APPARATUS AND METHOD FOR TRANSIENT THERMAL INFRARED EMISSION SPECTROMETRY

Abstract: A method and apparatus for enabling analysis of a solid material (16, 42) by applying energy from an energy source (20, 70) to a surface region of the solid material sufficient to cause transient heating in a thin surface layer portion of the solid material (16, 20) so as to enable transient thermal emission of infrared radiation from the thin surface layer portion, and by detecting with a spectrometer/detector (28, 58) substantially only the transient thermal emission of infrared radiation from the thin surface layer portion of the solid material. The detected transient thermal emission of infrared radiation is sufficiently free of self-absorption by the solid material of emitted infrared radiation, so as to be indicative of characteristics relating to molecular composition of the solid material.

Description: Description Apparatus and Method for Transient Thermal Infrared Emission Spectrometry. Technical Field The present invention relates to spectroscopic analysis of materials, and particularly, to non-contact, remote spectroscopic analysis of a quantity of moving or stationary material based on transient thermal infrared emission from the material. Background Art There are numerous types of analytical methods which currently are known for deriving information about materials. Spectroscopy is a well known and general method for analyzing materials. There are a number of types of spectroscopic methods which, in turn, are applicable to certain types of analyses and measurements, and which have advantages and disadvantages. Presently, there is a need for improvements in the ability to analyze materials, especially in those cases where such analyses need to be quick, efficient, and accurate. Additionally, there is a real need for such analyses for "in-process" situations; that is, directly on-line with respect to the manufacturing or the processing of materials. Presently, for many materials, there are a variety of generally conventional spectroscopic methods for analyzing the content and other characteristics of the materials. Some of those methods are infrared transmission, diffuse reflectance, photoacoustic, and emission spectroscopies. While generally these methods give satisfactory results, they are deficient because they require selective, and often destructive, sampling of the materials. Some materials (coal, for example) require grinding or pulverizing. The material must often be removed to a remote laboratory location where the testing and equipment requires time and resources to provide the results. Currently, no contemporaneous, non-destructive, on-line infrared analysis is reasonably possible for solid materials including semisolid materials such as flexible or rubber-like materials. Many of the aforementioned presently used methods also lack much flexibility in their use. While some of the methods do not require destructive sampling such as grinding or pulverizing, they may not be operable for materials of greater than minimal thickness, or for materials of varying thickness. Conventional transmission, reflection, or emission spectroscopies have problems because the optical density of many materials is too high to permit accurate and reliable measurement. That is, upon heating of a sample, such sample strongly reabsorbs the same wavelengths it strongly thermally emits as infrared radiation. When a thick sample is heated, the deep layers of the sample emit strongly at the preferred wavelengths and only weakly at other wavelengths. This deep-layer strong emission at preferred wavelengths, however, is greatly attenuated before leaving the sample since surface layers of the thick sample preferentially absorb those particular wavelengths and such process is termed "self-absorption". Self-absorption in optically-thick samples causes severe truncation of strong spec¬ troscopic bands and leads to emission spectra which closely resemble black-body emission spectra representative of an optically thick material being heated to a uniform temperature and which contain little spectral structure characteristic of the material being analyzed. Attempts have been made to solve this self-absorption problem by thinning sample materials. High-quality spectra of free-standing films and thin layers on low-emission substrates are routinely measured. However, this requires selective sampling and processing of the materials being analyzed. For other types of spectroscopic methods such as photoacoustic spectrometry which are less subject to optical density problems, deficiencies exist in that they are not easily performed on moving streams of εolid materials. Thus, there is a real need in the art for an apparatus and method which has the flexibility to be used both for moving and stationary materials; and for materials which may have significant optical densities. There is a further need for an apparatus and method which does not require the use of additive materials to or processing of the sample materials, and which can analyze non-destructively and remotely. For example, in some spectroscopic methods, the materials must be ground to fine powders and then diluted in a transparent matrix. Of course, any destructive processing or additive procedures would alter the beginning state of the material being analyzed. For an analytical apparatus and method to be used effectively in a production line, any fundamental change in the material must be avoided. For example, if variable-in-size crushed coal were being analyzed on a moving conveyor, no grinding or addition of any substance would be allowed, as the coal could not then be utilized for its intended purpose in its original state. Disclosure of the Invention It is therefore a principal object of the present invention to improve upon or overcome the deficiencies and problems in the art. Another object of the present invention is to provide an apparatus and method of thermal transient infrared emission spectroscopy which can be utilized on either moving or stationary materials. Another object to the present invention is to provide an apparatus and method as above described which can be accomplished generally without physical contact with the material. A further object to the present invention is to provide an apparatus and method as above described which can be done remotely from the material being analyzed. A further object to the present invention is to provide an apparatus and method as above described which can derive the molecular composition of a material, and various physical and chemical properties of the material that are related to molecular composi- tion. Another object to the present invention is to provide an apparatus and method as above described which can be utilized directly on production or processing lines which handle the materials. A still further object to the present invention is to provide an apparatus and method as above described which is non-destructive to the material being analyzed. A further object to the present invention is to provide an apparatus and method as above described which can also be utilized to analyze either large or small samples of the materials in laboratory settings. A still further object to the present invention is to provide an apparatus and method as above described which can be utilized with optically dense materials. A further object to the present invention is to provide an apparatus and method as above described which overcomes the spectroscopic problems caused by self-absorption of the emitted radiation from the material being analyzed. A further object of the present invention is to provide an apparatus and method as above described which can be utilized for stationary materials, or for an unknown quantity of moving material, on both a continuous and non-destructive basis. Another object of the present invention is to provide an apparatus and method as above described which can be directly utilized in-process for an unknown quantity of moving material. A further object of the present invention is to provide an apparatus and method as above described which is economical, efficient and reliable. Another object of the present invention is to provide an apparatus and method as above described which can operate within the extreme and changing conditions of a processing environment for materials, or within a laboratory setting. A further object of the invention is to provide an apparatus and method as above described, which can be combined with a computer system to derive information about the materials useful for processing, control, and understanding of the material. The present invention provides an apparatus and method for nondestructively analyzing either stationary or moving materials, particularly solid materials, by infrared spectroscopy. Energy is applied to a surface region of the material so as to cause transient heating in a thin surface layer portion of the material and enable transient thermal emission of infrared radiation from the thin surface layer portion. That is, a fraction of the energy is absorbed near the surface, heats a thin, near surface layer of the material, and causes thermal emission of infrared radiation. Substantially only the transient thermal emission of infrared radiation from the thin surface layer portion which is substantially free of self-absorption, is detected as an infrared spectrum by a spectrometer, for example. The spectrum contains information on the molecular composition of the material. Thereafter characteristics relating to the molecular composition of the material may be determined based upon the detected transient thermal infrared emission. In accordance with the present invention, the energy source supplies sufficient energy to the surface of the material to cause transient heating in the surface layer portion and may be a pulsed energy source such as a pulsed laser, electron, or ion beam, or other pulsed energy source. Alternatively, transient heating of the surface layer portion can also be created by utilizing a continuous energy beam directed at a moving material with the energy beams also being moved or oscillated, if desired. For example, in accordance with a feature of the present invention, a thermal energy source such as a hot gas jet is utilized to create transient heating in the material. Further, a heated roller or the like, may be utilized to contact the surface of the material so as to cause transient heating. Also, a strongly absorbed energy source, such as a laser, may be used to cause the transient heating and thermal infrared emission. Additionally, other energy sources can be utilized to create transient heating within the material such as electron or ion generators, operating in either continuous or pulse modes, hot gas jets, and heated rollers, as well aε others. In accordance with the present invention, the analysis and detection of the thermal infrared emission is accomplished by a spectrometer and detector which, in preferred embodiments, can be, for example, an optical filter spectrometer, a Fourier transform infrared spectrometer (FTIR)or a cooled HgCdTe infrared detector. The detector operates so as to detect subεtantially only the tranεient thermal emission of infrared radiation from the thin surface layer portion which is substantially free of self-absorption by the material of emitted infrared radiation. Such detection may be achieved in the case of a pulsed energy source applied to a stationary material by controlling the operation of the detector in accordance with the application of the pulsed energy so that the detector is activated for a predetermined period. That iε, a period of time after the tranεient thermal emiεεion of the infrared radiation from the thin εurfaσe layer portion, the self-absorption by the material of emitted infrared radiation results in essentially black-body emission spectra to be detected which black-body emisεion spectra prevents a determination of characteriεtics relating to the molecular composition of the material. Accordingly, the operation of the detector is controlled to detect the desired transient thermal emisεion and exclude other emissions as by gating of the detector in synchronization with the pulsed energy application to the surface of the material. Alternatively, if a continuous energy beam is applied to the surface of the material and the material is moving, the field of view of the detector is set in relation to movement of the material so that subεtantially only the transient thermal emisεion of the infrared radiation from the surface layer portion appears within the field of view of the detector as a result of movement of the material. Control arrangements may also be provided so as to be able to detect the transient thermal emisεion of the infrared radiation from the surface layer portion by suitable processing of the detected spectra. That is, by suitable filtering of the detected wavelengths, it is posεible that the wavelengthε of the applied energy source are present within the field of view of the detector and filtered out so that the transient thermal infrared emission is appropriately detected and recognized. In accordance with the present invention, the detector may be controlled by a control arrangement and/or provide an output to a control arrangement including a processor having appropriate software for deriving different characteristics from the detected and selected spectra of the infrared radiation from the material. Additionally, such control arrangement or proceεεor may include appropriate computer memory, storage, and printer or graphic components. The invention can be utilized aε a non-contact, remote analytical apparatus and method for measuring infrared absorbance spectra of materials, either in a moving stream or in a stationary setting. The heating of the thin, near surface layer of the material can be accomplished by either pulsing energy in time onto the material, or moving the material relative to the energy, or both. Thermal infrared emission from the thin layer is analyzed by the detector to obtain infrared absorbance spectra of the material utilizing Kirchhoff's law. The infrared abεorbance εpectra are then uεed to determine molecular composition and other correlated properties. The invention therefore provides efficient and accurate emisεion spectroscopic analysiε of materials. It eliminates the selective sampling, grinding, or other preparation required by presently utilized systems. The invention can also be adapted to a variety of situationε. It can be utilized in laboratory settings for a variety of different types and sizes of materials, and it can be utilized on-line in production settings. By utilizing computer software and hardware, it can form an integral part of process control by being able to derive this information during process¬ ing, without contact or sampling, or deεtruction of the material being processed. It can also be used to assist in controlling how the processing of the material proceeds. These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying drawings. Brief Description Of The Drawings Figure 1 is a schematic depiction of an embodiment of the invention. Figure la is similar to Figure 1 except it utilizes an alternative heating source to the laser of Figure 1. Figure 2 is a schematic depiction of a further embodiment of the invention. Figure 2a is a schematic depiction similar to Figure 2 except it utilizes a different heating source than the laser of Figure 2. Figure 3 is a graphical depiction of observed e issivity spectra for a phenolic-plastic disk sample. Resultε utilizing the invention with the sample rotating, and stationary, are compared to a reference photoacoustic absorption spectrum of phenolic-plastic. Figure 4 is similar to Figure 3, except it graphically depictε emiεεivity εpectra of a coal sample. Figure 5 is εimilar to Figureε 3 and 4 except it graphically depicts emissivity spectra of blue-green paint on aluminum, and shows spectra of both a stationary sample, and a sample rotating at two different εpeedε, compared to a reference photoacouεtic abεorption εpectrum of the paint. Figure 6 iε similar to Figure 5, except it graphically depicts emissivity spectra of electrical tape. Best Mode For Carrying Out The Invention With respect to the drawings, a detailed description of the preferred embodiments of the invention will now be described. This deεcription is by way of example only and is not intended to limit the scope or applications of the invention. A preferred embodiment of the invention iε εhown at Figure 1. The embodiment of Figure 1 can be used to analyze either stationary or moving materialε. By referring to Figure 1, it can be seen that in this embodiment the sample or material to be analyzed is designated by reference numeral 16. A sample position controller 18 can comprise either a material transporter such as a conveyor or some other type of positioner. For example, controller 18 could be a rotary table. Still further, controller 18 could εimply be a poεitioning table (for example an X-Y-Z poεitioning table) which can be controlled to accurately position a stationary sample material. The type of sample position controller 18 used depends upon the selected operation of the invention. The energy source in Figure 1 consists of a laser 20 which can be operated either in a pulsed or CW (continuous wave) mode. Laser beam 22 from laser 20 would be directed towards material 16. Focusing optics 24 would serve to pinpoint the laser beam 22 onto material 16. Collection optics 26, εuch aε are known in the art, are used to focus infrared radiation emitted by sample material 16 onto spectrometer/detector εystem 28. System 28 generates an electrical εignal aε a function of the wave number of the emitted radiation. The embodiment of Figure 1 alεo includeε computer εyεtem 30 which controlε the εpectrometer/detector system 28 aε well as controller 18, 34 and 38 and processes the spectrometer/detector εystem 28 signalε in order to obtain the chemical or physical information required from the analysis. The computer system 30 also controls measurement components, displayε reεultε, and commands auxiliary systems. Reference numeral 32 refers generally to a computer display, printer, and/or plotter. Laser 20 is powered by a laser power supply in controller 34. Moreover, in the embodiment in Figure 1, there is shown a microscope system 36 having appropriate optics for viewing the analysiε area on sample material 16; that is, the area of intersection of focused laser beam 22 onto material 16. Additionally, a cooling jet εupply and controller 38 can be operatively associated with the embodiment of Figure 1 to be able to supply a cooling jet of gas to the general area of inter-section of laser beam 22 with material 16. Reference numeral 40 designates communication connections or links between computer system 30 and other components and controls of the embodiment. Computer syεtem 30 can therefore, by appropriate software, operate laser power supply controller 34, sample position controller 18, spectrometer/detector system 28, microscope system 36, and cooling jet supply and controller 38. Other components and controls can also optionally be operated by computer syεtem 30, according to desire. The embodiment of Figure 1 is flexible and adaptable to be used for different materialε, and different analytical procedures. This embodiment can analyze moving or stationary solid materials. It imposes a surface absorbed energy beam upon material 16 causing transient heating in a thin surface layer of material 16 by pulsing the energy beam over time, or by rapid relative motion between the beam 22 and material 16; or by a combination of both. The transient thin layer heating causeε emiεεion of infrared radiation from a shallow enough depth to be sufficiently free of self-absorption to allow an infrared spectrum of material 16 to be meaεured. The emitted radiation from the thin layer is detected and measured by spectrometer/detector εyεtem 28. Computer system 30 then processes the signal to obtain molecular concentrations or other physical or chemical information through correlation techniques as required for any number of different operations, such as a process control, quality control, analytical chemistry, or non-destructive evaluation applicationε. Laεer 20 can be a UV laεer (a strongly absorbed pulsed energy source) . Spectrometer/detector syεtem 28 can be one or more optical filter εpectrometerε with infrared detectorε, a Fourier tranεform infrared (FTIR) spectrometer, or other spectrometers with an infrared detector such as an ambient temperature infrared detector or a HgCdTe or high T c superconducting detector operating at liquid nitrogen temperature. Computer system 30 can include appropriate computer software and complementary data for deriving different material characteristicε from infrared emiεεion εpectra. Additionally, it can uεe appropriate software, displays, complementary data and servo syεtems to make decisionε and εend and execute commandε based on the infrared spectra. Operation of the embodiment of Figure 1 can first be described with regard to a sample 16 which is in motion with respect to laser beam 22. In this case, laser 22 is focused by opticε 24 on sample 16, which for example, could be coal moving on a conveyor belt, or coal attached to a rotating disc, such as is generally indicated at reference numeral 18. The relative motion between the laser beam 22 and εample 16 produceε a tranεient heating effect in a thin εurface layer of the sample which is sweeping past the focal spot of the laεer beam 22. It iε to be underεtood that a similar transient may be generated by pulsing the intensity of the laser beam as a function of time if sample 16 were stationary. The maximum layer thickness observed by spectrometer/detector 28 for the moving εample 16 can be estimated by (4Dr/v) 1 /2 where D is thermal diffuεivity of εample 16, r is the object diameter from which emitted radiation is focused into the spectrometer/detector 28, and v iε the sample velocity. In comparison, in the case where laεer 20 iε pulsed and sample 16 is stationary, the maximum layer thicknesε expreεεion is (4Dτ) 1 /2 where r is the laser repetition period. It is also to be understood that depending on the application, it may alεo be desirable to both move the sample and pulse the beam. If long term temperature build-up occurs in the sample, cooling jet supply 38 can be employed to remedy this situation. Radiation emitted by the thin heated layer iε focuεed by collection optics 26 on the infrared spectrometer detector system 28. System 28 and computer system 30 measure the emitted radiation intensity as a function of wave number in terms of an electrical signal. The computer output peripherals (display, printer, and/or plotter 32) display and record data. The computer 30 procesεeε the infrared data to determine various material properties. The computer 30 useε communication or command links 40 to control various components of the measurement εyεtem, for example the laεer power εupply 34; and to control other systems, such as processing equipment (not shown) baεed on material propertieε determined by the on-line meaεurements. The embodiment of Figure 1 can also be utilized on a εtationary sample 16. The stationary measurement mode is appropriate for use in analytical laboratories where a moving stream of material iε not preεent. In thiε case, laεer 20 iε pulsed with a pulse time which is εhort on the εcale of the pulεe repetition time. Furthermore, cooling jet 38 iε employed to prevent the long term build-up of heat in the εample 16. Microεσope viewing εyεtem 36 can be employed to poεition the focal spot of the laser 20 at a precise location on the sample 16. This allows for microanalysiε. In other regardε the stationary εample measurement mode is similar to the moving εample embodiment described above. In the case of a pulsed laser 20 and a εtationary or rotating εample 16, due to the pulεed nature of the transient thermal emisεion from the thin surface layer portion produced by the laser irradiation, it is preferable to synchronize the laser firing with the spectrometer/detector εyεtem 23 sampling under control of the computer syεtem 30. The εpectrometer/detector syεtem may include a detector preamplifier and an A/D converter and by inserting a gated integrator between the output of the detector preamplifier and the A/D converter, a pulse utilized to trigger the A/D converter and the integrator may also be utilized to fire the laser and control sampling by way of the computer system 30. In this manner the detection by the spectrometer/detector iε gated in accordance with the firing of the laser to detect the transient thermal emission of infrared emission from the thin surface layer of the material for a short time period after each laser pulse thereby avoiding detection of emisεions affected by self-abεorption. Alternatively, a continuous laser whose beam is chopped may be utilized with a detector system having a lock-in amplifier so that εelective meaεurement of a signal occurring at the chopping frequency is effected. The use of a lock-in amplifier may also produce a less noiεy signal since it eliminates most noise at all frequencies other than the one it monitors. Additionally, if the chopping frequency is much higher than the sampling rate of the A/D converter in the spectrometer/detector, εynchronization of the chopping and the. εpectrometer/deteσtor may not be neceεεary. Alεo, it iε possible to effect relative movement of the energy beam and the field of view of the spectrometer/detector with respect to the surface of the sample material independent of sample material velocity. For example, by providing an oscillating mirror in the energy beam path and the field of view of spectrometer/detector with -a linear moving sample material, the energy beam and field of view are moved by oscillation of the mirror perpendicular to the direction of sample material motion and such results in a zig-zag track for the beam and field of view. Furthermore, the spectrometer/detector can be provided with suitable filters so that even though the energy beam is within the field of view, the wavelength thereof is filtered out. Figure la is essentially the same as Figure 1 except that a heating jet supply and controller 70 is utilized to impose heat energy upon sample 16, instead of laser 20 of Figure 1. Heating jet supply and controller 70 is configured to impose a jet of heated gaε onto material 16 to produce transient heating in the surface of material 16. As with Figure 1, the remaining components serve to collect the infrared radiation and procesε the same. It is to be noted that heating jet supply and controller 70 is operatively connected to computer syεtem 30 and can be controlled accordingly. Heating jet εupply and controller 70 produceε a hot gas heating jet which can be imposed on εample 16 to generate transient heating. This embodiment is especially useful for materials that are not strongly absorbing of energy sources such as laεers. It is also economical and non-complex. This alternative method of generating tranεient heat in material 16 highlights the fact that a number of different sources can be utilized to create such transient heating for purposeε of the invention. As previouεly mentioned, other energy bea ε εuch as electron and ion beams can be utilized, both either in a continuouε or a pulεed mode. Further, a heated roller or the like may be utilized to contact the εurface of the material εo aε to enable tranεient heating. The type of heating source can be selected according to desire and depends upon a number of factors specific to each situation. These factors include but are not limited to the characteristics and parameters of various energy sources, as well as the type of material being analyzed. Figure 2 shows another embodiment of the present invention for a better understanding of the invention. This embodiment was used to generate the spectra illustrated in Figures 3 through 6. A sample material 42 is contained on and secured to a rotating disk 44. A motor 46 has an axle 48 which spins disk 44. A fixed CW laser beam 50 is directed to a mirror 52 which in turn directs beam 50 through focusing optics 54, which can comprise a focusing lens. Another mirror 56 is adjustable to direct beam 50 to a desired location on disk 44. For the purpose of recording the spectra in Figures 3 through 6, disk 44 was either made of or covered with the sample material 42 and mounted on the axle 48 of variable-speed motor 46, and placed at the normal source position of a Perkin-Elmer 1800 Fourier Transform Spectrophotometer 58. Beam 50, from an argon-ion laser operating in the multi-line mode at up to 3.5 W, was focused on the disk 44 at a 45" angle to a εpot approximately 0.8 mm in diameter positioned 3.9 cm from the center of the disk 44. The spectrometer 58 observed the sample 42, normal to the sample surface with the laser focus centered in the spectrometer's 8 mm diameter field of view. The entry port of the spectrometer 58 was 5 cm from the disk 44 and was covered with a salt window 60. No special additional optics were used to better match the small source size to the 8 mm wide field of view. The spectrometer 58 waε fitted with a wide-band liquid nitrogen-cooled HgCdTe detector and accumulated 256 scans in single-beam mode with a 1.50 cm/s optical-path- difference velocity and 4 cm~l nominal resolution. In some caseε, a εample cooling jet 64 of chilled helium gas was used. A coil 66 of 1.6 mm diameter stainlesε-steel tubing carrying helium was immersed in liquid nitrogen. The open end of the tubing 66 directed the jet onto the disk 44 0.5 cm from the laser focus εo that the rotation of the disk 44 carried the area irradiated by the laser 50 into the jet 64 immediately after εuch area left the εpectrometer 58 field of view. It can therefore be seen that the embodiment of Figure 2 showε, in detail, another configuration for practicing the preεent invention. By uεing a laεer beam or other energy εource that will be abεorbed very near the εurface of the εample material 42, the invention provideε that only a thin surface layer is directly heated. In this embodiment, utilizing the rotating disk 44, the layer is transient since thermal diffusion will cauεe the heated layer to thicken and cool rapidly. On the other hand, for comparison, if a pulsed laser is used on a stationary sample, the layer is present in the sample at the poεition of the laser beam for a short time immediately following the laser pulse. It is to be underεtood that if a continuouε laser is used, it must be either scanned acrosε the εample εur¬ face, which would require a εcanning apparatuε, or the εample must be translated through the beam path. The transient layer then exists in the beam track across the εample at and immediately behind the beam position. Figure 2a depictε eεεentially the same embodiment as Figure 2, except that instead of utilizing a laser beam 50 to produce heating on sample 42, a heating jet supply and controller 70, such as discuεεed with respect to Figure la, could be utilized. This embodiment, utilizing the rotating sample, could readily be adapted to position the hot gaε jet upon a portion of sample 42 as it rotates by. This would create the transient heating which would produce the transient thermal emission of infrared radiation from the thin surface layer of the εample material which would be detected by spectrometer 56. Figure 2a schematically depicts one configuration for heating jet εupply and controller 70. It is to be understood that this is but one configuration, and others can be used while staying within the scope of the invention. For example, it is possible to in effect pulse the temperature of the gas jet by rapidly switching between a hot and a cold flow through the nozzle. Alternatively, the jet can be εcanned or oεcillated in a direction perpendicular to the εample motion by providing a jet in the form of a εerieε of nozzleε along a rim of a rotating wheel which nozzles apply the gas jet to the εample. The εpectrometer/detector would then εample the emiεεion from the εample material aε each jet paεsed through its field of view. Here again, it is posεible. to alternate hot and cold jets from the εerieε of nozzles. In accordance with Figure 2a, to supply the jet of heated gas to the sample, the pressurized gas canister 72 is operatively connected by hose 74 to heating chamber 76. The pressurized gas iε heated in heating chamber 76 and channeled into jet nozzle 78, which iε poεitioned to direct the gaε jet.onto the εample 42. In this configuration, gas canister 72, such as is well known in the art, has an on/off valve, and can also have a regulator 80, a flow meter 82, and an electrical valve 84 connected in line to gas canister 72. Heating of the gas in heating chamber 76 can be accompliεhed by poεitioning an electrical reεiεtance heating element 86, such as is well known in the art, within heating chamber 76. Heating element 86 would in turn be operatively connected to a variable transformer 88, which can be electrically controlled to adjuεt the current through heating element 86, to in turn control how much heat iε generated. Tranεformer 88 would also be operatively connected to an electrical power source 90. In the configuration shown in Figure 2a, compressed nitrogen gas is utilized, but it is to be understood that other gases, including air, could also be used. Still further, other methods of heating the gas are possible. This same configuration could be used for heating jet supply and controller 70 in figure la. It is to be understood, as indicated in Figure 2a by reference numerals 40, that each of the motor 46, εpectrometer 58, electrical flow valve 84, variable tranεformer 88, and flow meter 82 could be operatively connected to a system controller such as a computer. With respect to all of the embodiments described, once the thin heated surface layer is created, the emisεion from it iε analyzed by an FTIR or other infrared εpectrometer to obtain an emission spectrum which can be converted by spectrum analysis, according to Kirchhoff'ε Law, to an abεorbance εpectrum. Spectral information obtained from the invention, therefore, iε εimilar to that yielded by other typeε of infrared meaεurements (transmission, photoacoustic, or diffuse reflectance) . Like infrared absorption, the invention iε able to determine nondeεtructively not only molecular propertieε, but also many other material properties that are related to molecular εtructure. Such determinations can be made with the aid of existing software which correlates properties with infrared εpectral structure. Spectra from the invention of heterogeneouε samples can be expected to have some band intensity differences relative to spectra of the other measurement techniques due to differences in the heating ef iciency of the excitation beam for different components. Thiε effect can be compenεated for in the data treatment. It iε to be underεtood, that intenεities of observed emission spectra may fall off with increasing wavenumber in the same manner as black-body emisεion curveε. Additionally, εourceε other than the εample 42 (εuch as the spectrometer 58 itself) may contribute a background emisεion. Theεe may be corrected for by converting the emiεsion spectra to emissivity spectra. According to Kirchhoff's Law, emiεεivity iε proportional to the fraction of light abεorbed when it strikes the surface of a body and so an emisεivity εpectrum iε analogouε to an abεorbance εpectrum. Although Kirchhoff'ε Law strictly applies only at thermal equilibrium, emisεivity εpectra based on the emission spectra of the invention closely resemble absorbance spectra. An emissivity spectrum e may be calculated from an emisεion εpectrum uεing the equation ε = (Sχ-S2)/(Bι-B2) , where S and B are the observed sample and black-body emission spectra and the subscripts refer to two temperatures, Ti and T2, Tj is an effective elevated temperature which is induced by the transient laεer heating and T2 is ambient. S2 and B2 correct for background emisεion while the division by B compensateε for the Planck black-body modulation and for the reεponεe curve of the spectrometer and detector. In practice, S2 B^ and B2 are determined by complementary measurements. A comparison of emissivity derived from the present invention and photoacoustic absorbance spectra of a variety of samples has shown that emiεεivity spectra of the invention are nearly identical to absorbance spectra aε predicted by Kirchhoff's Law. Co parisonε of the reεultε of the invention were made with infrared absorption spectra recorded using photoacoustic detection. These results are shown in Figures 3-6. An MTEC Model 200 Photoacoustic Cell was mounted in the FTIR spectrophotometer (with the spectrophotometer's normal light source) and 32 scanε were accumulated at 0.05 cm/ε optiσal-path-difference velocity and 8cm -1 nominal reεolution. The times required to record a reference photoacoustic εpectrum and a εpectrum according to the invention were both about three minutes. It is understood that the determination of molecular properties and other material properties related to molecular propertieε does not require conversion of observed emisεion spectra to emisεivity εpectra aε deεcribed above. Such conversion is only necessary to produce spectra analogous to absorbance εpectra. Figures 3-6 depict emissivity spectra for phenolic plastic, coal, blue-green paint, and electrical tape respectively. In Figure 3, emisεivity εpectra (curveε A and B) are shown for a smooth-surfaced, 3.0 mm thick, red-colored, filled-phenolic-plastic (Synthane brand) diεk and compared to a reference abεorption spectrum (C) obtained photoacoustically. Curve A repreεentε an emiεεivity curve for a rotating sample at 75 revo- lutions per minute (rpm) with the spectrometer observing the transient thermal emisεion from the thin, heated layer aε detailed in the above embodiment deεcription. Curve B, by contrast, is the emiεεivity curve produced by the εame sample when stationary. With a stationary sample, no transient emitting layer exiεts since the bulk of the sample is heated by the laser beam, and so the resulting emissivity curve is very similar to that observed by traditional emisεion εpectroscopy. The superiority of Curve A over Curve B (that is, the greater similarity of Curve A to the reference spectrum, Curve C) is the direct result of the method of this invention. Coal was used as a second example of the improvements attained by observing transient rather than steady-state emission (see Figure 4) . Because of the roughness of the coal, its surface varied in and out from the spectrometer entry port by about 2 mm when the disk was rotating. This roughness caused fluctua¬ tions in the signal intensity obεerved by the εpectrometer, but did not appreciably inσreaεe the noiεe in the resulting spectrum. Again, curve A relates to emisεivity baεed on the coal εample being rotated at 75 rpm, and curve B relates to emissivity from a stationary εample. Curve C again iε a reference curve based on photoacoustic absorption measurements of coal, and again Curve A is more similar to Curve C than Curve B is. Figure 5 depicts emisεivity curves measured according to the present invention with respect to a 3 mm thick aluminum plate coated with blue-green baked-enamel paint. The paint tested how well the invention could handle a low-signal sample and a very thin sample. The low signal results both because the blue-green color of the paint made it a good reflector of the laser light and because the paint was a thin layer on aluminum, whose high thermal conductivity would diffuse the laser-depoεited heat rapidly. Curve A is an emissivity curve based on a stationary sample. Curves B and C are emiεεivity curves where the sample was rotated through the laser beam, at different rotational speeds with the laser at different powers. Finally, Curve D is again a reference curve based on photoacoustic absorption measurement of the sample. Figure 6 depictε the reεultε meaεured according to the present invention for conventional electrical tape compriεing a 0.18 mm thick (excluding adheεive) , pigmented, plasticized-polyvinyl-chloride sheet. The tape was attached by its own adhesive to a 1.6 mm thick aluminum disk (44 in Figure 2) . This tape has a lower thermal-decomposition threshold than the other samples. (The maximum service temperature for plasticized polyvinyl chloride iε typically 80 to 105*C) . The εpectra derived from the invention demonstrated that the invention's technique effectively reduces the saturation in the emission from optically thick sampleε to levelε comparable to photoacouεtic absorption spectra. The variety of lesε-than-ideal εamples presented (plastic, coal, paint, electrical tape) , show that the invention is potentially widely applicable and can be uεed on materialε with high reflectivity, irregular εurfaceε, and moderate thermal εtability. The results have good signal-to-noiεe ratios. The invention provideε a number of advantageε over other methods. Especially important is the fact that it does not require any εample preparation, unleεs the composition of the surface layer probed is not related in a known way to the bulk conεtituentε of intereεt. The invention iε alεo applicable to a very wide range of situations. Depending on the analysiε geometry, it can be applied to either a moving stream of material or a stationary object of any size. With a pulsed laser on a stationary target it may be posεible to perform high resolution infrared icroεcopy of microεampleε by tightly focuεing the excitation beam. At the other extreme, it is poεsible, using a moving stream of material, to derive measurements by averaging over much more material than would be practical by conventional infrared methods. For example, with a 1 m/s material velocity, a 1 mm diameter laser spot, and a probe depth of 50 μm, a measurement by the present invention can potentially examine in an hour the same amount of material as approximately 2 X IO 6 KBr-pellet infrared transmission analyses, assuming the spectrometer ac- quires spectra at every inεtant. Furthermore, the actual volume of εample diluted in a pellet iε aεεumed to be 8 X 10~ 5 cm 3 (10~ 4 cm thick and 10 mm diameter) , which would produce εaturation-free tranεmiεεion spectra. If pellets were made that produced spectra with roughly as much εaturation aε the εpectra discussed with regard to examples herein, each pellet could contain ten to a hundred timeε more εample. The included preferred embodimentε are given by way of example only, and not by way of limitation to the invention, which is solely described by the claims herein. Variations obviouε to one εkilled in the art will be included within the invention defined by the claims. For example, it is to be understood that the present invention iε applicable to on-line analyεis of other compoundε having bands in different spectral regionε. It additionally iε applicable to analysis of a wide range of solid materials under both moving (e.g., remote on-line) and εtationary sampling conditions. Alεo, different types of energy sources can be substituted for a laser, if they meet the required criterion to produce the infrared emissions. It is also to be understood that the preεent invention could be applied in scanned image IR microscopy. Using this process, chemical compound concentrations can be imaged at higher resolution because a shorter wavelength focus beam can be used to excite a responεe at longer wavelength. This can be extremely advantageous in the infrared spectra where diffraction limitationε prevent high spatial resolution in conventional microscopy. The type of instrumentation used can be tailored to meet the specific measurement requirements. A spectrometer based on a number of filters and detectors which operate simultaneouεly can collect εpectral data 100% of the time from a flowing εtream of material. An FTIR spectrometer, on the other hand, can provide much more detailed spectral information but does not collect data at every inεtant. Detailε of the excitation method can alεo be selected to match the particular situation. If a laser is used, laser type (pulse vs. continuous), wavelength, power, beam size, and optical geometry can all be adjuεted to produce the optimum reεult, and yet a εingle set of- choices can have broad applicability. Likewise .the field of view of the spectrometer/detector and/or the times of detection can be controlled or selected. The examples reported herein successfully examined several disparate materials, yet were all done with a single laser, a single beam size, and a single optical geometry. It is also to be understood that the invention can be uεed with factor-analyεiε computer εoftware to predict a wide variety of physical and chemical material properties from infrared spectra obtained by the invention. Additionally, it is to be understood that the present invention can be used to measure infrared εpectra of discrete microsampleε or of microscopic areas of larger samples in a stationary scanned imaging mode by focusing the energy beam used for heating to a small spot size. Higher resolution can be obtained using this procesε because the diffraction limit associated with infrared wavelengths can be avoided. It is further to be understood that the present invention allowε adjustment of the sampling depth from which spectra and hence information is obtained from a material by either changing the relative velocity between the sample and heating source or by changing the relative positioning of the heating source and the spectrometer/detector field of view or by changing the detection window times of the detector to achieve different sampling depthε. For example, aεεuming a two layered εample, if the energy source applied heat energy to the surface of the sample directly within the spectrometer/detector field of view as discuεεed above, then the transient thermal emission of infrared radiation will come from only the upper layer of the sample so that the upper layer can be analyzed subεtantially free from any interference from the material of the underlying layer or base, if the energy source is then moved upstream on the εample flow εo that the field of view of the εpectrometer/detector then views a portion of the material a longer distance from the heating point, then the observed transient thermal emission of infrared radiation comes from both the upper layer, which haε been previously detected and which is an optically thin upper layer, and the lower or base material layer, and such combined emisεionε are detected. Since the thickness of the heated lower layer or base material is greater than the thickness of the optically thin heated upper layer, the spectrum of the lower layer or base material may dominate in the combined spectrum. However, by subtracting the spectrum detected at the first position from the combined spectrum detected at the second position, a spectrum indicative of the spectrum of the lower layer or base material is obtained which is subεtantially free of interferences of the upper layer. Similar approaches may be utilized to focus on specific components in more complicated structureε. Alεo, direct co pariεon of the spectra detected at the positions may be utilized to examine inhomogeneities. For example, if an additive designed to improve extrudability of a plastic can only perform properly if it tends to collect at the surface of the plastic, a direct spectral comparison can indicate whether or not the additive concentration is higher near the surface of the plastic. Furthermore, it is to be understood that the apparatus and method of the invention can sample considerably larger volumes of material than would be practical by other methods and can provide time for altering proceεsing system parameters by locating the meaεurement εyεtem of the invention a εufficient distance upstream from processing systems. Moreover, it iε to be understood that it may be possible in certain applications to selectively heat and thereby measure particular components in a material by tuning a heating laser absorption to that of a particular component. Additionally, it iε to be underεtood that the proceεεor and graphicε componentε of the invention can be those which are conventionally known to personε of skill in the art. Also appropriate software can be written to operate on the received data to derive the desired results of the invention. It is further to be understood that the present invention can be altered to vary the sample depth that is sensed by adjusting the timing or position of the heat source and the spectrometer observation. Depth profile samples can be compiled by acquiring spectra at different timings or posi ionings corresponding to sensing depths of interest, and then performing εpectral subtractions. Although photoacoustic methods of materials analysis can also vary the sample depth which is sensed, the present invention will provide more frequency and consequently more depth range for profiling, if a laser is used, because a laεer εource iε more readily functional and can provide the high power necessary for high frequency measurements when utilized in the present invention. Additionally, photoacoustic methods require that the εample material be εealed in a chamber εo that εuch methods are not applicable to moving materials.

Claims: Claims 1. A method for enabling analysis of a solid material comprising the steps of: applying energy to a surface region of the solid material sufficient to cause transient heating in a thin εurface layer portion of the solid material so as to enable transient thermal emisεion of infrared radiation from the thin εurface layer portion; and detecting εubstantially only the transient thermal emisεion of infrared radiation from the thin εurface layer portion of the εolid material, the detected transient thermal emission of infrared radiation being sufficiently free of self-absorption by the solid material of emitted infrared radiation, so as to be indicative of characteriεticε relating to molecular composition of the solid material. 2. A method according to claim 1, further comprising the step of determining characteristicε relating to the molecular compoεition of the εolid material in accordance with the detected tranεient thermal emiεεion. 3. A method according to claim 1, wherein the εtep of applying energy to a εurface region of the εolid material includeε utilizing one of a laεer, ion beam generator, electron beam generator and hot gas source, and hot roller to apply the energy. 4. A method according to claim 3, wherein the step of applying energy includes applying one of pulsed energy and continuous energy to the surface region of the εolid material. 5. A method according to claim 1, wherein the solid material is one of a stationary material and a moving material. 6. A method according to claim 5, wherein the step of detecting includes detecting the transient thermal emission for a predetermined period of time upon application of the energy to the surface region. 7. A method according to claim 5, wherein the material iε a moving material, and the εtep of detecting includes setting a field of view for detection, moving the material into the field of view upon application of the energy to the εurface region of the material εo as to enable detection of the transient thermal emisεion of infrared radiation from the εurface layer portion, and then moving the εurface layer portion tranεiently emitting the infrared radiation out of the field of view of detection. 8. A method according to claim 1, further compriεing the εtep of additionally detecting a combination of the tranεient thermal emiεsion of infrared radiation from both the thin surface layer portion of the solid material and an adjacent lower layer portion of the εolid material. 9. A method according to claim 8, further comprising the εtep of determining characteriεticε relating to the molecular composition of the solid material in accordance with the detected transient thermal emission from the thin surface layer portion and the detected transient thermal emission from the lower layer portion of the solid material. 10. A method according to claim 9, wherein the step of determining characteristics includes at leaεt one of comparing εpectra of the detected tranεient thermal emiεεionε from the thin εurface layer portion and the lower layer portion and εubtracting spectra indicative of the detected tranεient thermal emiεεion from the εurface layer portion from the spectra indicative of the detected combination of the transient thermal emisεion from the thin surface layer portion and the lower layer portion. 11. A method according to claim 1, wherein the εtep of detecting the tranεient thermal emiεεion includes utilizing spectrometer means for detecting. 12. A method according to claim 11, wherein the spectrometer means includes filter means for filtering out at least one predetermined wavelength. 13. A method according to claim 11, further comprising the step of determining characteristics relating to the molecular composition of the solid material in accordance with the detected transient thermal emission by utilizing processing means coupled to the spectrometer means for proσeεεing the detected tranεient thermal emission. 14. A method according to claim 13, wherein the processing meanε provideε an output indicative of the characteriεticε relating to the molecular compoεition of the solid material, and further comprising the step of displaying the output of the proceεεing meanε. 15. A method according to claim 13, further compriεing the εteps of controlling at least one of the applying of energy to the surface region and the detecting of the transient thermal emission by the processing means. 16. An apparatus for enabling analysiε of a εolid material compriεing: meanε for applying energy to a εurface region of the solid material sufficient to cause transient heating in a thin surface layer portion of the solid material so as to enable transient thermal emisεion of infrared radiation from the thin εurface layer portion; and means for detecting subεtantially only the transient thermal emisεion of infrared radiation from the thin surface layer portion of the solid material, the detected transient thermal emisεion of infrared radiation being sufficiently free of self-absorption by the solid material of emitted infrared radiation, so as to be indicative of characteristics relating to molecular composition of the εolid material. 17. An apparatuε according to claim 16, further compriεing means for determining characteristics relating to the molecular composition of the solid material in accordance with the detected transient thermal emiεεion. 18. An apparatuε according to claim 16, wherein the meanε for applying energy to a εurface region of the εolid material includeε one of a laser, ion beam generator, electron beam generator, hot gas source, and hot roller. 19. An apparatus according to claim 18, wherein the applying energy meanε includeε meanε for applying pulεed energy and continuouε energy to the εurface region of the solid material. 20. An apparatus according to claim 16, wherein the detecting means include means for detecting the transient thermal emission for a predetermined period of time upon application of the energy to the surface region. 21. An apparatuε according to claim 21, further compriεing meanε for moving the εolid material. 22. An apparatus according to claim 20, wherein the detecting means include meanε for setting a field of view for detection, the moving means moving the material into the field of view upon application of the energy to the surface region of the material by the applying energy means so as to enable detection of the transient thermal emisεion of infrared radiation from the εurface layer portion, the moving means thereafter moving the surface layer portion transiently emitting the infrared radiation out of the field of view of detection of the detecting means. 23. An apparatus according to claim 16, further comprising controlling means for controlling at leaεt one of the applying energy means and the detecting means. 24. An apparatus according to claim 23, further comprising means for moving the solid material, the controlling means controlling the moving means. 25. An apparatus according to claim 23, wherein the controlling means includeε proσeεεing means for determining σharacteristicε relating to the molecular composition of the solid material in accordance with an output of the detecting means and for providing an output indicative thereof. 26. An apparatus according to claim 25, further compriεing diεplay meanε for diεplaying the output of the processing means.

Inventors: JONES ROGER W, MCCLELLAND JOHN F, JONES, ROGER, W., MCCLELLAND, JOHN, F.
Applicants: JONES ROGER W, MCCLELLAND JOHN F, UNIV IOWA RES FOUND, IOWA STATE UNIVERSITY RESEARCH FOUNDATION, JONES, ROGER, W., MCCLELLAND, JOHN, F.
Application year: 1990
Main classification: G01N21
IPC codes: G01N21, G01N25