Thermal Coefficients of Materials

1.0 INTRODUCTION

Thermal properties are of interest in many different applications as a computer chip cannot operate and perform its required function if it overheats. Dissipation of heat while the chip is operating is dependent on convection through the surrounding air, radiation, and conduction through other components adjacent to the chip. Higher thermal conductivity of these parts is vital to quickly dissipate the heat from the chip. By determining this thermal property for different materials, selection of the appropriate material for specific conditions is possible.  In this report the experimental methods for determination of thermal conductivity will be discussed.

2.0 EXPERIMENTAL METHODS

2.1 Thermal Conductivity Apparatus Method

The TD-8561 Thermal Conductivity Apparatus may be used determine the thermal conductivity. The procedure for measuring thermal conductivity using the stated apparatus is straightforward.  A slab of the material to be tested is clamped between a steam chamber, which maintains a constant temperature of 100 °C, and a block of ice, which maintains a constant temperature of 0°C. A fixed temperature differential of 100 °C is thereby established between the surfaces of the material. The heat transferred is measured by collecting the water from the melting ice. The ice melts at a rate of 1 gram per 80 calories of heat flow (the latent heat of melting for ice).

The thermal conductivity, k, is therefore measured using the following equation:

  

where distances are measured in centimeters, masses in grams, and time in seconds.

The Thermal Conductivity Apparatus includes the following equipment as:

• ü  Steam chamber with hardware for mounting sample
• ü  Ice mold with cover
• ü  Materials to test: Glass, wood, lexan, masonite, and sheet rock (The wood, masonite, and sheet rock are covered with aluminum foil for waterproofing.)

Equipment Included with the Thermal Conductivity Apparatus

 

2.2 Pulsed or Periodic Regime

This method uses the heat conduction through metals through the Single thermal pulse, applied to both ends, and subsequent evolution through the three monitored points;

The material to be tested for thermal conductivity in the form of bar are wrapped in a heating resistor to some length.

Montagem: Mineral wool sandwich covering the metal cylinders.

Three batches of thermometers are placed on different location on the bar. The heat sink keeps one of the ends at room temperature and is the reference point for the position axis. The bar is mounted in the middle of two layers of thermal insulation material (mineral wool) that prevent heat convection and minimize thermal losses.

In a pulsed or periodic regime, an electric current is applied to the heating resistor that heats that end through Joule's effect. The heat generated will run through the bar and be dissipated at the opposite end in sink.

In the periodic mode, the temperature reading has two components:

ü  One comes from the oscillation of the heating itself, its period being the same as the heat source (on and off);

ü  The other is the average heating of the bar as a whole, which is almost exponential.

Through a graphical fitting of a function to the average temperature, the oscillating values can be extracted by subtracting the average. By analyzing the oscillating data, the heat propagation constant may be determined by Fourier analysis or a simple sinusoidal fitting.

2.3 ANGSTROM’S METHOD

Angstrom developed a method of determining the thermal conductivity of a metal rod by applying an alternating heat pulse to one end while leaving the other end at room temperature. Doing this causes a heat wave to propagate down the rod and creates an observable temperature difference between two points on the rod. This also creates a varying phase relationship between the measured temperature recorded at the first and second points.

The thermal conductivity of the rod can be determined if the temperature of these two points is measured as a function of time. Since the temperature changes are periodic, the measurements of the power input used to heat the system are not required. Because of this, absolute measurements of the temperature are not required so that only relative changes in magnitude of temperature as a function of time and position must be recorded. The thermistors may be used to respond linearly over changes of a few degrees.