Thermal conductivity measurement apparatus and method

Abstract

A thermal conductivity measuring apparatus comprises a heat source member 3 and a heat sink member 5 between which members a sample 4 is clamped. The heat source member is connected to a clamping forc

Description

The thermal conductivity measurements are divided into two main classes: (i) steady-state and (ii) transient methods. Steady-state approaches - and corresponding conventional measurement systems - are considered to be more accurate. The method relies on measuring the heat flow through the sample sandwiched between heating and cooling members with known and constant temperatures. The thermal conductivity coefficient is then estimated according to Fourier's law as k = where Q is the thermal energy transferred through the sample having the thickness h and cross-section area A during time t at temperature difference AT. There are essential drawbacks of the steady-state thermal conductivity measurements: the long time (tens of minutes) needed for the heat flux stabilization, and very strict requirements for the sample geometry: extremely plain and extremely parallel to each other ‘sandwiched surfaces’ as well as a requirement for large samples. The mentioned geometry requirements are caused by two factors. First, the contacts between the sample and the apparatus's cooling and heating sides have a comparatively high thermal resistivity. Therefore, to present reliable measurements one needs to increase the sample thickness (the distance between its hot and cold sides) in such a way that the thermal resistivity of the sample is significantly higher than that of the contacts. The recommended thickness should be several centimetres even for materials with moderate thermal conductivity of 10 100 W / m K. The sample faces should be extremely flat as the imperfections would cause significant errors. Second, to provide a reliable measurement of the heat flux, it is often necessary that both the heating and cooling side holding rods were at least by an order of magnitude longer than the sample thickness. The mentioned elements should be arranged as a well-aligned column to ensure non-biased heat flow through the heating and cooling rods and the sample. The latter requires both sample sides' parallelism and the column (and sample) diameter to be comparable to the column length. Therefore, eventually, the sample required for steady-state thermal conductivity measurements is a disc with a diameter of more than 5 cm and thickness of more than 1 cm. The sample faces should be extremely flat and parallel. Such a sample is challenging and expensive to make, especially when hard materials - metals or ceramics - are to be measured. The mentioned long time required for the heat flux stabilization further contributes to the measurement expenses. Background Patent Documents Patent Document 1: CN101303320A(B), 12.11.2008 Patent Document 2: CN100516852C (A), 22.07.2009 Patent Document 3: KR102257190B1 (A), 01.06.2021 Patent Document 4: JP5379760B2 (A), 25.12.2013 Patent Document 5: US10775329B2 (Al), 15.09.2020 Patent Document 6: US10768127B2 (Al), 08.09.2020 Summary of the Invention Problem to be Solved The purpose of the present invention is to provide an apparatus capable of accurate and fast measurement of the thermal conductivity of a small sample with possible face non-parallelism of up to 10 degrees. The Invention Description To achieve the objections, the Invention presents the thermal conductivity measuring apparatus comprising (see Fig. 1): a clamp consisting of a heat source member and a heat sink member; a heat source member (3) clamping the measured sample (4) and joint to the upper support of the force application system (7) with either a gimbal lock or a universal joint positioning the heat source member aligning with the sample face; a heat sink member (5) as a single part with a thermostat (6) clamping the sample; a force application system (7) comprising the upper support, lower support, pressing agent, and guides mounted to the main support (9) via linear bearings and clamping the sample; a heater (1); a heat transfer rod (2) contacting a distal face of the heat source member (3) and transferring heat from the heater; a thermally conductive material for contacting surfaces compliance and low thermal contact resistance; at least two temperature sensors (8) attached to (i) the heat source member (3) in contact with the sample face and (ii) the heat sink member (5) in contact with the sample face and measuring the corresponding sample face temperatures; at least one temperature sensor (8) attached to the thermostat; at least one temperature sensor (8) measuring the thermostat ambient to take into account the thermostat heat loss. Measurement principles and the apparatus embodiment Before the measurement starts, the sample 4 contact faces should be covered with a thermally conductive material and clamped between the heat source member 3 and the heat sink member 4 (see Fig. 1). A gimbal lock of the heat source member 3 allows its self-adjustment in line with the sample surface adapting the heat flow assembly (heat source member - sample - heat sink member) to a possible non-parallelism of the sample contact faces (See Fig. 2). The clamping force should be applied to the assembly with the force application system (7). Then, the heat transfer rod 2 should be brought into contact with the distal face of the heat source member 3. The heat transfer rod transfers thermal energy from the heater 1 to the member 3 which serves as a heat source for the sample. The member 5 connected to a thermostat (6) is a heat sink collecting the thermal energy from the sample. The temperatures of the members 3 and 5, the thermostat 6, and the ambient are measured by the temperature sensors (8) e.g. thermocouples. After the heat transfer rod contacts the heat source member the member temperature starts rising. It results in two opposite processes. First, the sample's temperature gradient across the sample starts increasing together with the heat flux through it. Second, the temperature difference between the heat transfer rod and the heat source member decreases alongside the heat transfer power. Eventually, the heat flux from the heat source member through the sample to the heat sink member equals the heat flux from the heating rod, and the sample temperature gradient stabilizes (See Fig. 3). The thermodynamic process of the heat through the sample flow can be then described as quasistatic with the constant temperature gradient (see Fig. 4) but slowly raising sample temperature. The thermostat heating during the process provides information about the quantity of heat Q transferred via the sample, while the temperature difference between the heat source and the heat sink members AT together with the sample heights h allows calculating the temperature gradient. This provides all the information required for the thermal conductivity coefficient calculations: tAAT where t is the heat transfer time and A is the sample cross-section area. Effects of the Invention The use of the self-adaptable joint (e.g., universal joint or gimbal lock) for the heat source member allows the use of the sample with non-parallel faces up to 10 degrees. The stability of the central measurement assemblage of the heat source member - the sample - the heat sink member no longer depends on the sample diameter allowing the latter to be decreased up to 10 mm and even smaller. The use of the thermal conductive material on the sample interfaces decreases the interface thermal resistance significantly and allows the sample thickness to be as small as 3 5 mm; it also tolerates some deviations of the sample face flatness. The use of the described thermal conductivity measurement approach allows for a decrease in the measurement time to below 10 minutes. This time is smaller than the duration of the conventional steady-state experiment by almost 10 times. Description of Drawings Fig. 1. Configuration diagram of the invented thermal conductivity measurement apparatus: 1 - the heater 2 - the heat transfer rod 3 - the heat source member 4 - the sample 5 - the heat sink member 6 - the thermostat 7 - the force application system 8 - the temperature sensors 9 - the apparatus frame Fig. 2. The central part of the invented apparatus embodiment showing the gimbal lock connecting the force application system with the heat source member, the sample clamped between the heat source and heat sink members, the thermocouples attached to the heat source and heat sink members, and the thermostat. Fig. 3. The time dependence of the temperature dependence AT across the sample. Fig. 4. The time dependence of the quantity of heat Q transferred via the sample.

Claims

02 10 241. The thermal conductivity measuring apparatus which implements a quasistatic thermal conductivity measurement method by utilizing constant heat flow provided by a heat source assembly through a sample of interest into a heat accumulator.The apparatus comprises:a heat assembly containing a heater, heat transfer road and a heat source member in a sample clamp;a heater which heats a heat transfer road;a heat transfer rod that moves inside the heater and contacts the heat source member in the heat clamp by touching the latter at the beginning of the measurement;a sample clamp consisting of a heat source member and a heat sink member;a heat source member connected to the force application system by a gimbal lock that aligns the heat source member face with a sample face;a user sample that is exchangeable by the apparatus user;a force application system containing a force application agent and the clamping force measuring sensor;a force application system where the clamping force can be varied in line with measurement requirements;the heat sink member being a part of a heat accumulator with a known thermal capacity; a clamping force application system rigidly connected to the heat accumulator;a material with high surface conformity and heat conductivity applied to the sample clamp faces.at least two temperature sensors in contact with the clamp faces;at least one temperature sensor attached to the heat accumulator;at least one temperature sensor measuring the heat accumulator ambient temperature.

2. The thermal conductivity measuring apparatus according to claim 1 with data acquisition, visualisation and data processing systems.

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