Environmental Scanning Electron Microscopy as Tool to Study Shrinkage Microcracks in Cement-Based Materials
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been used in the past to detect microcracks on a specimen cross-section in concrete have one major drawback: the concrete is dried during microscope sample preparation or in the vacuum of the SEM [3,4,5]. This drying causes formation of additional microcracks on the sample surface. Therefore, studying original microcracks is difficult, because microcracks generated by sample preparation or in the SEM obscure the original crack-pattern. Unlike conventional SEM, Environmental Scanning Electron Microscopy is a promising visual tool for the study of shrinkage or mechanical microcracking in cement-based materials. In ESEM water vapor is used as a signal-amplifying gas to create an image. The presence of water vapor in the ESEM can be used to control the humidity in ESEM. The phase diagram of water (Fig. 1) shows how the relative humidity (RH) of a system depends on the water vapor pressure and the temperature. By controlling both the water vapor pressure and the temperature of the sample in the ESEM-chamber, it is possible to control the relative humidity above the sample. Thus, by creating a relative humidity of 90-100% above the sample it is possible to prevent the sample from drying during examination in the ESEM. In this paper the principles and equipment to control relative humidity by regulating both the pressure and the temperature are described. A 'wet' sample preparation method is described and an ESEM pumpdown-sequence without drying of the sample will be given. Preliminary results are presented which illustrate the visibility of shrinkage microcracks on a 'wet' specimen cross-section. For detailed principles and functions of ESEM reference is made to Uwins [6]. 141
Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
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Figure 1: Phase diagram of water at low pressures showing lines of relative humidity of the water vapour. 1 Torr = 133.3 Pa. EXPERIMENT Pressure Control A Phillips XL30 ESEM (Tungsten filament) is used in the present microcrack studies. The water vapor pressure in the sample chamber is controlled by the vacuum system of the microscope. The water vapor pressure of the chamber can be adjusted at any value between 0.1 and 20 Torr with an accuracy of 0.1 Torr. To obtain the required pressure the system will be either in the pump or flood mode. The equilibration mode is maintained by a needle valve which is controlled by an automatic control system (the pressure servo). The water vapor pressure is an important factor influencing the image clarity. At a working distance of 10 mm the optimum GSE-detector gain is at a vapor pressure of about 3.5 Torr. The practical maximum water vapor pressure at which still satisfactory images can be acquired is about 6 Torr at a 10 mm working distance. This means that at a temperature of 220C in the ESEM-chamber a maximum practical relative humidity of 30% can be achieved (Fig. 1). To obtain higher humidities, while keeping the pressure below 6 Torr, the samp
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