Crucible alumina

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

Mpa pressure, and subsequently fired in air at 1000 °C for 24 h in a covered alumina crucible. 

Sato et al. " measured the viscosities of some binary and ternary alkali carbonates. Since melt creep must be prevented, a highly sintered alumina crucible was used instead of a gold-plated nickel crucible. Homogeneity of a mixture sample was achieved by gas bubbling. A laser beam is combined with a computer-assisted time counter to obtain the logarithmic decrement. Roscoe s equationi3i has been used for calculation of the viscosity, while it has been claimed by Abe et al. that the viscosities calculated from Roscoe s equation are 0.6-1.5% lower than those from more rigorous equations. 

In a typical reaction 100 - 200 mg of metal [Cr or Ni] was evaporated from a preformed alumina crucible over a period of 60 - 90 min and deposited into a mixture of 2 in poly(dimethylsiloxane) [Petrarch Systems 0.1 P.] within a rotary solution metal vapor reactor operating at 10 4 torr. The reaction flask was cooled to approximately 270 K by an iced-water bath. For a description of the apparatus see Chapter 3 of reference 4. The product in each case was a dark orange viscous liquid and was characterized as obtained from the reaction vessel. 

Aliquots (0.3-0.5g) of dry sediment (passing 100 mesh) were weighed and then transferred into 10cm3 Coors alumina crucibles. The uncovered crucibles, contained in a suitable tray, were placed into a warm muffle furnace and ignited at 550°C. The samples were maintained at 550°C for 1.5h, then removed, allowed to cool, and transferred into 100ml calibrated flasks 50ml of 1.0M hydrochloric acid were then added to the flasks. The mixtures were next shaken for 14-18h at about 22°C. 

Depending on the material of the crucible, various cleaning agents may be used. Whenever possible, water and mild abrasives like sand are preferred. Cleaning with acids is usually applicable to platinum crucibles, but should be avoided in the case of alumina crucibles. Platinum crucibles can be cleaned especially thoroughly in a melt of potassium pyrosulfate. In all such operations it is important for the crucibles... 

Fig. 52. Typical growth of alumina whiskers in an alumina crucible...

Synthesis of sulfo-selenide Chevrel phases Phases ofM Mo x xSex composition (M = Cr, Mn) were prepared by the reaction of stoichiometric mixtures of MoS2 and MoSe2 binary powders with Mo and Cr, or Mn, metallic powders (Mantjour-Billah and Chevrel 2004). The mixtures were pressed into pellets and then (inside an alumina crucible) sealed in evacuated silica tubes. After heating to 800°C and then to 1000°C for 24 hours, two further grinding and annealing (at 1000°C) treatments were performed. Powder X-ray diffraction methods were used to study the solid solutions, the trend of the lattice parameters, etc. 

Thermolysis on insulating wool. Kaowool or other refractory wools are valuable for reduced radiative heat losses from a hot crucible. However, their use causes some increase in the extent of pyrolysis of substrate vapor by the crucible assembly and this, in rare instances, may spoil a metal atom synthesis. Only one example is known at present. The reaction of palladium atoms with benzyl chloride gives very low yields of t73-benzylpalladium chloride when the palladium is evaporated from an alumina crucible insulated with Kaowool, but a 30-50% yield with an uninsulated crucible. It has been established that this is due to enhanced formation of product-destroying radicals on the hot Kaowool. 

Copper is best evaporated from an alumina crucible of 2-5 mL capacity, resistance heated either by an external molybdenum or tungsten wire spiral or, preferably, by imbedded molybdenum or tungsten wire (See Sec. 11). If the crucible is insulated with a 10-mm layer of a refractory wool, for example, Kaowool (see Sec. 11), a power input of 180-250 W will heat the crucible to 1400° and evaporate the copper. The power required to give the desired rate of evaporation has to be determined for a new crucible by a trial evaporation in the absence of BC13. The volt and amp settings then found should be valid for many runs with the same crucible. An ordinary grade of soft copper rod is satisfactory... 

During reactions at high temperatures, the ceramic sample must be in contact with a container. Commonly used containers include alumina or zirconia boats or crucibles or noble metal foil-lined ceramic boats. It is important to be aware of possible reactions between the material being synthesized and the container which may be a source of foreign ions. For example, Al+S ions may be incorporated if alumina crucibles are employed. Forming the material into a pellet minimizes the surface area and helps limit reactions with containers. Both alumina and zirconia crucibles and boats can be cleaned with mineral acid washes and reused. 

The synthesis of the n=2 phase is similar to that described above for the n=l system. In general, stoichiometric portions of oxides and carbonates (or nitrates) are ground and reacted in a high density alumina crucible at temperatures between 800 and 820°C. The product is crushed and pressed into a pellet for further heat treatments at 840-870°C. The firing temperature must be raised incrementally to prevent melting of the sample. 

The procedure is to take stoichiometric amounts of the binary oxides, grind them in a pestle and mortar to give a uniform small particle size, and then heat in a furnace for several hours in an alumina crucible (Figure 3.1). 

Solutions were mixed with a magnetic stirrer and heated up to 300°C to evaporate the solvents and to obtain a crisp powder. These organo-metallic precursors were ground and calcined in pure alumina crucibles at 700°C, 800°C, 900°C, 1,000°C, 1,050°C, 1,100°C, 1,150°C, or 1,200°C in a box furnace (in air) with 10°C/min heating rate. After reaching the final temperature the furnace was turned of immediately and the powders were allowed to cool in the furnace. 

The SiC coating is processed based on the reaction of SiO vapor and carbon materials. Commercial SiO powders (99.9% pure) are provided as the silicon source. The carbon materials are placed on the SiO powder bed via a carbon felt as illustrated in Fig. 10.1. This assembly is covered with carbon sheets in an alumina crucible to keep the SiO gas pressure in the crucible, and heated in a vacuum furnace at various temperatures from 1150 to 1550°C in vacuum (about 0.03 Pa) for periods of time between 1 and 90 minutes. It is necessary to heat at a temperature greater than 1150°C for the vaporization of solid SiO. 

Based on these analyses on the SiC coating, the growth mechanism of the SiC layer on diamond is considered as follows. In the early stage of the SiC formation on diamond, a very thin SiC layer is formed on the diamond surface according to reaction (10.2) between diamond and SiO(g). Once the SiC layer is formed, this reaction does not proceed due to the protective layer of SiC. The carbon sheet and felt in an alumina crucible act as the carbon source. The reaction of C02(g) with these carbon sources will produce further CO(g) and deposit SiC(s) by reaction (10.7). Thin j3-SiC whiskers are observed on the surface of the SiC-coated diamond, suggesting the vapor growth of SiC. 

Two types of assembly were used for the SiC coating to investigate the growth mechanism of the SiC layer. In the first method, the SiO powders are set on the bottom of an alumina crucible and MWCNTs are placed upon SiO powders via a carbon felt, as shown in Fig. 10.6(a). In the second method, an alumina plate with a center hole is used instead of the carbon felt to separate the MWCNTs from SiO powders, as shown in Fig. 10.6(b). These assemblies... 

La3o-3iFei3-i4Sb57-55 (HfCuSi2-type, a = 0.44028-0.44035, c = 1.00119-1.00113). The alloys were obtained by arc melting under low electric current to minimize weight losses by vaporization of Sb, which were compensated beforehand by extra amounts of Sb. The samples were placed in alumina crucibles, sealed in evacuated quartz tubes and annealed for 7 days at 1070 K. After the heat treatment the alloys were quenched by submerging the silica tubes in water. The materials used were 99.9% pure. 

A special flux consisting of the halides of alkali metals with the addition of sodium and aluminium fluorides was employed both to protect the aluminium melt from oxidation and to pre-heat the specimen to the required temperature. First, the flux was melted in a 26 mm inner diameter alumina crucible. Melting started at about 350°C. The height of the flux column was around 15 mm. Aluminium pieces were then melted under the flux layer. The amount of aluminium taken was equivalent to a volume of... 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

Preparation of the Electrolytic Cell. The cell (Fig. 12) consists of a 50-ml. alumina crucible fitted with a platinum cap. The anode is a rectangular platinum strip X 2f X 0.01 in. suspended in the middle of the melt, approximately 1 in. from the bottom of the crucible. The cathode is a l-in.-diam. platinum disk, 0.01 in. thick, placed on the bottom of the cell. 

For post-type DTA s in which thermocouple junctions measure the temperature of the container of the sample (e.g. platinum or poly crystalline alumina crucibles), good mechanical contact between the sample and the bottom of the crucible will improve instrument sensitivity to transformations. Surface contact may be optimized by using samples shaped to match the crucible, or finely crushed granules, as opposed to more spherical or odd-shaped chunks. Optimum mechanical contact minimizes the lag time between when a reaction occurs and when heat propagates to/from the point of temperature measurement, and the reaction is recorded. 

An additional 15 g of the lead oxide was mixed with 10.9 g of Sb20s (molar ratio 2 1) in a ball mill for 30 min. This material was then heated in an alumina crucible at 950°C for 2 hr with an air flow of 0.8 SCFH directly on top of the melt. The resulting material was identified as Pb2Sb207 by x-ray diffraction ... 

Two methods for the evaporation of precursors may be employed - resistance heating and electron beam collision. The first method employs a simple alumina crucible that is heated by a W filament. Temperatures as high as 1,800°C may be reached inside the chamber, which is enough for some metals or metal salts to vaporize. Deposition rates for this method are 1-20 A s . The use of an electron beam to assist in the precursor evaporation results in temperatures on the order of 3,000°C, being more suited for the deposition of refractory metals/alloys and metal oxides such as alumina, titania, and zirconia. Since the temperature of the chamber interior is much higher than the walls, the gas-phase ions/atoms/molecules condense on the sidewalls as well as the substrate this may lead to film contamination as the nonselective coating flakes off the chamber walls. 

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