Crystal Pillars Research Articles

Comparison between YAP : Ce and CsI(Tl) multipillar matrices

The characteristics of segmented CsI(Tl) and YAP : Ce scintillators coupled to a position-sensitive photomultiplier tube (PSPMT Hamamatsu R2486) have been measured and compared to evaluate the potential use of such scintillators as precise X- and γ-ray imagers. For such configurations, we obtained good spatial resolution and linearity over their entire 40×40 mm 2 Fields of View (FOVs). The scintillating matrices consist of orthogonal arrays of crystal pillars with 0.6×0.6 mm 2 cross sections and thicknesses of 3 and 10 mm. The 3 mm thick CsI(Tl) matrix shows a light yield of 44% compared to NaI(Tl), an energy resolution of 22% FWHM at 122 keV, a single pillar spatial response of 0.5 mm FWHM, and a spatial resolution of 0.83 mm.

Single crystal galena pillars as highly anisometric dissolution forms

AbstractRemarkable single crystal galena formations in the unique form of cylindrical or steep conical ‘pillars’ with ‘capitals’ of small rhombododecahedral single crystals of sphalerite, are described. It is suggested that the pillars result from a highly specific process of anisotropic mineral dissolution controlled by a system of internal channels in the original skeletal crystals and represent a kind of dissolution whiskers.

Multi-crystal YAP: Ce detector system for position sensitive measurements

Yttrium aluminum perovskite (YAP:Ce) scintillation crystal has a light efficiency of about 40% relative to NaI. Because of the yttrium atomic number ( Z = 39) and the relatively high density (5.37 g/cm 3) it has a good gamma-ray absorption. Furthermore it is not hygroscopic and is inert. Its peculiarity consists of material processing that provides us with crystal pillars down to 0.3 × 0.3 mm 2 aperture size and up to some centimeters in length. An array consisting of 11 × 22 YAP:Ce elements was made where each crystal has an aperture of 0.6 × 0.6 mm 2 and a length of 7 mm. Each scintillation crystal is optically separated by a reflective material resulting in a separation layer between elements of about 5 μm. The multicrystal detector was optically coupled to a Hamamatsu Position Sensitive Photomultiplier Tube (R2486). The intrinsic spatial resolution of the PSPMT is better than 0.3 mm but it is strongly dependent on the Point Spread Function (PSF) generated on the photocathode. The multicrystal detector very well matched the PSPMT characteristics resulting in a spatial resolution of about 0.7 mm at 140 keV ( 99mTc) gamma irradiation.

Preparation of (Cr 0.45Fe 0.55) 5Si 3 crystals by vapour phase siliconizing of stainless steel 310 and their properties

New ternary compound crystals of (Cr 0.45Fe 0.55) 5Si 3 have been obtained by vapour phase siliconizing of SUS 310 plate; the growth conditions and some of the properties of this material were examined. Hollow pillar crystals of 3–5 mm in length and 0.15–0.35 mm in thickness were easily obtained at 1000–1100°C for 1 h. The crystals were much more corrosion resistant against concentrated HCl solutions than SUS 310 plate.

Single crystal growth of Cr3Si and Cr5Si3 using in-situ CVD process

Whiskers and pillar crystals of Cr 3 Si and Cr 5 Si 3 have been in-situ grown by the reaction of chromium powder, hexchlorodisilane and hydrogen at a temperature range of 850–1200°C on the inner wall of the quartz boat in which chromium powder was dispersed. The pillar crystals obtained have generally a hollow which penetrated the crystal axis. It is supposed that pillar crystals with square or regular hexagonal cross sections are Cr 3 Si (cubic) and tetragonal or deformed hexagonal is Cr 5 Si 3 (tetragonal). These chromium silicide crystals are supposed to deposit mainly on the boat by the following in-situ reactions: (1) Si 2 Cl 6 + H 2 → SiCl x + HCl; (2) Cr powder + HCl → CrCl 2 + H 2; (3) CrCl 2 + SiCl x + H 2 → Cr 3 Si + Cr 5 Si 3 + HCl. The maximum size of the crystals obtained is 5–6 mm in length and 10–20 μm in thickness at 1200°C for 60 min.

Crystal growth of new ternary compounds (Cr1−xFex)5Si3 on vapour-phase-siliconized stainless steel

Abstract Well-formed hollow pillar crystals of new ternary compounds (Cr1−xFex)5Si3 were obtained on the surface of stainless steel plate which was vapour-phase siliconized using a gas mixture of Si2Cl6 and H2 at 1100 °C. The crystal structure was determined to be the same as that of the Fe5Si3 phase (hexagonal, P63/mmc), and the lattice constants are a = 6.788 A and c = 4,730 A .

Growth of aluminium nitride pillar crystals by chemical vapour deposition

Aluminium nitride (AIN) pillar crystals were grown at 900–1000 °C on a graphite substrate by chemical vapour deposition (CVD) using the reactant gas system of AlBr 3-NH 3-H 2. Pillar crystal of AIN parallel to the C axis of the hexagonal AIN were deposited in a dilute atmosphere of NH 3 by its direct formation reaction with AlBr x on the substrate surface. Hexagonal prisms of AlN single crystals were grown at 900 °C radially form the impurity roots of the substrate, which suggested a VLS growth mechanism of these prismatic shaped crystals.

Iron oxide and hydroxide precipitation from ferrous solutions and its relevance to Martian surface mineralogy

The presence of red magnetic dust in the Martian regolith raises questions as to how surface processes on Mars might differ from those on Earth. Experiments were carried out to determine physico-chemical conditions under which specific iron oxides and hydroxides would precipitate from ferrous salts in aqueous solution. The results indicate that mineral substrates as well as solution chemistry control the precipitation of iron phases. Particular attention was given to identifying conditions favoring the formation of either lepidocrocite or goethite. All iron precipitates were identified and cross checked by using both infrared and magnetic analyses. Specifically, aqueous ferrous sulfate solutions oxidized rapidly at room temperature under neutral conditions yield lepidocrocite; this occurs both with and without fine-grained mineral substrates whose isoelectric points (IEPs) are less than 5 (except quartz) In contrast, mineral substrates with IEP>7 (and quartz, with IEP≤2) yield mixtures of lepidocrocite and goethite. Additional experiments tested the importance of charged interactions in the formation of iron oxyhydroxides by varying the dielectric constant of aqueous solvent. In solutions with a high dielectric constant (>60), lepidocrocite forms during rapid oxidation at neutral pH. In solutions with a low dielectric constant (≤20), goethite is the predominant phase. This implies that electrostatic effects in the aqueous solution and at the substrate surface strongly influence the deposition of iron oxyhydroxides. With reference to Mars, our results suggest that lepidocrocite, in addition to goethite, is more likely to precipitate from low-temperature aqueous solutions in a basalticre golith (assuming sufficient liquid water was present on Mars in its past history). Dehydration of lepidocrocite to maghemite is then an explanation for the ubiquitous, red magnetic phase in the Martian regolith. In contrast, goethite and hematite are the predominant secondary iron phases in the quartzose continental soils of planet Earth. They had a house of crystal pillars on the planet Mars by the edge of an empty sea, and every morning you could see Mrs K eating the golden fruits that grew from the crystal walls, or cleaning the house with handfuls of magnetic dust which, taking all the dirt with it, blew away on the hot wind.

Chemical vapour growth of CrB 2 and CrB crystals

Crystals of CrB 2 and CrB were grown from a gaseous mixture of CrCl 2, BCl 3, H 2 and Ar in a temperature range of 1050–1100°C. Hexagonal planar crystals of CrB 2 grew to a size of 6 mm in diagonal and 0.1 mm in thickness at 1050°C for 2 h. Hollowed pillar crystals of CrB (1–10 μm thick and 30–100 μm long) grew on a platinum dispersed substrate at 1100°C. The growth mechanism of quasi-nonagonal pillars of CrB was supposed to be repeated twinning across the (110) planes. The composition of CrB 2 was confirmed by gravimetric analysis to be nearly stoichiometric. The microhardness of CrB 2 and CrB deposits was measured to be as 3800–4000 and 1450–1500 kg/mm 2, respectively.

Pillar crystal growth of boron phosphide from the vapour in the presence of nickel impurity

Pillar crystals of boron monophosphide were grown from an atmosphere of BC1 3-PC1 3-H 2-Ar on an inner surface of a quartz tube in the temperature range of 1060–1090°C. Fine particles of nickel dispersed on the substrate induced pillar growth. The maximum length of the crystals was limited by the radius of the substrate tube. The cross section of the pillar crystals varied with growth time from circle to polygonal (tri-, hexa-, octa-, and nonagonal). The growth axis was considered to be the 〈111〉 direction, and the side faces to be {110}; and {211} forms. The growth mechanism was supposed to be initial VLS growth along the axis, followed by thickening.

ChemInform Abstract: IMPURITY METAL‐ACTIVATED CRYSTAL GROWTH OF NIOBIUM MONOPHOSPHIDE FROM GAS PHASE

AbstractIn Gegenwart verschiedener metallischer Fremdbeimengungen (Konzentration auf der Substratoberfläche ca. 0,05 mg/cm2) werden auf Quarzsubstraten NbP‐Einkristalle aus einer Gasmischung von NbCl5, PCl3, H2 und Ar abgeschieden.

Impurity Metal-Activated Crystal Growth of Niobium Monophosphide from Gas Phase

Abstract Niobium monophosphide single crystals were obtained by chemical vapor deposition from a gas mixture of NbCl5, PCl3, H2, and Ar on a quartz substrate using metallic impurity. The existence of Au, Pt, Pd, and Ni resulted in the growth of whiskers or pillar crystals. Maximum whisker length of 5 mm and thickness of 30 μm were obtained at 1150 °C for an hour with the use of Pt impurity. Platelet crystals (400×1000×20 μm, max.) were obtained with the use of Pt at 1240 °C. The growth directions of the whiskers and some platelet crystals were assigned to 〈001〉 and a direction in (001) plane, respectively. The optimum growth conditions for the pillar crystals in the case of Ni impurity: substrate temperature 1000–1020 °C, gas flow ratio PCl3/(NbCl5+PCl3) of 0.7–0.75. From observation of the tip and base of pillar crystals grown with Ni, the growth mechanism seems to be base VLS followed by VS.

Anomalous pillar-shaped crystal growth of zirconium disulfide

Anomalous pillar-shaped crystals of zirconium disulfide (1–2 mm in lengthand 20–40 μm in diameter), which were abnormal in the sense that the whole lateral surface was a (001) plane, were grown together with platelets from a gas mixture of ZrCl 4, H 2S, H 2 and Ar on a quartz substrate at 800°C. The pillar-shaped crystals grew among a colony of platelet crystals. Hexagonally winding steps were observed among the pillar crystals. Preferable flow ratios of H 2S ( H 2S + ZrCl 4 ) to g row platelets and pillar-shaped crystals were determined to be 0.4–0.8 and 0.6–0.9, respectively. The conductivity along the pillar axis was measured to be 0.14 and 0.22 ω -1 cm -1 at 25 and 160°C, respectively. The thermoelectric power was + 730 and +600 μV/°C at 25 and 200°C, respectively.

Chemical vapour deposition of niobium diboride (NbB2)

Niobium diboride was prepared by chemical vapour deposition from a gas mixture of NbCl5, BCl3, H2 and Ar on a quartz substrate heated to a temperature of 950–1200°C. Homogeneous films were formed from 950 to 1050°C, and pillar crystal from 1050 to 1200°C. Niobium diboride was found to react with the quartz substrate above 1200°C to form a silicide, Nb5Si3. The optimum molar compositions of the gas mixture used to grow crystals of NbB2 were 2.5–5.0 vol% of BCl3, and 2.5–5.5 vol% of NbCl5. In the presence of impurities of nickel regular hexagonal shaped pillar crystals of maximum 10 μm in a edge and 50 μm in length were obtained.

Chemical vapor deposition of tungsten carbide dendrites

Tungsten carbide was obtained by chemical vapor deposition from a mixture of WCl 6, C 3H 8, H 2, and Ar on a graphite substrate in the temperature range of 1200 to 1500 °C. The influence of various factors on the crystal growth was investigated. The geometry in the furnace proved to be an important factor for the process of crystallization. The adherent coating of α-W 2C could be obtained at relatively high flow rates of H 2 and WCl 6. The appropriate gaseous concentrations for the growth of WC were as follows: WCl 6: 1.0–1.5 mole%; C 3H 8: 1.7–2.8 mole%; H 2: 10–20 mole%; and Ar: 75–85 mole%; and the total flow rates was 1.78 ml/sec. Various crystal morphologies composed of uniform films, networks, dendrites and pillars with diameters of 2 to 40 μm were deposited on the WC-plated graphite. The dendritic or pillar crystals were obtained at temperatures between 1250 and 1400 °C, while the network-like crystals were formed at lower temperatures of 1200 to 1300 °C on the substrate surface.

Single Crystal Growth of Titanium Carbide by Chemical Vapor Deposition

Various forms of titanium carbide crystals were deposited on graphite substrates from the vapor phase consisting of hydrogen, titanium tetrachloride, and a hydrocarbon in the temperature range of 1200°–1350°C. The correlations between the crystal morphology and the growth conditions (temperature, gas flow‐rates, concentrations, impurities, etc.) were investigated. The decomposition of the hydrocarbon was found to play an important role on the growth process. By x‐ray diffraction, it was concluded that the growth direction of these crystals was mainly <111> although sometimes kinked toward . The growth mechanism of the single crystal was further studied by observation of the etching figures of the cross section which showed hexagonal growth layers. Finally the tensile strength of needle or pillar crystals was measured and a linear relation (on log‐log scale only) was confirmed between the diameter and the strength.

The diffraction of thermal neutrons by single crystals

Experimental measurements of the intensity of neutron reflexion by large single-crystal slices are shown to be in agreement with previously published calculations. It is demonstrated that accurate comparison can be made of the structure factors of planes in a zone by the use of cylindrical or pillar crystals, sufficiently small in diameter to give negligible secondary extinction effects. The use of single crystals for structure analysis is discussed.