Thermal measuring and testing – Determination of inherent thermal property – Thermal conductivity
Reexamination Certificate
2001-06-08
2002-12-24
Fulton, Christopher W. (Department: 2859)
Thermal measuring and testing
Determination of inherent thermal property
Thermal conductivity
C374S010000, C374S033000
Reexamination Certificate
active
06497509
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of using Differential Scanning Calorimetry (“DSC”) to measure the thermal conductivity of materials.
BACKGROUND OF THE INVENTION
Thermal conductivity characterizes the ability of a material to conduct heat. Traditional methods for measuring the thermal conductivity of materials comprise imposing a temperature gradient upon a material of known geometry, and measuring the heat flow through the material. The heat flow is measured by, for example, measuring the temperature drop across a sheet of material having a known thermal conductivity. Traditional methods of measuring thermal conductivity are limited in that they assume that thermal contact of a sample is highly reproducible and determinable from a calibration measurement. The present invention overcomes this limitation by using a simultaneous thermal contact measurement. No method has been heretofore proposed to determine the actual thermal contact and the thermal conductivity simultaneously from a single measurement of a heat capacity spectrum.
Thermal conductivity data are in great demand by industry, for use in polymer injection molding, in encapsulation of electronic devices and, in general, in modeling of different processes. Nowadays commercial techniques often measure thermal diffusivity or effusivity and calculate thermal conductivity using heat capacity values, measured separately.
DSC is a commercially available and widely used technique to measure heat capacity of samples of milligram size in a wide temperature range. Therefore it would be opportune to add to DSC instruments a feature to measure thermal conductivity of typical DSC samples.
Prior art of interest includes P. G. Knibbe, J. Phys. E: Sci. Instrum., vol. 20, pp. 1205-1211 (1987) which describes a “hot wire” technique for measuring the thermal conductivity of a material. This technique uses a temperature-sensitive resistor wire embedded in a sample of the material. The resistor wire serves the dual function of supplying heat to the specimen, and measuring the temperature change at the wire. This rate of change is related to the thermal conductivity of the sample of the material.
D. G. Cahill and R. O. Pohl, Phys. Rev. B. vol. 35, p. 4067 (1987), and D. G. Cahill, Rev. Sci. Instrum. vol. 61(2), pp. 802-808 (1990), describe a “3&ohgr;” technique for measuring thermal conductivity. This technique uses a temperature sensitive resistive metal film evaporated as a narrow line onto the surface of the sample to simultaneously heat the sample and detect the flow of heat away from the metal line. A current at angular frequency &ohgr; heats the metal line at a frequency of 2 &ohgr;. Because the resistance of a metal increases with increasing temperature, and this temperature is modulated by the sample thermal conductivity, this produces a small oscillation in the resistance of the metal line, resulting in a voltage across the resistor at a frequency of 3 &ohgr;. The thermal conductivity of the sample is then calculated from the amplitude of the 3&ohgr; voltage oscillations.
J. H. Flynn and D. M. Levin, Thermochimica Acta, vol. 126, pp. 93-100 (1988), describes a thermal conductivity measurement method, suitable for measuring the thermal conductivity of sheet materials, based upon first-order transitions in a sensor material. A film of the sensor material is placed on a surface of the sheet material. The thermal conductivity measurement is made at the temperature at which the sensor material undergoes a first order transition. For example, if indium is used as the sensor material, the measurement is made at the melting point of indium, i.e., at the temperature at which indium undergoes a first order transition. The flow of heat into the sensor material must match the transition enthalpy. The thermal conductivity of the sheet material is obtained by comparing the data obtained with only the sensor material in the heater of a differential scanning calorimeter, to the data obtained with the sensor material on top of the sheet material in the differential scanning calorimeter.
T. Hashimoto, Y. Matsui, A. Hagiwara and A. Miyamoto; Termochimica Acta vol. 163, pp. 317-324 (1990), describes a method to obtain thermal diffusivity by an AC calorimetric method. The AC current is passed through the heater; the periodical heat flow generates and diffuses to the rear surface of the sample. The variation of the temperature at the rear surface was detected. A sputtered gold layer on both surfaces was used as the heater and the sensor of temperature variation. The thermal diffusivity of four polymers were measured over the temperature range 20-200° C.
S. M. Marcus and R. L. Blaine, Thermochimica Acta, vol. 243, pp. 231-239 (1994) (herein incorporated by reference), describes a thermal conductivity measurement method where thermal conductivity is measured without modification of the commercially available DSC cell. A calculation of thermal conductivity is determined from a ratio of apparent and true heat capacities measured from a thick (about 3 mm) and a thin (about 0.5 mm) sample, respectively, in temperature-modulated mode. An additional calibration step takes heat losses through the purge gas surrounding into account. This method is based on the assumption that the face of the specimen at the heat source follows the applied temperature modulation, which means no thermal resistance between the sample and the furnace.
S. L. Simon and G. B. McKenna, J. Reinforced Plastics Composites, vol. 18, pp. 559-571 (1999) (herein incorporated by reference), describes two problems in the aforesaid method of Marcus and Blaine. First, the equation relating the apparent heat capacity to the thermal conductivity is limited in range due to an approximation made in their derivation. Second, a thermal resistance between the sample and the furnace can have significant effect on the measured apparent heat capacity. This reference also teaches that when calculating a value for thermal conductivity it is necessary to know the heat transfer coefficient, for without it, an accurate value cannot be obtained.
S. L. Simon and G. B. McKenna, J. Reinforced Plastics Composites, vol. 18, pp. 559-571 (1999), as well as U.S. Pat. No. 5,335,993 to Marcus et al. (herein incorporated by reference), additionally describe a method of determining thermal conductivity by obtaining the value from a single run at several frequencies, i.e. it is not necessary to measure two samples. In U.S. Pat. No. 5,335,993 the method described therein has insufficient sensitivity, in part; because it measures the conductivity of massive bodies brought into thermal contact with thin film wafers and does not fully eliminate interface thermal resistance. The basic problem with this patent as well as S. L. Simon and G. B. McKenna, J. Reinforced Plastics Composites, vol. 18, pp. 559-571 (1999), is that the actual thermal contact between sample and oven is not considered in the calculation of thermal conductivity. Theses references teach that thermal contact is considered to be highly reproducible and determined from the calibration measurement.
U.S. Pat. No. 5,244,775 to Reading et al. and U.S. Pat. No. 5,439,291 to Reading utilize frequency measurement resulting from the application of heat to determine phase transition of materials.
U.S. Pat. No. 5,439,291 to Reading (herein incorporated by reference) describes a technique for determining physical properties of a sample using thermal modulation techniques. Two identical samples are used, with one experiencing a linear temperature ramp and the other experiencing the same ramp with a temperature oscillation imposed. A chopped light source can be used to provide the energy necessary for the temperature oscillation. Thermocouples attached to each sample measure the temperature of each sample. Light is used as a radiation source to heat the temperature-modulated sample.
Of interest is Pat. No. 5,688,049 to Gorvorkov, which teaches a device and method for measuring the thermal conductivity of a thin film by determining the change in
Merzliakov Mikhail
Schick Christoph
De Jesús Lydia M.
Fulton Christopher W.
PerkinElmer Instruments LLC
St.Onge Steward Johnston & Reens LLC
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