Mechanical strength of silica aerogels

Pure silica aerogels are obtained by hypercritical evacuation of the solvent. The strength is measured by the three-point flexural test on monolithic parallelepipedic samples and by a diametral compression test on cylindrical samples. The stress-strain curve shows a perfect elastic behaviour and the “conchoidal” fracture morphology indicates that the material is as brittle as a conventional glass. The mechanical properties are followed as a function of the bulk density. Aerogels with the highest porosity (P > 95%) reveal a maximum flexural strength lower than 10⁻² MPa. A model is proposed to account for the obtained mechanical properties.     

The electrical properties of coal slag

The inorganic constituents of coal remaining after high temperature combustion in a MHD (Magnetohydrodynamic) power plant combustor form an iron-rich, “dirty” glass whose electrical properties are important in the operation of the MHD generator. In particular, alkali “seed” (K₂CO₃) is added to enhance the conductivity of the plasma so the slag layer which coats the walls and electrodes of the generator is rich in K₂O. We present results of a systematic study of the electrical conductivity of a Rosebud coal ash with graded amounts of K₂CO₃ added. At high temperatures, the conductivity curves are smooth with many ions contributing. At lower temperatures the curves become more complex with the presence of crystalline phases in the glass.     

Low-temperature synthesis of starting materials for β-barium metaborate (β-BBO) crystal growth

Synthesis of starting materials for β-barium metaborate (β-BBO) crysal growth by a low-temperature “carbonate” method has been investigated. Materials systhesized below the σ-β phase transformation temperature of BBO have been subjected to thermogravimetry, X-ray difraction and chemical analysis. The results show that an almost carbonless stoichiometric β-phase can be obtained without the need for the widely employed high-temperature “carbonate” synthesis. Some advantages of using the β-BBO material synthesized by the low-temperature “carbonate” method are emphasized.     

Organic inclusion complex strategy for designing nonlinear optical crystals for ultraviolet applications

Based on Molecular Engineering and Crystal Engineering concepts, a new method for designing nonlinear optical (NLO) crystal materials, the Organic Inclusion Complex (OIC) method has been demonstrated. In defining an appropriate optical transparent range with respect to the Molecular Engineering method, small organic molecules containing n-π conjugation were selected as “second order harmonic generation (SHG)-active units”(guests). Together with the Crystal Engineering method, chiral molecules were used as “molecular scaffolding” (hosts) to combine with the “SHG-active units” by hydrogen bonds. The former can provide a nonlinear optical effect. The latter leads to the OIC with a noncentrosymmetric structure and are expected to enhance the macroscopic nonlinearity in a synergistic mode of guest and host by inducing the guest molecular dipole alignment as well as other properties, such as thermal stability, mechanical strength and ease of growth. Here, we report two new inclusion complex crystals, urea-(d)tartaric acid (UDT) and urea-(dl)tartaric acid (UDLT) (here, d means dextral and dl means racemic). UDT and UDLT crystals belong to P2,2,2, and P2, space group, respectively. Bulk size crystals with high optical quality were successfully obtained from aqueous solution by using a temperature-lowering method. The experimental results show that these two crystals demonstrate higher NLO effects and shorter wavelength cutoff.     

GaSb/InAs heterojunctions grown by MOVPE: Effect of gas switching sequences on interface quality

The GaSb/InAs interface can be grown in two quite different ways either with In and Sb atoms forming the interface “InSb-like” or Ga and As atoms forming the interface “GaAs-like”. This is a result of both the Group III and Group V atoms changing at the interface. Different interfaces have been achieved in GaSb/InAs heterojunctions grown by atmospheric MOVPE using different gas switching sequences and the consequent changes in the electrical behaviour have been assessed using low field magnetotransport measurements. The results range from very poor (“GaAs-like”) to excellent (a particular “InSb-like”) interface. A further comparison is made to a previously used growth sequence for these structures. The effect of pauses during the interface sequence has also been investigated.     

Dynamic stability analysis of the solidification of binary melts in the presence of a mushy region: changeover of instability

A dynamic linear instability analysis of steady-state binary melt solidification with a mushy region has been carried out. Such an instability differs from a conventional morphological one of a planar solid–liquid interface and is connected with the perturbations in the steady-state solidification velocity. Solidification with a narrow mushy region has been revealed to be absolutely unstable with a monotonous instability of a “hard” type. An increase of the mushy region width leads to an instability changeover from the “hard” type to the oscillatory “soft” one. Both the critical changeover width and the neutral stability curves have been determined as the functions of relevant physical and operating parameters. The steady-state solidification regime with a broad mushy region is absolutely stable. Thus, the mushy region width has been shown to represent a stabilizing factor.     

An improved apparatus for measuring the inhomogeneity of glasses

An improved apparatus capable of quantatively measuring the inhomogeneity of glasses within several minutes was developed as a prototype for practical use. Inhomogeneities were measured for several kinds of glass articles on the market, such as plate, bottle and optical glasses etc. and a “relative homogeneity” has been proposed for the purpose of deciding a ranking of glasses measured in comparison to the powdered single crystal of CaF₂ as a standard material of uniformity.     

Low lying electronic tail spectrum of the attractive δ-potential random system by the path integral method and coherent state representation variational method

By using the path integral method to study the “partition function” and therefore the density of states of the attractive Frisch-Lloyd random system, it is found that the “partition function” is divergent. Therefore, we modify the Frisch-Lloyd model to a system with regular lattice sites on which the atoms are randomly distributed. Path integral and coherent state representation variational methods are applied to this random system; by using the effective Hamiltonian theory of a periodic potential with slowly varying envelope, the low lying tail spectrum which is consistent with the Lifshitz conjecture for a random system is obtained.     

Effect of steady ampoule rotation on radial dopant segregation in vertical Bridgman growth of GaSe

For vertical Bridgman growth of the nonlinear optical material GaSe in an ampoule sufficiently long that flow and dopant transport are not significantly influenced by the upper free surface, we show computationally that steady rotation about the ampoule axis strongly affects the flow and radial solid-phase dopant segregation. Radial segregation depends strongly on both growth rate U and rotation rate Ω over the ranges 0.25 μms⁻¹U3.0 μms⁻¹ and 0Ω270 rpm. For each growth rate considered, the overall radial segregation passes through two local maxima as Ω increases, before ultimately decreasing at large Ω. Rotation has only modest effects on interface deflection. Radial segregation computed using a model with isotropic conductivity (one-third the trace of the conductivity tensor) predicts much less radial segregation than the “correct” model using the anisotropic conductivity, with the segregation decreasing monotonically with Ω. Consideration of a model in which centrifugal acceleration is deliberately omitted shows that, as Ω increases, diminution and ultimately disappearance of the “secondary” vortex lying immediately above the interface is due to centrifugal buoyancy, while axial distension of the larger “primary” vortex above is due to Coriolis effects. These results, which are qualitatively different from those accounting for centrifugal buoyancy, suggest that several earlier computational and analytical predictions of rotating vertical Bridgman growth are either limited to rotation rates sufficiently low that centrifugal buoyancy is unimportant, or are artifacts associated with its neglect. The overall radial segregation depends approximately linearly on the product of and the growth rate U for the conditions considered, where is the segregation coefficient.     

Stress measurements in sol-gel films

Thin solid films of a wide variety of materials have received increased attention during the past decade. These films have been instrumental in the growth of numerous technologies. Until recently, “thin films” have primarily described layers of metallic or dielectric materials deposited onto substrates by evaporation, electron beam or ion beam techniques.

Advances in sol-gel technology have extended film research to include “glassy” materials of either crystalline, or amorphous nature. Sol-gel films can be tailor-made to accommodate a diverse range of applications due primarily to flexibility in chemical make up which determines the respective film's structure. One important characteristic of such films is their inherent residual stress. This inherent stress, and the stress the film introduces to the substrate as it is deposited, can result in a complex stress profile. While “thin” in the case of sol-gel films generally means <1 μm in thickness, large (10–100s of nm of retardance) inherent stress per unit thickness can severely limit a film's performance and subsequent application.

We describe our efforts to quantify the residual stress in silica-based sol-gel films as a function of several processing parameters.     

Anomalous paramagnetism in pressure quenched CdS

Very large “paramagnetic” on positive magnetization has been observed in “pressure quenched” samples of CdS. Pressure quenching is a formative process involving pressure release rates ≈10⁶ bar s⁻¹.

The pressure quenched samples were prepared by pressure quenching at room temperature from above 30 kbar, i.e. from above the insulation-to-metal like transition. magnetization as a function of magnetic field was measured at 293 and 77 K using a vibrating specimen magnetometer. A linear M versus H behavior is observed in fields above a few hundred gauss, with values of χ = (M/H) ≈ 10^-4 cgs units. In some specimens saturation occurs, while in others the magnetization passes through a maximum. The maximum value of the magnetic moment M observed is of the order of tens of gauss.     

Temperature quenching mechanisms for photoluminescence of MBE-grown chlorine-doped ZnSe epilayers

We report on a detailed investigation on the temperature-dependent behavior of photoluminescence from molecular beam epitaxy (MBE)-grown chlorine-doped ZnSe epilayers. The overwhelming neutral donor bound exciton (Cl⁰X) emission at 2.797 eV near the band edge with a full-width at half-maximum (FWHM) of 13 meV reveals the high crystalline quality of the samples used. In our experiments, the quick quenching of the Cl⁰X line above 200 K is mainly due to the presence of a nonradiative center with a thermal activation energy of 90 meV. The same activation energy and similar quenching tendency of the Cl⁰X line and the I₃ line at 2.713 eV indicate that they originate from the same physical mechanism. We demonstrate for the first time that the dominant decrease of the integrated intensity of the I₃ line is due to the thermal excitation of the “I₃ center”-bound excitons to its free exciton states, leaving the “I₃ centers” as efficient nonradiative centers. The optical performance of ZnSe materials is expected to be greatly improved if the density of the “I₃ center” can be controlled. The decrease in the luminescence intensity at moderately low temperature (30–200 K) of the Cl⁰X line is due to the thermal activation of neutral-donor-bound excitons (Cl⁰X) to free excitons.     

Crystallization of the E. coli polyamine-induced protein: a novel procedure based on the concept of ionic strength reducers

Nucleation and growth of macromolecular crystals occur in supersaturated solutions the properties of which depend on numerous parameters that influence macromolecular solubility. Detailed knowledge of the effects of those parameters is essential for crystallization. The concept of the so-called “ionic strength reducers” provides insight into the changes of solubility induced by organic solvents and hydrophilic polymers in aqueous electrolytic solutions. A simple and efficient procedure is presented which exploits the properties of ionic strength reducers in the crystallization of proteins. Using this procedure in the crystallization of the E.coli polyamine-induced protein, superior crystals compared to conventional techniques have been obtained. The procedure combines microseeding with dialysis techniques and is applicable to other proteins, particularly in cases where conditions favoring both for nucleation and growth cannot be found, or in cases where excessive nucleation leads to the growth of a large number of very small crystals.     

Low-frequency internal friction of metallic glasses near Tg A critique of the use of the torsion pendulum

The application of the torsion pendulum to internal friction measurements on metallic glasses near the glass transition is critically analysed. It is pointed out that a difference between the measured damping Q_t⁻¹ and the real internal friction Q_s⁻¹ of the mterial must be taken into account; quantitative expression are given for the inverted and Collette types of torsion pendulum. The Collette pendulum is found particularly useful for measuring the internal friction of metallic glasses during slow heating through the glass transition. The results obtained with this instrument on six different Pd- and Ni-based glasses, as well as theoretical evidence, suggest that the extremely large internal friction “peak” above T_g is generally due to the intervention of crystallization after a large eand monotonous increase of viscoelastic damping in the supercooled liquid. In particular, there is no vidence for a real internal friction peak due to the glass transition. This conclusion is contradictory to some internal friction results found in the literature which have been obtained with a conventional inverted torsion pendulum. For this type of pendulum, the difference between Q_t⁻¹ and Q_s⁻¹ has obviously not been sufficiently tajen into account.     

Study of PbO films on sodium borate glasses part II: Formation of interdiffusion layers at the glass surface

By selecting carefully controlled conditions for the thermal treatment of sodium borate glasses coated with PbO films, it is possible to prevent Pb²⁺ ions from penetrating deeply into the glass. For low alkali glasses, an interdiffusion layer can be formed, which sub-surface cation profiling (by ion beam induced radiation) shows is a solid solution of Na₂O and B₂O₃ in PbO which acts as solvent. Experiments with 18.0 mol-% Na₂O glass show that it is possible to transform such an interdiffusion layer into a second type in which Na₂O and B₂O₃ together act as solvent and the concentration of PbO solute varies through the layer. For both types of layer the Na₂O : B₂O₃ ratio is different from that of the glass substrate, and for the production of the second type of layer it is shown that an important factor connected with penetration of PbO into the glass surface is a “sweating” process in which thermal treatment of the glass, even in the absence of PbO, results in migration of Na⁺ ions so that the glass surface has a Na₂O content higher than that of the bulk glass. Changes in the UV spectra of the Pb²⁺ ion are correlated with the formation of the interdiffusion layers, and results show that types of layer have optical basicities significantly greater than that of the glass substrate, through either the high PbO or high Na₂O content.     

LPE growth of GaAs by yo-yo solute feeding method

The effect of gravity on both dissolution and growth of GaAs in the Ga---As system has been investigated using a horizontal “sandwich” system consisting of a substrate-solution-substrate configuration. Remarkable differences were observed in growth and dissolution between upper and lower substrates. These phenomena were attributed to the solutal convection driven by a concentration gradient. Based on these facts, a layer with a thickness of about 80 μm was successively grown by the yo-yo solute feeding method with 8 yo-yo repetition times between 700 and 650°C.     

Long and short period growth rate variations in potash alum crystals

Recent studies of the growth rate of the different faces of potash alum crystals have revealed that these grow with variable growth rates. This phenomena can be considered on two time scales. In the long term “constant” growth remains for period of hours followed by growth arrests and so on. Superimposed on this are short term (minutes) fluctuations. The potential causes for these variations were discussed.     

Highly uniform growth in a low-pressure MOVPE multiple wafer system

A novel horizontal metal organic vapor phase epitaxy (MOVPE) system, which is capable of handling six 3 inch wafers or eighteen 2 inch wafers mounted on a 10 inch diameter susceptor, has been developed for the growth of III–V compound semiconductors. The characteristic features in this system are “triple flow channel” gas injection and “face-down” wafer setting configuration. The inlet for the source gas flow is divided into three zones (upper, middle and lower flows for hydrides, organometals and hydrogen, respectively) to control the concentration boundary layer and the growth area. The wafers are placed inversely to prevent thermal convection and particles on the growing surface. The independent controlled three-part heating system is also adopted to achieve a uniform temperature distribution over an 8 inch growing surface. The thickness and the doping of GaAs, Al_0.3Ga_0.7As, In_0.48Ga_0.52P and In_0.2Ga_0.8As grown by this system are uniform within ± 2% over all 3 inch wafers.     

Microstructure and growth rate of chemical vapour deposited TiN films in a vertical cold wall reactor

TiN films were grown on SUS304 substrates heated by an induction furnace in a vertical cold wall reactor. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of films obtained at different deposition conditions (temperature, gas flow rate and gas composition). Film structures obtained in the present vertical reactor had the following features compared with those in the tubular reactor: (1) Abnormally grown “star-shaped” crystals were observed on the surfaces of films deposited in the following ranges of total gas flow rate (Q_T), temperature (T) and partial pressures (P): 9.0×10⁻⁶ ≤ Q_T ≤ 1.6×10⁻⁵ m³ s⁻¹, 1223 ≤ T ≤ 1273 K, 0.92 ≤ P_TiCl4 ≤ 6.18 kPa, P_H2 = P_N2. The matrix grains were responsible for (211) preferred orientation. (2) Surface morphologies did not vary so much with P_TiCl4. On the other hand, a drastic change was brought about by adding HCl to the source gas, i.e., plate-shaped crystals dominated and the large “star-shaped” crystals were no longer present. (3) The apparent activation energy for deposition reaction was 230 kJ/mol (1173 ≤ T ≤ 1273 K) and 76.5 kJ/mol (1273 ≤ T ≤ 1373 K) at P_TiCl4 = 2.43 kPa and P_H2 = P_N2 = 49.45 kPa.