Background on Multi-Properties Apparatus

The multiproperty apparatus is designed to measure a number of thermophysical properties, either concurrently or sequentially, on the same sample to very high temperatures. The upper temperature limitation is imposed by the sample's material characteristics, i.e., vapor pressure, melting temperature, softening temperature, etc. In general the material must be a reasonably good electrical conductor as direct Joulean heating using low voltages and high currents is employed to control the sample temperature. Metals, alloys, and graphitic materials have been measured accurately.

The thermophysical properties which have been measured with multiproperty apparatus and the symbols used in this manual to represent these properties are given in Table 3. Other properties that could be measured with this apparatus, but which have not been incorporated as routine measurements as yet, include Peltier coefficient, Seebeck coefficient, and Richardson coefficient.

The multiproperty apparatus has developed as a logical outgrowth of direct heating methods. The various direct heating methods are outlined in several publications [3,4]. The present technique has been extensively evaluated [5] and a number of reports and publications detailing this method and giving results for various materials have been prepared.

General Description

The multiproperty apparatus is designed to measure the thermophysical properties listed in Table 3 of rod-shaped samples of materials which are reasonable electrical conductors. Metals, alloys, and graphitic materials have been measured extensively using this device. Measurements of most of these properties can be made from room temperature to about 1000 C using thermocouples for temperature measurement. However, the apparatus is primarily a high temperature (>1000 C) device using optical pyrometry for temperature determinations.

The advantages and disadvantages of direct heating methods are described in Ref. [5]. The most obvious advantages are related to the fact that, in general, external furnaces are not used. Thus, thermal inertia is quite small and measurements can be made over large temperature regions in very short times.

The multiproperty apparatus (Figure 10) consists of a high vacuum system (10^-7 torr), large bell jar equipped with two long windows, interior piping and sample holders with provisions for sample expansion and contraction, and regulated DC power supplies. An automatic optical pyrometer and elevating stand are required, and twin telemicroscopes and stand are used for thermal expansion. Samples in the form of thin rods, tubes, or wires are supported vertically between water-cooled movable electrodes. The electrode separation distance is adjustable between 0 and 14 inches. Sample expansion and contraction is maintained stress-free through a spring network mounted on a movable "c-cell" equipped with strain gauges. The bell jar which covers the sample support system is raised and lowered by a hoist. The bell jar rests on a feed-through collar which contains rotary feed-throughs for instrumentation leads, electrical connections, water lines and for protecting the window and for moving the c-cell. Usually instrumentation readout is accomplished using a minicomputer based digital data acquisition system. The bank of regulated power supplies is equipped with remote controls, reversing switches and calibrated current shunts. Temperature measurements are made using the automatic optical pyrometer mounted on a positioning stand external to the vacuum system and viewing the sample through an optical window. The system is shown schematically in Figure 11. Linear thermal expansion measurements use twin telemicroscopes mounted on a second stand and viewing the sample through a second window.

Figure 12 shows a close-up view of the test enclosure. A sample (A) is mounted eccentrically (to reduce back-scattered radiation) between two copper electrodes (B, C). The lower is joined to a thermal expansion take-up assembly (D) and the upper fixed to a movable stainless steel plate (E). The samples can extend through each electrode.

The plates supporting the electrodes are clamped to hollow stainless-steel water-cooled columns (F) with ceramic rings as electrical insulation. Regulated DC power flows to the electrodes through water-cooled copper lines. Water lines to the electrodes are of copper and those to the stainless-steel columns are of stainless steel. All copper parts, except the movable parts of the lower electrode, are nickel plated to facilitate cleaning and attainment of good vacuum. Also, all bolts are slotted.

The test cell is enclosed by a water-cooled steel bell jar (G) (raised in Fig. (12) 18 inches in diameter and 36 inches high and fitted with two 13 x 2 inch vertical windows. It is operated by a hoist, and, in the closed position, rests on an O-ring seal fitted to the universal feed-through collar (H). A shutter (I) protects the window when not in use for optical pyrometer measurements.

All control mechanisms, leads, and water feeds operate through seals in the collar (H). The unit of Figure 12 is mounted on an automatic high-vacuum facility, capable of maintaining the bell-jar enclosure in the mid-10^-7 torr range.

The governing equation for Joulean heated long thin rods in vacuum subjected to radiation loss from the surface is

where P is the circumference, simga is the Stefan-Boltzmann constant, T_0 is the temperature of the vacuum enclosure, d is the density, Z is the length coordinate (using polar coordinates), t is the time, T is the surface temperature at position Z, and the symbols for the thermophysical properties are lambda for thermal conductivity, rho for electrical resistivity epsilon for total hemispherical emissivity, u for Thomson coefficient, and C_p for specific heat at constant pressure. Measured values for lambda, rho, epsilon , u, and C_p are based on Eq. (4). Linear thermal expansion (delta L/L_0) is measured by observing the displacement of fiducial marks on a uniformly heated portion of the sample. From these various results, the coefficient of linear expansion (alpha_exp), enthalpy (delta H), thermal diffusivity (alpha), and Wieddemann-Franz-Lorenz ratio (L_0) are derived. Normal spectral emissivity (epsilon) as a function of wavelength can be measured using the high temperature emissometer described in a separate section.

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