Investigation of Thermal Expansion Coefficient of Metals from Light Diffraction Pattern

Abstract

In this work, an experimental set for determining the linear thermal expansion coefficient (α) of a metal was designed and constructed. It was divided into three parts: metal rod holder, light source, and heating system. A cylindrical plastic pipe with an inner diameter of 16 cm was used as a metal rod holder. The sample metal rod with unknown linear thermal expansion coefficient was inserted vertically into a holder and covered by the first razor blade at one end, while the other end was fixed. The second razor blade was attached with posts. Two razor blades were aligned and separated with a distance of 0.05 cm; therefore, two razor blades were acted as a thin single slit. The steam generated from heating system was transferred into metal rod holder; subsequently, the length of metal rod increased with increasing temperature. The design of the apparatus for this proposed method based on decreasing the width of single slit with the same amount of increasing the metal rod length due to linear thermal expansion. A He-Ne laser with a wavelength of 632.8 nm used as a light source was passed through a single slit and produced a diffraction pattern on a screen. The decrease in slit width was determined by measuring the fringe width. The value of linear thermal expansion coefficient of a metal can be calculated from alteration of fringe width. In our case, stainless steel (314), copper (UNS C11000), and aluminium (6063) rods with the original length of 45.3 cm were used as samples. The experimental values of α for stainless steel, copper, and aluminium are 15.25 × 10-6, 17.74 × 10-6, and 23.13 × 10-6 (°C)-1, respectively. These values are in good agreement with the standard values as reported by the National Metal and Materials Technology Center (MTEC), Thailand. The error of this proposed method is found to be less than 1.2%.