Seeded vapor transport

Abbreviations and symbols denote ZNT ZN Technology Inc. (formerly Eagle Pitcher), Brea, USA Cl Cermet Inc., Atlanta, USA ML Mineral Ltd., Alexandrov, Russia TDC Tokyo Denpa Co., Tokyo, Japan s-CVT seeded chemical vapor transport PM pressurized melt HT hydrothermal T growth temperature P growth pressure d dimension p resistivity Nb,a - donor or acceptor concentration p Hall mobility Nas dislocation density... 

Large diameter (2-inch diameter) boules have been reported to be grown at Eagle Picher at 1000-1200 °C by seeded chemical vapor transport (SCVT) in a nearly closed horizontal tube using H2 as a carrier gas and a small... 

Polycrystalline zeolite membranes consist of inter-grown zeolite crystals with no apparent cracks or pinholes (Fig. lA). These films are composed of only zeolite (i.e., there are no non-zeolite components such as amorphous silica or polymer). They are normally supported on a substrate although free-standing films have also been synthesized. Membranes can be prepared on different substrates such as silicon wafer, quartz, porous alumina, carbon, glass, stainless steel (SS), gold, etc. Polycrystalline films are primarily prepared by hydrothermal synthesis methods including in situ crystallization, seeded growth,and vapor transport, " and have potential use in all of the applications discussed in this entry. 

A series of bioassays established the credibility of the route of volatile terpenes produced by the plants to the soil. Toxin production from roots was eliminated in the preliminary experiments, but leaf material, in contrast, was highly toxic when assayed in direct contact with germinating seeds and also when crushed leaves were present in a sealed bioassay chamber in which only vapor transport of toxins to seeds was possible. Avena fatua root growth was reduced 50% by volatiles from 0.5 g of Artemisia califomica, and 64% by 0.5 g of S. leucophylla (Muller et al. 1964). Volatile inhibitors were also indicated by observations that the bare and inhibition zones were similar in extent on both the uphill and downhill sides of S, leucophylla stands (Muller 1966). 

On the other hand, bulk crystals of 4H-SiC and 6H-SiC are made by a physical vapor transport (seeded sublimation growth) technique known as the modified-Lely method [32]. High-quality bulk materials (threading dislocation... 

From infrared cyclotron resonance in n-type ZnO grown by seeded chemical vapor transport method (Eagle—Picher). The value in parentheses (0.29) is the effective polaron mass [78]. 

A method that produces very high-quality bulk ZnO wafers is based on seeded chemical vapor transport. In this method, the reaction takes place in a nearly closed horizontal tube [28]. Pure ZnO powder used as the ZnO source is placed at the hot end (hot zone) of the tube that is kept at about 1150 °C. The material is transported to the cooler end of the tube, maintained at about 1100 °C, by using H2 as a carrier gas. 

Figure 2.3 ZnO wafer grown by seeded chemical vapor transport with chemical mechanical polish finish on both faces (Courtesy of J.-J. Song, ZN Technology, Inc.)...

Table 2.1 Chemical impurities (ppma) in ZnO wafers grown by seeded chemical vapor transport technique.

Reynolds et al. [23] (bulk crystals grown by a seeded physical vapor transport method). Thonke et al. [45] (bulk crystals grown by seeded chemical vapor transport). 

One of the reports of dielectric functions of ZnO is that by Ashkenov et al. [135] who characterized thin films grown by pulsed laser deposition on c-plane sapphire and a single-crystalline sample grown by seeded chemical vapor transport method. The static dielectric constant was obtained from infrared spectroscopic ellipsometry measurements. The high-frequency dielectric constant was calculated through the Lyddane-Sachs-Teller (LST) relation, (Equation 1.31), using the static constant and the TO- and LO-phonon mode frequencies. The results are compared with the data from some of the previous studies in Table 3.8. 

In a circulating-liquor crystallizer, as the liquor flashes in the vapor head, it becomes supersaturated (path a-b). Ideally, the liquor will remain at a constant supersaturation (b-c) as it is transported to the seed bed. Spontaneous nucleation, however, will tend to lower the supersaturation (b-d). The remaining supersaturation is released on the crystals in the seed bed (d-e). As hot feed is mixed with the recirculating liquor, the mixture becomes unsaturated (e-a). The cycle recommences as the liquor is transferred to the vapor head. 

Specifically for the preparation of zeolite films, zeolite nanoparticles could be used directly to form zeolite films by self-assembly. They could also be combined nicely with the seeded growth and vapor phase transport method to produce high quality polycrystalline zeolite films. There is clear evidence that small zeolite nanoparticles are preferred for producing compact continuous films. 

The story related here about HCB is one of consequences, unintended and intended. The scientists who recommended seed treatment with HCB likely had the best of intentions. They recognized that wheat smut could easily destroy half a wheat crop. The new and very remarkable compound HCB could avert famine. But those scientists could not foresee that the compound s high vapor pressure and resistance to degradation would result in global transport of HCB, or anticipate the possible consequences of exposure. Alarms began to sound in the late 1940s about the possible dangers of exposure to pesticides, such as this editorial in the Journal of the American Medical Association in 1948 [122, pp. 1604-1605] ... 

Mayville et at. [117] extended this procedure to produce titania particles with coatings of polyurea up to 0.25 fim. thick, to provide a steric barrier when the particles were subsequently dispersed. They transported the aerosol of titania into a chamber containing vapor of hexamethylenedi-isocyanate this vapor condensed on the particles and was then polymerized by exposure to vapor of ethylenediamine. The electrophoretic mobility of the coated particles was found to be the same as for the pure polymer. It may be possible to produce monodisperse aerosols at a rapid rate without seeding by use of a properly designed nozzle [118], but this has not yet been demonstrated experimentally. 

In the jet vapor deposition (JVDtm) process, evaporated atoms/molecules are seeded into a supersonic jet flow of inert carrier gas that expands into a rapidly pumped vacuum chamber. The jet transports the atoms/molecules to the substrate surface where they are deposited. The vapor source can be in the form of thermal evaporation or sputtering and is located in the jet nozzle. The deposition chamber pressure is about 1 Torr and is pumped using high capacity mechanical pumps. The JVD process can be combined with high current ion bombardment for in situ control of the film properties. 

---