Thermoelectric material and sensor utilizing the same material

Abstract: In a fine structure of a thermoelectric material, fine particles of a material exhibiting Seebeck effect are electrically linked in a loosely contact state one another without fusing, having spaces formed at clearances among the fine particles. A method of manufacturing the thermoelectric material comprises a step of compacting fine particles made of a material exhibiting Seebeck effect through a cold pressing. Also, disclosed is a sensor for quantitatively sensing a substance, which comprises a pellet (5) of a powder thermoelectric material, where a temperature difference is generated between two points inside the piece of thermoelectric material. The sensor further includes thermocouples (8) connected to a heater plate (6) and a cooling plate (7), and a controller (9) which is electrically connected in the loop circuit of the thermocouples (8) for detecting thermoelectric current corresponding to the temperature difference, thereby to control the heating of the heater plate (6).

Description: The present invention relates to a sensor for measuring gas pressure, said sensor using a thermoelectric material; it further relates to a method of manufacturing said sensor. The Seebeck effect is a phenomenon by which, when a temperature difference arises between two different points of an electrically conductive material, an electromotive force is developed in proportion to the temperature difference. The electromotive force consists of two components, i.e., an electromotive force due to distribution of charge carrier density caused by a migration of charge carriers for keeping constant free energy of the charge carriers within the material, and another electromotive force due to an interaction between charge carriers and heat flow, namely, phonon flow from higher- to lower-temperature portions. These charge carriers and phonons behave differently between a surface portion (up to about 10 nm (100 Å) in depth) and an internal portion of the material. Therefore, thermoelectric effects of the material such as thermal electromotive force (i.e., Seebeck electromotive force) are different between its surface portion and its internal portion. For thin films having a thickness on the order up to 1 µm or fine particles having a diameter on the order up to 1 µm, the surface portion (interface portion) occupies larger part relative to the internal portion so that the thermoelectric effect in the surface portion can no longer be an ignorable phenomenon. Generally, a surface of a thermoelectric material has atmospheric gasses such as oxygen, nitrogen, steam, carbon dioxide and the like adsorbed thereon, which, it is considered, would affect the thermoelectric effects on the surface portion (interface portion). Obeying Langmuir's adsorption isotherm, the adsorped amount of atmospheric gas increases with increasing pressure of the atmospheric gas so that the higher the pressure of the atmospheric gas, the greater the change in the thermal electromotive force. Thermoelectric materials capable of interconversion between temperature difference and electromotive force have been manufactured heretofore, as bulk materials, by melt-casting a thermoelectric material or calcining a powdery material at a high temperature, and their resulting fine structures are shown in Figs. 6 and 7, respectively. Referring to Figs. 6 and 7, in the conventional material prepared by melt-casting a thermoelectric material, crystal grains are arranged densely and continuously although a small amount of cracks 13 and pores 14 may exist as shown in Fig. 6. In the conventional powder product calcined at a high temperature, the particles are partly fused and electrically linked to each other as shown by reference numeral 15 in Fig. 7. On the other hand, there can be obtained thin-film type thermoelectric materials formed through deposition on a glass substrate or organic membrane or other methods. In either case of the conventional methods, thermoelectric materials have been provided in a structure as dense as possible so that their electrical conductivity and mechanical strength as a bulk material would be enhanced for increased practicability. Sensors using such thermoelectric materials are in most cases implemented by making use of a function of detecting temperature difference. Examples of such sensors include temperature sensors or electric thermometers incorporating alloy thermocouples such as iron/gold - chromel and alumel - chromel thermocouples. An example of the sensors can be found in an application for detecting the extinction of a pilot burner of a boiler by sensing the change in temperature difference. Although there have already been developed semiconductor sensors using changes in electric resistance and sensors using enzyme electrode reactions as gas-pressure sensors or specified-substance oriented sensors, no sensors have been proposed yet to which any thermoelectric phenomenon has been applied. Conventional thermoelectric materials have been used as materials for converting temperature difference into electricity due to thermal electromotive force or for obtaining temperature difference by conducting an electric current through the materials. As for sensors using these materials which have been applied only for detecting temperature difference, there is a difficulty in measuring other physical quantities such as gas pressure, concentration of a substance in a solution. Document FR-A-2 532 786 discloses a cold pressed thermoelectric powder material exhibiting the Seebeck effect. The particles of the material are small crystallites which are spaced from each other, leaving an open space between their boundaries. This reference also discloses (Bi,Sb) 2 Te 3 as a thermoelectric material, the relative quantities of Bi and of Sb being 0 to 100 %. Document EP-A-0 335 213 discloses a method for producing thermoelectric elements. It is reported that thermoelectric elements with excellent thermoelectric characteristics such as Seebeck coefficient thermoelectromotive force and thermal conductivity can be produced by molding a powder of metal or metal alloy as a raw material and then sintering. Ultrafine powder material is obtained containing Fe and Si as main components and having a main particle diameter of 50 to 5000 Angström. It is the object of the present invention to provide a novel sensor allowing measurement of gas pressure. This object is obtained by a sensor for measuring gas pressure as defined in claim 1 and by a method for manufacturing a gas sensor element as defined in claim 12. Preferred and suitable embodiments of the sensor and of the method are described in the respective subclaims. The thermoelectric material of the sensor of the present invention comprises a structure such that fine particles of a material exhibiting the seebeck effect are electrically linked to one another in a loosely contact state. The term "loosely contact state" means a semipressure-contact state of fine particles being electrically linked without fusing and having spaces formed at clearances between any adjacent fine particles when formed in a cold pressing or sintering process. The method according to the present invention comprises the step of compacting fine particles exhibiting the Seebeck effect through cold press forming. Further, the sensor for measuring gas pressures according to the present invention comprises: a pellet of a thermoelectric material in which fine particles exhibiting the Seebeck effect are electrically linked to one another in a loosely contact state with spaces formed at clearances between adjacent fine particles; means for generating a temperature difference between two points inside the pellet of the thermoelectric material; and means for sensing a thermal electromotive force generated between the two points in the material. The fine particles constituting the theromoelectric material may be combined of coated with a material exhibiting selective adsorption or reaction. In the above structure, the material having adsorption specificity or reaction specificity is preferably formed in a film-like state mainly on the surface of the particles of thermoelectric material. According to a feature of the present invention, gas pressure and concentration of a specific substance can be detected correctly in a form of voltage using a thermoelectric material in which fine particles of a material exhibiting the Seebeck effect are electrically linked to one another in a loosely contact state with spaces formed at clearances between adjacent fine particles. According to another feature of the present invention, the arrangement of the sensor for measuring gas pressure, particularly when lower than the air pressure, detects a change in the thermal electromotive force when the gas pressure of the atmosphere is varied, thereby to measure the atmospheric gas pressure. Furthermore, by constituting a sensor by combining various types of materials exhibiting selective adsorption or reaction along with a thermoelectric material, the concentration of a specific substance in a solution or an atmosphere can be detected. When thermoelectric materials for use in sensors are so constructed that fine particles of thermoelectric materials are electrically linked partially to one another in a loosely contact state, high sensitivities of the sensors can be enhanced. Obeying Langmuir's adsorption isotherm, the amount of adsorbed atmospheric gas increases with increasing the pressure of the atmospheric gas. Therefore, the higher the pressure of the atmospheric gas, the greater the change in the thermoelectric force. Further, when the thermoelectric material is combined with various types of materials exhibiting selective adsorption or reaction, adsorption of a specific substance onto the material will cause a change in the thermal electromotive force generated in the material. Furthermore, the cold-pressed thermoelectric material according to the present invention is more strongly affected by the adsorbed gas since charge carriers and phonons traverse the adsorption layer. The objects and features of the description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a cross sectional view of a fine structure of a thermoelectric material according to the present invention; Fig. 2 is a graph representing the distribution of Seebeck coefficient of the thermoelectric material with respect to an atmospheric gas pressure according to the present invention; Fig. 3 is a schematic perspective view of a sensor for detecting a gas pressure or a specific substance according to the present invention; Fig. 4 is a flow chart showing a procedure of producing a sensor for quantitatively detecting a specific substance in an atmospheric gas; Fig. 5 is a cross sectional view of a fine structure of a thermoelectric material having Fe-Mn-Si particles coated with a thin-film like polyacrylamide gel including enzyme; Fig. 6 is a cross sectional view of a fine structure of a conventional melt-casted thermoelectric material; and Fig. 7 is a cross sectional view of a fine structure of a conventional powder-calcined thermoelectric material. The following describes preferred embodiments of the present invention with reference to the accompanying drawings. First, a method suitable for producing the thermoelectric material will be explained. In the first step, specified amounts of raw materials of Fe, Mn and Si each having a purity of not less than 3N (> 99.9%) were melted in an arc furnace and then crushed. In this case, Fe and Si are used as basic materials while Mn is used as a doping material. After repeating the cycle of melting/crushing process three times, the composition of the resultant material was analyzed and the ratio of the raw materials was adjusted to obtain a composition of Fe 0.985 Mn 0.015 Si 2 . Next, in order to obtain a β-phase, the material having a composition of Fe 0.985 Mn 0.015 Si 2 was put into a vacuum sealed glass ample and annealed at 850 °C for eight hours in the glass ample. Then the annealed material was milled in a planetary ball mill for five minutes under an Ar gas atmosphere, and shifted through a mesh to obtain a powder material consisting of particles each having a size of about 5 µm in diameter. Then, the powder material was molded by cold pressing under a pressure in the range of 0.1 to 0.3 MPa (1000 to 3000 kg/cm 2 ) at room temperature so that the powder material was pelletized. A small amount of ethanol or acetone was added to the powder to enhance the rigidity of the pellets. That is, ethanol or acetone serves as a lubricant to remove cracks or the like in the pellets. Then, the Seebeck coefficients for the pellets of the resultant material were measured under various pressures of atmospheric gas. Although Mn was used as a doping material, in this case, other materials such as Co, Cr and Al may be used as a doping material. Fig. 1 shows the fine structure of the thermoelectric material obtained by the above mentioned processes. Each particle of the thermoelectric material having a composition of Fe 0.985 Mn 0.015 Si 2 coheres together in a loosely contact state without fusing so that atmospheric gas can be adsorbed onto a surface 3 and fed to be adsorbed to a loosely contact part 4 through a cavity 2. That is, substantially no solid or liquid material exists in cavity 2. Seebeck coefficients of the pellets of the which the conditions of atmospheric gas (i.e., pressure, species of the gas) can be adjusted. Fig. 2 shows the distribution of Seebeck coefficients depending on the atmospheric gas pressure when measured under Ar and He gaseous atmospheres. Seebeck coefficient was found to largely depend upon the pressure of atmospheric gas, particularly in a range of the pressure below 0.01 MPa (100 mmHg). Also, the Seebeck coefficient was found to be different between species of He and Ar gaseous atmospheres even at the same pressure. Therefore, the thermoelectric material appears to have a function of identifying species of atmospheric gas to some degree. The following describes an arrangement of a sensor for detecting gas pressure with reference to Fig. 3, which is assembled by using a thermoelectric material according to the present invention. Referring to Fig. 3, a pellet 5 is provided with a rectangular shaped plate having dimensions of 2 mm x 2 mm x 10 mm, which is made of the thermoelectric material having a composition of Fe 0.985 Mn 0.015 Si 2 . A heater 6 is mounted to one end of the pellet 5 for heating, while a cooling plate 7 serving as a heat sink is attached to the other end of the pellet 5 for discharging heat. Electric current is applied from a power source (not shown) to the heater 6 for heating through a controller 9. The applied electric current is controlled by the controller 9 in such a manner that the temperature difference between the both ends of the pellet 5 is kept constant at a difference value of 10°C by providing 8 is connected to the heater plate 6 and the cooling plate 7, where the controller 9 is electrically connected in the loop circuit of the thermocouples 8 to control the heating of the heater 6 by detecting thermoelectric current corresponding to the temperature difference. A pair of electrodes 10A and 10B, each of which is made of silver paste, are provided in the both end portions of the pellet 5 for measuring thermal electromotive force developed in the pellet 5, where the electrode 10A is positioned in the left side in the figure corresponding to the heater plate 6 while the electrode 10B is positioned in the right side in the figure corresponding to the cooling plate 7 as shown in Fig. 3. The voltage due to the thermal electromotive force in accordance with the temperature difference is measured by means of a voltmeter 12 which is electrically connected across the both electrodes 10A and 10B via silver lead wires 11. Ar gas was introduced into a chamber or container and it was repeatedly confirmed that the Seebeck coefficients correspond to the gas pressures in a one-by-one manner. Next, the following describes a method of producing a sensor for quantitatively detecting a specific substance in an atmospheric gas with reference to a flow chart shown in Fig. 4. It is to be noted here that the basic structure of the sensor for detecting a specific substance is the same as that of the gas-pressure sensor shown in Fig. 3. As shown in Fig. 4, a powder thermoelectric material having a composition of Fe 0.985 Mn 0.015 Si 2 was previously obtained by the production method as described before, which material was processed into granules each having a size of 5 to 10 µm in diameter. Then, in order to obtain a material exhibiting selective adsorption or reaction, formaldehyde dehydrogenase enzyme was mixed into polyacrylamide sol and further mixed with the above processed powder thermoelectric material of Fe 0.985 Mn 0.015 Si 2 , thereby to obtain a slurry state, where the polyacrylamide sol is used as a supporting member. The mixed slurry material was atomized in a dried Ar gas atmosphere so that the polyacrylamide sol was gelatinizated and then excessive solvent was removed by evaporation. In the resultant powder material obtained by the process mentioned above, there was contained a thin-film like polyacrylamide gel including enzyme, which was formed on the surface of the Fe 0.985 Mn 0.015 Si 2 particles, and the film thickness was 0.5 - 5.0 µm as shown in Fig. 5. Referring to Fig. 5, the particles 51 of the powder material of Fe 0.985 Mn 0.015 Si 2 were coated with a thin-film like polyacrylamide gel 52 including enzyme. In this fine structure of the powder material, there were formed spaces 53, i.e., cavities among the particles. Then the obtained powder material was dispersed in an organic solvent and formed on a glass substrate through a printing method. In the same manner as that for the gas-pressure sensor, a temperature difference of 10°C was kept constant and the variation in thermal electromotive force was traced. The measurement values of electromotive force for atmospheres of Ar gas containing formaldehyde of 1 ppm, 10 ppm and 100 ppm were respectively 2350 µV, 2550 µV and 2700 µV, which were increased by 300 µV, 500 µV and 650 µV respectively comparing to the value (2050 µV) for pure Ar gas. The foregoing example is an application of enzyme reaction for sensing the concentration of a specific substance in a gas atmosphere. By using other enzyme systems or antigen-antibody reactions as reactions having specificities, concentration of a specific substance in a solution can also be detected and a wide variety of detectable substances can be selected. As a method for supporting these materials having adsorption specificities, there can be used a cellulose membrane. Another method can be also adopted by fixing antibodies or enzymes directly on the surface of fine thermoelectric particles through a chemical reaction. On the other hand, although the sensitivity was not very high, gas pressures and concentrations of specific substances could be detected by using a thin film of approximately 1 µm thick instead of fine thermoelectric particles. As described hereinabove, according to the sensing material in the embodiment, the material has a fine structure in cross section in which fine particles 1 cohere together in a loosely contact state without fusing and an adsorption of atmospheric gas or of a specific substance through an enzyme reaction occurs on the surface 3 and the loosely contact part 4 through the cavity 2 as shown in Fig. 1. When a substance is adsorbed on the surface (the interface), the thermal electromotive force changes depending upon the amount of adsorption and then the system acts as a sensor. Accordingly, a gas pressure and a concentration of a specific substance can be detected in a form of a voltage by using thermoelectric materials. As described above, according to the present invention, gas pressures and concentrations of specific substances can be detected correctly in a form of voltage by using a thermoelectric material of the present invention by virtue of the structure that fine particles of a material exhibiting the Seebeck effect are electrically linked in a loosely contact state while spaces are formed at clearances among the particles. The sensor of the present invention is very profitable in the industry since gas pressures or concentrations of specific substances can be detected as voltage values. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention as defined by the appended claims, they should be construed as included therein.

Claims: A sensor for measuring gas pressure comprising a pellet (5) of a powder of a thermoelectric material having a structure in which fine particles (1) of a material exhibiting the Seebeck effect and made by cold pressing are electrically linked to one another in a loosely contact state without fusing, with cavities (2) being formed between the fine particles (1) through which atmospheric gas can be fed to be absorbed at loosely contact parts (4) of said fine particles (1); means (6, 7, 8, 91 for generating a temperature difference between two points inside said pellet of the thermoelectric material; and means (10A, 10B, 11, 12) for sensing a thermal electromotive force generated between the two points inside said pellet. The sensor of claim 1, wherein said thermoelectric material has the composition Fe 0.985 Mn 0.015 Si 2 and has the shape of a rectangular plate. The sensor of claim 1 or 2, wherein said means for generating a temperature difference includes a heater plate (6) attached to one end of said rectangular plate (5) and a cooling plate (7) attached to the other end of said rectangular plate (5). The sensor of anyone of the preceding claims, wherein said means for generating a temperature difference further includes control means (9) for controlling the temperature difference between said two points to be kept constant. The sensor of claim 4, wherein said means for generating a temperature difference further includes thermocouples (8) connected to the heater plate (6) and the cooling plate (7), and wherein said control means (9) is electrically connected in the loop circuit of said thermocouples (8) for detecting a thermoelectric current corresponding to the temperature difference, thereby to control heating of the heater plate (6). The sensor of anyone of the preceding claims, wherein said means for sensing a thermal electromotive force includes first and second electrodes (10A, 1 OB) made of silver paste which are respectively provided at the end portions of said thermoelectric plate (5), said first electrode (10A) being located in a position corresponding to the heater plate (6) while said second electrode (10B) being located in a position corresponding to the cooling plate (7). The sensor of claim 6, wherein said means for sensing a thermal electromotive force further includes a voltmeter (12) which is electrically connected across said first and second electrodes (10A, 10B) via silver lead wires (11) for measuring a voltage due to the thermal electromotive force in accordance with the temperature difference. The sensor of anyone of the preceding claims, wherein said fine particles (1) are coated with a material exhibiting selective adsorption for the gas pressure. The sensor of claim 8, wherein said coating material exhibiting selective adsorption is an enzyme. The sensor of claim 9, wherein said enzyme is formaldehyde dehydrogenase mixed into polyacrylamide sol. The sensor of anyone of claims 8 to 10, wherein said coating material is formed in a film-like state mainly on the surface of said fine particles (1). A method for manufacturing a sensor element for quantitatively detecting a specific substance contained in an atmospheric gas, characterized by the steps of a) producing a thermoelectric powder material having a composition of Fe 0.985 Mn 0.015 Si 2 by a method wherein fine particles of a material exhibiting the Seeback effect are compacted by cold pressing, said fine particles being made by the steps of melting specified amounts of raw materials of Fe, Mn and Si each having a purity of not less than 99,9 % in an arc furnace to be then crushed; repeating the melting/crushing step three times; analyzing the composition of the resultant material; and adjusting the ratio of the raw materials to derive a composition of Fe 0.985 Mn 0.015 Si 2 ; b) processing the resultant thermoelectric powder material into granules having a size of 5 to 10 µm in diameter; c) mixing an enzyme exhibiting selective adsorption towards said specific substance into a polyacrylamide sol and mixing the sol with the processed thermoelectric powder material of Fe 0.985 Mn 0.015 Si 2 to obtain a slurry; d) atomizing the slurry in a dried argon gas atmosphere to gelatinizate the polyacrylamide sol and removing excessive solvent by evaporation; e) dispersing the obtained gel material in an organic solvent and forming the dispersion obtained on a glass substrate by printing The method of claim 12, characterized in that said enzyme is formaldehyde dehydrogenase and said specific substance is formaldehyde. The method of claim 12 or 13, characterized in that in step d) a thin film-like polyacrylamide gel including enzyme is formed on the upper surface of the Fe 0.985 Mn 0.015 Si 2 particles having a film thickness adjusted to 0,5 to 5,0 µm.

Inventors: GYOTEN HISAAKI, NAKAGIRI YASUSHI, YAMAMOTO YOSHIAKI, GYOTEN, HISAAKI, NAKAGIRI, YASUSHI, YAMAMOTO, YOSHIAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01L35
IPC codes: G01N27, H01L35, G01K7