Development of a laser interferometric dilatometer Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wh246v724

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  • Thermal expansion is one of the most frequently measured physical properties of materials. One of the important methods of detecting and measuring these changes is dilatometry, which is measuring the thermal expansion of the specimen and relating the specimen's expansion to its temperature. Four major types of dilatometers are commonly used: The precision micrometric method, the Fizeau-Pulfrich interferometric dilatometer, the quartz tube and dial indicator apparatus, and the autographic optical lever method. These methods have various shortcomings and are not particularly suited for high temperature application. The development of the laser, which provided a high intensity monochromatic light source, prompted the investigation of interferometric configurations as a dilatometer technique. The interferometer has distinct advantages over other systems in that the measurement is absolute, that is, calibration is not needed as measurements are made directly in terms of a wavelength of light and are relatively contactless in nature. The purpose of this investigation was to determine the feasibility of a dilatometer using an interferometric configuration with a gas laser light source and to construct a working model interferometers characteristically break a light beam into two wave trains which, when reunited, will interfere if a phase shift has occurred. The dilatometer configuration developed suspends the specimen in the center of a one inch by eight inch long tantalum heating element such that the specimen is contained in one of the beams of the interferometer, As the specimen changes length, the path containing the specimen changes length, causing a change in the point of interference. The interference bands are counted as they pass in front of a photoconductive cell orifice. The only direct contact between the specimen and the dilatometer is at the supporting wires. The feasibility of using an interferometric configuration utilizing the polished ends of the specimen has been demonstrated by various experiments, one of which was to produce fringes at 1,000°C using a polished steel specimen. Various experiments using a Michelson interferometer and a gas laser produced clear, stable fringes which could be easily counted. Preliminary measurements have been made to 500°F with the dilatometer. The specimen and support system were heated to 1,000°C for one hour. The performance of the vacuum system and the cooling system was satisfactory. Subsequent inspection of the support system, etc., showed that it was undamaged. Calculations indicate an upper limit of the present configuration with the particular laser light source to be about 1,800°C. Preliminary data indicate the accuracy of the apparatus to be about five per cent compared to a probable accuracy for an improved model of about two per cent. The dilatometer measurement errors can be grouped in optical errors, errors due to differential thermal expansion in the apparatus, and readout errors. Most of the errors are independent of the coefficient of expansion of the specimen; thus, the percentage error will be largely determined by the coefficient of thermal expansion of the material measured. All of the errors except the errors due to the movement of the specimen and its support system can be reduced or eliminated by proper design and use of materials which have low coefficients of thermal expansion. Movement of the specimen causes a series of second order errors, estimated in the aggregate to be less than twenty millionths of an inch per 100°C. The use of the gas laser light source in an interferometric configuration for dilatometry is feasible and has definite high temperature potential. A working model has been built and some preliminary measurement capability demonstrated.
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