Zero creep technique

Table 4.2. Solid/vapour surface energies of pure metals measured by the zero-creep technique (Eustathopoulos and Joud 1980).

Techniques to measure the surface tension of solids are notoriously difficult and known for their inaccuracies. Reliable surface tension data requires not only a reliable measurement technique but careful control over parameters such as sample purity and the gaseous atmosphere in which the experiments are conducted. TTie zero creep technique is considered one of the most accurate and reliable of these techniques since it requires only a simple length measurement(8). Samples can be either wires or thin foils. Hondros(9) has postulated that the use of thin foils increases the sensitivity of the technique and thus allows more accurate measurements. The thinner the foil, the more it approximates a surface. Wire gauges are limited due to the loads required to strain the sample. Table I lists some of the results obtained using the zero creep foil technique. It should be pointed out that the terms surface tension and surface free energy are often used interchangeably, though they are not equivalent(9,10). 

The technique used to measure the surface tension of foils in this work couples two well Imown technologies the zero creep technique for foils, and the technique of laser interferometry. The theory behind each of these techniques will be discussed briefly in order to develop the expressions necessary to generate the desired surface tension data which will be used to calculate the surface free energy. 

The Zero Creep Technique. The zero creep technique was developed by Udin, Shaler, and Wulff(8) to measure the surface energy of Cu wires. The technique was later extended for use with thin foils by Hondros(16). Very thin foils, approximating a surface, are readily available. When shaped into a cylinder, the sample will tolerate large loads without necking. Since necking does not occur, the stress can be considered constant throughout the experiment. Figure 1 shows a schematic of a foil and the associated stress under an applied load... 

Table I. Summary of investigators using the zero creep technique including their method of length measurement...

Figure 8.31 Data of Gregg and Rhines (55) for the sintering stress versus relative density for 30 xm copper particles, determined by the zero-creep technique.

Measurement of surface and grain-boundary energies 4.4.1 The zero-creep technique... 

Figure 4.20 A schematic diagram of the zero-creep technique for measuring the surface energy of solids.

Most experiments on the surface free energy which have been discussed in the preceding sections have been performed on polycrystalline material. One exception is the zero creep technique when applied to foils which can be prepared as single crystal surfaces [67Hon], Absolute values for the surface free energy of a specific orientation have also been obtained by the cleavage technique, see data in section 4.4.7.1. 

Fig. 13. Surface free energy of Ag as measured by the zero creep technique in dependence of the oxygen partial pressure. The slope of the solid curve indicates...

Fig. 16. Variation of the surface free energy of copper with oxygen pressure. Filled circles represent data measured by the zero creep technique at 927 °C [73McL], open squares are from measurements at...

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