Background

In our daily life, we contacted with materials with different thermal conduct character everywhere. Usually, we use good or poor medium for conducting heat to differentiate them. But this is not enough in researching or production, we need exact values about them.

Basic Theory

Heat will transmit from higher temperature T_H to lower temperature T_L. This is called thermal conduction, one of the basic thermal transmission methods. The heat transported perpendicularly to the cross-section area in unit time is given by Joseph Fourier equation:

,

where λ is the thermal conductivity. Now we use steady-state measurement to get λ of a rubber plate.

The measured sample―the rubber plate is placed between a cylindrical heater and a copper plate. The heater is heated by electricity so that there is enough heat can be transmitted to the sample. When the same amount of heat transporting to the sample is transmitted to the copper plate, this process is called a steady-state thermal conduction.

In steady state, the heat desorbed by copper is . So there must be the same amount of heat ΔQ is gotten from the sample, that is . From the equation of the rate of heat desorption , thermal conduction rate  will be obtained through measuring the cooling rate ΔT/Δt near a steady temperature T_L. Considering the correction for heat desorption rate from copper, the thermal conductivity is given by

,

where d_c, h_c are diameter and thickness of the copper plate, m_c and c_c the mass and specific heat of copper, d_b, h_b the diameter and thickness of the round sample.

Procedures

1. Place the copper plate and sample on the holder. Then put down the heater and adjust it so that their bottoms are co-axial. Make as close as possible the small holes on heater and copper plate’s sides, which are used to be connected with thermoelectric couples. Adjust the screws on the holder so that the heater, sample and copper plate have a tight contact.

2. Slowly increase the voltage of the regulator for heater from 0v to 100v. Make the connection of thermoelectric couples as shown in the picture below. The temperature of the heater and copper can be read (in mV) from the voltmeter, respectively by shifting a switch on the bottom of the apparatus. The larger one is the temperature reading of the heater.

3. If temperature readings T_H (the larger one) and T_L (the smaller) keep almost constant for 20s, the state could be regarded as a steady state. Record T_H and T_L, and zero the voltage of regulator. Then take away the sample and make the copper plate directly connect with the heater. When T_L is increased by about 0.3mV, lift the heater and hold it by the pothook quickly. Let the copper plate cool down naturally. Record the temperature reading of the copper plate every 20s until 1 reading smaller than T_L. Select the last 2 readings to fill in the table as below.

4. Calculate the thermal conductivity as below on you paper with the measured parameters.

=__________________kJ・kg^-1・℃^-1

Precautions

1.      Read the 3^rd step in procedures carefully, so that you can operate proficiently.

2.    Keep distance from the electric source.