the Si-O vibration bands of MMT occur almost immediately after mixing with the solvent regardless of the polarity of the solvent. However, the extent and the rate of changes in H-O-H bending and Si-O stretching are dependent on the polarity of the solvent. Results show a very good correlation between the polarity of the solvent and the shift in H-O-H bending of interlayer water, and also between the polarity of the fluids and the d(001) spacing of the MMT-solvent samples. Low polar fluids such as methanol tend to make weak electrostatic interactions with clay surface oxygen and interlayer-water molecules, which result in an increase in interlayer spacing. Although, the alteration of the Si-O structure due to high polar molecules such as formamide is a continuous process, the influence of nonpolar fluids such as TCE (trichloroethylene) on the Si-O structure is almost instantaneous, which may result in high hydraulic conductivity in the clay.

Understanding the rates at which fluid flows into clay interlayers at the molecular level is fundamental to designing an effective clay barrier system. In this work, molecular interactions at the Na-montmorillonite (MMT)-water interface, emphasizing the flow properties of the clay interlayer, have been studied at the molecular and nanoscale level using polarized Fourier transform infrared (FT-IR) spectroscopic and X-ray diffraction (XRD) techniques. Clay-water slurries were smeared on inert gold-coated metal substrates for FT-IR experiments and slurries were smeared on quartz plates for XRD experiments. By analyzing the O-H stretching and H-O-H bending vibrations in clay slurries, it was concluded that the molecular behavior of interlayer water is significantly different from the molecular behavior of bulk water. With increasing clay-water interaction time, it was also seen that the Si-O stretching bands of clay are being significantly altered by the water molecules in the interlayer. Using these spectroscopic techniques we have estimated the time required for water to flow into the clay interlayer. Further, by analyzing the particle size of the clay using atomic force microscopy (AFM) imaging, we were able to estimate the flow velocity of the water in the clay interlayer. This velocity is found to be 3.23 x 10(-9) cm/s. This flow velocity was found to be of the same order of magnitude as the hydraulic conductivity of smectite-type clay reported elsewhere. Also described in this work is the correct positioning of the Si-O out-of-plane vibration band of MMT at the two-layer saturation level in the interlayer. This band was only observed in p-polarized spectra at 1211 cm(-1). Thus, we attribute this band to the Si-O out-of-plane vibration band.