Sinking materials

Electronic. Diamonds have been used as thermistors and radiation detectors, but inhomogeneities within the crystals have seriously limited these appHcations where diamond is an active device. This situation is rapidly changing with the availabiHty of mote perfect stones of controUed chemistry from modem synthesis methods. The defect stmcture also affects thermal conductivity, but cost and size are more serious limitations on the use of diamond as a heat sink material for electronic devices. 

To remove this unwanted heat (also known as thermal management), it is often necessary to use hybrid circuits and bulky heat-dissipation devices or complicated and expensive refrigeration. Metals with good thermal conductivity, such as copper or aluminum, are presently used as heat sink materials but, since being metals they also have high electrical conductivity, they require an electrical-insulation barrier. 

Beryllia (BeO) is an excellent heat-sink material which is presently widely used but is being phased out because it presents acute safety problems.It is being replaced by aluminum nitride which extensively produced by CVD, mostly in Japan (see Ch. 10, Sec. 2).P2]... 

Diamond is an electrical insulator with the highest thermal conductivity at room temperature of any material and compares favorably with beryllia and aluminum nitride. P3]-P5] jg undoubtedly the optimum heat-sink material and should allow clock speeds greater than 100 GHz compared to the current speed of less than 40 GHz. 

Specific gravity (SG) of a liquid is used to determine whether a spilled material that is insoluble will float or sink. Materials heavier than water have SGs greater than 1 and materials lighter than water have SGs less than 1. 

G.A. Lane, J.S. Best, E.C. Clarke et al., Solar Energy Subsystems Employing Isothermal Heat Sink Materials, ERDA Contract No. NSF-C906. The Dow Chemical Company, Midland, MI, 1976. 

The previous equations describing the adsorption/desorption behavior of gases lead to models describing sink effects in indoor environments. In addition, transport of molecules within the sink material can have a major impact on desorption rates thus, models accounting for internal diffusion have been developed for indoor sinks. Models based on fundamental theories are preferred over empirical approaches, but some studies rely on experimental data to fit empirical models [21-23]. 

In order to use sink models, important parameters must be available. For example, the Langmuir adsorption model requires information on the rates of adsorption and desorption. Diffusion models require information on the diffusion coefficients. These parameters are dependent upon the characteristics of both the VOC (or SVOC) and the sink material, and fundamental data are generally not available. Thus, experimental studies are required to determine the values of the important parameters of the sink models. 

A number of test methods have been used to determine sink model parameters. The most common test protocol uses a dynamic, flow-through chamber and involves challenging a test sink material with a test gas [20, 31, 35, 36]. Details on this technique are presented later. Other methods include static tests and microbalance measurements. Borrazzo et al. [37 ] took a fundamental physical chemistry approach and used static equilibrium tests to determine partition coefficients for trichloroethylene and ethanol vapors and several types... 

The most common method of evaluating indoor sinks involves placing a sample of the sink material in a chamber and then flowing a fixed concentration of challenge gas through the chamber at a known flow rate. This is shown schematically in Fig. 1. 

Figures 1,2, and 3 are provided to illustrate one protocol often used to evaluate sink materials [20,32,42-47] however, other methods are also used. For example, Krebs and Guo [48] reported on a unique method involving two test chambers in series. The first chamber is injected with a known concentration of a pollutant (in this case, ethylbenzene). The outlet from the first chamber provides a simple first-order decay that is injected into the inlet of the second chamber that contains the sink material (gypsum board). Thus, this method exposes the sink test material to a changing concentration typical of many wet VOC sources. The sink adsorption rate and desorption rate results are comparable to one-chamber tests and are achieved in a much shorter experimental time. Kjaer et al. [31] reported on using a CLIMPAC chamber and sensory evaluations coupled with gas chromatography retention times to evaluate desorption rates. Finally, Funaki et al. [49] used AD PAG chambers and exposed sink materials to known concentrations of formaldehyde and toluene and then desorbed the sinks using clean air. They reported adsorption rates as a percentage of concentration differences.

Experimental studies to determine adsorption and desorption rates show wide differences in values depending on the sink material and the VOC adsorbate. Because of the large munber of possible combinations of indoor surfaces and VOCs, only a few of these combinations have been evaluated. References that give values of k and kj for a variety of indoor sink surfaces and indoor pollutants based on dynamic small chamber testing are provided in Table 1. 

A review of the references cited in Table 1 would show a wide difference in ka and kj values depending on the sink material and pollutant. This is illustrated in Table 2, where a few representative values are provided. 

Table 1 Sink materials and indoor pollutants evaluated for and kj...

Reference Sink material Indoor pollutant fc3(mh- ) fcjm)... 

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