Zone-refining Technique Research Articles

Development of novel crystal scintillators for lunar surface science

The development of novel crystal scintillators for a lunar landed mission near future is reported. On that mission, we aim to assess the radiation dose, biological effect, water, and other resources on the lunar surface. To accomplish this, a Lunar Vehicle Radiation Dosimeter (LVRAD) made up of several radiation detectors has been proposed. New scintillation crystals are being developed for neutron and gamma measurements. Stilbene and undoped LaCl3 crystal were selected for fast neutron/gamma spectrometers. The mutual combination of organic and inorganic detectors benefits from direct fast neutron detection via 35Cl(n, p)35S reaction as well as the good gamma resolution of LaCl3 and the high detection efficiency of stilbene. LaCl3 crystals with the diameter of 38.1 mm were grown by a homemade modified Bridgman furnace. For epithermal neutron spectrometer, 6LiI:Ag crystal was selected due to its pulse shape discrimination capability and highest detection efficiency in terms of size for both thermal and epithermal neutron among scintillation materials. Enrich 6LiI was purified using the zone-refining technique. The crystal was then grown by the same Bridgman furnace.

Integrated analysis and design optimization of germanium purification process using zone-refining technique

Germanium (Ge) is a preferred material in the fabrication of high-performance gamma radiation detector for spectroscopy in nuclear physics. To maintain an intrinsic region in which electrons and holes reach the contacts to produce a spectroscopic signal, germanium crystals are usually doped with lithium (Li) ions. Consequently, hyperpure germanium (HPGe) should be prepared before the doping process to eliminate the interference of unexpected impurities in the Li dopant. Zone-refining technique, widely used in purification of ultra-pure materials, is chosen as one of the purification steps during detector-grade germanium production. In the paper, numerical analysis has been conducted to analyze heat transfer, melt flow and impurity segregation during a multi-pass zone-refining process of germanium in a Cyberstar mirror furnace. By modifying the effective redistribution coefficients, axial segregations of various impurities are investigated. Marangoni convection is found dominant in the melt. It affects the purification process through modifying the boundary layer thickness. Impurity distributions along the ingot are obtained with different conditions, such as pass number, zone travel rate, initial impurity concentration, segregation coefficient, and hot-zone length. Based on the analysis, optimization of the purification process design is proposed.

Limiting impurity distribution during zone refining

The problem of the limiting distribution of the impurity removed using the zone-refining technique is considered for different distribution factors and molten zone sizes. An analytical solution is suggested and some examples are discussed in the approximation of a long finite-length ingot with a uniform cross section and constant molten-zone size.

Integrated study of lead iodide crystal growth and doping processes

Due to its high atomic number, high density and wide band-gap energy, lead iodide is a promising nuclear-radiation detector material. The radiative transition in PbI2 is mainly due to donor–acceptor pair (D–A) recombination associated with impurities, which suggests improving scintillating performance of lead iodide by doping certain impurities. Before the doping study, high-purity PbI2 with the least undetermined impurities is obtained by purifying the commercial PbI2 with zone-refining technique. After well mixed with certain dopants (AgI and LaI3), the purified PbI2 is used as starting material for single crystal growth by vertical Bridgman method. Numerical studies are conducted to study temperature fields in both zone-melting and Bridgman furnaces. The solid/liquid interface is determined according to the heat balance on the growth front. The results are found to agree well with experimental measurements. Distribution of impurity concentration in purified ingot is studied experimentally, while that in grown crystal is studied analytically. By comparing analytical solutions with experiments, the effective redistribution coefficients of dopants in lead iodide crystal are determined. We further conclude that diffusion-dominated mechanism of dopant redistribution accounts for the uniform distribution of dopants in crystal, and that the suppression of natural convection in the melt is critical. By integrated study of heat and mass transfer phenomena during purification and growth processes of lead iodide, important information is achieved for the understanding and improvement of hot-zone design in the furnaces as well as for the parametric optimization of both processes.

Ethyl propionate at 185 K

The title compound, C5H10O2, was prepared by a modified zone-refining technique, and has a layered structure with a high degree of disorder within the layers.

Structural phase transitions in melt-grown PbI2 crystals under laser light

PbI2 crystals of various purities were grown using zone-refining technique. Structure of all the crystals was found to be 12R but in few cases in coalescence with 4H. In very pure crystals the phase transition 12R→2H has been observed under laser light. The findings have been explained on the structural considerations of PbI2.

Electron transport and energy-band structure of InSb

Various n-InSb crystals, pure and Te doped, were prepared using the horizontal zone-refining technique. The electrical conductivity and Hall coefficient of these samples were measured as functions of temperature from 90 to 470 K. By self-consistently fitting the theoretical transport parameters to the experimental data, a set of parameters was refined and/or determined. They include the optical band-gap variation E0(T) = [0.235 − 3.2× 10−4T2/(220 + T)] eV, the heavy-hole effective mass m*hh = (0.20 ± 0.02)me, and the acoustic deformation potential Ed = (13 ± 1) eV. Various scattering mechanisms and impurity phenomena are also discussed.

Growth of CdBr 2 single crystals by zone refining

CdBr 2 has been purified using a 12-pass zone-refining technique. Single crystals grown from the starting material showed arcing in the X-ray oscillation photographs whereas this is absent in the crystals grown from zone refined material. These crystals are found to be 6R type. In order to study the effect of impurities, DC conductivity studies were made on the crystals at various stages of purification. Surface morphology of the crystals have been studied using an optical microscope.

Photoelectronic properties of PbI2 using dye-lasers

Pure PbI2 single crystals have been grown by the zone-refining technique and the spectral distribution of the photocurrent by using different dye-lasers has been recorded withE‖c andE⊥c. The fundamental absorption edge appears at 2.36 eV (E⊥c) and is found to be dichromatic in nature with a splitting of 0.05 eV. The decay pattern of the photocurrent at different voltages, temperatures and intensities of illumination has been used to calculate the lifetimes of charge carriers. The photocurrent-voltage characteristics have been used to calculate the trap concentration and mobility of the charge carriers. The lux-ampere characteristics at the band edge frequency have been studied at different applied voltages.

Radioisotopes in Zone Refining—A Student Experiment

Illustration of redistribution of impurities by zone-refining could be achieved by adding a colored, radioactive impurity to a bar of material and surveying the bar, before and after passing the zone, by a counting system. Stepwise details of the experiment are given. The main conclusions of the theory of zone-refining are summarized. Comparison between the experimental results and the theoretical predictions proved success of the zone-refining technique and demonstrated the advantage of using radioisotopes as a nondestructive means of analysis.

Purification of metal chelates by a zone-refining technique

Metal chelate compounds may be purified by a zonerefining technique. As an example, zone refining of tris(acetylacetonato)chromium-III is described. It has been proved that the central metal ion can be purified by this technique.