Alumina tubes

Any sihcate that forms thermally and chemically stable residual compounds as its oxygen content is reduced provides a suitable source of siUcon for this reaction. A typical process consists of alternating aluminum, siUca, and graphite plates separated by 2—4-cm thick graphite spacers stacked in a graphite-lined alumina tube and heated to 1400°C for 12 h in a nitrogen atmosphere. After cooling for approximately 6 h the fibers are removed. 

A typical converter is made up of multiple furnaces, each of which contains 8 to 10 reactors. Each reactor is made up of 10 to 30 sintered alumina tubes lined with platinum. The furnaces are direct fired with natural gas to 1200—1300°C. A typical furnace can produce about 125 t per month of hydrogen cyanide. Catalyst life is approximately 10,000 h. 

The green cokes were calcined by placing a weighed amount of green coke into an alumina tube. The tube was fitted with end caps to allow for a constant purge of nitrogen. The alumina tube was then inserted into a high-temperature furnace and the temperature raised to about 1000°C for a period between 30 and 60 minutes. The furnace was turned off, cooled to room temperature, and the product recovered to determine the calcined coke yield. 

Hollomon s ethos, combined with his ferocious energy and determination, and his sustained determination to recruit only the best researchers to join his group, over the next 15 years led to a sequence of remarkable innovations related to materials, including man-made diamond, high-quality thermal insulation, a vacuum circuit-breaker, products based on etched particle tracks in irradiated solids, polycarbonate plastic and, particularly, the Lucalox alumina envelope for a metal-vapour lamp. (Of course many managers besides Hollomon were involved.) A brilliant, detailed account of these innovations and the arrangements that made them possible was later written by Guy Suits and his successor as director, Arthur Bueche (Suits and Bueche 1967). Some of these specific episodes will feature later in this book, but it helps to reinforce the points made here about Hollomon s coneeption of broad research on materials if I point out that the invention of translucent alumina tubes for lamps was... 

The/3" —alumina tube inserted in a steel cell case which forms the negative terminal. The space between the ceramic tube and the cell case is the sodium compartment. 

The overall manufacturing process of / "-alumina tubes can be subdivided into the production stages powder preparation,... 

Life and reliability data of / "-alumina tubes have been reported by Barow[24], Table 7 includes the dimensions and physical properties of / "-alumina tubes, and Table 8 shows the resistivity of / " -alumina tubes at 300 and 350 °C. The resistivity at 350 °C reported by Heavens [25] is somewhat lower. The resistivity of / " -alumina remains nearly constant even when zirconia is added (Fig. 14). 

Table 7. Dimensions and physical properties of p" -alumina tubes (laboratory-made)...

The preparation method also influences the resistivity of f3"-alumina tubes. Bug-den and Duncan [26] showed that the resistivity of tubes made from spray-dried powder is lower (4.3Qcm) than that of tubes of vacuum-dried powder (5.5 Qcm). 

Compared with the ionic conductivity of p" —alumina tubes, the electronic conductivity is negligible. The electronic resistivity was found to be 109Qcm at 300 °C and 7x lO3 Qcm at 1000 °C [27]. 

The zeolite-alumina tube is no more gas-tight after the thermal treatment. The presence or absence of the V-AI2O3 toplayer in the starting support does not influence the gas transport properties of the final zeolite-alumina tube. 

Methyl radicals were produced by pyrolysis of azomethane (CH3N2CH3). Azomethane was synthesized as describe earlier [18]. It was purified periodically by fteeze-pump cycles at 77 K, and the gas purity verified by RGA. The methyl radical source was similar to that developed by Stair and coworkers. [10, 11] The source was made of a quartz tube with 3 mm OD and 1 mm ID, resistive heating was supplied by means of a 0.25 mm diameter tantalum wire wrapped outside the quartz tube. The len of the heating zone was 4 cm, recessed from the end of the tube by 1 cm. An alumina tube around the outside of the heating zone served as a radiation shield. Azomethane was admitted to the hot tube at a pressure of 1x10-8 to 1x10-7 Torr via a high-vacuum precision leak valve. The pyrolysis tube was maintained at about 1200 K, adequate to decrease the major peaks in the mass sp trum of the parent azomethane at 58 and 43 amu by at least a factor of 100. 

Indium also has many of the characteristics that make Al and Ga very useful for such applications. Particularly important is its capacity to dissolve Si, Ge and several lanthanide and transition metals, producing highly reactive forms of the elements. Moreover In does not form binaries with Si and Ge and has a low-melting point. RNiGe2 compounds, for instance, were prepared from stoichiometric quantities of the components in fine powder mixed with a 10 fold quantity of In in alumina tubes. These, flame sealed in fused silica tubes, were slowly heated to 1000°C, held at this temperature for a few hours, ramped down to 850°C, held for an additional 4 days and finally cooled down to room temperature over the course of another 4 days. Compound isolation from the In excess was performed by centrifugation at 300°C through a coarse frit. Further purification was carried out by a 15-minute submersion and sonication in 6 M aqueous HC1 (Salvador et al. 2004). 

Porous alumina tube (in double pipe configuration), wall thickness 1 mm, length 200 mm, outer diameter,... 

Porous alumina tube externally coated with a MgO/PbO dense film (in double pipe configuration), tube thickness 2.5 mm, outer diameter 4 mm, mean pore diameter 50 nm, active film-coated length 30 mm. Feed enters the reactor at shell side, oxygen at tube side. Oxidative methane coupling, PbO/MgO catalyst in thin film form (see previous column). r-750X,Pr ed 1 bar. Conversion of methane <2%. Selectivity to Cj products > 97%. Omata et al. (1989). The methane conversion is not given. Reported results are calculated from permeability data. 

The electrolyte surrounding the central positive electrode is sodium tet-rachloroaluminate, which melts at 157 °C and acts as a solvent for the nickel(II) chloride, and this is separated from the molten sodium in the outer compartment by a /1-alumina tube which, again, serves as a fast ion conductor for Na+. The cell operates at 300 °C and delivers up to 2.58 V. 

The radioactive oxides which served as the vapor sources were contained in a small platinum crucible. This crucible was placed on top of an assembly of two porous alumina plugs, mounted in tandem on a small, gas-tight alumina tube. The whole assembly could be inserted through the bottom opening of the furnace tube into the low temperature zone. 

A dry air stream was introduced into the furnace via a hole in the small alumina tube just below the top alumina plug. The air flowed up and around the top alumina plug where it mixed with the radioactive oxide vapor, and the air-radioactive oxide vapor mixture flowed up through the high temperature zone and left through the top of the furnace. 

The cell design is shown schematically in Fig. 8.22. Two nickel components are sealed to a metallized j3-alumina collar by thermocompression bonding the 0-alumina tube is then attached to the alumina collar by a glass seal. This sub-assembly can then be welded to the prismatic metal can. A metal wick inserted in the steel can, before the sub-assembly is welded in place, acts as the cathode for sodium deposition when the cell is charged. The positive electrode is added as a mixture of nickel powder, salt and aluminium powder, and after addition of the molten sodium chloro-aluminate the cell is sealed by welding on the positive terminal... 

Physically, these gas reference electrodes consist of an alumina tube containing carbonate melt with a gold wire current collector. Provisions are made for purging the melt in the tube with the appropriate gas mixture. Complete details about these reference electrodes are summarized in the review by Selman and Maru [5]. 

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