GB2284932A

GB2284932A - Temperature sensor having cermet resistor and calcined subtrate

Info

Publication number: GB2284932A
Application number: GB9424697A
Authority: GB
Inventors: Nobuhide Kato; Nobukazu Ikoma
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1993-12-15
Filing date: 1994-12-07
Publication date: 1995-06-21
Anticipated expiration: 2014-12-07
Also published as: GB2284932B; DE4444594C2; JPH07167714A; GB9424697D0; DE4444594A1

Description

2284932

TITLE OF THE INVENTION

TEMPERATURE SENSOR HAVING CERMET RESISTOR AND METHOD OF PRODUCING THE TEMPERATURE SENSOR BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to a temperature sensor and a method of producing the temperature sensor, and in particular to such a temperature sensor which includes a cermet resistor made of metal and ceramic, and which can be favorably used for a prolonged time at a consi ' derably high temperature.

Discussion of Related Art There has been known a temperature sensor which has a resistor and is adapted to measure a temperature by detecting changes in the resistance value of the resistor.

An example of such a temperature sensor is produced by 1) printing a resistor made principally of platinum on a ceramic substrate, 2) baking the resistor on the substrate, 3) trimming the resistor so as to adjust its resistance, and 4) covering and protecting the resistor with glass, as disclosed in JP-A-4-279831, for example. It is also known to use a ceramic material in the form of a paste or slurry for covering and protecting the resistor formed on the ceramic substrate. In this case, the temperature sensor is obtained by applying the ceramic material to the ceramic substrate so as to cover the resistor, and then firing the ceramic material on the substrate.

2 In the conventional temperature sensors as described above, the resistor formed on the ceramic substrate is made principally of platinum and has a relatively small thickness. When the sensor is used at a considerably high temperature, therefore, the platinum resistor is likely to be sintered due to the high temperature, thereby causing undesirable changes in its resistance value, which result in deteriorated measuring accuracy of the sensor. In the case where the resistor is covered and protected with glass as described above, the temperature sensor cannot be used in a temperature range that exceeds 10001C, for example, due to a limit arising from the melting point of the glass. Further, the temperature sensor suffers from changes in the resistance value due to migration of impurities, such as Na and Ca, which are contained in the glass. The sensor also suffers from thermal shocks due to a difference in the coefficient of thermal expansion between the glass and the ceramic substrate, resulting in lowered operating reliability.

Where a ceramic paste or slurry is used to cover the resistor formed on the ceramic substrate, and then fired to give a ceramic covering layer for the resistor, cracks are likely to be formed in the ceramic covering layer, due to a limit by the ceramic substrate to the amount of shrinkage of the ceramic covering layer upon firing thereof.

Further, the thus formed temperature sensor may suffer from peeling-off of the covering layer during its use, for 3 example, due to a relatively small bonding strength between the ceramic covering layer and the ceramic substrate.

SUMARY OF THE INVENTION

It is therefore a first object of the present invention to provide a temperature sensor which includes a resistor having a reduced variation in its resistance value, and which exhibits increased bonding strength between the resistor and a ceramic covering layer, without suffering from cracks, thus assuring sufficiently high durability when used at a high temperature.

It is a second object of the invention to provide a method of producing such a temperature sensor having excellent features as described above.

is According to a first aspect of the present invention there is provided a temperature sensor comprising: a ceramic substrate; a cermet resistor comprising metal and ceramic; and a ceramic covering layer formed on the ceramic substrate so as to cover at least the cermet resistor, the ceramic covering layer and ceramic substrate containing the same ceramic material as a major component, the ceramic covering layer being formed on a calcined body which gives the ceramic substrate and on which the cermet resistor is formed and trimmed so as to provide a predetermined resistance value, the ceramic covering layer and the calcined body being fired together.

4 According to a second aspect, the Invention Sprovides a method of producing a temperature sensor, i comprising the steps of: preparing a ceramic green body which is formed principally of a ceramic material and gives a ceramic substrate; printing a cermet resistor comprising metal and ceramic on the ceramic green body; calcining the ceramic green body with the cermet resistor to provide a calcined body; trimming the cermet resistor on the calcined body so as to adjust a resistance of the resistor to a predetermined value; covering. at least the cermet resistor with a ceramic covering layer which contains as a major component the ceramic material used for forming the ceramic green body; and firing the calcined body with the ceramic is covering layer.

According to the method of the present invention as described above, the cermet resistor is trimmed on the calcined body of the ceramic substrate, and then the ceramic covering layer containing as a major component the ceramic material used for the ceramic substrate is applied to and fired together with the calcined body of the ceramic substrate. Accordingly, the ceramic covering layer is free from cracks which would otherwise be formed upon firing thereof. Further, the covering layer and the ceramic substrate formed principally of the same ceramic material are formed into an integral body, assuring a sufficiently high bonding strength therebetween. Moreover, the present temperature sensor has as high resistance to thermal shocks as the ceramic substrate itself, thus assuring sufficiently high operating reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and optional objects, features and advantages of the present invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:

Fig. 1 is a perspective view showing a laminar structure which includes three ceramic sheets that are laminated on each other, and a plurality of resistor patterns that are printed on one of the ceramic sheets so as to provide a plurality of temperature sensors; Fig. 2 is a series of views showing process steps for producing one embodiment of a temperature sensor of the present invention, using a sensor green body which is obtained by cutting the laminar structure as shown in Fig.

1.

Fig. 3 is a series of views showing process steps for producing another embodiment of the temperature sensor of the present invention; and Fig. 4 is a diagram illustrating another example of the process of producing the temperature sensor of the type as shown in Fig. 3 according to the present invention.

6 PREFERRED EMBODIMENTS OF THE PRESENT INVENTION Referring f irst to Figs. 1 and 2, there will be described by way of example a process of manufacturing a plurality of temperature sensors constructed according to the present invention. Initially, a plurality of planar resistor patterns 4 which are prepared from a paste made of metal and ceramic are formed by printing on a ceramic sheet 2a, so as to provide the plurality of temperature sensors at the same time. Each of the resistor patterns 4 consists of a temperature-sensing portion 4a which will be located in a f luid whose temperature is to be measured by the sensor, a terminal portion 4b which will be connected to an exterior detecting device, and a lead portion 4c which connects the temperature-sensing portion 4a with the terminal portion 4b.

is Further, a plurality of ceramic sheets 2b, 2c (two sheets in this embodiment) are laminated on the ceramic sheet 2a on which the resistor patterns 4 are printed, such that these ceramic sheets 2a, 2b and 2c are f ormed into a laminar structure which has a sufficiently high mechanical strength. This laminar structure is cut into a plurality of unf ired ceramic substrates 2 on which the respective planar resistor patterns 4 are printed. In this manner, a plurality of sensor green bodies 6 as shown in Fig. 2 are obtained.

While the ceramic sheets 2a, 2b, 2c which provide the ceramic substrate 2 can be formed of a material selected f rom alumina, steatite, mullite, silicone nitride, zirconia and others, it is particularly desirable that the material 7 for the ceramic substrate 2 provides highelectrical insulation at a high temperature, and has a relatively low coefficient of thermal conductivity. It is recognized that zirconia has a relatively low coefficient of thermal conductivity but is unsatisfactory in its electrical insulation performance at a high temperature, while alumina exhibits sufficiently high electrical insulation at a high temperature but has an undesirably high coefficient of thermal conductivity. To make use of the advantageous features of these two materials, an alumina layer on which a resistor layer is printed can be formed on a zirconia sheet, and an alumina layer and a zirconia covering layer can be f ormed in this order on the resistor layer af ter it is trimmed.

is The resistor pattern 4 formed on the ceramic sheet 2a is a cermet resistor which is formed of a cermet material which contains a metal (conductor) as a major component, and a ceramic. The ceramic contained in the resistor is pref erably the same material as that used f or f orming the ceramic sheet 2a (ceramic substrate 2), so that the substrate 2 and the resistor 4 can be formed into an integral body upon simultaneous f iring thereof, assuring a high bonding strength therebetween and sufficiently high resistance to thermal shocks. The metal contained in the resistor 4 is required to exhibit a high positive temperature coefficient of resistance, and may be selected from platinum, rhodium, silver, tungsten, nickel and others, 8 f or example. In particular, platinum, rhodium and tungsten having high heat resistance may be favorably used. The ratio of the metal to the ceramic in the cermet material is suitably selected depending upon the pattern or shape and desired resistance value of the resistor.

Since the cermet resistor made of metal and ceramic is used for measuring the temperature in the present temperature sensor, the obtained resistor is given a relatively high specific resistivity, and its resistance value can therefore be made high, resulting in an enhanced detecting accuracy of the sensor.

Subsequently, the sensor green body 6 obtained by cutting the laminar structure shown in Fig. 1 is further processed as shown in Fig. 2 to provide a desired is temperature sensor. More specifically, the individual green body 6 is first calcined at a temperature that is determined depending upon the ceramic material used f or the green body 6, so as to provide a calcined body 8. In this calcining operation, the upper limit of the calcining temperature is desirably determined so that firing shrinkage of the ceramic sheets 2a, 2b, 2c of the green body 6 is held to be not larger than 15%, preferably, not larger than 10%, so as to satisfactorily achieve the objects of the present invention.

This is because cracks are likely to be formed in a ceramic covering layer (which will be described) upon firing of the sensor green body 6 if the f iring shrinkage at the time of calcining is excessively large. On the other hand, the 9 smaller the f iring shrinkage at the time of calcining, the smaller a change in the resistance value of the resistor at the time of subsequent firing of the calcined body 8.

Therefore, the lower limit of the calcining temperature is desirably determined so that the variation of the resistance value is controlled to be a desired value. The firing shrinkage measured upon calcining of the green body 6 can be suitably adjusted by selecting the material used for the ceramic sheets 2a, 2b, 2c which gives the ceramic substrate 2, the particle size of the material, the ratio of a binder to the material and other parameters.

In the next step, the resistor pattern 4 of the calcined body 8 obtained by calcining the sensor green body 6 is subjected to an ordinary trimming operation by means of a laser beam, for example, so that the resultant temperature sensor is provided with a resistor having a desired resistance value. This results in a reduced variation in the resistance values of the temperature sensors produced according to the present invention. In particular, the resistor pattern 4 is trimmed through feedback control of the resistance value of the pattern 4, so as to achieve a desired resistance value with high accuracy. Further, the resistance value is adjusted so as to accommodate a change in the resistance value due to sintering of the metal and ceramic of the cermet resistor upon subsequent firing of the calcined body 8, assuring a significantly reduced variation in the resistance value after the firing of the calcined - body 8. The trimming of the resistor pattern 4 can be effected by trimming the planar temperature -sensing portion 4a, so as to provide a desired resistor. It is also possible to f orm by printing a trimmed portion made of a cermet material as well as the portions 4a, 4b, 4c of the resistor pattern 4, and then trim the trimmed portion as needed so as to provide a desired resistor.

On the calcined body 8 with the resistor pattern 4 trimmed, there is formed a ceramic covering layer 10 having a suitable thickness so as to cover at least the temperature-sensing portion 4a of the resistor pattern 4, as shown in Fig. 2. This covering layer 10 is f ormed of the same ceramic material as the ceramic substrate 2 P that is, the sensor green body 6. Therefore, the ceramic covering layer 10 has firing shrinkage percentage which is approximate to that of the ceramic substrate 2, and thus exhibit high density when it is fired later. More specifically, the firing shrinkage of the covering layer 10 upon f iring thereof is desirably controlled to be within 10%, preferably, 5% of that of the ceramic substrate 2.

The firing shrinkage measured upon firing of the ceramic covering layer 10 is suitably adjusted by selecting the particle size of the ceramic material used for the layer 10, the ratio of a binder to the material, and other parameters.

The ceramic covering layer 10 may be applied to the calcined body 8 by any one of the following and other methods: (1) immersing the calcined body 8 in a suitable ceramic slurry or paste, (2) coating the calcined body 8 with the ceramic slurry or paste, (3) printing, (4) attaching a ceramic sheet made of the same material as the ceramic substrate, to the calcined body 8, and integrating the ceramic sheet with the calcined body 8. When the method (4) is employed, the resultant temperature sensor exhibits a relatively high durability since the resistor is substantially embedded in the ceramic substrate 2.

Subsequently, the calcined body 8 provided with the ceramic covering layer 10 is fired or sintered according to an ordinary firing method suitable for the ceramic material used. Consequently, a desired temperature sensor is obtained in which the resistor 4 has a significantly reduced variation in its resistance value, and the ceramic covering layer 10 adheres to the ceramic substrate 2 and resistor 4 with high strength, and is free from cracks. The thus obtained sensor assures sufficiently high durability when used at a relatively high temperature.

The advantageous features of the temperature sensor as described above will be easily understood from comparison between an example of the temperature sensor produced according to the above-described process of the present invention, and a comparative example of a conventional temperature sensor.

To produce the temperature sensor according to the method of the present invention, an alumina ceramic sheet 2a was initially formed, and a plurality of planar resistor 12 - patterns 4 formed from a platinum paste made of platinum and alumina were applied by printing to the ceramic sheet 2a.

Then, two other alumina ceramic sheets 2b, 2c were laminated on the major surface of the ceramic sheet 2a remote from the resistor patterns 4, so as to provide an integral alumina ceramic substrate 2 which has a sufficiently high mechanical strength. Then, the laminar structure of the ceramic sheets 2a, 2b, 2c was cut into a plurality of sensor green bodies 6 on which the respective planar resistor patterns 4 are printed. Each of the thus obtained green bodies 6 was calcined at 12000C, causing almost no firing shrinkage, so as to provide a calcined body 8.

Thereafter, the temperature-sensing portion 4a of the resistor pattern 4 of the calcined body 8 was trimmed by a laser beam so as to achieve a desired resistance value.

During this trimming process, the resistance value of the resistor pattern 4 was detected by a monitor, and the length of the trimmed resistor pattern 4 and the number of segments produced by the trimming were controlled by signals received from the monitor, so that the resistance value was accurately controlled to a desired level through feedback control of the trimming. Then, the calcined body 8 with the trimmed resistor was immersed in an alumina slurry such that at least the temperature-sensing portion 4a of the resistor pattern 4 was covered with the alumina slurry 10. Then, the calcined body 8 was fired at 16001C, to provide an intended temperature sensor.

When the thus obtained temperature sensor was subjected to 1000 cycles of a thermal shock test in which the sensor was alternately exposed to an atmosphere having 1200C and a room temperature in each cycle, no cracks were found in a ceramic covering layer 10 formed from the alumina slurry. In another test in which the temperature sensor was kept exposed to a high temperature of 1200C for 1000 hours, the resistance value of the sensor was varied to an extremely small extent, that is, as small as 1%.

According to a conventional method, on the other hand, an alumina ceramic substrate was fired, and a platinum cermet resistor was printed and. fired on the substrate.

Then, an alumina ceramic paste which gives an alumina covering layer was printed on the resistor formed on the ceramic substrate, and fired. In this manner, a plurality of conventional temperature sensors were produced. Some of the thus obtained temperature sensors had cracks formed in the alumina covering layers. Other temperature sensors which did not suffer from cracks upon firing thereof were subjected to the above-described thermal shock test, and cracks appeared in the covering layers of these sensors. Another comparative example was prepared by using glass for forming the covering layer. In this case, the resistance value of the sensor was varied to a great extent when the sensor was kept exposed to a considerably high temperature for a long time.

While the ceramic substrate 2 of the temperature sensor of the illustrated embodiment takes the form of a 14 - sheet or plate, a ceramic green body 12 in the f orm of a stepped pipe may be used to provide a ceramic substrate which takes the form of a stepped pipe having a large-diameter portion and a small-diameter portion, as shown in Fig. 3. In this case, a suitable resistor pattern 4 is printed on a circumferential surface of the pipe-like ceramic green body 12. After calcining the green body 12 and trimming the resistor pattern 4, another pipe-like green body which gives a ceramic covering layer 14 is f itted on the small-diameter portion of the calcined body 12 so as to cover at least the temperature -sensing portion 4a of the resistor pattern 4. Then, the calcined body 12 with the resistor pattern 4 and the covering layer 14 is f ired to provide a pipe-like temperature sensor. This embodiment is is advantageous in that the ceramic covering layer 14 completely adheres to and is f ormed integrally with the ceramic substrate 2 upon f iring thereof, since the f iring shrinkage of the green body for the covering layer 14 is larger than that of the calcined body 12 when it is f ired into the ceramic substrate 2. Further, the provision of the temperature- sensing portion 4a of the resistor 4 on the small-diameter portion of the ceramic substrate 2 advantageously permits the substrate 2 to have a relatively small heat capacity.

In addition to the pipe-like sensor as shown in Fig. 3, the temperature sensor may take the form of a solid cylindrical bar, or a solid or hollow bar having a rectangular cross section. To assure an improved sensing accuracy of the temperature sensor, it is preferable to form the resistor having a hollow temperature sensing portion, or diminish the dimensions of the temperature sensing portion, as in the embodiment of Fig. 3, so as to reduce heat conduction toward a non-sensing portion of the resistor.

Referring next to Fig. 4 showing a process of producing the temperature sensor of the stepped-diameter type as shown in Fig. 3, a f irst ceramic pipe having a relatively small diameter and a second ceramic pipe having a relatively large diameter are f ormed by extrusion, using a suitable ceramic material, and a predetermined resistor pattern is then printed on a circumferential surf ace of the first ceramic pipe. After calcining the first pipe and is trimming the resistor pattern, the second ceramic pipe is f itted on the calcined f irst pipe with the resistor, and then calcined. Thereafter, an end portion of the second ceramic pipe is subjected to a cutting operation so as to provide a desired stepped configuration as shown in Fig. 3.

That is, the diameter of cut end portion of the second ceramic pipe is reduced to provide a temperature - sensing portion. Then, the first and second pipes are fired, to thus provide an intended temperature sensor. According to this method, the cutting operation can be effected on the second pipe which has been calcined, with enhanced dimensional accuracy, as compared with the case where the cutting 16 - operation is effected on a green structure having plasticity.

While the present invention has been described in its preferred embodiments, for illustrative purpose only, it is to be understood that the invention is not limited to the details of the illustrated embodiments, but may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art,

Claims

CLAIMS:

1. A temperature sensor comprising a ceramic substrate (2), a resistor (4), and a ceramic covering layer (10) f ormed on said ceramic substrate so as to cover at least said resistor, characterized in that: said resistor is a cermet resistor comprising metal and ceramic; said ceramic covering layer and said ceramic substrate contain the same ceramic material as a major component; and that said ceramic covering layer is formed on a calcined body which gives said ceramic substrate and on which said cermet resistor is formed and trimmed so as to provide a predetermined resistance value, said ceramic covering layer and said calcined body being fired together.

2. A temperature sensor as defined in claim 1, wherein said ceramic substrate comprises a ceramic plate (2) which consists of at least one ceramic sheet (2a, 2b, 2c).

3. A temperature sensor as defined in claim 1 or 2, wherein said cermet resistor comprises a temperature - sensing portion (4a), a terminal portion (4b), and a lead portion (4c) which connects said temperaturesensing portion with said terminal portion, said 18 ceramic covering layer covering at least said temperature-sensing portion of said cermet resistor.

4. A temperature sensor as def ined in any one of claims 1-3, wherein said ceramic of said cermet resistor consists of the same ceramic material as said main component of said ceramic substrate.

5. A temperature sensor as def ined in any one of claims 1-4, wherein said metal of said cermet resistor is selected f rom the group consisting of platinum, rhodium, silver tungsten and nickel.

6. A temperature sensor as def ined in any one of claims 1, 3, 4 and 5, wherein said ceramic substrate takes the form of a stepped pipe which has a large-diameter portion and a small-diameter portion, said cermet resistor having a temperature-sensing portion which is formed on an outer circumferential surface of said small-diameter portion.

7. A method of producing a temperature sensor comprising the steps of:

preparing a ceramic green body (2a, 2b, 2c) which is formed principally of a ceramic material and gives a ceramic substrate; printing a cermet resistor (4) comprising metal and ceramic on said ceramic green body; calcining said ceramic green body with said cermet resistor to provide a calcined body (8); trimming said cermet resistor on said calcined body so as to adjust a resistance of the resistor to a predetermined value; covering at least said cermet resistor with a ceramic covering layer (10) which contains as a major component said ceramic material used for said ceramic green body; and firing said calcined body with said ceramic covering layer.

8. A method according to claim 7, wherein said ceramic material for said ceramic green body is used as said ceramic contained in said cermet resistor.

9. A method according to claim 7 or 8, wherein said ceramic green body is formed by laminating a plurality of ceramic green sheets (2a, 2b, 2c) on each other.

10. A method according to any one of claims 7-9, wherein said ceramic green body is calcined in a temperature range whose upper limit is determined such that - f iring shrinkage of said ceramic green body is held to be not larger than 15%.

11. A method according to claim 10, wherein said upper limit of said temperature range is determined such that said f iring shrinkage of said ceramic green body is held to be not larger than 10%.

12. A method according to any one of claims 7-11, wherein said ceramic covering layer is fired such that firing shrinkage of the ceramic covering layer is controlled to be within 10% of that of said ceramic substrate.

13. A method according to claim 12, wherein said firing shrinkage of said ceramic covering layer is controlled to be within 5% of that of said ceramic substrate.

14. A temperature sensor substantially as any herein described with reference to and as shown in the accompanying drawings.

15. A method of producing a temperature sensor substantially as any herein described with reference to and as shown In the accompanying drawings.