GB1593425A - Thermal conductivity of materials - Google Patents

Description

(54) THERMAL CONDUCTIVITY OF MATERIALS (71) We, FOSECO INTERNATION AL LIMITED, a British company, of 285 Long Acre, Nechells, Birmingham B7 5JR do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to the determination of thermal conductivity of insulating materials. Apparatus for this purpose is known but to make measurements with these designs of apparatus is usually a prolonged and expensive job to be done by a skilled technician. It is an object of this invention to provide apparatus for the purpose specified which can be used with improved speed and with improved accuracy i.e. with high precision.

According to the invention apparatus for the determination of the thermal conductivity of a material, comprises a pair of spaced apart metal plates which between them define a space to receive a specimen of the material the thermal conductivity of which is to be measured, a pair of cushions of compressible material, preferably a refractory fibrous composition within the space and each in contact with one respective plate, a heat flow sensor in contact with one of the plates, and means for measuring the temperature on each side of the specimen when received in the space, and means for raising the temperature of one of the plates with respect to the other.

The apparatus is based on the design described by Dickson, 1963 J. Physics & BR< Scientific Instruments, 6, p 1074-76 but includes the additional presence of the cushions of compressible material separating the thermocouples from the hot plate. It has been shown by evaluations that by the presence of the cushions improved accuracy of results can be obtained. These evaluations have been done at temperatures of up to 800"C and in respect of materials having thermal conductivities in the range of 0.030.6 Wm-lK-'.

This range covers materials such as insulation materials, fireclay and silica brick refractories and the like. The measurements were made rapidly (of the order of 90 minutes to 120 minutes for each particular temperature setting).

The cushions of compressible material are preferably made from a material having a thermal conductivity of a similar order of magnitude (e.g. t 50it) to that of materials whose thermal conductivity is to be measured using the apparatus; When used the refractory fibrous composition is preferably made of aluminosilicate fibre but other materials may be used, examples being calcium silicate, asbestos, alumina, silica, zirconia or carbon fibres. The fibres may be bonded for example using an inorganic binding agent such as colloidal silica sol, colloidal alumina sol, sodium silicate, potassium silicate, ethyl silicate or a metallic phosphate or borate.

The heat flow sensor which quantitatively senses the heat flow through the specimen is preferably a differential thermocouple mounted on a very thin plastics film and is preferably of the type supplied by Keithley Instruments Ltd. (see also Hager, 1965 Review of Scientific Instruments 36 156470).

An embodiment of the invention is illustrated by the accompanying drawings in which: Figure 1 is a perspective view of one form of apparatus and Figure 2 is a sectional view showing a detail of the thermal conductivity measurement apparatus.

The apparatus of Figure 1 comprises a test assembly 1, connected to secondary apparatus comprising a digital voltmeter 2, mounted on a chart recorder 3 having a socket panel 4 for thermocouple plugs.

Associated with the recorder are a voltage regulator 5, an ammeter 6 and a panel light 7 and switch 8.

The test assembly shown in detail in Figure 2 comprises a lower highly conductive metal plate 20 (such as copper) and an upper metal plate 21 (such as nickel alloy).

The lower plate 20 is mounted on a housing 22, the upper surface of which supports an arm 23 on to which the upper plate 21 is pivotally mounted.

The housing 22 contains a cooling system generally indicated as 24 for the passage of cooling water to the lower plate 20. The walls of the housing are covered by an insulation 25.

The upper plate 21 contains an electrical heating element (not shown). The plate is mounted in a lid 26 to which the arm 23 is attached and also has an insulation 25.

A respective cushion of ceramic fibre 30 is mounted on each of the plates 20, 21. A miniature heat flow sensor 31 is mounted on the plate 2() below the fibre cushion 30. A pair of 0.5 mm thick mineral insulated thermocouples 32 are laid adjacent the fibre cushions 3() axially above the heat sensor 31.

In use, the lower cushion 30 is laid on the lower plate 2() and a specimen 33 the thermal conductivity of which is to be measured, is placed on top. The upper cushion 31 is then superimposed followed by the upper plate. Cooling water is passed through the system 24 and heat is applied to the upper plate 21. When the apparatus has reached thermal equilibrium (11/2 - 2 hrs) readings are taken and thereafter the thermal conductivity is determined according to Fourier's law. The presence of the cushions ensures good contact of the thermocouples with the adjacent surfaces, and minimises the effects of any irregularities in the surfaces of the specimen.

Thermal conductivity measurements at temperatures below the dew point may be affected as a result of the specimen picking up moisture from the atmosphere. Measurements may also be affected by moisture generated from the specimen by heat from the hot plate. To overcome these problems the test assembly may be enclosed by a hood and means be provided for permanent removal of moisture from within the hood, for example the air in the hood may be drawn through silica gel and recirculated. by means of a fan.

The degree of precision of the apparatus of the invention is high, reproducibility being of the order of + 1.5% compared with approximately + 4% for apparatus presently available.

WHAT WE CLAIM IS: 1. Apparatus for the determination of the thermal conductivity of a material, comprising a pair of spaced apart metal plates which between them define a space to receive a specimen of the material the thermal conductivity of which is to be measured, a pair of cushions of compressible material within the space and each in contact with one respective plate, a heat flow sensor in contact with one of the plates, and means for measuring the temperature on each side of the specimen when received in the space, and means for raising the temperature of one of the plates with respect to the other.

2. Apparatus according to claim 1 wherein the compressible material is a refractory fibrous composition.

3. Apparatus according to claim 2 wherein the refractory fibrous composition comprises aluminosilicate fibres, calcium silicate fibres, asbestos or alumina, silica, zirconia or carbon fibres.

4. Apparatus according to claim 3 wherein the fibres are bonded by means of an inorganic bonding agent.

5. Apparatus according to claim 4 wherein the bonding agent is colloidal silica sol, colloidal alumina sol, sodium silicate, potassium silicate, ethyl silicate, or a metallic phosphate or borate.

6. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. socket panel 4 for thermocouple plugs. Associated with the recorder are a voltage regulator 5, an ammeter 6 and a panel light 7 and switch 8. The test assembly shown in detail in Figure 2 comprises a lower highly conductive metal plate 20 (such as copper) and an upper metal plate 21 (such as nickel alloy). The lower plate 20 is mounted on a housing 22, the upper surface of which supports an arm 23 on to which the upper plate 21 is pivotally mounted. The housing 22 contains a cooling system generally indicated as 24 for the passage of cooling water to the lower plate 20. The walls of the housing are covered by an insulation 25. The upper plate 21 contains an electrical heating element (not shown). The plate is mounted in a lid 26 to which the arm 23 is attached and also has an insulation 25. A respective cushion of ceramic fibre 30 is mounted on each of the plates 20, 21. A miniature heat flow sensor 31 is mounted on the plate 2() below the fibre cushion 30. A pair of 0.5 mm thick mineral insulated thermocouples 32 are laid adjacent the fibre cushions 3() axially above the heat sensor 31. In use, the lower cushion 30 is laid on the lower plate 2() and a specimen 33 the thermal conductivity of which is to be measured, is placed on top. The upper cushion 31 is then superimposed followed by the upper plate. Cooling water is passed through the system 24 and heat is applied to the upper plate 21. When the apparatus has reached thermal equilibrium (11/2 - 2 hrs) readings are taken and thereafter the thermal conductivity is determined according to Fourier's law. The presence of the cushions ensures good contact of the thermocouples with the adjacent surfaces, and minimises the effects of any irregularities in the surfaces of the specimen. Thermal conductivity measurements at temperatures below the dew point may be affected as a result of the specimen picking up moisture from the atmosphere. Measurements may also be affected by moisture generated from the specimen by heat from the hot plate. To overcome these problems the test assembly may be enclosed by a hood and means be provided for permanent removal of moisture from within the hood, for example the air in the hood may be drawn through silica gel and recirculated. by means of a fan. The degree of precision of the apparatus of the invention is high, reproducibility being of the order of + 1.5% compared with approximately + 4% for apparatus presently available. WHAT WE CLAIM IS:

1. Apparatus for the determination of the thermal conductivity of a material, comprising a pair of spaced apart metal plates which between them define a space to receive a specimen of the material the thermal conductivity of which is to be measured, a pair of cushions of compressible material within the space and each in contact with one respective plate, a heat flow sensor in contact with one of the plates, and means for measuring the temperature on each side of the specimen when received in the space, and means for raising the temperature of one of the plates with respect to the other.

2. Apparatus according to claim 1 wherein the compressible material is a refractory fibrous composition.

3. Apparatus according to claim 2 wherein the refractory fibrous composition comprises aluminosilicate fibres, calcium silicate fibres, asbestos or alumina, silica, zirconia or carbon fibres.

4. Apparatus according to claim 3 wherein the fibres are bonded by means of an inorganic bonding agent.

5. Apparatus according to claim 4 wherein the bonding agent is colloidal silica sol, colloidal alumina sol, sodium silicate, potassium silicate, ethyl silicate, or a metallic phosphate or borate.

6. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.