Thermal insulating oxide materials

Thermal insulating oxide materials generally are of one of the following types ... 

The self-heating temperature (effectively the AIT) of a 50 50 mixture of ethylene oxide and air is reduced from 456°C on passage through various thermal insulation (lagging) materials to 251-416°C, depending on the particular material (of which 13 were tested). 

Aluminium phosphate with a surface area of -330 mVg can be prepared using ethylene oxide as in (5.77), and heat treating the precipitate. Alnmininm orthophosphate can be fabricated in the form of lightweight foamed blocks which make good thermally insulating building materials. When heated with sodium nitrate, alumina is obtained (5.78), while suitable reductiai gives elemental phosphorus (4.6). 

As thermal insulation cover material, LDPE competes with polyvinyl chloride. Sometimes the covers disintegrate at the areas of contact with the highest temperature. Copper catalyses oxidative failure. 

Thermal oxidizers must be built to provide the residence time and temperatures to achieve the desired destruction efficiency (DE). As such, thermal oxidizers are comparatively larger than catalytic oxidizers since their residence time is two to four times greater. Historical designs of thermal oxidizers were comprised of carbon steel for the outer shell and castable refractory or brick as the thermal liner (a refractory is like a cement, which is put on the inside of the rector shell to act as a thermal insulation barrier). Modern units are designed and built using ceramic fiber insulation on the inside, which is a lightweight material, and has a relatively long life. Old refractory would tend to fail over a period of years by attrition of expansion and contraction. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

One final note is appropriate for this section. Dne to the fact that many oxide ceramics are used as insulating materials, the term thermal resistivity is often used instead of thermal conductivity. As will be the case with electrical properties in Chapter 6, resistivity and conductivity are merely inverses of one another, and the appropriateness of one or the other is determined by the context in which it is used. Similarly, thermal conductance is often used to describe the thermal conductivity of materials with standard thicknesses (e.g., building materials). Thermal condnctance is the thermal conductivity divided by the thickness (C = k/L), and thermal resistance is the inverse of the prodnct of thermal conductance and area R = 1/C A). 

It is usual to operate an aqueous-medium fuel cell under pressure at temperatures well in excess of the normal boiling point, as this gives higher reactant activities and lower kinetic barriers (overpotential and reactant diffusion rates). An alternative to reliance on catalytic reduction of overpotential is use of molten salt or solid electrolytes that can operate at much higher temperatures than can be reached with aqueous cells. The ultimate limitations of any fuel cell are the thermal and electrochemical stabilities of the electrode materials. Metals tend to dissolve in the electrolyte or to form electrically insulating oxide layers on the anode. Platinum is a good choice for aqueous acidic media, but it is expensive and subject to poisoning. 

The two most common substrates for thin film electrodes are various types of glass—soda-lime, Pyrex, and various forms of quartz or fused silica—and silicon wafers that have been treated to produce an insulating surface layer (typically a thermally grown oxide or nitride). Other possible substrates include mica, which can be readily cleaved to produce an ordered surface, and various ceramic materials. All of these materials can be produced in very flat, smooth... 

Magnesium oxide, MgO, is pressed into sheets for use as a thermal insulating material in oven walls. 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

Hot lines and equipment are usually thermally insulated to conserve energy and to ensure safe service at temperatures above the oxidation limits of the materials. Care must be taken to ensure that the process stream chemistry is either nonoxidizing or that the process-side surface is protected by an insulating or refractory liner. In such cases, the limiting factor will be the availability of a code maximum allowable stress. 

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