Patent Description:
Conventionally, a temperature sensor has been widely used that uses a thermistor of which the electric resistance value (hereinafter simply referred to as a resistance value) changes according to a temperature, as a heat sensitive body. The characteristics of the thermistor are generally shown by the resistance value and a temperature coefficient of resistance (temperature dependence of the resistance value). The resistance value characteristics of the thermistor are different depending on a material constituting the element, and various materials have been developed which show the resistance value characteristics according to the purpose of use.

An average temperature coefficient of resistance (hereinafter referred to as B constant) can be obtained by the following expression; <MAT>.

The thermistor is a substance which detects a temperature based on a change in the resistance value, and when the resistance value becomes too low, cannot accurately detect the temperature. Accordingly, a thermistor which is used in a wide temperature range is required to have a small B constant.

As is disclosed in <CIT> and <CIT>, a thermistor is known which has an NTC (Negative Temperature Coefficient: negative temperature coefficient of resistance) characteristics having a B constant of <NUM> or lower in a temperature range of <NUM> to <NUM>. It is disclosed that the thermistors in <CIT> and <CIT> are formed of an oxide sintered body of which the composition is Y, Cr, Mn and Ca, and that a typical composition of Y, Cr, Mn and Ca is Y: <NUM> mol%, Cr: <NUM> mol%, Mn: <NUM> mol% and Ca: <NUM> mol%.

According to <CIT>, this thermistor sintered body has resistance values of <NUM> kΩ, <NUM> kΩ, <NUM> kΩ, <NUM> kΩ and <NUM> kΩ at <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, respectively; and has the B constant of <NUM> in a temperature range between <NUM> and <NUM>. It is disclosed that this thermistor can be used for controlling a temperature to <NUM> or lower.

<CIT> uses a thermistor chip formed of an oxide sintered body having the same composition as that of <CIT>, and achieves a mechanically, thermally and chemically stable thermistor element over a wide temperature range from -<NUM> to approximately <NUM>. However, <CIT> achieves the stable thermistor element in the above described wide temperature range, by covering a joined portion between the thermistor chip and a lead wire, with a covering material that is formed of the metal oxide and a sintering accelerator which does not have a conductivity enhancing function. In other words, a different point between <CIT> and <CIT> exists in a material of the covering material, and the B constant of the oxide sintered body which is disclosed by <CIT> and has the same composition as that of <CIT> is equivalent to that of <CIT>.

<CIT> discloses a method for manufacturing high-temperature thermistor materials having stable thermistor properties and a high-temperature thermistor. The document discloses a thermistor material that is a mixed powders of (Mn. Cr) O<NUM> spinel powder and Y<NUM>O<NUM> powder and is fired the mixed powder at a temperature in a range of <NUM> to <NUM>, so as to cause the components of the mixture to react with each other, thereby generating (Mnx. Cry) O<NUM> spinel and Y(Cr. Mn) O<NUM> perovskite.

<CIT> discloses a highly accurate reduction resistant thermistor exhibiting stable resistance characteristics even under conditions where the inside of a metal case of a temperature sensor becomes a reducing atmosphere, wherein when producing the thermistor comprised of a mixed sintered body (M1 M2)O<NUM>. AOx, the mean particle size of the thermistor material containing the metal oxide, obtained by heat treating, mixing, and pulverizing the starting materials, is made smaller than <NUM> and the sintered particle size of the mixed sintered body, obtained by shaping and firing this thermistor material, is made <NUM> to <NUM> so as to reduce the grain boundaries where migration of oxygen occurs, suppress migration of oxygen, and improve the reduction resistance.

<CIT> discloses how to stabilize the characteristic of a high-temperature by thermistor, mixing a (Mn~NCr)O<NUM> spinal powder with Y<NUM>O<NUM> powder and baking the mixed raw-material powder at a specific temperature to make both the components react with each other, and by obtaining a high-temperature thermistor material made of a specific spinel and a specific perovskite. This document suggests that mixing of Cr<NUM>O<NUM> with MnO<NUM> is conducted to make the mole ratio of Ce/Mn equal to <NUM>, the mixture is baked temporarily to obtain (Mn. Cr)O<NUM> spinel powder. Mixing the 50mol% (Mn. Cr)O<NUM> spinel powder with a 50mol% Y<NUM>O<NUM> powder, both a sintering assistant and an organic vehicle are added to the mixture to obtain a thermistor paste. Then, printing a Pt on a ceramic green sheet to change into a substrate after burning and printing further thereon the thermistor paste, another green sheet to change into a cover after baking is laminated thereon. Heating this laminate at a temperature of <NUM>-<NUM> to bake it, both are made to react with each other to obtain a mixed sintered compact made of (Mnx. Cry)O<NUM> spinel (<NUM> < x, y < =<NUM>, x+y=<NUM>) and Y(Cr+Mn)O<NUM> perovskite.

<CIT> discloses how to allow a wide selective range of resistive values and temperature coefficients of resistance by sintering a material obtained from a compound of high resistive, high temperature-coefficient (Mn. Cr)O<NUM> and low resistive, low temperature coefficient YCrO<NUM>. According to this document, a material consisting of <NUM>. 3mol% YCrO<NUM> and <NUM>. 7mol% Mn<NUM> Cr<NUM>O<NUM> indicates a high resistivity and a high temperature coefficient of resistance. If YCrO<NUM> is added more percentage, a resistivity and the temperature coefficient of resistance are lowered. When the content percentages of Mn<NUM>Cr<NUM>O<NUM> and YCrO<NUM> are changed, the resistivity can be changed in a wide range and also the temperature coefficient in a wide range.

It is required for thermistors to be used in a higher temperature range, and in the circumstance, it is required to control the B constant of the thermistor itself so that the B constant can cope with this high temperature range.

For this reason, an object of the present invention is to provide a thermistor sintered body which can control the B constant to the same extent as that of the conventional wide range type even at <NUM>, and to provide a thermistor element.

The above object is achieved by a thermistor sintered body according to claim <NUM> and by a thermistor element according to claim <NUM>. The dependent claims are directed to different advantageous aspects of the invention.

According to the present invention, it is possible to control the B constant (B(<NUM>/<NUM>)) to <NUM> or lower, in the thermistor sintered body formed of a composite structure which includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase or the YMnO<NUM> phase, by controlling the composition so that <NUM> ≤ Cr/Mn < <NUM> holds, in other words Mn is relatively rich to Cr. Thereby, the present invention can provide a thermistor sintered body that can accurately detect a temperature in a wide temperature range from -<NUM> to <NUM>.

Embodiments of the present invention will be described below with reference to the attached drawings.

As is shown in <FIG>, a thermistor sintered body according to the present embodiment is an oxide sintered body having a composite structure including the Y<NUM>O<NUM> phase (<NUM>) and the Y(Cr, Mn)O<NUM> phase (<NUM>). This thermistor sintered body corresponds to the previously described form A1 and form A2.

The Y<NUM>O<NUM> phase has properties of an electrical insulator, and affects a resistance value of the thermistor sintered body. In addition, the Y(Cr, Mn)O<NUM> phase has a property of a semiconductor, and affects a B constant of the thermistor sintered body.

The thermistor sintered body has a structure of a sintered body, which has the Y<NUM>O<NUM> phase of which the resistance value and the B constant are high, and the Y(Cr, Mn)O<NUM> phase of which the resistance value and the B constant are low. The thermistor sintered body is more occupied by the Y<NUM>O<NUM> phase than by the Y(Cr, Mn)O<NUM> phase, and more than <NUM>% by volume to <NUM>% by volume is occupied by the Y<NUM>O<NUM> phase, and the remaining portion (<NUM>% by volume or more, and less than <NUM>% by volume) is occupied by the Y(Cr, Mn)O<NUM> phase.

As one example, a microstructure photograph (<NUM> magnitudes) of the thermistor sintered body is shown in <FIG>. The thermistor sintered body typically has a sea-island structure, and has a composite structure in which the Y(Cr, Mn)O<NUM> phase that forms a sub-phase is dispersed in the Y<NUM>O<NUM> phase that forms a main phase. The thermistor sintered body preferably contains <NUM> to <NUM>% by volume of the Y<NUM>O<NUM> phase, and more preferably <NUM> to <NUM>% by volume.

When the thermistor sintered body has the sea-island structure, grain boundaries cannot be clearly identified in some cases, but the Y<NUM>O<NUM> phase has an average grain size (d50) of approximately <NUM> to <NUM> and the Y(Cr, Mn)O<NUM> phase has an average grain size of approximately <NUM> to <NUM>.

Each of the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase in the thermistor sintered body has been subjected to a composition analysis.

The results are shown in <FIG>, and it has been confirmed that Ca dissolves in the Y(Cr, Mn)O<NUM> phase. Ca dissolves in the Y(Cr, Mn)O<NUM> phase, which is considered to thereby contribute to lowering the B constant of the Y(Cr, Mn)O<NUM> phase.

In the case of a composition system which does not contain Cr, as in the form B, the YMnO<NUM> phase is formed instead of the Y(Cr, Mn)O<NUM> phase, and Ca dissolves in the YMnO<NUM> phase.

The thermistor sintered body exhibits thermistor characteristics of NTC, and has the following characteristic: the B constant (B(<NUM>/<NUM>)) in a temperature range between <NUM> and <NUM> is <NUM> or lower. Thus, according to the thermistor sintered body, the B constant can be lowered in a wide temperature range. Besides, the thermistor sintered body according to the present invention can control a resistance value at <NUM> to <NUM> kΩ or larger, and preferably to <NUM> kΩ or larger; and also can exhibit a resistance value of <NUM> kΩ at room temperature or lower, and preferably below the freezing point, for example, at -<NUM> or lower. Thereby, by using the thermistor sintered body, it becomes possible to accurately detect a temperature in a wide temperature range ranging from a temperature below the freezing point to <NUM>, more specifically, -<NUM> to <NUM>.

The thermistor sintered body which has the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase and corresponds to the form A1 has a chemical composition of Cr, Mn, Ca and Y excluding oxygen, which contains Cr: <NUM> to <NUM> mol%, Mn: <NUM> to <NUM> mol%, Ca: <NUM> to <NUM> mol%, and the balance being unavoidable impurities and Y.

In addition, the thermistor sintered body which includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase and corresponds to the form A2 has a chemical composition of Cr, Mn, Ca and Y excluding oxygen, which contains Cr: ≤<NUM> mol% (excluding <NUM>), Mn: <NUM> to <NUM> mol%, Ca: <NUM> to <NUM> mol%, and the balance being unavoidable impurities and Y.

In addition to adopting the above described composition range, the thermistor sintered body has a composition in which Cr/Mn that is a ratio of Cr to Mn is smaller than <NUM>, in other words, Mn is relatively rich to Cr. The thermistor sintered body can lower the B constant by adopting this Mn-rich composition. Because of this, in the thermistor sintered body, in the form A1, the range of Cr is set at <NUM> to <NUM> mol%, but on the other hand, the range of Mn is set at <NUM> to <NUM> mol%.

A preferable range of Cr is <NUM> to <NUM> mol%, and a more preferable range of Cr is <NUM> to <NUM> mol%.

In addition, a preferable range of Mn is <NUM> to <NUM> mol%, and a more preferable range of Mn is <NUM> to <NUM> mol%.

It is preferable for the Cr/Mn to be <NUM> to <NUM>, is more preferable to be <NUM> to <NUM>, and is further preferable to be <NUM> to <NUM>.

Also in the form A2, the Mn-rich composition is adopted; but a range of Cr is ≤<NUM> mol% (excluding <NUM>), and on the other hand, a range of Mn is set at <NUM> to <NUM> mol%.

It is preferable for Cr/Mn to be <NUM> or smaller, and is more preferable to be <NUM> or smaller.

The form B which adopts the most Mn-rich composition does not contain Cr, and sets a range of Mn at <NUM> to <NUM> mol%. The preferable range of Mn is similar to that in the form A2.

Ca has a function of lowering the B constant of the thermistor sintered body by dissolving in the Y(Cr, Mn)O<NUM> phase. Accordingly, when the amount of Ca is increased, the B constant can be lowered even though the Cr/Mn has been increased.

In the form A1, a preferable range of Ca is <NUM> to <NUM> mol%, and a more preferable range of Ca is <NUM> to <NUM> mol%.

The form A2 and the form B contain <NUM> to <NUM> mol% of Ca, which is more as compared with that of the form A1; and a preferable range of Ca is <NUM> to <NUM> mol%, and a more preferable range of Ca is <NUM> to <NUM> mol%.

Next, one example of a method for producing a thermistor sintered body will be described with reference to <FIG>.

As is shown in <FIG>, the production method according to the present embodiment includes steps of weighing of raw material powders, mixing of the raw material powders, drying of the raw material powders, calcining, mixing/grinding after calcining, drying/granulating, compacting and sintering. Each of the steps will be described below in sequence.

In the present embodiment, yttrium oxide (Y<NUM>O<NUM>) powder, chromium oxide (Cr<NUM>O<NUM>) powder, manganese oxide (MnO, Mn<NUM>O<NUM>, Mn<NUM>O<NUM> or the like) powder and CaCO<NUM> powder are used as raw material powders. When a thermistor sintered body according to the form B is produced, the chromium oxide (Cr<NUM>O<NUM>) powder is removed.

The above described raw material powders are weighed so as to form the above described chemical composition.

The Y<NUM>O<NUM> powder contributes to the formation of a Y<NUM>O<NUM> phase, and the Y<NUM>O<NUM> powder, the Cr<NUM>O<NUM> powder and the Mn<NUM>O<NUM> powder contribute to the formation of a Y(Cr, Mn)O<NUM> phase. The CaCO<NUM> powder functions as a sintering aid; and in addition, dissolves in the Y(Cr, Mn)O<NUM> phase in a form of Ca, and contributes to lowering a B constant.

As for the raw material powders, powders are used of which the purities are <NUM>% or higher, preferably <NUM>% or higher, and more preferably <NUM>% or higher, in order to obtain a thermistor sintered body having high characteristics.

In addition, the particle size of the raw material powder is not limited as long as calcination proceeds, but can be selected from a range of <NUM> to <NUM> by an average particle size (d50).

The Y<NUM>O<NUM> powder, the Cr<NUM>O<NUM> powder, the Mn<NUM>O<NUM> powder and the CaCO<NUM> powder which have been weighed by predetermined amounts are mixed. The mixing can be performed, for example, by using a ball mill to mix the mixed powders converted into a slurry by addition of water. A mixer other than the ball mill can also be used for the mixing.

It is preferable to dry and granulate the slurry which has been mixed, by a spray dryer or other equipment, and to form a mixed powder for calcination.

The mixed powder for calcination, which has been dried, is calcined. By the calcination, a calcined body having a composite structure of the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase is obtained from the Y<NUM>O<NUM> powder, the Cr<NUM>O<NUM> powder, the Mn<NUM>O<NUM> powder and the CaCO<NUM> powder.

The calcination is performed by placing the mixed powder for calcination, for example, in a crucible, and holding the mixed powder in a temperature range of <NUM> to <NUM> in the air. If the calcination temperature is lower than <NUM>, the formation of the composite structure is insufficient, and if the calcination temperature exceeds <NUM>, there is a possibility that a sintering density lowers and/or the stability of the resistance value lowers. Therefore, the holding temperature in the calcination is set in the range of <NUM> to <NUM>.

A holding time period in the calcination should be appropriately set according to the holding temperature, but if the holding temperature is in the above described temperature range, the purpose of the calcination can be achieved by the holding time period of approximately <NUM> to <NUM> hours.

The calcined powder is mixed and pulverized. The powders can be converted into slurry by addition of water, and be mixed and pulverized with the use of a ball mill, in the same manner as before the calcination.

It is preferable to dry and granulate the pulverized powder, by a spray dryer or other equipment.

The granulated powder after the calcination is compacted into a predetermined shape.

For compacting, press compacting with the use of a die, and besides a cold isostatic press (CIP) can be used.

The higher the density of the compacted body is, the higher density a sintered body easily obtains; and accordingly, it is desirable to enhance the density of the compacted body as highly as possible. For that purpose, it is preferable to use the CIP which can obtain the high density.

Next, the obtained compacted body is sintered.

The sintering is performed by a procedure of holding the temperature range of <NUM> to <NUM> in the air. If the sintering temperature is lower than <NUM>, the formation of the composite structure is insufficient; and if it exceeds <NUM>, the sintered body melts, and/or a reaction occurs with a sintering crucible and/or the like. The holding time period in the sintering should be appropriately set according to the holding temperature, but if the holding temperature is in the above described temperature range, a dense sintered body can be obtained by a holding time period of approximately <NUM> to <NUM> hours.

It is preferable to subject the obtained thermistor sintered body to annealing, in order to stabilize the thermistor characteristics. The thermistor sintered body is annealed by being held, for example, at <NUM> in the air.

A specific example of a thermistor <NUM> will be described to which the thermistor sintered body obtained in the above described way is applied.

As is shown in <FIG>, the thermistor <NUM> includes a thermistor element <NUM> and a covering layer <NUM>.

The thermistor element <NUM> is used together with a detection circuit for extracting a change of a resistance value as a change of voltage, thereby detects a temperature of an environment in which the thermistor element <NUM> is placed, and generates a temperature detection signal consisting of an electrical signal.

The covering layer <NUM> seals the thermistor element <NUM> to keep the thermistor element <NUM> in an airtight state, thereby prevents the occurrence of chemical and physical changes of the thermistor sintered body in particular, based on the environmental conditions, and also mechanically protects the thermistor element <NUM>.

As is shown in <FIG>, the thermistor element <NUM> in this example includes a flat plate-like thermistor sintered body, electrodes 12A and 12B, connection electrodes 13A and 13B, and lead wires 14A and 14B.

The electrodes 12A and 12B are each formed in a film shape on the whole area of both the front and rear surfaces of the plate-like thermistor sintered body. The electrodes 12A and 12B are formed from platinum (Pt) or another noble metal.

The electrodes 12A and 12B are formed as thick films or thin films. The thick film electrodes 12A and 12B are formed by applying a paste which has been prepared by mixing an organic binder with platinum powder, to both the front and rear surfaces of the thermistor sintered body, and by drying and then sintering the paste. On the other hand, the thin film electrode can be formed by vacuum deposition or sputtering.

The thermistor sintered body on which the electrodes 12A and 12B have been formed is worked into a predetermined dimension.

The connection electrodes 13A and 13B are formed of metal films which are formed on the surfaces of the electrodes 12A and 12B, respectively. Also the connection electrodes 13A and 13B are formed from platinum (Pt) or another noble metal.

One ends of the lead wires 14A and 14B are electrically and mechanically connected to the electrodes 12A and 12B via the connection electrodes 13A and 13B, respectively. The other ends of the lead wires 14A and 14B are connected to an external detection circuit. The lead wires 14A and 14B are formed of a wire material which has heat resistance and is formed from, for example, platinum or an alloy of platinum and iridium (Ir).

The lead wires 14A and 14B are connected to the electrodes 12A and 12B in the following way.

A paste containing a platinum powder which will form the connection electrodes 13A and 13B is applied to one end sides of the lead wires 14A and 14B beforehand. The platinum paste is dried in a state in which sides of the lead wires 14A and 14B, to which the platinum paste has been applied, are in contact with the electrodes 12A and 12B, respectively, and then the platinum powder is sintered.

For the covering layer <NUM> shown in <FIG>, the glass can be used which contains, for example, SiO<NUM>, CaO, SrO, BaO, Al<NUM>O<NUM> and SnO<NUM> as the raw materials. By such glass, the thermistor element <NUM> and the one end sides of the lead wires 14A and 14B are sealed.

A method for sealing the thermistor sintered body and the like by the covering layer <NUM> can be arbitrarily selected; and it is possible to seal the thermistor sintered body and the like by covering the thermistor sintered body and the like with, for example, a glass tube which is made from glass and becomes the covering layer <NUM>, and then by melting the glass tube.

The thermistor <NUM> is preferably subjected to annealing treatment, after having been sealed by glass and cooled. By this annealing treatment, it becomes possible to prevent the resistance of the thermistor element <NUM> from decreasing.

Next, another form of the thermistor <NUM> will be described with reference to <FIG>.

The thermistor <NUM> includes a thermistor element <NUM> and a covering layer <NUM> as shown in <FIG>, and is similar to the thermistor <NUM> in appearance. The thermistor element <NUM> and the covering layer <NUM> have similar functions to those of the thermistor element <NUM> and the covering layer <NUM> of the thermistor <NUM>, respectively.

As shown in <FIG>, the thermistor element <NUM> in this example includes a flat plate-like thermistor sintered body, electrodes 22A and 22B, connection electrodes 23A and 23B, and lead wires 24A and 24B.

The thermistor element <NUM> has features in portions of the thermistor sintered body and the electrodes 22A and 22B, as compared with the thermistor element <NUM>. As shown in a middle stage of <FIG>, the thermistor sintered body and the electrodes 22A and 22B constitute a thermistor chip <NUM>, in the thermistor element <NUM>. The thermistor chip <NUM> is produced in the following way.

Into the previously described pulverized calcined powder, for example, an ethyl cellulose-based binder is mixed, and the mixture is formed into a sheet shape. The conditions of the calcination are as previously described.

Next, a predetermined dimension of the sheet is punched from the formed sheet, and is sintered. The conditions of the sintering are as previously described. Then, a wafer obtained by the sintering is polished and a wafer <NUM> having a predetermined thickness is obtained as is shown in the middle stage of <FIG>. After that, a paste for forming an electrode is applied to both the front and rear surfaces of the polished wafer <NUM> (thermistor sintered body) by printing, and then is sintered; and a wafer <NUM> is obtained on which electrode films have been formed. The electro-conductive material to be contained in the paste is selected from platinum (Pt) and other noble metals. When the platinum has been selected, the sintering is performed at approximately <NUM>. After that, the wafer <NUM> is cut so as to have a predetermined dimension, as shown in the middle stage of <FIG>, and thereby the thermistor chip <NUM> is obtained which has the film-like electrodes 22A and 22B formed on the front and back surfaces, respectively.

Next, the lead wires 24A and 24B are joined to the electrodes 22A and 22B on both the front and rear surfaces of the thermistor sintered body, respectively, with the use of a Pt paste, then the resultant thermistor sintered body is subjected to baking treatment to have the connection electrodes 23A and 23B formed thereon, and the thermistor element <NUM> shown in the lower part of <FIG> is produced.

Next, the covering layer <NUM> is formed; and for the covering layer <NUM>, the previously described glass can be used, or the covering material can also be used which is formed of a constituent material similar to the thermistor sintered body.

For this covering material, a composite oxide can be used that has been obtained by sintering a powder which has been obtained by calcining Y<NUM>O<NUM>, Cr<NUM>O<NUM> and Mn<NUM>O<NUM> disclosed in the above mentioned <CIT> and a B<NUM>O<NUM> powder. In other words, the covering layer of the thermistor sintered body in the present invention can be arbitrarily selected as long as the purpose can be achieved.

Next, a thermistor sintered body of the present invention will be described based on Example <NUM>. Example <NUM> corresponds to the previously described form A1.

Raw material powders having the following average particle sizes were prepared, and thermistor sintered bodies having various compositions shown in <FIG> were produced according to the above described production steps. In this table, Nos. <NUM> to <NUM> are samples in which Cr/Mn is smaller than <NUM> and Mn is rich relative to Cr; and Nos. <NUM> and <NUM> are samples in which Cr/Mn is <NUM> or larger, in other words, Cr and Mn are in equal amounts, or Cr is rich. Calcination was performed under conditions of <NUM> and <NUM> hours, and sintering was performed under conditions of <NUM> and <NUM> hours, both in the air.

The B constant was determined for each of the obtained sintered bodies. The results are collectively shown in <FIG>. Incidentally, the B constants in <FIG> are values between <NUM> and <NUM> (B25/<NUM>).

As is shown in <FIG>, it is understood that the B constants (B25/<NUM>) of thermistor sintered bodies (Samples Nos. <NUM> to <NUM>) in which Mn is rich relative to Cr become low, as compared to thermistor sintered bodies (Samples Nos. <NUM> and <NUM>) in which Cr/Mn is <NUM> or larger.

Referring to the relationship between Cr/Mn and the B constant, as shown in the Sample No. <NUM>, for example, when Mn is too excessive with respect to Cr, the B constant B25/<NUM> becomes high. Therefore, in order to lower the B constant (B25/<NUM>), it is preferable for Cr/Mn to be controlled in the range of <NUM> to <NUM>, and is more preferable to be controlled in the range of <NUM> to <NUM>.

Ca contents are different between Samples Nos. <NUM> to <NUM> and Samples Nos. <NUM> to <NUM>, and are <NUM> mol% and <NUM> mol%, respectively; and the B constants of Samples Nos. <NUM> to <NUM>, of which the Ca contents are high, can be lowered, as compared to those of Samples Nos. <NUM> to <NUM>.

The same can be inferred from the comparison between Samples Nos. <NUM> to <NUM> and Samples Nos. <NUM> to <NUM>.

Next, the B constants in a plurality of temperature ranges were determined by a procedure of measuring resistance values in the range of -<NUM> to <NUM>, about several samples in <FIG>. The B constants were determined about the following four types of which the Rn and Rm were different in the following expression. Incidentally, the B constants were similarly determined for Sample Nos. <NUM> and <NUM>, which corresponded to Comparative Example.

It is understood that B(<NUM>/<NUM>) of the Samples Nos. <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> which correspond to the present invention are <NUM> or lower, and the B constants are controlled to be lower in a wide temperature range of - <NUM> to <NUM> than those of Samples Nos. <NUM> and <NUM>, which correspond to Comparative Example.

B(-<NUM>/<NUM>), B(<NUM>/<NUM>) and B(<NUM>/<NUM>) are the B constants which were determined by dividing the range of -<NUM> to <NUM> into a low temperature range (-<NUM> to <NUM>), a middle temperature range (<NUM> to <NUM>) and a high temperature range (<NUM> to <NUM>).

It is understood that the B constants of Samples Nos. <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> which correspond to the present invention are controlled to be lower than those of Samples Nos. <NUM> and <NUM>, which correspond to Comparative Example, in any temperature range of the low temperature range, the middle temperature range and the high temperature range.

In <FIG>, the resistance values at <NUM> (<NUM>, in case of Comparative Example) are shown, and according to the thermistor sintered body according to the present invention, the resistance value at <NUM> can be increased to <NUM> kΩ or larger, and further <NUM> kΩ or larger. In Sample No. <NUM> of which the resistance value at <NUM> is <NUM> kΩ or larger, the resistance value does not fall below <NUM> kΩ even at <NUM>; and Sample No. <NUM> can achieve accurate temperature detection at <NUM>.

In addition, in <FIG>, a temperature at which a resistance value of <NUM> kΩ is exhibited is also shown, but the thermistor sintered body in the present invention can exhibit a resistance value of <NUM> kΩ at a temperature equal to or lower than room temperature such as -<NUM> and even -<NUM>.

As described above, it is possible to achieve the accurate temperature detection in a wide temperature range from -<NUM> to <NUM>, by using the thermistor sintered body of the present invention.

The graphs of <FIG> show a relationship between a temperature and a resistance value (R-T curve) of Samples Nos. <NUM>, <NUM>, <NUM> and <NUM> in the range of -<NUM> to <NUM>. However, the temperatures of Sample Nos. <NUM> and <NUM> are <NUM> or lower. The above description is similar also in <FIG> which will be described later.

In Sample No. <NUM> and Sample No. <NUM> which correspond to the present invention, the R-T curve is positioned in a middle of the R-T curves of Sample No. <NUM> and Sample No. <NUM>. In addition, Sample No. <NUM> and Sample No. <NUM> show a resistance value equivalent to Sample No. <NUM> which is directed to a low temperature to a medium temperature, at a low temperature of -<NUM>, and show a resistance value equivalent to Sample No. <NUM> which is directed to a medium temperature to a high temperature, at a high temperature of <NUM>. In other words, Sample No. <NUM> and Sample No. <NUM> enable the accurate temperature detection in a wide temperature range from the low temperature to the high temperature.

Next, the graphs of <FIG> show R-T curves in the range of -<NUM> to <NUM> of Samples Nos. <NUM>, <NUM>, <NUM> and <NUM>.

Sample No. <NUM> has a high resistance value in a range from a medium temperature to a high temperature, enables the temperature detection in a wide temperature range from a low temperature to a high temperature, and is suitable for accurately detecting a temperature particularly in the high temperature range.

Next, a thermistor sintered body of the present invention will be described based on Example <NUM>. Example <NUM> corresponds to the previously described form A2 and form B.

Example <NUM> is further richer in Mn relative to Cr than Example <NUM>, and includes even a thermistor sintered body (form B) which does not contain Cr.

Thermistor sintered bodies having chemical compositions shown in <FIG> were produced by a similar method to that in Example <NUM>. Samples Nos. <NUM> to <NUM> in <FIG>, in which Cr/Mn is <NUM>, is richer in Mn than Sample No. <NUM> in which Mn is richest in Example <NUM> and Cr/Mn is <NUM>. Furthermore, Samples Nos. <NUM> to <NUM> in <FIG> do not contain Cr, and can be referred to as examples in which Mn is richest relative to Cr. In addition, Samples Nos. <NUM> to <NUM> contain Ca in the range of <NUM> to <NUM> mol%, and contain more Ca than Sample No. <NUM> and the like that contain <NUM> mol% of Ca, which is highest in Example <NUM>.

For the thermistor sintered bodies of Samples Nos. <NUM> to <NUM>, the B constant was measured in a similar way to that in Example <NUM>. The results are shown in <FIG>. As for Sample Nos. <NUM> to <NUM>, the B constants equivalent to those in Example <NUM> were obtained, and as for Samples Nos. <NUM> to <NUM>, the B constants which surpassed those in Example <NUM> were obtained. The measurement results are out of the cognizance of the person skilled in the art, as will be described below.

Conventionally, in regard to Cr/Mn and the B constant and in regard to the Ca content and the B constant, a thermistor sintered body which includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase and contains Ca has been recognized in the following way.

A low B constant can be obtained in a sintered body that includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase, in a range in which Cr/Mn is in the vicinity of <NUM>. When Mn exceeds this range and becomes rich, the B constant becomes high.

A low B constant can be obtained in the sintered body which includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase, in a range in which the content of Ca is in the vicinity of <NUM> mol%. When the sintered body contains Ca beyond this range, the B constant becomes high.

According to the above described conventional recognition, the B constants of Sample Nos. <NUM> to <NUM> are high in which Mn is rich relative to Cr and Ca exceeds <NUM> mol%. However, as shown in <FIG>, the B constants of Samples Nos. <NUM> to <NUM> are low. This is understood to be because crystal structures of these samples are concerned.

It has been considered that when the thermistor sintered body which includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase has the crystal structure of an orthorhombic crystal system, the thermistor sintered body exhibits stable characteristics (B constant) until reaching a high temperature. On the other hand, YMnO<NUM> which does not contain Cr has a crystal structure of a hexagonal crystal system, and accordingly shows a high B constant. In addition, when YMnO<NUM> becomes the Y(Cr, Mn)O<NUM> phase by containing Cr, the crystal structure becomes the orthorhombic crystal system.

However, according to the study of the present inventors, even though Mn is rich relative to Cr, the crystal structure of the orthorhombic crystal system can be obtained by containing Ca in an amount exceeding conventional recognition, and the B constant can be lowered.

Specifically, as is shown in <FIG>, the crystal structures of Samples Nos. <NUM>, <NUM>, <NUM> and <NUM> in Example <NUM> are the orthorhombic crystal system as are those of Samples Nos. <NUM>, <NUM> and the like in Example <NUM>.

In <FIG>, the bottom is a chart of a sintered body which is formed of a composite phase of the Y<NUM>O<NUM> phase and the YMnO<NUM> phase and does not contain Ca, and the second from the bottom is a chart of a sintered body which is formed of the composite phase of the Y<NUM>O<NUM> phase and the YMnO<NUM> phase and contains <NUM> mol% of Ca. Furthermore, the third from the bottom is a chart of a sintered body which is formed of the composite phase of the Y<NUM>O<NUM> phase and the YMnO<NUM> phase and contains <NUM> mol% of Ca.

An enlarged view of a portion in which these three charts are surrounded by a rectangle in <FIG> is shown in <FIG>. A single-phase sintered body of the YMnO<NUM>, which does not contain Ca, has a crystal structure of only a hexagonal crystal system.

In a sintered body which further contains <NUM> mol% of Ca in the above sintered body, a peak of the crystal structure of the orthorhombic crystal system is detected, but a peak of the crystal structure of the hexagonal crystal system is detected; and the sintered body has a crystal structure in which the orthorhombic crystal system and the hexagonal crystal system are mixed.

In a sintered body in which the content of Ca has increased to <NUM> mol%, a peak of the crystal structure of the hexagonal crystal system disappears; and the sintered body is formed of a crystal structure of only the orthorhombic crystal system.

The results are as described above, and the single-phase sintered body of the YMnO<NUM> has the crystal structure of the hexagonal crystal system. However, the crystal structure of the orthorhombic crystal system can be obtained by containing a considerable amount of, here <NUM> mol% of Ca, even though the sintered body is the single-phase sintered body of the YMnO<NUM>. The reason why the B constant of Example <NUM> (Sample Nos. <NUM> to <NUM>) is low is attributed to that the crystal structure is the orthorhombic crystal system.

As has been described in the above Example <NUM> and Example <NUM>, the thermistor sintered body having a composite structure which includes the Y<NUM>O<NUM> phase and the Y(Cr, Mn)O<NUM> phase, or the Y<NUM>O<NUM> phase and the YMnO<NUM> phase can control the B constant to be low and can control the lowering of the resistance value particularly in a high temperature range, by controlling Mn to be rich relative to Cr. Accordingly, according to the present invention, the accurate temperature detection can be achieved in a wide temperature range.

The present invention has been described above based on the preferred embodiments and Examples, but the configurations included in the above described embodiments can be selected, or be appropriately changed to other configurations, insofar as they do not deviate from the scope of the invention as disclosed in the appended claims.

The thermistor sintered body and the thermistor of the present invention can be used over a wide temperature range from -<NUM> to approximately <NUM>, and accordingly can be widely used as a temperature sensor for automotive exhaust-gas treatment devices, and for the measurement of a high temperature in a water heater, a boiler, an oven range, a stove and the like.

Claim 1:
A thermistor sintered body comprising a sintered body that comprises:
more than <NUM>% by volume to <NUM>% by volume of a Y<NUM>O<NUM> phase as a main phase;
<NUM>% by volume to less than <NUM>% by volume of a Y(Cr, Mn)O<NUM> phase as a sub-phase dispersed in the main phase; and
Ca dissolved in the Y(Cr, Mn)O<NUM> phase,
characterized in that the sintered body has a composition in which <NUM> ≤ Cr/Mn < <NUM> so that the sintered body is rich in Mn relative to Cr,
the sintered body has a crystal structure of an orthorhombic crystal system, and
the sintered body satisfies one of the following conditions (a) to (b):
(a) the sintered body comprises a Y(Cr, Mn)O<NUM> phase as the sub-phase, and comprises <NUM> to <NUM> mol% of Cr, <NUM> to <NUM> mol% of Mn, <NUM> to <NUM> mol% of Ca, and a balance of Y, wherein Cr/Mn is <NUM> to <NUM> and calculation of the molar percentages of Cr, Mn, Ca and Y does not take into account the amount of oxygen comprised by the sintered body; or
(b) the sintered body comprises a Y(Cr, Mn)O<NUM> phase as the sub-phase, and comprises ≤<NUM> mol% (excluding <NUM>) of Cr, <NUM> to <NUM> mol% of Mn, <NUM> to <NUM> mol% of Ca, and a balance of Y, wherein Cr/Mn is <NUM> or smaller (excluding <NUM>) and calculation of the molar percentages of Cr, Mn, Ca and Y does not take into account the amount of oxygen comprised by the sintered body;
such that a B constant (B(<NUM>/<NUM>)) of the sintered body determined by the following expression is <NUM> or lower and <NUM> or more: <MAT> wherein
ln: natural logarithm,
Rm: resistance value at <NUM>,
Rn: resistance value at <NUM>,
Tm: <NUM>, and
Tn: <NUM>.