Asbestos matrix

In the conception of Bjorkman [35], an extended area graphite cathode is comprised of a packed, flow-through bed. The electrolyte, in this case aqueous HC1, was immobilized in an asbestos matrix. Chloride ions are transported to the graphite anode where pure Cl2 is liberated. 

The impregnation of porous nickel discs with CoPc was difficult because of the limited solubility of the chelate in the usual solvents. CoPc cathodes with carbon as substrate were therefore prepared for use in H2/O2 fuel cells. A mixture of 72 mg CoPc and 48 mg acetylene black, with PTFE as binder, was pressed into a nickel mesh of area 5 cm2. Electrodes of this type were tested in an H2/O2 fuel cell with 35% KOH electrolyte in an asbestos matrix at 80° C. Figure 5 compares the current/voltage characteristics of CoPc cathodes (14 mg/cm2) with those of other catalysts, including platinum (9 mg/cm2), silver (40 mg/cm2), and pure acetylene black (20 mg/cm2). An hydrogen electrode (9 mg Pt/cm2) was used as the anode in all tests. To facilitate comparison of the activity of different cathodes, the pure ohmic internal resistance of the cells (of the order of 0.02 ohm) was eliminated. 

Alkaline fuel cells (AFCs) were the first type of fuel cell to be widely used in space exploration applications-for example, in NASA s Apollo and space shuttle flights. Figure 1.8 shows a schematic of an AFC stmcture. AFCs use H2 and 02 as fuel and oxidant, respectively. The electrolyte is a concentrated KOH solution absorbed into an asbestos matrix. The temperature for AFCs ranges from 100-250°C and the efficiency can be > 60%. OH ions are transported through the electrolyte from cathode to anode. The reactions are as follows ... 

Mechanisms based on electron transfer and active oxygen species have been proposed to explain asbestos-induced toxicity and lung disease. Fisher et al. (1987) studied the effect of heat treatment on chrysotile asbestos toxicity. The in vitro study showed that heat treatment reduced cytotoxicity. Infra red spectra indicated a reduction of external hydroxyl group population, which repopulated after irradiation. There is, apparently, an electron transfer from the asbestos matrix to biological receptors. In an earlier study, Fisher and coworkers (1985) reported that irradiation of chrysotile samples heated to 400°C (752°F) restored the biological activity to near-control values. X-ray diffraction pattern showed no change in the crystal structure. Brucite, present as a surface contaminant, was removed by heating. 

Other workers gradually went to less concentrated alkali (30-40% KOH) than found in Bacon s and P W s batteries. For the space shuttle program, United Technology Corporation (UTC-Power) developed a battery of alkaline fuel cells where 35% KOH immobilized in an asbestos matrix was used as the electrolyte. The electrodes contained a relatively large amount of platinum catalysts, so that at a temperature of 250°C it was possible to work at very high current densities, of up to 1 A/cm. ... 

In 1962, a research group in the American company Allis-Chalmers started developing a new type of hydrogen-oxygen fuel cell with an alkaline electrolyte. The distinguishing feature of this cell was to use, instead of a fi eely flowing liquid electrolyte (KOH solution or melt, as described above), a quasi-solid electrolyte in the form of potassium hydroxide solution immobilized in an asbestos matrix. Asbestos... 

Electrolyte Hydrated Polymeric Ion Exchange Membranes Mobilized or Immobilized Potassium Hydroxide in asbestos matrix Inunobilized Liquid Phosphoric Acid in SiC Inunobilized Liquid Molten Carbonate in LiA102 Perovskites (Ceramics)... 

Figure 1.7 shows an alkaline fuel cell with stationary electrolyte. In this system, the electrolyte is held in asbestos matrix material. 

Hence, unlike mobile electrolyte AFC, the electrolyte does not need to be pumped out. Moreover, there is no problem of an internal short-circuit, which makes it more favourable for space applications. The KOH electrolyte in an asbestos matrix possesses excellent porosity, corrosion resistance and strength. Due to the stationary nature of the electrolyte, it becomes mandatory that pure oxygen should be supplied at the cathode. In case CO2 is present in the air, carbonate formation takes place and the complete fuel cell needs to be rebuilt. Hydrogen is pumped at the anode and circulated to remove water. The cooling system comprises fluorinated hydrocarbon dielectric liquid. The product water is used for humidification of the cabin, cooking and drinking in space orbiter systems. As the water is used at the cathode, its level must be kept sufficiently high at the anode side. 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

A sintered friction material is composed of a metal matrix, generally mainly copper, to which a number of other metals such as tin, zinc, lead, and iron are added. Important constituents include graphite and friction-producing components such as siHca, emery, or asbestos. 

During the late 1960s and 1970s, the finding of health problems associated with heavy exposure to airborne asbestos fibers led to a strong reduction (or ban) in the use of asbestos fibers for thermal insulation. In most of the current applications, asbestos fibers are contained within a matrix, typically cement or organic resins. 

The choice of a particular mining method depends on a number of parameters, typically the physical properties of the host matrix, the fiber content of the ore, the amount of sterile materials, the presence of contaminants, and the extent of potential fiber degradation during the various mining operations (33). However, most of the asbestos mining operations are of the open pit type, using bench drilling techniques. 

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

The main characteristic properties of asbestos fibers that can be exploited in industrial appHcations (8) are their thermal, electrical, and sound insulation nonflammabiUty matrix reinforcement (cement, plastic, and resins) adsorption capacity (filtration, Hquid sterilization) wear and friction properties (friction materials) and chemical inertia (except in acids). These properties have led to several main classes of industrial products or appHcations... 

Finally, the combined reinforcing effect and high absorption capacity of asbestos fibers have been exploited in a variety of appHcations to increase dimensional stabiHty, typically in vinyl or asphalt tiles and asphalt toad surfacing. Figure 9 summarizes, as of 1984, the various classes of application for asbestos fibers in combination with other materials. The diagram shows that in recent years, most industrial appHcations have evolved towards composite materials where the fibers are bonded within an organic or inorganic matrix. 

Future brakes must satisfy health standards and most vehicle manufacturers have moved toward removing all asbestos from brakes. Lighter weight rotors and caUpers based on aluminum-based metal-matrix materials are also on the horizon for lighter vehicles requiring a whole new family of compatible friction materials. 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

There are two main types of proficiency testing scheme. First, there are those set up to assess the competence of a group of laboratories to undertake a very specific analysis, e.g. lead in blood or the number of asbestos fibres in air collected on membrane filters. Secondly, there are those schemes used to evaluate the performance of laboratories across a certain sector for a particular type of analysis. Because of the wide range of possible analyte/matrix combinations it is not practicable to assess the performance of laboratories when analysing all the possible sample types. Instead, a representative cross-section of analyses is chosen (e.g. determination of different pesticide residues in a range of foodstuffs or the determination of trace levels of metals in water samples). 

Alkaline Fuel Cell (AFC) The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature ( 250°C), or less concentrated (35-50 wt%) KOH for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered with the alkaline cell. 

AMs are richly endowed with endoplasmic reticulum (ER). Following exposure to cigarette smoke hydrocarbons in the presence of asbestos, the ER synthesizes the enzyme aryl hydrocarbon hydroxylase. This enzyme converts biologically inactive hydrocarbons to metabolically active forms that, when excreted from the cell, react with the extracellular matrix materials. 

The AFC type was originally created for the Apollo program, after that a modernized version has been developed and is even now in use to provide electrical power for shuttle missions. The electrolyte in this fuel cell is KOH, concentrated (85 wt %) for fuel cells operated at relatively high temperatures, that is, around 250°C, and less concentrated (35-50 wt %) for cells operated at lower temperatures, that is, less than 120°C [6,9,11], In the construction of these fuel cells, the electrolyte is retained in a matrix, typically asbestos, and a wide range of catalysts, for example, Ni, Ag, metal oxides, and noble metals, can be used for both the hydrogen and the oxygen electrodes [8,9],... 

One material that has been used to replace asbestos in certain applications is wollas-tonite or calcium metasilicate. This is also a fibrous mineral filler but with a lower aspect ratio than asbestos. Surface-treated versions are available to improve adhesion to the epoxy matrix. It can be applied at relatively high loading levels to provide for high strength and improvements in moisture resistance.25... 

Fillers present a potential inhalation and dermal contact hazard. They can cause mechanical damage to the skin, which may aggravate the irritant effect of other chemicals and additives. When fillers are handled in a liquid epoxy matrix or in a cured epoxy, their inhalation hazard is low. However, inhalation exposure to fillers can occur when they are handled in the dry state or when one is machining or grinding cured epoxy products. Inhalation exposure to fillers such as crystalline silica or fiberglass may result in delayed lung injury. Asbestos fillers have long been abandoned from use for these reasons. 

Since ancient times, natural fibers have been used to reinforce brittle materials. For example, thousands of years ago, Egyptians began using straw and horsehair to reinforce and improve the properties of mud bricks. In more recent times, large-scale commercial use of asbestos fibers in a cement paste matrix began with the invention of the Hatschek process in 1898. However primarily due to health hazards associated with asbestos fibers, alternate fiber types have been investigated and introduced throughout the 1960 s and 1970 s. 

---