Poor Transport Properties Research Articles

Rare-earth-activated wide bandgap materials for scintillators

Open f-shell rare-earth (RE) ions in wide bandgap host materials are usually characterized by closely spaced electronic levels due to various electron configurations and charge states. These levels provide convenient luminescent transitions that can be excited by efficient recombination of charge carriers generated in the host material by ionizing radiation. Therefore, it is the area of ionizing radiation detectors, where search for new, fast and efficient scintillator materials for high-energy physics and nuclear medicine, has yielded much of the recent advances in the understanding of radioluminescence and scintillation mechanism in some solid state, UV and VUV luminescent, RE-activated materials. In this paper we shall present selected results of basic experiments such as radioluminescence, VUV spectroscopy, time profiles and thermoluminescence, on barium fluoride (activated with Ce, Pr, Nd, Tb) and two aluminum perovskites, YAlO 3 and LuAlO 3, activated with Ce. We shall demonstrate that these results point to consecutive carrier capture and recombination at RE ions as the basic mechanism of radioluminescence and scintillation in these materials, despite the strong self-trapping and poor charge transport properties. Consequently, various electron and/or hole traps that intercept and retain for some time the recombining charge carriers play an active role influencing both the scintillation light yield and time profiles of scintillation pulses in these and many other wide bandgap RE-activated luminescent materials.

Heterologous expression and topography of the main intrinsic protein (MIP) from rat lens

Wild type rat lens main intrinsic protein (MIP) and MIP mutated (F73I, F75L) to resemble the glycerol facilitator of Escherichia coli in the region of the NPA1 box were used to investigate the topology of MIP in the membrane of Spodoptera frugiperda (Sf21) cells using the baculovirus expression system and expression in mouse erythroid leukaemia cells (MEL C88). Differential fixation for staining was used, with paraformaldehyde for externally exposed antigenic sites, and acetone for both externally and internally exposed protein antigenic sites. Immunofluorescence using antibodies to synthetic MIP peptides showed that wild type MIP had a six transmembrane topography. The N- and C-termini were intracellular in both expression systems, and both NPA boxes were found to be extracellular. These results show that residues around the NPA1 box can influence the folding of the MIP in the membrane, and provide structural evidence for the poor water transport properties of MIP, as the NPA boxes lie outside the plane of the membrane.

A Perspective of GaAs1-xNx and GaPxN1-x as Heavily Doped Semiconductors

The behavior of GaAs1—xNx and GaPxN1—x is contrasted with that of AlxGa1—xAs and InxGa1—xAs with respect to irregular and regular alloy behavior. It is proposed that GaAs1—xNx and GaPxN1—x behave as heavily nitrogen doped semiconductors rather than dilute nitride alloys and that their abnormal or irregular alloy behavior is associated with impurity band formation that manifests itself in the giant bowing and poor transport properties characteristic of these materials.

Overcoming limitations in semiconductor alloy design

The phenomenon of giant band gap ‘bowing’ recently observed in several III–V dilute nitride alloys is promising for increasing the flexibility in choice of semiconductor band gaps available for specified lattice constants. However, the poor electrical transport properties that these materials exhibit seriously limit their usefulness. It is proposed that these materials behave as heavily nitrogen-doped semiconductors rather than dilute nitride alloys and that the abnormal or irregular alloy behavior is associated with impurity band formation that manifests itself in the giant bowing and poor transport properties. The potential for regularizing the alloy behavior using isoelectronic co-doping is discussed.

Simulation of the collection properties of CdTe strip detectors

CdTe is an attractive material for X-ray and gamma-ray detectors, but the poor transport properties of holes affect the performance by introducing a low energy tailing in the observed spectra. A possible solution to this problem is to optimize the electrode geometry, for example, by reducing the dimension of the anode with respect to the cathode. In this way, the charge signal in the external circuit is mostly due to the electrons moving toward the anode that is where the weighing field becomes localized. The optimization of the electrode geometry can be approached by a numerical analysis as the problem of charge collection is strictly connected to trapping and detrapping processes, which are difficult to analytically treat in nonuniform electric fields. In this paper, we present a numerical simulator based on a finite difference numerical method (which follows the weighing field approach) and on a Monte-Carlo procedure, which is able to analyze, in two dimensions, the effect of different electrode configurations: single strip, multiple strip, a single strip with a lateral extended cathode, and, for comparison, the uniform geometry. The results are analyzed in terms of the maps of the local charge collection efficiency and their histograms, equivalent to spectra due to high-energy X-rays.

Transport Properties and Thermal Expansion of Sr0.97Ti1−xFexO3−δ (x=0.2–0.8)

Increasing iron concentration in perovskite-type solid solutions Sr0.97Ti1−xFexO3−δ(x=0.2–0.8) was found to decrease the unit cell volume and to increase the partial ionic and electronic conductivities as well as the thermal expansion. The oxygen permeation through Sr0.97(Ti,Fe)O3−δ membranes is limited by both bulk ionic conduction and surface exchange rates. Prolonged stabilization under oxygen chemical potential gradients suggests formation of ordered microdomains in the perovskite lattice. The activation energy for the ionic conductivity, evaluated from oxygen permeation and Faradaic efficiency data, is nearly independent of iron content, varying in the range 97–104 kJ/mol. Thermal expansion coefficients of Sr0.97Ti1−xFex O3−δ (x=0.4–0.8) vary from (11.7–13.8)×10−6 K−1 at temperatures of 300–720 K to (16.6–27.0)×10−6 K−1 at higher temperatures. In comparison with the Sr(Ti, Co)O3−δ system, strontium titanates ferrites possess somewhat poorer transport properties, but lower thermal expansion.

Electrode design for coplanar-grid detectors

The coplanar-grid charge sensing technique provides a method for improving the spectral response of gamma-ray detectors based on compound semiconductors, which typically have poor charge transport properties. The technique functions by effectively modifying the charge induction characteristics of the detector such that the dependence of detector signal on the depth of radiation interaction is minimized. The effectiveness of this technique however can be compromised by non-uniform charge induction characteristics across the detector. This paper examines such non-uniformity due to fringe effects near the detector edges. Alternate electrode configurations are studied that provide effective compensation for such effects. Results from experimental measurements and computer simulations are presented.

Signal generation in CdZnTe strip detectors

The energy resolution of CdTe and CdZnTe detectors is usually limited by the poor transport properties of holes. Devices segmented into small pixels have been observed to exhibit improved energy resolutions. Simulations have shown that this small pixel effect is due to the fact that small pixels are mostly sensitive to carriers moving close to the pixel, within a distance of the order of the pixel size. In this paper, signals are calculated for CdZnTe strip detectors in order to determine to what extent a similar small electrode effect is produced by strips. The free carrier density distributions following the absorption of a /spl gamma/-ray are calculated by solving the continuity equations. Combined with the strip weighting field, this provides the signal induced in the strip. Simulations are made for various detector geometries and for both the anode and cathode strips. Simulated signals are compared with actual signals measured on CdZnTe detectors.

The use of pulse processing techniques to improve the performance of Cd/sub 1-x/Zn/sub x/Te gamma-ray spectrometers

Cd/sub 1-x/Zn/sub x/Te (CZT) has recently shown great promise for use as a room temperature gamma-ray detector material. The availability of large volume (>1 cm/sup 3/) high resistivity CZT crystals has allowed the demonstration of detectors much larger than can be built with the similar material CdTe. However, CZT-like many other room-temperature materials-suffers from the poor transport properties of holes. The poor hole drift properties of CZT cause the characteristic "hole tailing" in gamma-ray pulse height spectra. We have applied pulse processing methods to reduce the hole tailing effects and improve the energy resolution of CZT detectors. We have used two signal processing methods to reduce hole tailing in CZT detectors: digital rise-time compensation and dual time-constant sampling. We discuss the implementation of these techniques, demonstrate results obtained in the laboratory, and compare the performance obtained with other detector systems.

Case histories of peptidomimetics: Progression from peptides to drugs

AbstractThere is a growing number of biologically active peptides which have potential for the development of new therapeutics. However, native peptides are only rarely directly usable as drugs, due to inherent limitations which include rapid proteolysis and metabolism, poor transport properties, rapid excretion by the liver and kidneys, and low oral activity. Furthermore, peptides are often aselective in their actions owing to their flexible structure. In efforts to address these limitations, peptides are modified into mimetics with specific physical, chemical and biological characteristics. These so‐called peptidomimetics are derived from peptides by partly or completely removing the amide bonds while retaining essential amino acid side chains in a defined, spatial relationship. They can be developed in a rational design cycle which starts either from a lead compound obtained in a screening programme or from the parent peptide. In the latter approach, which is the focus of this overview, first the smallest, active sequence is found, and the significance of each amino acid for the biological activity is determined. In the following steps the bioactive conformation is defined by the introduction of local and global constraints. In the final step the essential amino acid side chains are positioned onto a scaffold, preferably a small, polyfunctional ring of defined stereochemistry, using structural information obtained in earlier steps. Five examples of the application of the peptidomimetic design cycle in medicinal chemistry are described here, illustrating the progression from native peptides to therapeutically useful drugs.

Enhancement of Single-Phase Heat Transfer and Critical Heat Flux From an Ultra-High-Flux Simulated Microelectronic Heat Source to a Rectangular Impinging Jet of Dielectric Liquid

Jet impingement is encountered in numerous applications demanding high heating or cooling fluxes. Examples include annealing of metal sheets and cooling of turbine blades, x-ray medical devices, laser weapons, and fusion blankets. The attractive heat transfer attributes of jet impingement have also stimulated research efforts on cooling of high-heat-flux microelectronic devices. These devices are fast approaching heat fluxes in excess of 100 W/cm[sup 2], which have to be dissipated using coolants that are both electrically and chemically compatible with electronic components. Unfortunately, fluids satisfying these requirements tend to possess poor transport properties, creating a need for significant enhancement in the heat transfer coefficient by such means as increased coolant flow rate and phase change. The cooling problem is compounded by a need to cool large arrays of heat sources in minimal volume, and to reduce the spacing between adjacent circuit boards. These requirements place severe constraints on the packaging of jet impingement cooling hardware.

Ion‐Transporting Composite Membranes: III . Selectivity and Rate of Ion Transport in Nafion‐Impregnated Gore‐Tex Membranes Prepared by a High‐Temperature Solution Casting Method

In a previous paper we discussed the rate of ion transport in Nafion™,2 impregnated Gore‐Tex (NIGT) membranes. The membranes described in this previous paper were prepared by immersing the Gore‐Tex in a solution of Nafion and then allowing the solvent to evaporate at room temperature. Nafion films which are cast at room temperature show poor mechanical and transport properties. This paper describes the transport properties of NIGT membranes prepared via a high‐temperature solution casting method. We have found that these high‐temperature‐cast NIGT membranes show better physical and mechanical properties than the low‐temperature‐cast NIGT investigated previously. The double diffusion‐pathway model, described in the previous paper, quantitatively accounts for the transport data for the high‐temperature‐cast membranes. At the ca. 10% (or higher) Nafion loading level, both the high‐ and low‐temperature‐cast membranes show excellent cation permselectivity. Furthermore, these membranes show cation transport rates for which are an order of magnitude higher than transport rates in homogeneous Nafion.

Density of States of Amorphous Germanium Thin Films Deposited by the Pecvd of H2-Diluted Germane

ABSTRACTThe disorder in a-Ge:H thin films produced by the plasma-enhanced chemical vapor deposition (PECVD) technique is strongly reduced when the GeH4 gas is diluted at 1% in H2 and the radiofrequency power density is increased to 0.1 W.cm−2. This improvement is attributed to a better surface passivation by the hydrogen atoms during the growth. However, the poor transport properties indicate a still high defect density. The midgap defect absorption and the Urbach energy, obtained from the photothermal deflection spectra calibrated with optical data, both decrease as a function of the film thickness. The optical defect density is calibrated with EPR spin measurements. For a-Ge:H films thicker than 2 μm, obtained at a deposition temperature Ts ranging from 150 to 250°C, the Urbach tail parameter E° is lower than 50 meV and not sensitive to Ts ; the dangling bond density is around 4.107 cm−3, which is higher by a factor of 100 than in a-Si:H. Preliminary transport measurements indicate that the Fermi level density of states is larger than 1018 cm−3.eV−1.

The quantum efficiency of CdS/CuIne solar cells

This paper is concerned with the analysis of the external quantum efficiency of CdS/CuInSe 2 and In 2O 3:Sn/InP solar cells. Measurements have been made at several values of reverse bias and the data have been fitted to a simple window layer heterojunction model of the devices. This appears to be most successful for the CdS/CuInSe 2 cells, but for the In 2O 3:Sn/InP cells there are significant differences between theory and experiment. However, good agreement can be established by modeling the latter devices as very shallow homojunctions with a type-converted surface having rather poor transport properties.

Parallel pulse processing for mercuric iodide gamma-ray detectors

Mercuric iodide, a high- Z, wide band-gap semiconductor material, is a room temperature gamma-ray detector. Poor hole transport properties limit the detector performance. By parallel pulse processing schemes we are exploring this problem and making corrections to enhance resolution. This yields interaction depth information which is used to estimate hole collection deficits. Deficit estimates are used as corrections to pulse height to enhance resolution, or to reject pulses from high-deficit regions. These techniques have been implemented by several schemes for filtering the pulses using off-the-shelf amplifiers and computer simulation of amplifiers on detectors from 1–3 mm thick. Hardware and simulated results are compared and improved spectral performance is discussed. A resolution better than 7% for correction and 2.5% for rejection of hole deficit is obtained.

Electrohydrodynamic heat pipes

An electrohydrodynamic heat pipe of radical design is proposed which substitutes polarization electrohydrodynamic force effects for capillarity in collecting, guiding, and pumping a condensate liquid phase. The discussed device is restricted to the use of dielectric liquids as working fluids. Because of the relatively poor thermal transport properties of these liquids, capillary heat pipes using these liquids have not been high performance devices. The employment of the electrohydrodynamic concept should enhance this performance and help fill the performance gap that exists in the temperature range from 250 F to 750 F for 'conventional' capillary heat pipes.

Influence of water on evaporated ZnSe films: O Busmundrud, Phys Stat Sol (a), 2 (4), Aug 1970, K227– K230

We have studied the effects of structural vacancies on the thermoelectric properties of the ternary compounds (Cu2Te)1−x(Ga2Te3)x (x=0.5, 0.55, 0.571, 0.6, 0.625, 0.667 and 0.75), which are solid solutions found in the pseudo-binary phase diagram for Cu2Te and Ga2Te3, and possesses tunable structural vacancy concentrations. This materials system is not suitable due to the cost and scarcity of the constituent elements, but the vacancy behavior is well understood and will provide a valuable test case for other systems more suitable from the standpoint of cost and abundance of raw materials, which also possesses these vacancy features, but whose structural characterization is lacking at this stage. We find that the nominally defect free phase CuGaTe2 possess the highest ZT (ZT=S2T/ρκ, where S is the Seebeck coefficient and ρ is the electrical resistivity κ is the thermal conductivity and T is the absolute temperature) which approaches 1 at 840 K and seems to continuously increase above this temperature. This result is due to the unexpectedly low thermal conductivity found for this material at high temperature. The low thermal conductivity was caused by strong Umklapp (thermally resistive scattering processes involving three phonons) phonon scattering. We find that due to the coincidentally strong scattering of carriers by the structural defects that higher concentrations of these features lead to poor electrical transport properties and decreased ZT.