Patent Description:
Thermal diffusivity is an intrinsic property of all materials. It describes the rate with which heat spreads through a material, from a hotter to a colder region. Thermal diffusivity plays an important role in thermal design and modeling.

An instrument for determining thermal diffusivity of disc shaped opaque solid or quasi solid materials using a high intensity short duration flash of light from a single LED, a planar LED array, or laser diode source is disclosed. This instrument comprises an axially and radially indexed cylindrical sample holder able to accommodate a plurality of test samples and sequentially bring them into a designated testing position to expose one face of each sample to the flash of light while the obverse face of the disc is monitored by a temperature measuring device, for the purpose of recording the attendant thermal excursion. An improved calculating method, based on empirical data observed during each test, is used for calculating thermal diffusivity.

This disclosure relates to the field of thermal diffusivity and more specifically an instrument and method for measuring thermal diffusivity of materials. The disclosure includes, in a first example, an instrument for measuring thermal diffusivity of materials, the instrument consisting of a system for measuring thermal diffusivity comprising a light emitting diode (LED) light source, wherein at least one light pulse from the LED light source impinges on one face of an opaque solid material sample of uniform thickness, L, producing a time dependent temperature evolution, or thermogram, on the opposite side of the sample, and wherein a means to calculate included in the system analyzes the thermogram and calculates the time t<NUM>/<NUM> it takes for the temperature rise to reach <NUM>/<NUM> of its maximum value and calculates the thermal diffusivity α of the sample, according to the formula: α = c · L<NUM>/t<NUM>/<NUM>, where c = <NUM> for the t<NUM>/<NUM> point on said temperature rise. This instrument may comprise at least one LED, a light concentrator, a light columnator, and a light transmission device. The LED light source or at least one laser diode is controlled by a controller with a program which commands production at least one light pulse. When at least two light pulses of predetermined, and different duration and intensity is produced, and wherein the program calculates thermal diffusivity using the at least two light pulses, denoted P<NUM>, P<NUM>, each having a t<NUM>/<NUM> associated with the at least two light pulses, denoted (t<NUM>/<NUM>)<NUM>, (t<NUM>/<NUM>)<NUM>, and having the at least two said t<NUM>/<NUM> values subject to a regression analysis versus their corresponding pulse durations, the resultant function then extrapolated to a pulse duration that is shorter than <NUM> (milliseconds), to obtain the corresponding t<NUM>/<NUM>, which is then used in the formula: α = c · L<NUM>/t<NUM>/<NUM>, where c = <NUM>. In another instance, the LED light source or laser diode may be commanded to provide a plurality of pulses according to proportional-integral-derivative (PID) control principles to heat said sample to a desired temperature prior to the above-mentioned time pulses for the thermal diffusivity measurement.

Before explaining the disclosed embodiments of the present disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of the particular arrangement shown, since the disclosure is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.

One of the most elegant and frequently used method of evaluating thermal diffusion was first developed by <NPL>). This method is illustrated schematically in <FIG>. The method uses a high intensity light pulse <NUM> from a Xenon light source or pulse laser <NUM>, which falls on one face (face A) of a small disk-shaped sample <NUM> and measures the temperature evolution <NUM> of the opposite face (face B) which is temperature rise as a function of time. This temperature evolution data is used to create a thermogram T, illustrated in <FIG>.

Various points obtained from this characteristic thermogram can be used to derive thermal diffusivity. For example, once the maximum temperature excursion point (P) is determined in terms of (△T), it is projected <NUM> to the △T axis, intercepts it where marked MAX, then <NUM>% of this △T value, marked MAX/<NUM> is calculated and shown as represented by point (R), then it is projected back onto the thermogram <NUM>, intercepting it at point (S). The time at which point (S) occurs is called "half-max time" as projected <NUM> to the TIME axis to point (W), yielding t<NUM>/<NUM>. L is the thickness of the sample. This characteristic time was shown to relate to thermal diffusivity (α), as <MAT> where c = <NUM> for t<NUM>/<NUM>.

Other points on the thermogram have analogous relationships, with correspondingly changing values for constants c. Equation <NUM> is based on a theoretical model assuming an infinitesimally short pulse and no thermal losses from the sample.

This method is commonly used and referred to as the "flash method" for determining thermal diffusivity. It is applicable for opaque, homogeneous solid, or quasi-solid samples of uniform thickness, where quasi-solid is meant to refer to materials that show no dimensional deformation, as long as mechanical forces acting upon it remain constant, or encapsulated non-solids and transparent materials, made to mimic solid samples. It is a highly favored for being fast, accurate, and most importantly, not needing the knowledge of the quantity of heat conveyed by the pulse <NUM>. Since thermal diffusivity (α) is related to thermal conductivity (k), specific heat capacity (Cp) and density (ρ), as shown in Equation (<NUM>), <MAT> the flash method fast became the method of choice for derivative measurement of thermal conductivity.

Typically, for testing at temperatures other than ambient, the specimen is enclosed in a thermally controlled environment, such as furnace or cryostat <NUM>, <FIG>. With elevated temperatures, heat losses from the sample promote non uni-axial heat flow after the pulse energy is adsorbed such as radiative losses from the surface of face A, <FIG>, the surface of face B, and edge losses C, each causing deviations from the basic relationship in Equation <NUM>.

Thermally Controlled Environments are used to transfer heat into the sample, and often to the sample holder via radiation, conduction, and convection. In one instance for testing molten droplets of metals high frequency induction heating was used to elevate the temperature of the levitated sample from within.

Between <NUM> to <NUM> Joule pulse energy produces an analyzable thermogram which is a depiction of temperature rise as a function of time for face B of the sample <NUM> opposite of the face A, upon which the energy of the light pulse is deposited. This temperature evolution data is referred to as a thermogram, on a sample having <NUM> in diameter and <NUM> in thickness. The presently disclosed embodiment, using an at least <NUM>,<NUM> lumen LED source, yields sufficient energy with a <NUM> duration pulse.

Common practice with light sources showed that pulse widths extending into hundreds, or thousands of milliseconds may distort the thermogram obtained from face <FIG>, B, and adversely affect the resulting data. For this reason, a number of analytical corrections appeared in the literature, generally referred to as pulse width corrections. These were all based on analyzing and manipulating the test result obtained from the thermogram to closely approximate a theoretical thermogram, which assumes an infinitesimally short pulse and no heat losses. Severe losses from faces A, B and surface C may also lead to further data degeneration. Analytical corrections have also been published to correct this distortion as well. Further complicating matters is the fact that every material and the surface preparations of each sample greatly influence its emissivity, and consequentially radiated heat losses from it. As a result, a correction obtained for one sample may differ from the correction to be applied for another sample, even if the material is the same and certainly even more at different temperatures. Over the decades, these corrections were based on theoretical predictions and were applied to the data derived from thermograms at the end of the test. They provided no way to vary test parameters in real time, only manipulate obtained data after testing. It is a known fact that the thermal diffusivity of all materials changes with temperature, therefore there is the need to test them at temperatures other than ambient and over a range of temperatures. In any practical device for testing at other than ambient, first the sample must be enclosed in a precisely controlled thermal volume, such as what is within the uniform portion of the hot zone of a furnace and provide a way to firmly support the sample.

In prior art, the xenon light source <NUM>, <FIG>, directly shined upon the sample seated in a holder. This proved to be very limited in operation, as such a source has nearly hemispherical light emission, and only a very small portion of its energy was reaching the sample. This was remedied with the use of properly positioned elliptical or parabolic concentrating mirrors, coupled with other collimating components. Also, pulse lasers, ruby, YAG, and Nd-glass, became the choice of light source, due to their inherently collimated beam, extremely high-power density and comparatively short pulse. Collimation without large divergence is a practical necessity to reach into the furnace or cryostat. Both types of sources brought with them very high electrical noise, radiated and conducted through wiring, emanating from the high voltage (> <NUM> V) discharge of very large capacitors and a very high voltage (><NUM>,<NUM> V DC) trigger pulse through the xenon flash lamp, for both the direct xenon flash or the xenon pumped pulse laser. Flash diffusivity instruments based on the use of either type pulse source have been available for a while, all of them suffering from the extensive electrical noise emission, requiring complex mitigative hardware and software measures, at great cost.

<CIT> discloses such an instrument for measuring the thermal diffusivity of materials.

In a practical device, one of the fundamental requirements for any sample holder is to physically support the solid sample while exposing one face A, <FIG> of the sample to the pulse from the pulse source <NUM> and presenting the other face B to the temperature rise detector <NUM>. Initially measuring instruments were equipped with a holder for a single sample. For purposes of the disclosure herein a solid may be a solid or quasi solid. It takes only a few seconds to obtain data to develop the thermogram produced by a single pulse, but it takes several magnitudes longer to reach equilibrium temperature for the sample before the pulse is triggered. For example, a test cycle to reach a maximum temperature of <NUM>ºC with <NUM> to <NUM> intermediate temperatures, can easily take <NUM>-<NUM> hours. To make testing more time efficient, equipment with multi-specimen sample holders were devised. As shown in <FIG>, the first version used a circular platform <NUM> with as many as six radially spaced samples <NUM> in the plane of the platform, with the rotational <NUM> indexing around axis Z, which is perpendicular to the plane of the samples <NUM> and with the plane of the sample holder <NUM>. These samples were then sequentially pulsed <NUM> and the rear temperature evolution <NUM> recorded. In another prior art version shown in <FIG>, a single or multiple linear sequence of samples <NUM> in the plane of a platform <NUM> was devised.

In all cases, the planes of all sample faces (A) are in a plane parallel with the plane of the holder and that of the indexing <NUM>, lateral displacement <NUM> and longitudinal displacement <NUM>. In contrast, the present disclosure introduced a rotational indexer.

The flash diffusivity apparatus of the present disclosure comprises a thermally controlled environment, a multi-sample holder moved via a concurrent radial and axial indexer, a temperature rise detector, a LED light pulse source to raise the temperature of a sample by the light pulse emitted from the LED, electronic circuitry connected to a computer with operating software to control the device, and a new analysis algorithm, based on an empirical model evaluation in real time, after pulse enabling the ability to vary test parameters during a test, rather than hypothetical analysis and corrections applied post test data, as in prior art.

The schematic representation in <FIG> shows for illustrative purposes, and to place the elements of the disclosure in context, means to produce a thermally controlled environment <NUM> having a highly uniform (at least +/- <NUM>ºC/in) and stable (at least +/- <NUM>ºC/min) thermal volume <NUM> within its hot zone <NUM> at any set temperature, after sufficient equilibration time. The thermally controlled environment comprises a temperature sensor, heater, and controller. It is also possible to heat the sample directly by prolonged light pulses applied directly to the sample <NUM>, using a proportional integral derivative PID control algorithm for meeting a desired temperature without a separate heater <NUM>, and then to apply the light pulse for the purpose of the measurement. The thermally controlled environment <NUM>, which may also be termed a furnace or cryostat, comprises an outer shell <NUM> housing the insulation <NUM>, and a heater or heating element <NUM>. The thermally controlled environment depicted for the present disclosure is capped on one longitudinal end with a movable end plate <NUM> which also serves as base or the sample support structure, comprising a sample holder and sleeve, and indexer <NUM>. The thermally controlled environment including the end plate/base <NUM> is enclosed by the outer shell <NUM> and insulation <NUM> to maintain internal temperature although the enclosure wall opposite of the end plate <NUM> is not shown in the diagram for purposes of explanation.

The thermally controlled environment <NUM> is equipped with two sets of passages that allow light to pass therethrough, <NUM> and <NUM>, each set cutting through shell <NUM>, insulation <NUM> and heating element <NUM>. The two sets are coaxial with each other, as well as the optical axis Y of the light pulse source <NUM>, and the temperature rise sensor <NUM>.

When the longitudinal axis of disk-shaped sample <NUM> is brought into coincidence with axis Y, the light pulse <NUM> can impinge on the face A, the illuminated face, of said sample. This position will be heretofore referred to as test position <NUM>.

In the present disclosure, multiple light pulses of varying intensity in close sequence, and distributed according to Proportional-Integral-Derivative (PID) control principles, may be used to heat the sample to a desired temperature before the test pulse is issued. This direct heating involves no heat transfer from an external heater, therefore it is a means for generating a thermally controlled environment.

A primary feature of the disclosure is a compound of axial translation and rotary indexed sample holder structure <FIG>, <NUM>. As shown in <FIG>, the sample holder structure <NUM> is comprised of a multi-sample holder <NUM> and a sleeve <NUM> surrounding it. The sample holder <NUM> may comprise a cylindrical body as shown, or have a polygonal cross section, the polygon having at least three sides, at least four sides, at least five sides, or at least six sides or more. The sleeve <NUM> serves to hold the samples in their places as also shown in <FIG>, except in the test position, as shown in <FIG>. The sleeve <NUM> is rigidly mounted to the movable end plate/sample holder base <NUM> which separates the interior environmental space of the thermally controlled environment <NUM> from the ambient space of the surroundings <NUM>. One may envision that the base <NUM> forms part of a wall of the thermally controlled environment <NUM>. The sample holder <NUM> and its function is further described in <FIG>. It comprises at least one sample placement cavity <NUM>, on the cylindrical surface of the sample holder, each continuing with a coaxial light passage <NUM> having an opening on one side of the cylinder (first face) that is of smaller diameter than the opening to the opposite surface, the second face, of the cylinder. The larger cavity opening is of slightly larger size yet same cross-sectional shape as the sample(s) <NUM>, allowing placement and removal without interference. The depth of each indentation is sufficient to accept said sample(s) <NUM> with none of the sample protruding and interfering with the sleeve <NUM> or impeding rotation of the sample holder <NUM> inside its cylindrical bore. and the openings are concentric with each other, and are along an axis with this axis being perpendicular to the longitudinal axis of the sample holder. These interconnected and concentric cavities having one opening larger than the other, placed <NUM>°from each other.

This disclosure shows cylindrical disk-shaped samples throughout for illustrative purposes, however, one may envision samples of other shapes, such as rectangular slabs, be employed in an embodiment, without impact on the otherwise disclosed details.

<FIG> presents an example of a configuration for a sample holder <NUM> for six solid samples <NUM>. The first row of samples <NUM> are in axial positions k, I and m, where the plane defined by k, I and m, respectively, and also includes the longitudinal axis of the cylindrical sample. While the preferred configuration for a cavity <NUM> comprises of a tubular indentation, a polygonal shaped indentations is also possible for samples other than circular disks, most likely coupled with a similar shaped light passages or cavities. Normally, samples below <NUM> inches (in) or above <NUM> in diameter are not encouraged, for practical reasons. Typical thicknesses vary from <NUM> to <NUM> in, depending on the material. A typical sample <NUM> is <NUM> or <NUM> inches in diameter and <NUM> to <NUM> inches in thickness.

Rows of cavities <NUM> in the sample holder <NUM> are arranged radially displaced from each other (R<NUM>, R<NUM>), as shown on <FIG> thereby having at least two samples radially displaced from each other. Adjacent cavities may vary in size as well as shape, as desired. The number of cavities in a row or the number of rows is only limited by practical considerations and could be as few as <NUM> or <NUM>, <NUM> or <NUM>, or six or more. In all configurations, adjacent cavities may not encroach in the space of any adjacent cavity. Thus, the number of such radially indexed rows is only limited by the circumference of the holder. Said rows may have the placements radially aligned or staggered for optimum placements per unit length of holder. An example embodiment of axially aligned two rows of three placements each is presented throughout this disclosure, while <NUM> to <NUM> cavities in a row and <NUM> to <NUM> rows, is considered practical, with no limitation on the number of cavities whatsoever. The sleeve <NUM>, <FIG>, surrounding the holder <NUM> is equipped with two slots <NUM> and <NUM> opposite of each other. The length of each slot is such as to fully expose all cavities in a row when the vertical axis of the cavities is coincidental with the optical axis Y.

Loading samples <NUM>, <FIG> into the sample cavities <NUM> is accomplished by retracting the sample holder structure <NUM>, <FIG> by moving the movable base <NUM> to which the sample holder structure <NUM> is attached far enough out of the thermal environment <NUM>, so as to fully expose and access slots <NUM> and <NUM>. In this position, <FIG>, samples <NUM> are placed <NUM> into individual cavities <NUM> coinciding with positions k, l, m, in rotational index R<NUM> through slot <NUM>. After rotating <NUM> said holder <NUM> a sufficient amount angularly to reach R<NUM>, bringing the next row of placements into coincidence with slot <NUM>, multiple samples <NUM> are loaded into this row of cavities, similarly as was described above for R<NUM>. Meanwhile, samples <NUM> in radial position R<NUM> are held captive in their respective cavities <NUM> by the inner wall of the sleeve <NUM>, as shown in <FIG>. The process is repeated for additional radial sample locations <NUM>. For illustrative purposes only, <FIG> shows <NUM> radial positions, R<NUM> and R<NUM> and axial positions k, l, and m. For unloading, the sample holder <NUM> is rotated 180º for each placement row, <FIG>, from its respective loading position to an unloading position, and the samples <NUM> are allowed to fall out <NUM> through slot <NUM>. During a test sequence, loaded samples <NUM> are never brought into coincidence with the unloading radial position. It may be envisioned that the sample holder may retract far enough out of the base <NUM> to load the samples without moving the base <NUM> in an alternate design.

While the coincidence of the X axis to horizontal is not critical, preferred operation is obtained close to horizontal, with the loading slot <NUM> on holder <NUM> being on top.

Using the previously defined designations, the placements along a longitudinal axis X as k, l, m. z in <FIG> and along circumferential orientation R<NUM>, R<NUM>, R<NUM>. Rn, <FIG>, a test sequence could consist of laterally translating <NUM>, <FIG>, the multi-sample holder structure <NUM> to bring R<NUM> into test position, as shown in context in <FIG>, <NUM>, then R<NUM>, R<NUM> etc., then R<NUM>, R<NUM> and so on. The above sequence may be done accomplished by either sequential or concurrent indexing.

Another notable advantage of this axial / radial indexing configuration is that it requires a considerably smaller thermal volume within the uniform heat zone of the thermally controlled environment <NUM> that any of the previous multi-specimen holders (such as the circular platform <FIG> cited earlier), thus reducing the per-sample thermal volume requirement, <FIG>, <NUM>, which saves energy and thereby offers greener operations. While a six-sample disk holder <FIG> requires at a minimum <NUM> in diameter x <NUM> in uniform thermal volume in the hot zone of a <NUM>ºC thermally controlled environment <NUM> (equal to <NUM> in<NUM> / sample), the disclosure's indexed sample holder structure <NUM> only needs a space of <NUM> in diameter x <NUM> in long thermal volume for <NUM> samples, corresponding to <NUM> in<NUM> / sample, a nearly <NUM>% reduction in needed energy, and thermal pollution of the environment.

Referring to <FIG>, The multi-sample holder structure, as described above, supports one or several samples, and with controlled axial and radial motion of the sample holder <NUM>,in reference to the stationary sleeve <NUM>, provides means to bring a selected samples axis, which is perpendicular to its face A, coincident with the axis of the light beam <NUM> emanating from the light source, whereby it provides means to align the first side of the selected sample 1A, facing the pulse source. Concurrently the same means allows the second side of the above sample, <NUM> B to face a suitable temperature rise detector <NUM>, to sense the radiated signal <NUM>, upon impingement of the light pulse <NUM>. By equipping the sample holder <NUM>, with several radially displaced rows of cavities as sample placements <NUM>, it also serves as means for having one or more radially displaced samples.

An illustrative example of an axial/radial indexer <NUM>, shown in detail in <FIG>, is located outside of the thermally controlled environment <NUM> in the ambient space <NUM>. It comprises a rod or arm <NUM> connected to the sample holder <NUM>, and it passes through the movable base <NUM> through a rotating bushing <NUM>. This bushing <NUM> is equipped with a pin <NUM> that engages a longitudinal groove <NUM> parallel with axis X of <NUM> in said extension <NUM>. Since the pin <NUM> is firmly attached to the bushing <NUM>, a rotation of the bushing <NUM> will correspondingly rotate said extension. To axially displace the extension, a linear actuator <NUM> is used. The axial displacement is indicated by a linear position transducer <NUM> coupled <NUM> to linear actuator <NUM>. The end of the extension <NUM> is terminated in a powerful disk magnet <NUM>, which is in contact with a corresponding size steel disc <NUM>, or another oppositely polarized disk magnet connected rigidly to the moving end <NUM> of the linear actuator <NUM>. The magnet <NUM> and steel disk <NUM> may be reversed. Inasmuch as the magnet <NUM> is strongly attracting the steel in line with X, but very weakly in any other direction, rotational slippage between the surfaces of <NUM> and <NUM> is permitted, but with no gap developing between them. In this coupling, an accurate axial X displacement <NUM> is ensured concurrent with unhindered and accurate rotation <NUM> around axis X for radial indexing. Rotary actuation is obtained with an actuator <NUM> such as a gear motor or a stepper motor, coupled to an internal or external rotary transducer <NUM>, such as a potentiometer or encoder, thereby also coupled to bushing <NUM> by connecting means <NUM> such as a timing belt, a gear, or an o-ring belt. Various parts may supplement the actuator, transducer, magnet functions, and their interaction, as are knowns in the art to accomplish a concurrent rotational and lateral actuation.

Thus, means for holding at least two samples comprises the sample holder, <FIG>, <NUM>, equipped at least two sample placement cavities, <FIG>, <NUM>, on its cylindrical surface radially displaced from each other, inside sleeve <NUM>, and connected to the axial/radial indexer <FIG>, <NUM>.

Still referring to <FIG>, the pulse source <NUM> may be located below the thermally controlled environment <NUM> shell <NUM> and temperature rise detector <NUM> on the top, or in reverse. Sideway orientation is possible but is generally more difficult to achieve. <FIG> shows <NUM>, the temperature rise detector, which may be an intrinsic or beaded thermocouple or an optical radiation detector (not shown), used for sensing the temperature rise of the face B, which is opposite the illuminated face, A, of a sample <NUM> in the test position, after receiving an energy pulse.

In the present disclosure, as illustrated in <FIG>, the light source is at least one Light Emitting Diode (LED) <NUM>, singularly or in an array. This LED light source, made up of one or several LEDs is the means providing a pulse source produces minimal electrical noise by only switching low voltage electric current (typically < <NUM> V DC) into a quasi-resistive load. Prior art devices which utilize lasers or Xenon flash tubes in the course of generating a short pulse of light by discharging large high voltage capacitor and very high voltage (><NUM>,000V), trigger pulses to initiate the discharge both of which generates a tremendous amount of electrical noise. In addition, high voltage lasers are hazardous to use. For lasers or Xenon flash tubes it is difficult to control the pulse width of the light pulse. In contrast, controlling the pulse width of an LED it is simple, precisely controllable as it turns on and off upon command with extremely sharp rise and decay times (<<NUM>), and produces variable intensity in response to controlled voltage levels. Use of LED also lowers cost, imparts added safety and reduces complexity of the device.

The energy density of most currently available LEDs is usually insufficient by itself, as compared to the needed energy for the test, therefore it may be considered to use a plurality of LEDs in an array. Using at least one LED <NUM>, the light emitted <NUM>, which may or may not be concentrated <NUM> first, is formed into a columnated beam consisting of bundle of parallel rays <NUM>. This highly columnated light beam travels through passages <FIG>, <NUM>, into the interior of the environmental space <NUM> and into the thermal volume <NUM>, as shown in <FIG>, without appreciable loss of energy delivered to face A of the sample lined up to its optical axis Y. In an alternate embodiment at least one laser diode may be incorporated in place of an LED.

From any combination of single or multiple LEDs or LED arrays <NUM>, placed on a base <NUM>, shown in <FIG>, the light emitted <NUM> may be concentrated or shaped by conventional optical means, such as a combination of lenses <NUM>, then columnating or transmitting members, such as optical fibers, or light pipes <NUM> to form a parallel ray bundle <NUM>. There may be multiple methods to concentrate with columnate, and transmit light as are known in the field.

These various configurations have one common denominator in that they are used to concentrate the light output <NUM> of a single LED or multiple LEDs in an array operated in a pulse mode, to a high-power density columnated light beam <NUM>, resulting in power density sufficient to produce the desired <NUM>-3º Celsius (C) thermal excursion of face B of the sample.

Due to the energy density of the light pulse being somewhat limited by what may be provided by an LED, or to other experimental reasons, it may be necessary to employ longer pulses of light, to obtain the necessary total energy input to the sample and produce a well measurable temperature rise, on face B, <FIG> of the sample <NUM>, ranging from <NUM> to <NUM>ºC for most materials.

The analysis in the operating and analysis software of the present disclosure largely overcomes the problems noted above, which were caused by heat losses and thermogram distortions due to finite length pulses. In prior art corrections for these were derived by manipulating post-test data, according to theoretical considerations. Since all of the factors are interacting, a correction for heat loss and a separate correction for finite pulse width rarely produce satisfactory results without numerous tests. The aim in these cases was to manipulate test data with the inclusions of theoretical corrections, to a point where the test data closely fits the theoretically perfect thermogram.

In contrast, the disclosed device is able to vary on command both the intensity and the duration of the pulse to produce a sequence of different width pulses which can be iterated to a desired outcome within the same test cycle, essentially in real time. Thus, the method is operating on all interacting factors combined by using sequential pulses of varied parameters, as they appear in real time. As shown in <FIG>, in a test cycle, for a particular material sample, a sequence of at least two pulses, designated P<NUM> and P<NUM>, but may be three or more with P<NUM> naming the third pulse and Px for those following, each with specific durations is used, then t<NUM>/<NUM> computed for each case and this t<NUM>/<NUM> data is stored in a database thereby providing means for calculating thermal diffusivity from the thermogram. The database may be housed, stored, or saved on a storage device of a computer. For example, the shortest pulse P<NUM> generates maximum temperature rise of Max<NUM>. From that, (t<NUM>/<NUM>)<NUM> is computed and is derived by the method described earlier. Similarly, (t<NUM>/<NUM>)<NUM> and (t<NUM>/<NUM>)<NUM> are derived for pulses P<NUM> and P<NUM>, respectively, wherein P<NUM> is the longest pulse and P<NUM> is greater than P<NUM>. The pulses used are of different durations, in increasing or decreasing order of duration magnitudes. Temporal displacements of the resulting thermograms and their corresponding t<NUM>/<NUM> are then analyzed in terms of their corresponding pulse widths at constant pulse intensity. By applying linear or higher order regression analysis to the sequence of t<NUM>/<NUM> values as a function of pulse duration, the resultant function is extrapolated to a very short pulse duration where t<NUM>/<NUM> is much less than <NUM> times the duration of the corresponding pulse, then the resultant t<NUM>/<NUM> is used in Equation <NUM>. The purpose of this procedure is to keep errors <NUM>% or below. The numbers of multiple pulses for this analysis is at least two. By varying the pulse intensity, the extent of the temperature rise of the rear face B can be further optimized as well and defined.

For real time analysis the evaluation of each thermogram associated with a pulse is analyzed individually right after the thermogram is defined. Then the regression analysis is performed after the last pulse in the desired sequence while the sample's thermal environment has not changed substantially. Based on the statistical parameters of the regression, and a determination of whether the results are acceptable, either further pulse sequence with altered parameters of intensity and pulse wide are iterated, or the test proceeds to the next temperature point. Alternately the analysis can be done posttest, but without the benefit of real time iterations using varied parameters as describe above.

Claim 1:
An instrument (<NUM>) for measuring thermal diffusivity of solid materials, the instrument comprising:
a. a furnace (<NUM>) for creating a thermally stable environment;
b. an LED array light source (<NUM>, <NUM>) capable of raising the temperature of at least one sample (<NUM>) by an emitted light pulse from it;
c. a sample holder (<NUM>, <NUM>) for holding at least two samples radially displaced from each other;
d. the LED array light source for illuminating a first side (A) of the at least one sample;
e. a temperature rise detector (<NUM>, <NUM>) for reading a temperature change of a second side (B) of the at least one sample and recording a thermogram after a light pulse to the first side is deposited;
f. a computer (<NUM>) for calculating the thermal diffusivity from the thermogram;
g. sample placements having the at least two radially displaced samples each part of a row of axially displaced samples,
wherein the furnace for creating the thermally stable environment further comprises programming to provide a plurality of pulses according to proportional-integral-derivative (PID) control principles to heat said sample to a desired temperature first, and reach a thermally stable condition before the measurement pulse is issued.