Thermal diffusivity measuring device

Abstract

A thermal conductivity measuring device includes a sample holder, a light source designed to emit a pulse of light of a predetermined impulse energy, an optical system for directing the pulse of light in a light path onto the sample in the sample holder, an infrared sensor for time-dependent detection of an infrared radiation intensity emitted by the sample, and an evaluation unit designed to automatically calculate the thermal conductivity of the sample on the basis of the time-dependent infrared radiation intensity. In addition, the light source has a continuous wave laser that is designed to emit an intensity-modulated laser beam, the intensity of which is modulated with an intensity modulation frequency. The light source is arranged to emit the laser beam along a light path. The evaluation unit is designed to automatically calculate the thermal conductivity of the sample on the basis of the time-dependent infrared radiation intensity and the intensity-modulation frequency.

Claims

1. A thermal diffusivity measuring device, comprising: a sample holder for accommodating a sample to be measured, a light source designed to emit a pulse of light of a predetermined impulse energy, an optical system for directing the pulse of light in a light path onto the sample, an infrared sensor configured for a time-dependent detection of an infrared radiation intensity emitted by the sample, and an evaluation unit for calculating a thermal diffusivity of the sample, wherein the light source comprises a continuous wave laser designed to emit an intensity-modulated laser beam, wherein the intensity-modulated laser beam has an intensity that is modulated with an intensity modulation frequency, and wherein the light source is arranged to emit the laser beam along the light path, and wherein the evaluation unit is designed to automatically calculate the thermal diffusivity of the sample on the basis of the time-dependent detection of the infrared radiation intensity and the intensity modulation frequency.

2. The thermal diffusivity measuring device according to claim 1, wherein the evaluation unit automatically calculates the thermal diffusivity of the sample using the time-dependent infrared radiation intensity caused by precisely one pulse of light and/or according to a light pulse method.

3. The thermal diffusivity measuring device according to claim 1 wherein the light source comprises a flash lamp, and wherein the intensity-modulated laser beam passes through the flash lamp.

4. The thermal diffusivity measuring device according to claim 1 further comprising a sample changer, wherein the sample holder comprises a first sample holder and at least one second sample holder, and where in the sample changer comprises both the first sample holder and the at least one second sample holder, and wherein the evaluation unit is configured to automatically carry out a method comprising: (i) measuring a thermal diffusivity of a first sample held by the first sample holder using the intensity-modulated laser beam or the pulse of light, (ii) controlling the sample changer so that at least one second sample held by the least one second sample holder is brought into the light path, (iii) measuring the thermal diffusivity of the at least one second sample using the intensity-modulated laser beam or the pulse of light used in step (i), and (iv) subsequently measuring the thermal diffusivity of the first sample and the at least one second sample using the intensity-modulated laser beam or the pulse of light, whichever was not used in step (i).

5. The thermal diffusivity measuring device according to claim 1 further comprising a sample changer, wherein the sample holder comprises a first sample holder for holding a first sample and at least one second sample holder for holding at least one second sample, and wherein the sample changer permits changing between the first sample holder and the at least one second sample holder, and wherein the evaluation unit is configured to automatically carry out a method comprising: (i) measuring the thermal diffusivity of the first sample using the intensity-modulated laser beam, (ii) controlling the sample changer such that at least one second sample held by at least one second sample holder is brought into the light path, (iii) measuring the thermal diffusivity the at least one second sample using the intensity-modulated laser beam, and (iv) subsequently measuring the thermal diffusivity using a pulse of light.

6. The thermal diffusivity measuring device according to claim 1, further comprising a furnace designed to set at least a first predetermined temperature and a second predetermined temperature different from the first predetermined temperature, wherein the sample holder is arranged in the furnace, wherein the evaluation unit is configured to automatically carry out a method comprising: (i) controlling the furnace so that the furnace is brought up to the first predetermined temperature, (ii) detecting that the furnace is at the first predetermined temperature, (iii) at the first predetermined temperature, measuring a first thermal diffusivity and a second thermal diffusivity of the sample in the sample holder respectively using (a) the intensity-modulated laser beam, and (b) the pulse of light, (iv) controlling the furnace so that the furnace is brought up to the second predetermined temperature, (v) detecting that the furnace is at the second predetermined temperature, and (vi) at the second predetermined temperature, measuring a first thermal diffusivity and a second thermal diffusivity of the sample in the sample holder respectively using (a) the intensity-modulated laser beam, and (b) the pulse of light.

7. The thermal diffusivity measuring device according to claim 1 further comprising a furnace designed to set at least a first predetermined temperature and a second predetermined temperature different from the first predetermined temperature, and a sample changer, wherein the sample holder comprises a first sample holder for holding a first sample and at least one second sample holder for holding at least one second sample, and wherein the sample changer permits changing between the first sample holder and the at least one second sample holder, and wherein the evaluation unit is configured to automatically carry out a method comprising: (i) detecting that the furnace is at the first predetermined temperature, (ii) measuring the thermal diffusivity of the first sample using either the intensity-modulated laser beam or the pulse of light, (iii) controlling the sample changer such that the at least one second sample is brought into the light path, (iv) measuring the thermal diffusivity of the at least one second sample using the intensity-modulated laser beam or the pulse of light used in step (ii), (v) subsequently measuring the thermal diffusivity of the first sample and the at least one second sample using the intensity-modulated laser beam or the pulse of light, whichever was not used in step (i), and (vi) repeating the steps (i) to (iv), except performing them at the second predetermined temperature.

8. The thermal diffusivity measuring device according to claim 1 wherein the evaluation unit is configured to automatically carry out a method comprising: (i) measuring thermal diffusivity by means of an intensity-modulated laser beam at at least two points on the sample, and (ii) determining a measurement uncertainty for the measurement of the thermal diffusivity measured in step (i) using a pulse of light, and if the measurement uncertainty exceeds a predetermined measurement uncertainty threshold value, then reducing the number of points measured in step (i).

9. The thermal diffusivity measuring device according to claim 1 wherein the evaluation unit is configured to automatically carry out a method comprising: (a) measuring the thermal diffusivity using a pulse of light to obtain a light pulse thermal diffusivity measured value, (b) performing time-resolved measurement of thermal diffusivity using an intensity-modulated laser beam at at least two points to obtain an Angstrom thermal diffusivity measured value, and (c) calculating an inhomogeneity parameter that describes an inhomogeneity of the thermal diffusivity of the sample from the light pulse thermal diffusivity measured value and the Angstrom thermal diffusivity measured values.

10. The thermal diffusivity measuring device according to claim 1 wherein the sample comprises two or more samples, and wherein the sample holder is designed to position the two or more samples in two positional degrees of freedom transverse to an optical axis of the infrared sensor.

11. The thermal diffusivity measuring device according to claim 1 further comprising a camera arranged to record an image of the sample.

12. The thermal diffusivity measuring device according to claim 1 wherein the optical system is designed to displace the light path transverse to an optical axis of the infrared sensor so that one or more lateral offsets between the optical axis of the infrared sensor and a strike point of the laser beam are adjustable.

13. A method for measuring thermal diffusivity by means a thermal diffusivity measuring device according to claim 1, comprising: (a) emitting from a light source a pulse of light of a predetermined impulse energy along a light path onto a sample to be measured, (b) time-dependent detection of an infrared radiation intensity emitted by the sample using an infrared sensor, (c) automatically calculating the thermal diffusivity of the sample on the basis of the time-dependent infrared radiation intensity, (d) emitting an intensity-modulated laser beam having an intensity which is modulated with an intensity modulation frequency onto the sample using a continuous wave laser which runs along the light path, and (e) automatically calculating the thermal diffusivity of the sample on the basis of the time-dependent infrared radiation intensity and the intensity modulation frequency.

14. The method according to claim 13, wherein the calculation of the thermal diffusivity of the sample is performed (i) on the basis of the time-dependent infrared radiation intensity caused by precisely one pulse of light and/or (ii) according to a light pulse method.

Description

DESCRIPTION OF THE DRAWINGS

[0056] In the following, the invention will be explained in more detail with the aid of the accompanying drawings. They show:

[0057] FIG. 1 a scaled cross-section through a thermal diffusivity measuring device according to the invention in accordance with an embodiment of the invention,

[0058] FIG. 2a a schematic drawing of the thermal diffusivity measuring device according to FIG. 1,

[0059] FIG. 2b a diagram in which the time-dependent infrared intensities in the light pulse method are depicted, and

[0060] FIG. 3a a diagram in which the time-dependent infrared intensities in the Angstrom method are depicted,

[0061] FIG. 3b a schematic view of a sample surface of the sample with the points at which is the laser beam strikes,

[0062] FIG. 4a a cross-section through a thermal diffusivity measuring device according to the invention in accordance with a second embodiment of the invention,

[0063] FIG. 4b a cross-sectional view of a flash lamp of the thermal diffusivity measuring device according to FIG. 4a,

[0064] FIG. 4c a cross-sectional view of a second embodiment of a flash lamp of the thermal diffusivity measuring device according to FIG. 4a and

[0065] FIG. 5 a perspective sectional view through a light source of a thermal diffusivity measuring device according to the invention in accordance with a second embodiment of the invention.

DETAILED DESCRIPTION

[0066] FIG. 1 shows a thermal diffusivity measuring device 10 that comprises a sample holder 12 for accommodating at least a first sample 14.1 and a second sample 14.2. During operation, each light source 16 emits individual pulses of light. Each pulse of light has an impulse energy W of preferably between W=5 J and W=50 J.

[0067] The pulses of light are directed on a light path 20 by means of an optical system 18. If the pulse of light strikes the sample 14.1, it emits infrared radiation 22. The infrared radiation 22 is bundled with collection optics and the infrared radiation intensity I.sub.22(t) of the resulting infrared beam measured by means of an infrared sensor 24 as a function of time t. The infrared radiation intensity I.sub.22(t) is detected by an evaluation unit 26.

[0068] The light source 16 has a pulse light source 27 in the form of a pulse laser 28, which may be, for example, an Nd:YAG laser, in particular a Q-switched laser.

[0069] The sample holder 12 is arranged in a furnace 30, which can be brought to a predetermined temperature T by a heating element 32, in particular an electrical heating element. The furnace 30 preferably features a thermometer 34, by means of which the temperature T of the samples 14.i (i=1, 2, . . . ) can be measured. On the basis of this temperature, the evaluation unit 26 or another control system regulate a heat output of the furnace 20 to the predetermined temperature T.

[0070] The infrared sensor 24 has a field of view that is defined by an aperture 34, particularly an iris, and extends about an optical axis A.sub.24. The sample 14.1 is located completely in the field of view S.sub.24 of the infrared sensor 24, so that the infrared sensor 24 only detects infrared radiation 22 that originates from the sample 14.1.

[0071] It is practical if the thermal diffusivity measuring device 10 comprises an adjustment unit 34, by means of which the furnace 10 can be adjusted relative to the light path 20.

[0072] The optical system 18 has a beam expander 36, for example a lens. In the present case, the optical system 18 also has a deflection mirror 38. The beam expander 36 is designed in such a way that the beam expanding effect is greater for the light of the pulse light source 27 than for the light of the continuous wave laser 46. In the present case, the lens only has an influence on the light of the pulse light source 27.

[0073] It is possible and provided for in the present embodiment, but not essential, that the deflection mirror is mounted by means of a guide 40 such that it can be displaced. It is therefore possible to displace the light path 20 transverse to the optical axis A.sub.24 of the infrared sensor 24 by a lateral offset s. After displacement, the optical axis A.sub.20 of the light path 20 extends parallel to the optical axis A.sub.24, but at a lateral offset.

[0074] The sample holder 12 constitutes part of a sample changer 41 on which the sample holders 12.i for the samples 14.i are configured. The sample holders 12.i can be rotated collectively by means of a rotary drive 42 about a sample holder rotational axis A.sub.12 and a predetermined angular position β of the sample changer 41 can be set. Furthermore, a schematically depicted positioning drive 44 can be used to set a distance r.sub.14.i of the samples 14.i from the sample holder rotational axis A.sub.12. As a result, each sample 14.i can be positioned in the two degrees of freedom β and r transverse to the optical axis A.sub.20.

[0075] FIG. 2a depicts a schematic view of the thermal diffusivity measuring device 10 according to FIG. 1. It shows that the light source 16 also comprises a continuous wave laser 46, by means of which an intensity-modulated laser beam 48 can be emitted. An intensity I.sub.48 of the laser beam can then be described using the formula

I.sub.48=I.sub.0I sin 2πf.sub.I  (1)

[0076] The laser beam 48 is injected into the light path 20 using a light injector 50. In the present case, the light injector 50 is formed by a dichroic mirror. If a dichroic mirror is used as a light injector, it is necessary for the two light sources to have two different wavelengths. Alternatively, the light injector 50 can be a beam splitter, for example. Instead of the dichroic mirror, the light injector may also be designed, for example, as a movable, for example folding, mirror that can be moved by a drive.

[0077] FIG. 2a also shows that a collection lens 52 is arranged behind the sample 14.1 that focuses the infrared radiation 22 emitted by the sample 14.1 onto the infrared sensor 24.

[0078] FIG. 2a also shows that the thermal diffusivity measuring device 10 can feature a camera 54 in whose field of view S.sub.54 the sample 14.1 is situated. In other words, the camera 54 can be used to record an image of the surface of the sample 14.1 onto which the pulse of light and the laser beam 48 fall.

[0079] FIG. 2b schematically depicts the dependency of a temperature T.sub.14 of the sample 14.1 measured by the infrared sensor 24 when a pulse of light 56 strikes the sample 14.1, which has a thickness L. At the time t=0, the pulse of light is emitted onto the sample 14.1. After the half rise time t.sub.1/2 the temperature T.sub.24 has risen to half the maximum value ΔT.sub.max that the temperature reaches at its maximum. On the basis of the formula

[00003]$\begin{array}{cc}\alpha =0.13879\frac{L^2}{t_{1/2}}& \left(2\right)\end{array}$

[0080] the thermal diffusivity α is calculated by the evaluation unit 26 from the half rise time t.sub.1/2.

[0081] The thermal diffusivity measuring device performs a method according to the invention in that the evaluation unit 26 first controls the light source 16 so that the pulse laser 28 emits a pulse of light. This strikes the sample 14.1 and illuminates a point S0, as is schematically illustrated in FIG. 3.

[0082] The infrared sensor 24 records the intensity of the infrared radiation 22 in a continuous and time-dependent manner, as shown in FIG. 2b. From this, the thermal diffusivity au is calculated from the thickness L of the sample 14.1 stored in the evaluation unit 26 and the half rise time t.sub.1/2 using the indicated drawing.

[0083] If there are at least two samples 14.i, the sample changer 41, in particular its rotary drive 42, is then controlled by the evaluation unit 26 in such a way that the second sample 14.2 is moved into the light path 20. Its thermal diffusivity α.sub.14.2 is then determined on the second sample using the pulse of light. The thermal diffusivites α.sub.14.i of the remaining samples are subsequently measured in the same way.

[0084] The evaluation unit 26 then controls the continuous wave laser 46 in such a way that it generates an intensity-modulated laser beam 48 that strikes the sample 14.1 at a first strike point, which can also be referred to as a first point S1. On the basis of the formula

[00004]$\begin{array}{cc}\theta =L\sqrt{\frac{\pi }{\alpha }}\sqrt{f_I}& \left(3\right)\end{array}$

[0085] the evaluation unit 26 determines the thermal diffusivity α from a phase shift θ between the intensity modulation of the laser beam 48 on the one hand and the infrared radiation intensity I.sub.22 on the other, the intensity modulation frequency f.sub.I and the infrared radiation intensity I.sub.22 as well as the thickness L and π (Pi). If the course of the measurement data deviates from the linear dependence of the phase shift θ on the root of the intensity modulation frequency f.sub.I, only measurement data θ(√{square root over (f.sub.1)}) in the interval in which the linear dependence exists are used.

[0086] The positioning drive 44 and/or the rotary drive 42 are subsequently controlled by the evaluation unit 26 in such a way that the laser beam 48 strikes at a second point S2. It can be seen that the diameters of the surfaces at the points S1, S2, which are illuminated by the intensity-modulated laser beam 48, are considerably smaller than the diameter of the surface of the point S0 illuminated by the pulse of light. In particular, the diameters of the surfaces at the points S1, S2 are at most half or at most a third, especially preferably at most a quarter, of the diameter of the surface illuminated by the pulse of light.

[0087] If the thermal diffusivity α is to be measured as a function of the temperature T, the evaluation unit 26 controls the furnace 30 in such a way that it approaches different temperatures T.sub.j one after the other. Of course, it is also possible that another control or regulation system carries out the control of the furnace 30. If the evaluation unit 26 detects that the respective temperature T is reached, it conducts the method described above.

[0088] To detect the position of the laser beam 48 on the sample 14.1, an image is recorded using the camera 54. It is possible, but not essential, for the laser beam 48 to fall on the sample in the process. If the camera 54 is sensitive in the infrared range, as provided for by a preferred embodiment, the position of the laser beam 48 on the sample 14.1 can be directly recorded. The at least one image can be stored in a digital memory of the evaluation unit 26. It is then possible to directly assign the location where the thermal diffusivity was measured to a location on the surface of the sample.

[0089] To measure the thermal diffusivity transverse to the optical axis A.sub.24 of the infrared sensor 24, the deflection mirror 38 is moved by means of a displacement drive 39 in such a way that it has a lateral offset s to the optical axis A.sub.24 of the infrared sensor 24.

[0090] The thermal diffusivity α.sub.q transverse to the optical axis A.sub.24 of the infrared sensor 24 is calculated by the evaluation unit 26 on the basis of the formula

[00005]$\begin{array}{cc}\theta =L\sqrt{\frac{\pi }{{\alpha }_q}}\sqrt{f_I}s& \left(4\right)\end{array}$

[0091] If the course of the measured data θ(s) deviates from the linear dependence of the phase shift θ on the lateral offset s, only measured data θ(s) in the interval in which the linear dependence exists are used.

[0092] A measurement uncertainty u.sub.ist is also calculated for each measurement of the thermal diffusivity. To this end, the mean squared error between the curves are calculated according to the formulas (3) or (4) or sections of the curves on the one hand and the measured values on the other.

[0093] Assuming an adiabatic heating, the curve according to FIG. 2b can be described as

[00006]$\begin{array}{cc}T\left(x=L,t\right)=\frac{q_0}{\rho c_{p}V}\left[1+2\underset{n=1}{\overset{\infty }{.Math.}}{\left(-1\right)}^n\exp \left(-\frac{n^2{\pi }^2\alpha t}{L^2}\right)\right]& \left(5\right)\end{array}$

[0094] In this case, q.sub.0 is the laser energy density, c.sub.p the specific thermal capacity, p the density of the sample and V the volume of the sample.

[0095] FIG. 3a depicts the dependence of the temporal course of the infrared radiation intensity I.sub.22(t) on the time tin arbitrary units on the one hand and the dependence of the temporal course of the laser radiation intensity I.sub.46(t) of the continuous wave laser 46 on the time t on the other. As illustrated, the phase shift θ an be determined, for example, from the temporal position of the respective maximum.

[0096] If the measurement uncertainty u.sub.ist exceeds a predetermined measurement uncertainty thresh-old value u.sub.max, a warning signal can be emitted. This warning signal can be emitted acoustically, optically or electrically.

[0097] It is possible that the number of points Si at which the thermal diffusivity is measured is reduced, in particular to zero, if the measurement uncertainty threshold value u.sub.max is exceeded.

[0098] FIG. 3b shows a schematic view of a sample surface of the sample 14.1 with the points S1, S2, S3, S4 at which is the laser beam 48 strikes for the spatially resolved measurement of thermal diffusivity. The point S0 denotes the area that is illuminated by the pulse of light 56. It can be seen that an averaging effect occurs due to the size of the point S0.

[0099] FIG. 4a shows a second embodiment of a thermal diffusivity measuring device 10 according to the invention in which the pulse light source 27 of the light source 16 is formed by a flash lamp 57. In the present case, the flash lamp 56 is a xenon flash lamp. The flash lamp 57 features a flash tube 58 that receives a current pulse from a charging circuit 60 and then emits a flash of light.

[0100] The light of the flash of light is reflected by a reflector 62 onto collection optics 64. The collection optics 64 form a pulse of light that expands along the light path 20. An opening 66 is designed in the reflector 62 through which the laser beam 48 passes. As a result, the laser beam 48 passes through the flash lamp 57.

[0101] FIG. 4b depicts a second preferred embodiment of the flash tube 58 which is curved and, in the present case, has a circular curvature. The laser beam 48 passes through the space surrounded by the curved flash tube 58. The reflector 62 is schematically illustrated.

[0102] FIG. 4c shows a third preferred embodiment of the flash tube 58, which has a U-shaped curvature. The laser beam 48 also passes through the space surrounded by the curved flash tube 58.

[0103] FIG. 5 depicts a sectioned perspective view of a light source 16 of a thermal diffusivity measuring device according to the invention. The light of the flash lamp 57 passes through a window 68 into a light guide tube 70. The light guide tube 70 may comprise a reflective coating on its inner side, for example made of gold or nickel.

[0104] For example, the light guide tube 70 is made of stainless steel, particularly Inconel.

REFERENCE LIST

[0105] 10 thermal diffusivity measuring device [0106] 12 sample holder [0107] 14 sample [0108] 16 light source [0109] 18 optical system [0110] 20 light path [0111] 22 infrared radiation [0112] 24 infrared sensor [0113] 26 evaluation unit [0114] 27 pulse light source [0115] 28 pulse laser [0116] 30 furnace [0117] 32 heating element [0118] 34 adjustment unit [0119] 36 beam expander [0120] 38 deflection mirror [0121] 39 displacement drive [0122] 40 guide [0123] 41 sample changer [0124] 42 rotary drive [0125] 44 positioning drive [0126] 46 continuous wave laser [0127] 48 laser beam [0128] 50 light injector [0129] 52 collection lens [0130] 54 camera [0131] 56 pulse of light [0132] 57 flash lamp [0133] 58 flash tube [0134] 60 charging circuit [0135] 62 reflector [0136] 64 collection optics [0137] 66 opening [0138] 68 window [0139] 70 light guide tube [0140] α Thermal diffusivity [0141] α.sub.q thermal diffusivity transverse to A.sub.24 [0142] β Angular position [0143] θ Phase shift between the intensity modulation and infrared radiation intensity [0144] A.sub.24 optical axis of the infrared sensor [0145] A.sub.20 optical axis of the light path [0146] A.sub.24 optical axis of the infrared sensor [0147] A.sub.12 rotary drive rotational axis [0148] f.sub.I intensity modulation frequency [0149] running index of samples [0150] I.sub.22 infrared radiation intensity [0151] I.sub.46 laser radiation intensity [0152] L thickness [0153] r.sub.14.i distance of sample 14.i from the rotary drive rotational axis [0154] s lateral offset [0155] S field of view [0156] t time [0157] T temperature [0158] t.sub.1/2 half rise time [0159] u measurement uncertainty [0160] u.sub.max measurement uncertainty threshold value [0161] W impulse energy