Method of producing a NTCR sensor

Abstract

The present invention relates to a method of producing a negative temperature coefficient resistor (NTCR) sensor, the method comprising the steps of: providing a mixture comprising uncalcined powder and a carrier gas in an aerosol-producing unit, with the uncalcined powder comprising metal oxide components; forming an aerosol from said mixture and said carrier gas and accelerating said aerosol in a vacuum towards a substrate arranged in a deposition chamber; forming a film of the uncalcined powder of said mixture on said substrate; and transforming the film into a layer of spinel-based material by applying a heat treatment step.

Claims

1. A method of producing a negative temperature coefficient resistor (NTCR) sensor, the method comprising: providing a mixture comprising uncalcined powder and providing a carrier gas in an aerosol-producing unit, the uncalcined powder comprising metal oxide components; forming an aerosol from the mixture and the carrier gas and accelerating the aerosol in a vacuum towards a substrate arranged in a deposition chamber; forming a film of the uncalcined powder of the mixture on the substrate; and transforming the film into a layer of spinel-based material by applying a heat treatment step.

2. The method in accordance with claim 1, wherein the heat treatment step is applied at a temperature below 1000 C.

3. The method in accordance with claim 2, wherein the heat treatment step is applied at a temperature in the range of 600 C. to 1000 C.

4. The method in accordance with claim 1, wherein the heat treatment step takes place in an atmosphere, wherein said atmosphere has a controlled partial oxygen pressure.

5. The method in accordance with claim 4, wherein the heat treatment step is applied at a temperature in the range of 780 C. to 1000 C.

6. The method in accordance with claim 1, wherein the carrier gas is selected from the group consisting of oxygen, nitrogen, a noble gas, and combinations thereof.

7. The method in accordance with claim 1, wherein the uncalcined powder comprises particle sizes in the range of 50 nm to 10 m.

8. The method in accordance with claim 1, wherein the layer of spinel-based material comprises a spinel composed of two or more cations from the group consisting of Mn, Ni, Co, Cu, Fe, Cr, Al, Mg, Zn, Zr, Ga, Si, Ge and L.

9. The method in accordance with claim 8, wherein the layer of spinel-based material comprises the chemical formula M.sub.xMn.sub.3-xO.sub.4, M.sub.xM.sub.yMn.sub.3-x-yO.sub.4, and M.sub.xM.sub.yM.sub.zMn.sub.3-x-y-zO.sub.4, wherein M, M and M are selected from the group consisting of Ni, Co, Cu, Fe, Cr, Al, Mg, Zn, Zr, Ga, Si, Ge and Li, and wherein the uncalcined powder comprises compounds of at least one of M, M, or M.

10. The method in accordance with claim 1, wherein the uncalcined powder comprises at least two different metal oxide components.

11. The method in accordance with claim 1, wherein the mixture comprises at least one filling material component.

12. The method in accordance with claim 1, further comprising forming at least one of a further layer or a structure on at least one of the substrate, the film before applying the heat treatment step, or the layer of spinel-based material.

13. The method in accordance with claim 12, further comprising the step of sintering the at least one of the further layer or the structure, wherein said heat treatment step is applied as a single heat treatment for transforming the film into the layer of spinel-based material and for sintering the at least one of the further layer or the structure.

14. The method in accordance with claim 12, wherein the at least one of the further layer or the structure is selected from the group consisting of an electrode, an electrically conducting layer or structure, an electrically insulating layer or structure, a protective film, a thermally conducting layer, and combinations of the foregoing.

15. The method in accordance with claim 12, wherein said at least one of the further layer or the structure is applied using at least one of a thick film technology, a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) process, a plasma-enhanced chemical vapor deposition (PECVD) process, a sol-gel process, or a galvanization process.

16. The method in accordance with claim 15, wherein the at least one of the further layer or the structure is structured by at least one of a laser beam, an electron beam, a sand jet, or a photolithographic process.

17. The method in accordance with claim 1, further comprising introducing at least one mask into the deposition chamber, the at least one mask being arranged between the aerosol-producing unit or the substrate.

18. A method in accordance with claim 1, further comprising adapting a resistance of the NTCR sensor by changing a size of the film formed on one of the substrate or of the layer of spinel-based material.

19. The method in accordance with claim 18, wherein, the change in size is effected by a mechanical trimming processes.

20. The method in accordance with claim 1, wherein the aerosol-producing unit comprises a nozzle via which the aerosol is accelerated towards the substrate, wherein the forming the film on the substrate comprises moving the substrate and the nozzle relative to one another to define an extent of the film.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

(1) Further embodiments of the invention are described in the following description of the Figures. The invention will be explained in the following in detail by means of embodiments and with reference to the drawing in which is shown:

(2) FIG. 1 a schematic view of an apparatus for forming NTCR sensors in accordance with the invention;

(3) FIG. 2 a schematic drawing highlighting the method steps used during a first embodiment of the invention;

(4) FIG. 3 a schematic drawing highlighting the method steps used during a second embodiment of the invention;

(5) FIG. 4 a schematic drawing highlighting the method steps used during a third embodiment of the invention;

(6) FIG. 5 an SEM image of the fractured surface of a NiOMn.sub.2O.sub.3 composite film on an Al.sub.2O.sub.3 substrate;

(7) FIG. 6 a photograph of two NTCR sensors after the completion of the third method step of the embodiment of the invention described in connection with FIG. 2;

(8) FIG. 7 an SEM image of the fractured surface of an NTCR sensor from FIG. 6 which is temperature-treated at 850 C.;

(9) FIGS. 8a and b the electrical characterization of the two NTCR sensors of FIG. 6, with FIG. 8a showing the .sub.25 specific resistance in dependence on temperature and FIG. 8b showing the B-constant of each sensor;

(10) FIGS. 9a and b the .sub.25 specific resistance (FIG. 9a) and the B-constant (FIG. 9b) of an NTCR sensor formed by means of the process described in connection with FIG. 2, both in dependence on tempering temperature;

(11) FIGS. 10a and b graphs similar to those of FIGS. 9a and 9b, but for an NTC resistor using a prior art method;

(12) FIG. 11 a drawing showing the measurement and tempering temperature cycle used to obtain FIGS. 9 and 10; and

(13) FIG. 12 an XRD spectrum of an NTCR sensor formed by means of the process described in connection with FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(14) In the following, the same reference numerals will be used for parts having the same or equivalent function. Any statements made having regard to the direction of a component are made relative to the position shown in the drawing and can naturally vary in the actual position of application.

(15) The principle of aerosol-based and vacuum-based cold deposition of NTCR sensors 17 (see FIG. 2) will be explained in the following with reference to FIG. 1. FIG. 1 shows an apparatus 1, in which a substrate 2 is provided. A mixture 3 of powder 8 and a carrier gas 9 is deposited as an aerosol 9 on the substrate 2 in a deposition chamber 4. The apparatus 1 can be evacuated using an evacuation apparatus 5, such as a vacuum pump or a system of vacuum pumps.

(16) An aerosol-producing unit 6 comprising the mixture 3 is connected to the deposition chamber 4. The mixture 3 is directed and accelerated towards the substrate 2. The acceleration of the mixture 3 is brought about as a consequence of the pressure difference between the aerosol-producing apparatus 6 and the evacuated deposition chamber 4. The mixture 3 is accelerated solely due to the applied vacuum and not because of any external fields, such as magnetic or electric fields. The mixture 3 is transported from the aerosol-producing unit 6 via a suitable nozzle 7 into the deposition chamber 4. The mixture is accelerated further due to the change in the cross-section of the nozzle 7. In the deposition chamber 4, the mixture 3 impacts the moving substrate 2 and forms a dense, scratch-resistant film there.

(17) The mixture 3 is composed of an uncalcined powder 8. This is significantly different to the prior art, where a calcined powder is ground prior to deposition on a substrate. The uncalcined powder 8 is then mixed with a carrier gas 9 (e.g. oxygen, nitrogen or a noble gas) in the aerosol-producing unit 6 such that the mixture 3 of powder 8 and aerosol 9 is formed.

(18) In this connection it should be noted that an uncalcined powder 8 relates to a powder of the individual metal oxide compounds 9.1, 9.2, 9.3, . . . 9.x used to form the NTCR sensors 17 (see FIG. 2). This uncalcined powder 8 has not been sub-jected to a heat treatment step during which a ceramic form of the desired composition of the NTCR sensor 17 is produced.

(19) The powder 8 in this respect in accordance with FIG. 1 comprises x powdery components 9.1, 9.2, 9.3, . . . 9.x (where x2) selected from the group of metal oxides. Thus, 9.1 denotes a first metal oxide component, 9.2 a second metal oxide component, 9.3 a third metal oxide component and 9.x an x.sup.th metal oxide component. The metal oxide powder 9.1, 9.2, 9.3, . . . 9.x typically has particle sizes selected in the range of 50 nm to 10 m.

(20) Due to the pressure difference between the aerosol-producing unit 6 and the deposition chamber 4, the particles 9.1 . . . 9.x (metal oxide component 1 . . . x) and the carrier gas 9 of the mixture 3 are transported via the nozzle 7 into the deposition chamber 4 and are accelerated towards the substrate 2. The particles 9.1 . . . 9.x and the carrier gas 9 of the aerosol 9 impact on the substrate 2 and form a firmly adhering, scratch-resistant composite film 10 on the substrate 2.

(21) In order to increase a surface area of the composite film 10 formed on the substrate 2, the substrate 2 is moved relative to the nozzle 7 in the x-direction and/or the y-direction. The spatial directions X, Y and Z are also indicated in FIG. 1.

(22) FIG. 2 shows a schematic drawing highlighting the method steps used during a first embodiment of the invention. In the first step of the method, a powder mixture 8 which is composed of x metal oxide components (where x2) is deposited on the substrate 2 (e.g. formed of Al.sub.2O.sub.3 or AlN) by means of an aerosol-based and vacu-um-based cold composite deposition process (as schematically described in connection with FIG. 1). The metal oxide components 9.1 to 9.x of the mixture 3 can comprise elements, such as Ni, Mn, Co, Cu or Fe.

(23) In this connection, it should be noted that the components are starting metal ox-ides of a composite that can be transformed preferably into a spinel-structure, i.e. into preferably a cubic crystal system well known for compositions comprising Mn. The spinel structure, i.e. the cubic structure of the composition, is not yet present in this starting material and is formed during the application of the subsequent method.

(24) The deposition is based on the fact that the powder mixture 8 is accelerated by means of the combination of the aerosol 9 and the vacuum present in the deposition chamber 4. The particles of the metal oxide component 9.1, the metal oxide component 9.2, the metal oxide component 9.3, . . . the metal oxide component 9.x, and the carrier gas 9 are directed via the nozzle 7 onto the substrate 2. On impact at the substrate 2, the particles 9.1, 9.2, 9.3 . . . 9.x break open, bond with one another and with the substrate 2, without changing their crystal structure in this respect, and form the firmly adhering composite film 10.

(25) Subsequently, in the second step of the method, two further layers 1 1 are applied on the composite film 10. In the present instance they are intended to form two electrode structures 12 that are applied to the surface of the composite film 10 by means of an appropriate film technology, e.g. by screen printing or stencil printing of conductive paste 11 on the composite film 10 of composite material.

(26) In the subsequent third method step, the composite film 10 having the conductive paste 11 present thereon is heat treated in a heat treatment step. The heat treatment step is carried out at a temperature below 1000 C., preferably in the range of 600 C. to 1000 C., in particular in the range of 780 C. to 1000 C., particularly preferably at 850 C. to 1000 C. The temperature depends on the desired composition of the layer 13 of spinel-based material. During this heat treatment step, several processes take place simultaneously.

(27) In this connection, it should be noted that the heat treatment step takes place in an atmosphere, such as air. Alternatively, the heat treatment step can also be carried out using an atmosphere having a controlled partial oxygen pressure.

(28) During this heat treatment step, two significant effects are achieved. On the one hand, the screen-printed conductive paste 11 is sintered forming the electrode structures 12 and, on the other hand, the metal oxides, e.g. oxides of Ni, Mn, Co, Cu or Fe, of the composite film 10 are crystallized into a common spinel structure, i.e., the film of composite materials is transformed into a layer 13 of spinel-based material.

(29) Generally speaking, a composition of the film 10 of composite material and of the subsequently formed layer 13 of spinel-based material is described for example by one of the following chemical formulas M.sub.xMn.sub.3xO.sub.4, M.sub.xM.sub.yMn.sub.3xyO.sub.4, and M.sub.xM.sub.yM.sub.zMn.sub.3xyzO.sub.4, where M, M and M are selected from the group of members consisting of Ni, Co, Cu, Fe, Cr, Al, Mg, Zn, Zr, Ga, Si, Ge and Li. In order to ensure this, the uncalcined powder comprises compounds of at least one of M, M and M. In this connection it should be noted that x, y and z can be any number between and including 0 and 3.

(30) On the other hand, the heat treatment affects grain growth and, at a moderate cooling rate, a reduction of the film strains such that an NTCR behavior of the NTCR sensor 17 is achieved which has long-term stability. The NTCR behavior is a consequence of the spinel-structure of the composition.

(31) Thus, the step of transforming said composite film 10 into said layer 13 of spinel-based material comprising the heat treatment step simultaneously transforms the at least one further layer, e.g. the two the screen-printed portions of conductive paste 11 into two electrode structures 12, while also forming the spinel-structure.

(32) The NTCR sensor 17 formed comprises the substrate 2, a spinel-based layer 13 and the sintered electrode structures 12. Alternatively to the thick film technology in the second method step, one or more electrodes and/or electrode structures 12 can also be applied to the spinel-based layer 13 using a PVD process, such as sputtering or evaporation. If the electrodes or electrode structures 12 are directly formed, then they can be applied after the heat treatment of the composite film 10.

(33) The electrodes or electrode structures 12 can optionally be structured by means of lasers or in a photolithographic manner.

(34) The NTCR sensors 17 work as desired due to the spinel structure of the layer 13 of spinel-based material. Without the transformation of the starting material to the spinel-based structure (see e.g. FIG. 12 in this connection), the desired properties of such NTCR sensors 17 would not be obtained.

(35) FIG. 3 shows a schematic drawing highlighting the method steps used during a second embodiment of the invention (NTCR sensor 18). In contrast to the embodiment shown in FIG. 1 an electrode or an electrode structure 12 is provided on the substrate 2 prior to the formation of the composite film 10. The electrodes or electrode structures 12 are applied to the substrate 2, e.g. with the aid of a PVD process (e.g. evaporation, sputtering), thick film technology, a galvanization process or similar and are optionally structured by means of a laser beam or an electron beam or a photolithographic process (not shown).

(36) In the second step, aerosol-based and vacuum-based cold composite deposition takes place, optionally using a suitable mask 14 (one-way stencils/multiway stencils, sacrificial material, etc.).

(37) Subsequently, a temperature treatment of the composite film 10 at temperatures up to 1000 C. takes place in the third step such that the desired spinel structure is formed and process-related film strains and grain boundaries are reduced.

(38) A subsequent trimming of the layer 13 of spinel-based material is possible, e.g. by means of a laser beam or an electron beam, to set the resistance value of the created spinel-based layer 13 in an exact manner.

(39) FIG. 4 shows a schematic drawing highlighting the method steps used during a third embodiment of the invention (NTCR sensor 19). The starting point is a conductive substrate or a substrate that is provided with a conductive film or electrode 12. The latter can, in analogy to FIG. 3, be applied e.g. by a PVD process, a CVD process, a PECVD process, thick film technology, a galvanization process, a sol-gel process or similar and can optionally be structured by means of a laser beam or an electron beam or in a photolithographic manner.

(40) In the second step, a composite film 10 is deposited onto this electrode or electrode structure 12 with the aid of the aerosol-based and vacuum-based cold composite deposition of a powder mixture 8.

(41) The powder mixture 8 in this respect not only comprises x metal oxide components (where x2) that form the later spinel-based layer 13, but also filler material components 15. The latter can indeed likewise belong to the group of metal oxides such as Al.sub.2O.sub.3, but are not installed into the spinel lattice, which is active with respect to NTCR, and thus serve to set/increase the resistance value in the later so-called sandwich structure.

(42) The powder mixture 8 is, as described in FIG. 1, mixed with the carrier gas 9 for the purpose of acceleration. The particles of the aerosol, i.e. the particles of the metal oxide component 1, 2, . . . x 9.1, 9.2 . . . 9.x, as well as the filling material particles 15, move out of the nozzle 7 at a higher speed and impact onto the electrode or electrode structure 12 located on the substrate 2. Suitable particles in this respect break open, deform plastically and form a firmly adhering, scratch-resistant composite film 10.

(43) It should be noted that the filling materials 15 can also be inactive with respect to the material of the layer 13 of spinel-based material of the NTCR sensor 19, such as Al.sub.2O.sub.3, and are included in addition to the starting metal oxides of the spinel.

(44) On the other hand, the filling material 15 can be a dopant material of the oxide material used to form the spinel-based structure. Such a dopant material can lead to improved or desired characteristics of the spinel-based layer 13 of the NTCR sensor 19.

(45) A conductive paste 11 is applied to the surface of the composite film 10 by means of thick film technology in the next step.

(46) In the subsequent temperature treatment step that takes place up to 1000 C., the sintering of the conductive paste 11, as well as the reduction of film strains and grain boundaries and the crystallization of some of the composite film 10 components in a common spinel structure take place simultaneously. The remaining part, this means the filling material grains 16 in the film, are present unchanged after the temperature treatment. Alternatively to thick film technology, the electrode 12 can also be applied subsequently, that is after the temperature treatment, by a PVD process such as sputtering or evaporation.

(47) The structure created in this manner on the substrate 2 comprises an electrode 12, the spinel-based layer 13 and the further electrode 12 to form a so-called sandwich structure. The filling material grains 16, which are present distributed finely in the spinel-based layer 13, form a simple possibility of raising or setting the resistance value, which is low due to the small NTCR film thicknesses of just a few m, in a defined manner.

(48) In view of the foregoing, it can thus be summarized that at least one further layer or structure can be formed on at least one of the substrate, the film and the layer of spinel-based material. In this connection, the at least one further layer or structure can be provided before the step of forming said film, following the step of forming said film or following the step of transforming said film into the layer of spinel-based material.

(49) It should further be noted that the at least one further layer or structure is selected from the group of members consisting of an electrically insulating layer or structure, an electrically insulating but thermally conducting layer or structure, an electrically conducting layer or structure, such as an electrode, a protective film and a thermally conducting layer.

(50) Depending on when and where the at least one further layer or structure is applied, said at least one further layer or structure can be applied using thick film technology, a CVD process, a PVD process, a sol-gel process and/or a galvanization process; with the at least one further layer or structure optionally being structured by means of a laser beam, an electron beam, a sand jet or a photolithographic process or similar.

(51) By way of example an NTCR sensor 17 can be formed by providing a Cu substrate 2, a layer of electrically insulating and preferably thermally conductive material, such as Al.sub.2O.sub.3, can be deposited directly on the Cu substrate 2. A composite film 10 of NiO and Mn.sub.2O.sub.3 is then deposited on this layer of preferably thermally conductive but electrically insulating material. One then proceeds as described in connection with FIG. 2 to form two electrodes 12 on this layer 10.

(52) Such an NTCR sensor 17 formed on a Cu substrate 2 can then be placed, e.g. directly in the vicinity of engine components in order to e.g. monitor the temperature in a cylinder of an engine (not shown) to carry out a high-precision temperature measurement of the cylinder and monitor the temperature development thereof in real time.

(53) FIG. 5 shows an SEM image of the fractured surface of a NiOMn.sub.2O.sub.3 composite film 10 on an Al.sub.2O.sub.3 substrate 2 in accordance with the first method step of an embodiment of the invention described in connection with FIG. 2. In this first step, a powder mixture comprising two metal oxide components 9.1, 9.2, namely NiO and Mn.sub.2O.sub.3, is formed on the Al.sub.2O.sub.3 substrate 2 by means of the aerosol-based and vacuum-based cold composite deposition process. The NiOMn.sub.2O.sub.3 composite film 10, which is produced in this respect and is shown in FIG. 5, has a high density, a good bonding with the Al.sub.2O.sub.3 substrate 2 and grains in the umpteen nm range.

(54) In FIG. 6, two possible NTCR sensors 17 are shown after the completion of the third method step of the embodiment of the invention described in FIG. 2. In accordance with this embodiment, an aerosol-based and vacuum-based cold composite deposition of a two-component metal oxide powder mixture of NiO and Mn.sub.2O.sub.3 onto an Al.sub.2O.sub.3 substrate 2 took place in the first step. An AgPd conductive paste 1 1 was subsequently applied by screen-printing onto the NiOMn.sub.2O.sub.3 composite film 10 in the second step. In the third step, a temperature treatment of the compound took place at 850 C.

(55) Then, as shown in FIG. 6, the electrode structure 12 is present as burned and an NTCR film (the layer 13 of spinel-based material) having a cubic NiMn.sub.2O.sub.4 spinel structure 13 is present. The electrodes 12 shown are so-called interdigital electrodes. They result in a low resistance of the NTCR sensor 17. Depending on the selection of the electrode form, the resistance value can be set in a wide range. A more detailed characterization of the NTCR sensors 17 shown in FIG. 6 is illustrated in FIGS. 7 to 9.

(56) FIG. 7 shows an SEM image of the fractured surface of an NTCR sensor 17 of FIG. 6 that is temperature-treated at 850 C. Following the deposition of NiO and Mn.sub.2O.sub.3 compounds, homogenous and scratch-resistant composite layers 10 having thicknesses in the range of approximately 1 to 3 m thickness could be produced.

(57) The lower half of the SEM image shows the Al.sub.2O.sub.3 substrate 2. The spinel-based layer 13, a cubic NiMn.sub.2O.sub.4 spinel, is located thereon. It has a good adhesion to the substrate 2, as well as a crack-free and uniform layer morphology. The crack-free and uniform layer morphology is still observed following a 10 minute sintering step carried out at 950 C. The screen-printed and subsequently sintered AgPd interdigital electrodes 12 are located on the spinel-based layer 13. The fractured image in this respect shows the cross-section of a finger of an AgPd interdigital electrode 12.

(58) The layer morphology has however changed from a dense, nanoporous AcD layer as shown in FIG. 5 to a closed pore layer without clearly recognizable pores as shown in FIG. 7. The effect of the pore formation on calcination of the composite layer 10 is presumably due to the reduction in volume as a consequence of the formation of the spinel-structure.

(59) An electrical characterization of the two NTCR sensors 17 that are shown in FIG. 6 is illustrated in FIGS. 8a and 8b. Both NTCR sensors 17 show the typical behavior of a ceramic thermistor having a B-constant of approximately 3850 K and a specific resistance .sub.25 at 25 C. of approximately 25 m. FIG. 8a in this regard shows the change in specific resistance with respect to temperature in C.

(60) Advantageously, both the B-constant (see FIG. 8b) and the specific resistance .sub.25 (see FIG. 8a) remain substantially constant at approximately 3850 K and 25 m despite temperature-treating the sensors at different temperatures in the range of 200 C. to 800 C. In order to confirm the stability of the NTCR sensors 17 with respect to resistance and temperature, the two NTCR sensors 17 were each subjected to one-hour lasting temperature treatments at T=200 C., 400 C., 600 C. and 800 C. (see e.g. FIG. 11 in this regard). Between each temperature treatment, the NTCR sensors 17 were allowed to cool down to room temperature at a cooling rate of 10 K/min.

(61) An electrical characterization of each of the two NTCR sensors 17 took place following each temperature treatment step. The results of these measurements are shown in FIGS. 9a and 9b. Both the B-constant (see FIG. 9b) and the specific resistance .sub.25 (see FIG. 9a) substantially maintain their values despite the various temperature treatments.

(62) It should be noted in this connection that on forming the actual NTCR sensors 17, 18, 19 a single heat treatment step of e.g. 850 C. is carried out. This means that one does not have to perform several independent heat treatment steps (as carried out for the stability evaluation) on the manufacture of NTCR sensors 17, 18, 19.

(63) In order to produce the graphs shown in FIG. 9 (NTCR sensor 17) and FIG. 10 (prior art NTCR sensor as explained below), the measurement and temperature cycle depicted in FIG. 11 was used.

(64) The NTC thermistors were measured both once they were deposited as the composite film 10 and subsequently sintered with the electrodes (in case of FIG. 9) or were deposited as spinel-based film 13 on electrode structures (in case of FIG. 10) and after the different heating steps in order to monitor at which temperature the transformation to the layer 13 of spinel-based material took place. The measurements took place in the constant temperature circulator described in the following. For the tempering the heating/cooling rate was 10 K/min and the temperature was maintained for 60 min at each temperature.

(65) In order to conduct the electric characterization of the NTCR sensors 17 as shown in FIGS. 8 to 10, the measurements were carried out in a constant temperature circulator (Julabo SL-12) at temperatures between 25 C. and 90 C. using a low viscosity silicone oil (DOW CORNING 200 FLUID, 5 CST) as a measurement liquid. A four-terminal sensing method was used for the investigations using a digital multimeter (Keithley 2700) to measure the electrical resistance in dependence on the temperature. The measurement temperature was detected in the direct vicinity of the NTC thermistor with the aid of a high-precision Pt1OOO resistor. The calculation of the specific resistance .sub.25 took place across the complete resistor at 25 C. and via the sensing geometry (electrode spacing, electrode width, number of electrode pairs, NTCR layer thickness). The B-constant was determined in accordance with the following relationship via the resistance at 25 C. and 85 C.

(66) $B=\ln \frac{R_{25}\text{/}{R}_{85}}{\frac{1}{298K}-\frac{1}{358K}}$
Comparative measurements using a different constant temperature circulator showed that the obtained results depicted in FIGS. 8 and 9 could be reproduced.

(67) FIG. 12 shows XRD spectra confirming that the film 10 of composite material of NiOMn.sub.2O.sub.3 is transformed into the layer 13 of spinel-based material having the desired cubic NiMn.sub.2O.sub.4-spinel in an air atmosphere on being subjected to a high temperature treatment.

(68) In this regard, FIG. 12a shows various XRD spectra of the composite film 10 respectively of the layer 13 of spinel-based material at different temperatures. The lowest spectra of FIG. 12 a shows the XRD spectrum of the composite film 10 prior to any heat treatment, the temperature is subsequently increased for each higher lying XRD spectrum up to a temperature of 800 C. following which the layer 13 of spinel-based material is cooled down again.

(69) The different spectra shown in FIGS. 12b to 12d relate to reference spectra of respective pure layers. FIG. 12b shows the XRD spectrum of a pure NiO layer having a cubic structure. FIG. 12c shows the XRD spectrum of a pure Mn.sub.2O.sub.3 layer having a cubic structure. FIG. 12d shows the XRD spectrum of a pure NiMn.sub.2O.sub.4 layer having a cubic structure.

(70) Specifically, following the deposition at 25 C. the composite film 10 has the reflexes of the starting material of NiO and Mn.sub.2O.sub.3, i.e. the peaks present in this XRD spectrum correspond to the dominant reflexes found in FIGS. 12b and 12c. The composite film 10 maintains these reflexes up to a temperature of 400 C. Thus, the deposition of the composite film 10 alone does not bring about a transformation to the layer 13 of spinel-based material. This phase change starts at a heating step in the range of 600 C. to 750 C., where the cubic structure of NiMn.sub.2O.sub.4 starts to become apparent, i.e. the dominant peak shown in FIG. 12d can first be seen in the XRD spectrum at 600 C. and the amplitude of this peak increases with an increase in temperature. In this intermediate state several NiMn-Oxides are present (cubic Mn.sub.2O.sub.3 (Bixbyit), orthothrombic NiMn.sub.2O.sub.3 (Ilmenite), tetragonal Mn.sub.3O.sub.4 (Hausmannite) and cubic NiMn.sub.2O.sub.4 (Spinel)) alongside one another. At a temperature of 800 C., the phase change is completed and only reflexes of the desired cubic NiMn.sub.2O.sub.4-Spinel are present. These reflexes, i.e. the cubic NiMn.sub.2O.sub.4 structure are/is maintained also after cooling (see FIG. 12a) at 500 C. and 30 C.).

(71) In the following, a discussion of the temperature behavior of NiMn.sub.2O.sub.4 layers formed using aerosol deposition as discussed e.g. in U.S. Pat. No. 8,183,973 B2 will be presented.

(72) As discussed in the foregoing, in U.S. Pat. No. 8,183,973 B2, a ground powder of completely calcined NiMn.sub.2O.sub.4 powder is deposited by means of Aerosol Deposition (AD) using an apparatus such as the one discussed in connection with FIG. 1. The completely calcined NiMn.sub.2O.sub.4 powder is deposited onto an Al.sub.2O.sub.3 substrate provided with a screen-printed AgPd-electrode structure. Following the generation of the film on the electrode structure, the complete structure is subjected to a heat treatment step. Following the different heat treatment steps carried out at the different temperatures the specific resistance .sub.25 and the B-constant of the material is measured. The results of these measurements are shown in FIGS. 10a and 10b. The results shown in FIG. 10 after the 800 C. tempering step (.sub.25, 800 C., B.sub.800 C.) are nearly identical to the measurement results (.sub.25, 800 C., B.sub.800 C.) shown in FIG. 9. However, the tempering behaviour of the sensors shown in FIG. 10 is markedly different to those depicted in FIG. 9. The curves in FIGS. 10a and 10b show a clear gradient with increasing tempering temperature, while the curves in FIGS. 9a and 9b are approximately constant. In this way the stability exhibited in the graphs shown in FIGS. 9a and 9b is not achieved, i.e. with respect to different heat treatments a more instable structure is obtained using the prior art method. Hence, the method described herein leads to the formation of NTCR resistors 17, 18, 19 having at least the same quality as those known from the prior art.

(73) It should be noted that the described heat treatment step used to induce the conversion of the film 10 into the layer 13 of spinel-based material and to induce the sintering of the conductive paste 11 to form the electrode structures 12 is carried out using thermal convection. Other forms of heat treatment step could be employed. In this connection, radiation from a specifically tuned laser or from a microwave source could be used to induce this change in state of the respective layer of structure. It is also conceivable, that if a thermally and electrically conductive layer is provided on the substrate or as a substrate that a sufficiently high current is applied at this layer to induce the desired transformation.

LIST OF REFERENCE NUMERALS

(74) 1 apparatus 2 substrate 3 mixture 4 deposition chamber 5 evacuation apparatus 6 aerosol-producing unit 7 nozzle 8 powder mixture having x metal oxide components (x2) 9 aerosol 9 carrier gas 9.1 particle of the metal oxide component 1 9.2 particle of the metal oxide component 2 9.3 particle of the metal oxide component 3 9.x particle of the metal oxide component x 10 composite film (from aerosol-based and vacuum-based cold composite deposition) 1 1 conductive paste 12 electrode/electrode structure 13 spinel-based layer 14 mask 15 filling material particle 16 filling material grain in layer 17 NTCR sensor having interdigital top electrodes 18 NTCR sensor having interdigital bottom electrodes 19 NTCR sensor having sandwich electrodes