Powder immersion technique

Powder compacts were carefully dried and sintered in air. During fast-heating-rate tests the thermocouple was embedded in powder (the same as that used to make the compact being tested), and the compact was placed over it. Specimens were not held a measurable time at temperature except when the time at temperature is specifically stated. The densities after sintering were obtained either by immersion techniques if the specimen was warped or by geometric measurements. 

Nelson and Eggertsen/in 1958, extended the Loebenstein and Dietz technique by continuously flowing a mixture of helium and nitrogen through the powder bed. They used a hot-wire thermal conductivity detector to sense the change in effluent gas composition during adsorption and desorption, when the sample cell was immersed into and removed from the bath, respectively. 

Future important contributions of heats of immersion will be made in the field of solution adsorption despite the necessity for more exacting experimentation. The common problem in solution adsorption has been to define the nature and extent of the interface between solid particles and mixed liquids. Specifically, more information is needed concerning the orientation and solvation of adsorbed molecules as well as the composition and practical boundary of the adsorbed phase. Direct adsorption measurements yield only net changes in concentration and indirect approaches must be taken (66). Much can be learned, however, by measuring the heats of immersion of powders into two component solutions of varying composition where the adsorption of one component is predominant. This technique, also, is the only available method for measuring the heat of adsorption of... 

In this technique,27 a laser beam is directed onto a solid microcrystalline ( powder ) sample that is sometimes immersed in an index matching liquid. The emitted second harmonic light is collected, filtered, detected, and compared with a standard (usually a urea powder for the organometallic complexes measured to date). This technique is crude The magnitude of the response depends on particle size, and care must be taken in preparation of samples (e.g., sieving samples to ensure a narrow particle size range). 

In principle, to carry out immersion microcalorimetry, one simply needs a powder, a liquid and a microcalorimeter. Nevertheless, it was early realized that the heat effects involved are small and the sources of errors and uncertainties numerous. Many attempts have been made to improve immersion microcalorimetric techniques. Before commenting on this type of experiment, we describe the equipment and procedure which has been found by Rouquerol and co-workers to be of particular value for energy of immersion measurements (Partyka et al., 1979). 

If properly used, immersion calorimetry is a versatile, sensitive and accurate technique which has many advantages for the characterization of porous solids and powders. An indication of these possibilities is given in Figure 5.7. The major areas of application are outlined in this section and reference made to specific examples which are discussed in more detail in other chapters. 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

In this technique, slices are produced by the repeated process of spreading layers of a powdered material and selectively fusing parts of each layer by laser beam. Here, the non-fused powder serves as support, resulting at the end of the building process, an object immersed in the non-fused powder. 

It was noted by MacDiarmid and Epstein as early as 1989 that PAn salts may also be deposited as films on a variety of substrates by immersing the substrate in the polymerization mixture.29 In fact, during standard chemical polymerization, one often observes the deposition of a thin, extremely adherent green emeraldine salt (ES) film on the walls of glass reaction vessels, as well as the bulk precipitation of PAn/HA powder. By judicious manipulation of the polymerization conditions such as reagent concentrations/ratios and modification of the substrate surface, one can maximize the surface deposition as opposed to polymer precipitation.30 This phenomenon has been developed into a widely useful in situ polymerization technique for the preparation of PAn films on a variety of insulating surfaces such as glass and plastics, as well as on fibers and fabrics. 

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