Parameter evaluation in automated digital deep-level transientspectroscopy (DLTS)

A method of directly evaluating the activation energy ΔE, capture cross section σ, and density N_T, of deep-level traps from the pulsed reverse bias capacitance transient is described. The main advantages of this technique are that it requires only a single temperature scan, and it can resolve nonexponential transients due to closely-spaced energy levels. The test samples used for this paper consisted of Schottky diodes fabricated on nonirradiated and 1-MeV electron-irradiated n-type VPE (vapor-phase epitaxy) GaAs wafers. The well known EL2 trap was identified with ΔE of 0.81 eV, and σ _n of 1.0×10^-13 cm² for the nonirradiated sample. These values were found to be in good agreement with published data using established, conventional DLTS techniques. For the irradiated samples a nonexponential capacitance transient was found in the EL2 range of temperatures. The discussed technique was able to resolve two closely spaced deep levels lying at E_c-0.81 eV and E_c-0.84 eV, and with capture cross sections of 1.5×10^-13 cm² and 2.5×10^-12 cm², respectively     

Channel characterization and television field strength measurements

The new standard in North America for the advanced television (ATV) transmission system will be selected in 1996. The new ATV system is expected to co-exist with the conventional NTSC television system for many years to come. Simulcasting of the two systems will require a good understanding of the channels used. In this paper, a method to characterize a channel using the ghost canceling reference (GCR) signal is presented as well as the method used to conduct field strength measurements of television channels. The channel characterization gives us the time and frequency response of channels. The field strength measurements show the power variation of the signals that are transmitted through a channel over a period of time. The results obtained from field tests for a number of channels and with different polarization of transmitter's and receiver's antennas are then presented. The results for the channel characterization of a cable network using the GCR signal are also presented     

A vibrating-tube densimeter for fluids at high pressures and temperatures

A vibrating U-tube apparatus has been developed for determining the densities of pure fluids and fluid mixtures at 10-200 MPa and 323-773 K. Measured parameters areP,T, andr (period of vibration). Fluids are injected into the U-tube at constantP andT. Three or more reference fluids are used to calibrate the response of the instrument. Fluid mixtures are produced by pumping pure fluids into T-junctions on the upstream side of the U-tube using high accuracy syringe pumps. An automated syringe pump is used to maintainP at setpoint ±0.01 MPa.T is controlled to ±0.01 K using a closed-loop, electronic signal amplification/feedback system. For mixtures, a statistically significant number of measurements of r are obtained to account for the effects of small heterogeneities in fluid composition (generally <0.005X;). Typically, density data for 15 fluids can be obtained in a 6- to 8-h period. Considering all of the potential sources of error in the experimentation, conservative estimates of uncertainty are as follows:P, ±0.02 MPa;T, ±0.05 K;p (pure fluids), ±0.0005g.cm^–3; andp (fluid mixtures), ±0.0005-0.0010g-cm^–3.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Influence of adsorption on the measurement of diffusion coefficients by Taylor dispersion

The effect of adsorption on the measurement of diffusion coefficients by the Taylor dispersion technique is investigated by modifying the governing equation to account for reversible, nonequilibrium adsorption. The resulting two-dimensional equations are solved by an explicit finite-difference technique. Experimental data for the acridine carbon dioxide system indicated that acridine adsorbs on the walls on the tubing and these data were investigated with this model. The influence of carious parameters including the number of sites and the rates of adsorption desorption was investigated by conducting a parametric sensitivity analysis on the model. It was found that adsorption of the solute on the wall of the tubing could produce an error as high as 35% on the measured diffusion coefficient compared to the actual diffusion coellicient. Examination of the influence of each of the parameters will enable Inure investigators to reduce the effect of adsorption in the measurement of diffusion coefficients by Taylor dispersion.     

Bone remodelling after total hip arthroplasty

Bone remodelling of the proximal femur following total hip arthroplasty (THA) is related to stress deviation with respect to physiological condition. The clinical relevance of this process is much debated with respect to its role in THA failure. In the present study a group of 475 An.C.A. anatomic cementless stems implanted in our institution were assumed as clinical reference. Of them, 294 had a short stem and 181 had a long stem. Stress shielding was X-ray evaluated in each patient. The survivorship analysis of this study group (negative events = stress shielding) showed significantly (p<0.05) lower survival rates at 25 months follow-up for patients with long-stem implants. A 3-D FEM model of the proximal femur was used to analyse the load transfer mechanism for the two types of stems in fully or proximally only bonding conditions. Little difference was predicted in the proximal stress magnitudes for the different stem lengths. On the contrary, stem-bone bonding leads to a notable increase in the stress shielding.     

Chemical characterization of in vivo aged zinc phosphate dental cements

The chemical composition of zinc phosphate dental cements aged in vivo was studied. Twenty-seven samples aged 2 to 43 years were investigated using X-ray diffraction, infrared spectroscopy, ³¹P nuclear magnetic resonance spectroscopy, thermogravimetric analysis and differential scanning calorimetry. Evidence for the presence of zinc oxide, amorphous zinc phosphate, water of hydration and crystalline zinc phosphate tetrahydrate was found. The latter was identified as hopeite; it was present in 92% of the cements studied. No correlation with time concerning either the chemical structure of the components or their relative amounts was found. Zinc phosphate dental cements show very good chemical stability on long-term use.     

Modified refraction-interference method for calculating the geometric and optical parameters of optical waveguides

We describe a ray method for analysis of the geometric and optical characteristics of two-layer fiber optical waveguides. We have analyzed the effect of the polarization of the illuminating beam on the overall pattern of the intensity distribution. We consider the characteristic types of rays for a single-layer and two-layer fiber optical waveguide. We have obtained definitive dependences of the beam intensity for each ray individually. We have developed a program for calculation of the scattering pattern in fiber optical waveguides illuminated by a broad beam. Translated from Izmeritel'naya Tekhnika, No. 3, pp. 39–42, March, 1996.     

Reprocessing of Microcrystalline Cellulose Spheres with Low Drug Concentrations

Spherical dosage forms have been reported to be an efficient and effective method for delivering drugs into the body and controlling their dissolution rate. Substantial work has been conducted in these laboratories illustrating the advantages of microcrystalline cellulose-based spheres for these purposes. Through various methodologies, but most routinely the extrusion and marumerization technique, it has been determined by Funck, et al. that not more than 50% drug can be incorporated into a sphere formulation without the addition of other binders.

Because of the nature of the extrusion and marumerization manufacturing process, the type of drug being processed and the resultant particle size requirements of the spheres, the percent of spheres falling outside the desired particle size can range from between 3% and 20%. For this reason and the possibility of operator errors, our objective was to determine the parameters which needed monitoring when reprocessing was necessary.