Resistance paste and resistor comprising the material

Abstract: An organic vehicle is added to and kneaded with a solid component comprising from 60 to 95 % by weight of a resistance material having a composition of La x Sr 1-x CoO 3 (x is from 0.40 to 0.60) and from 5 to 40 % by weight of glass frit to obtain a resistance paste. A substrate is coated with the resistance paste and baked to produce a resistor. The resistance paste can be baked in any of air, neutral and reducing atmospheres. The resistor has any desired resistance value within a broad range, and the reproducibility of the resistor with a desired resistance value is good.

Description: The present invention relates to a resistance paste, a method of producing a resistance paste, a resistor and a method of producing a resistor. More specifically, the present invention relates to a resistance paste which can be baked in an oxidizing, neutral or reducing atmosphere, and a resistor formed by the use of the resistance paste. Description of the Related Art: In general, a ceramic substrate such as alumina, zirconia-or the like is provided with circuit patterns for electrodes, resistors, etc., in order that various electronic parts can be mounted thereon. Electrodes (electrode patterns) are generally formed on the substrate by screen-printing a noble metal electrode paste comprising, silver, a silver-palladium alloy or the like as the conductive component, followed by baking the thus-printed paste in air. Resistors (resistor patterns) which are to connect the thus-formed electrode patterns with each other are usually also formed by printing a resistance paste comprising a resistance material of an oxide of a noble metal such as ruthenium at predetermined sites followed by baking it in air. However, a noble metal paste such as that mentioned above is not only expensive but also problematic in its migration resistance. Therefore the tendency for such an expensive noble metal paste to be replaced by a base metal paste comprising, as the conductive component, copper, nickel, aluminum or the like has become accepted in this technical field. Such a base metal paste can be screen-printed on a substrate and then baked in the neutral or reducing atmosphere to give an inexpensive and good electrode pattern. In this case, it is desirable that the resistance paste which is to form resistors (resistor patterns) on the substrate, by which the plural base electrodes as formed by baking the printed base metal paste are connected with each other, can also be baked in a neutral or reducing atmosphere. Therefore, various resistance pastes that can be baked in a neutral or reducing atmosphere to form resistors (resistor patterns) have been proposed. Of conventional resistance pastes, those that can be baked in an air atmosphere consist essentially of expensive noble metal oxides such as ruthenium oxide or bismuthruthenium composite oxide (pyrochroite type material), while a metal glaze resistance paste comprising silver-palladium is used on rare occasions only when resistors in low-resistance regions are formed. As materials that can be baked in a neutral or reducing atmosphere, various resistance pastes comprising LaB 6 , SnO 2 , silicides, SrRuO 3 , Nb x La 1-x B 6-4x or the like have been proposed and have already been put to practical use. However, the above-mentioned conventional resistance pastes may be baked in different atmospheres and, at present, there are known only a few resistance pastes that can be baked in either air or reducing atmospheres. In addition, conventional resistance pastes for thick resistor films are expensive because of comprising noble metal oxides. In the prior art, the resistance values of resistors are controlled (or varied) by changing the ratio of the resistance material to glass frit with which it is mixed. However, depending on the type of the resistance material to be used, the change in the mixing ratio often causes a too rapid change in the resistance values of the resistors formed and is therefore problematic in that it is difficult to attain_the desired resistance values and in that the reproducibility in the production of the desired resistors is extremely poor. The publication INORGANIC MATERIALS, vol. 17, no. 6, May 1981, pages 749-753, XP000609209, TOLOCHKO S.P. et al: "Electrical conductivity of complex oxides La 1-x Sr x CoO 3 (x=0.2-1.0)" discloses a resistance material which has the composition La 0.5 Sr 0.5 oO 3 which can be produced by sintering a raw material mixture of La, Sr and Co containing raw material substances at a temperature of 1100°C. SUMMARY OF THE INVENTION The object of the present invention is to provide a resistance paste which can be baked in any of air, neutral and reducing atmospheres to reliably give resistors having any desired resistance values within a broad range, a method for producing a resistance paste, a resistor which can be formed by the use of the resistance paste and which can realize resistance values within a broad range while the reproducibility of the realizable resistance values is good, and a method of producing a resistor. This object is achieved by a resistance paste according to claim 1 and by a method for producing a resistance paste according to claim 3. The resistance material forming part of the resistance paste has a composition of the general formula: La x Sr 1-x CoO 3 wherein x is from about 0.40 to 0.60. The resistance paste which the present invention provides so as to attain the above-mentioned object is characterized in that it comprises a solid component consisting of from about 60 to 95 % by weight of the resistance material and from about 5 to 40 % by weight of glass frit and an organic vehicle. The resistor and the method of producing such a resistor which the present invention also provides so as to attain the above-mentioned object are defined by claims 5 and 6. The resistor is formed by coating the resistance paste on a substrate and then baking it thereon in an air atmosphere or in a neutral or reducing atmosphere such as in nitrogen. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph showing the relationship between temperatures at which resistance materials were produced in the examples and the comparative examples mentioned hereinunder and the specific resistivity values of the materials. DETAILED DESCRIPTION OF THE INVENTION The resistance material used in the resistance paste of the present invention has a composition of a general formula: La x Sr 1-x CoO 3 wherein x is from 0.40 to 0.60. One embodiment of the resistance material is such that the material is produced by sintering a raw material mixture to be prepared by mixing a La-containing raw material substance, a Sr-containing raw material substance and a Co-containing raw material substance at a predetermined ratio, at a temperature falling between about 800°C and 1150°C. The resistance paste of the present invention comprises a solid component consisting of from 60 to 95 %, preferably about 65 to 90 %, by weight of the resistance material and from 5 to 40 %, preferably about 10 to 35 %, by weight of glass frit and an organic vehicle. One embodiment of the resistance paste is such that the glass frit is (a) a lead zinc borosilicate glass frit comprising from 15 to 25 mol% of B 2 O 3 , from 40 to 50 mol% of SiO 2 , from 15 to 25 mol% of PbO and from 7 to 13 mol% of ZnO, or (b) a calcium barium borosilicate glass frit comprising from 3 to 10 mol% of B 2 O 3 , from 35 to 45 mol% of SiO 2 , from 25 to 35 mol% of CaO and from 15 to 20 mol% of BaO. The resistor of the present invention is formed by coating the resistance paste on a substrate and then baking it thereon in an air atmosphere or in a neutral or reducing atmosphere such as in nitrogen. In the resistance material used in the resistance paste of the present invention, x falls between about 0.40 and 0.60. This is because, if x is less than 0.40 or more than 0.60, the resistance material has a much increased specific resistivity value (not lower than 10 -1 Ω•cm) and therefore loses electroconductivity. If so, the material cannot satisfy the object of the present invention where the material is one having electroconductivity in some degree. In the resistance paste of the present invention, the content of the resistance material in the solid component falls between 60 % by weight and 95 % by weight and that of the glass frit in the same falls between 5 % by weight and 40 % by weight. This is because if the content of the glass frit is less than 5 % by weight, the adhesiveness between a fired resistor and the substrate is lowered with the result that the resistor formed can be peeled from the substrate, but if it is more than 40 % by weight, the specific resistivity of the resistor formed is unfavorably too large. The particle size of the resistance material to be in the resistance paste of the present invention is preferably from about 0.1 to 5 µm, more preferably from about 0.5 to 3 µm. The particle size of the glass frit to be in the resistance paste of the present invention is preferably from about 1 to 10 µm, more preferably not larger than about 5 µm. To prepare the resistance paste of the present invention, an organic vehicle is added to and kneaded with a mixture (solid component) comprising the resistance material and glass frit, so that the resulting resistance paste shall have the necessary printability. For this, various organic vehicles which are generally used in ordinary resistance pastes for forming thick film resistors are employable and which are prepared, for example, by dissolving an ethyl cellulose resin or acrylic resin in a terpene solvent such as α-terpineol or in a high-boiling point solvent such as kerosene, butyl carbitol, carbitol acetate or the like. If desired, additives may be added to the paste so as to make it thixotropic. The present invention is explained in more detail with reference to the following examples, which, however, are not intended to restrict the scope of the present invention. Examples: Production of Resistance Material Samples: As raw material substances for resistance materials, powdery La 2 O 3 , SrCO 3 and Co 3 O 4 were weighed at predetermined proportions, mixed, put into a crucible and heated in air at predetermined temperatures. The raw material substances used are not be limited to only the abovementioned ones but carbonates may be used in place of the oxides or other oxides may be used in place of carbonates. As the case may be, any other substances (compounds) may also be used. In these examples, the raw material substances were weighed at such proportions that x in the general formula La x Sr 1-x CoO 3 representing the resistance material used in the paste of the present invention is 0.30, 0.40, 0.50, 0.60 or 0.70. The heat treatment for sintering the raw material mixtures to produce resistance materials was conducted in an air atmosphere at 650°C, 750°C, 800°C, 850°C, 950°C, 1050°C, 1150°C or 1180°C for 5 hours. The heating speed for the treatment was 5°C/min. As one comparative example, a raw material mixture which was not sintered was also prepared. The heat treatment for the other comparative examples was conducted at 950°C. Each product thus produced was put into a partial-stabilized zirconia pot and ground in a pure water medium along with grinding media therein, using a shaking mill, into a powder having a mean particle size of 2 µm or so. The powders were then dried to be resistance materials of the examples of the present invention and comparative examples. The thus-obtained resistance materials each were formed into green compacts and the relative specific resistivity was measured according to the method mentioned below. First, each resistance material to be measured was dried, and about 50 mg of the material was weighted and put into a mold, to which a load (50 kgf/cm 2 ) was applied for 10 seconds. A pellet having an outer diameter of about 0.4 cm was formed, and this was taken out of the mold. The shape (with respect to the outer diameter and the height) of the thus-shaped pellet was measured with a micrometer. Next, both surfaces of the pellet were coated with a thermosetting silver electrode composition and then baked. The resistance value and the specific resistivity value of the thus-obtained pellet (green compact) were measured. In addition, the resistance values and the specific resistivity values of green compacts of RuO 2 and CaRuO 3 , of which the specific resistivity values were known (monitors), were also measured under the same condition. On the basis of the values of the monitors, the relative specific resistivity value of each green compact sample pellet was calculated and presumed. The results obtained are shown in Table 1 below. In Table 1, sample No. (1) is the resistance material sample that had not been heat-treated. In Table 1 above, the samples with asterisk (*) are comparative samples not falling within the scope of the present invention. The "specific resistivity value measured" in Table 1 is the actually measured specific resistivity value (ρ) of the green compact sample, which is obtained according to the following equation. ρ = (R x A)/L wherein R is the actually measured resistance value (Ω), A is the cross sectional area (cm 2 ), and L is the length (cm). The "relative specific resistivity value calculated" in Table 1 is value calculated from the actually measured specific resistivity values of the individual samples, on the presumption that the ratio of the data (3.50 x 10 -5 Ω•cm) of the monitor sample RuO 2 in literature to the actual measured specific resistivity value (0.349 Ω•cm) thereof applied to the other samples. The reasonability of the relative specific resistivity value thus calculated is established by the fact that the data of CaRuO 3 in literature is 3.7 x 10 -3 Ωcm and is the same as the data in Table 1 which was calculated from the actual measured specific resistivity value thereof. Accordingly, it is understood that the method employed herein is reasonable for determining the relative specific resistivity value of each sample. The relationship between the temperature at which each sample was heat-treated and the specific resistivity value of each sample is shown in Fig. 1. From the data in Table 1 and Fig. 1, it is known that the composite oxides of La x Sr 1-x CoO 3 as produced by heat treatment at temperatures falling between 800°C and 1150°C can have a controlled relative specific resistivity value on a level of 10 -4 Ω•cm (partly on a level of 10 -3 Ω•cm). From the data in Table 1 and Fig. 1, it is also known that when the temperature for the heat treatment for producing the composite oxides is lower than 800°C or higher than 1150°C, the composite oxides produced have an extremely high specific resistivity value and are therefore unsuitable for practical use. Though not shown in Table 1 and Fig. 1, it has been confirmed that resistance materials with a desired specific resistivity value (electroconductivity) are also obtained when mixtures comprising raw material substances at a molar ratio (x) of Sr to La of being from 0.40 to 0.60 are heat-treated at temperatures falling between 800°C and 1150°C, like those having a molar ratio (x) of 0.50 as above. If, however, the molar ratio (x) of Sr to La is not between 0.40 and 0.60, for example, as in sample No. (10) (where x = 0.30) or in sample No. (11) (where x = 0.70), the relative specific resistivity values calculated of the products are on a level of 10 -1 Ω•cm or, that is, the products have an extremely large specific resistivity value and therefore lose electroconductivity. These could not be used as resistance materials and are unfavorable for the object of the present invention. Formation of Glass Frit Samples: Apart from the resistance material samples prepared above, a lead zinc borosilicate glass frit sample (hereinafter referred to as "glass frit A") and a calcium barium borosilicate glass frit sample (hereinafter referred to as "glass frit B") were prepared according to the methods mentioned below. First, raw materials for glass frit A, B 2 O 3 , SiO 2 , PbO and ZnO were mixed at a molar ratio of 21.5:46.2:21.5:11.8 and then melted at from 1200 to 1350°C to obtain a fused glass of B 2 O 3 -SiO 2 -PbO-ZnO. This fused glass was rapidly cooled by putting it into pure water and then ground, using a shaking mill, into particles having a mean particle size of not larger than 5 µm. Thus was obtained a glass frit sample (glass frit A). As the raw materials for glass frit A, also employable are carbonates, etc., in place of the above-mentioned oxides. On the other hand, raw materials for glass frit B, B 2 O 3 , SiO 2 , BaO and CaO were mixed at a molar ratio of 8.6:41.0:18.0:32.4 and then melted at from 1200 to 1350°C to obtain a fused glass of B 2 O 3 -SiO 2 -BaO-CaO. This fused glass was rapidly cooled by putting it into pure water and then ground, using a shaking mill, into particles having a mean particle size of not larger than 5 µm. Thus was obtained a glass frit sample (glass frit B). As the raw materials for glass frit B, also employable are carbonates, etc., in place of the above-mentioned oxides. Formation of Resistance Paste Samples: The resistance material sample produced at 1050°C and glass frit A were mixed at various ratios shown in Table 2 below, while the resistance material sample produced at 950°C and glass frit B were mixed at various ratios shown in Table 3 below. In Tables 2 and 3, the samples with asterisk (*) are comparative examples where the molar ratio (x) of Sr to La is outside the scope of the present invention or the proportion of the glass frit to the resistance material is outside the scope of the present invention. To the mixture (solid component) comprising the resistance material and the glass frit was added an organic vehicle as prepared by dissolving an acrylic resin in α-terpineol. The resulting blend was kneaded, using a mixer such as a three-roll kneader or the like, to obtain a resistance paste. The proportion of the solid component (mixture comprising resistance material and glass frit) to the organic vehicle was about 70:30 by weight. Formation of Resistor Samples: First, a silver-palladium paste or a copper paste was screen-printed on an insulating substrate of alumina and baked in an air atmosphere or nitrogen atmosphere to form electrodes thereon. Next, the resistance paste sample obtained in the manner as above was screen-printed between the electrodes as formed on the alumina substrate to form a pattern thereon, which partly covered both terminal electrodes and had a length of 1.5 mm, a width of 1.5 mm and a dry thickness of 20 µm. Then, this was leveled and thereafter dried at 150°C for 10 minutes. Next, the alumina substrate having the silver-palladium electrodes thereon was baked in a tunnel furnace having an air atmosphere at a peak temperature of 850°C for 10 minutes, whereby a resistor was formed on the substrate. On the other hand, the alumina substrate having copper electrodes thereon was baked in a tunnel furnace having a nitrogen atmosphere at a peak temperature of 900°C for 10 minutes, whereby a resistor was formed on the substrate. Thus, resistor samples were prepared. Evaluation of Characteristics: The sheet resistance value of each resistor sample prepared as above was measured. Table 2 above shows the data measured with the resistor samples that had been prepared by use of the resistance pastes comprising glass frit A, while Table 3 above shows those of the resistor samples that had been prepared by use of the resistance pastes comprising glass frit B. The sheet resistance value was measured at 25°C, using a digital volt meter. As shown in Table 2 and Table 3 above, the resistor samples that had been prepared by the use of the resistance pastes comprising the resistance material prepared in the above have somewhat different resistance values, depending on the type of the glass frit in the resistance paste used. With respect to the molar ratio (x) of Sr to La, the resistor samples produced in air (for silver-palladium electrodes) or nitrogen (for copper electrodes) all had a sheet resistance value falling within the practicable range when the molar ratio (x) falls within the range of the present invention of from 0.40 to 0.60. On the other hand, however, if the molar ratio (x) was less than 0.40 or more than 0.60, the specific resistivity value of the resistors was too large or was not lower than 1 GΩ so that the resistors could not be put to practical use (see sample Nos. 1, 11, 12, 22 in Table 2 and sample Nos. 23, 33, 34, 44 in Table 3). Regarding the mixing ratio of the glass frit to the resistance material, the resistor samples having a content of the resistance material falling between 60 % by weight and 95 % by weight (therefore having a content of the glass frit falling between 5 % by weight and 40 % by weight) that had been produced in air (for silver-palladium electrodes) or nitrogen (for copper electrodes) all had a resistance value falling within a practicable range. On the other hand, however, if the mixing ratio of the glass frit to the resistance material oversteps the above-mentioned range of the present invention, the resistor films formed peeled (see sample Nos. 4, 15 in Table 2, sample Nos. 26, 37 in Table 3) or had too large a specific resistivity value of not lower than 1 GΩ (see sample Nos. 8, 19 in Table 2, sample Nos. 30, 41 in Table 3) so that the resistors could not be put to practical use. In the above-mentioned examples, used were a lead zinc borosilicate glass frit comprising B 2 O 3 , SiO 2 , PbO and ZnO at a molar ratio of 21.5:46.2:21.5:11.8 or a calcium barium borosilicate glass frit comprising B 2 O 3 , SiO 2 , BaO and CaO at a molar ratio of 8.6:41.0:18.0:32.4. However, the components constituting the glass frit for use in the present invention and the composition ratios of the components are not limited to those illustrated in these examples. Needless-to-say, it is possible in the present invention to employ other glass frits comprising other components than the illustrated ones and glass frits having other composition ratios other than the illustrated ones. The above-mentioned examples have demonstrated the formation of the resistors on an alumina substrate. However, the substrate on which the resistors of the present invention are formed is not limited to such alumina substrates but the present invention is applicable to the formation of the resistors on other various substrates or bases made of other various materials. The present invention is not limited to only the above-mentioned examples with respect to the other various aspects. For example, the proportion of the organic vehicle to the solid component comprising a resistance material and glass frit in the resistance paste of the present invention and the temperature conditions and the atmosphere conditions for baking the resistance paste can be variously changed or modified within the scope of the present invention as claimed. As has been described in detail hereinabove, the resistance paste of the present invention is formed by adding an organic vehicle to a solid component comprising from 60 to 95 % by weight of the resistance material of the present invention which has a composition of a general formula La x Sr 1-x CoO 3 (where x is from 0.40 to 0.60) and from 5 to 40 % by weight of a glass frit, followed by kneading them, and this can be baked in any of air, neutral and reducing atmospheres. By coating a substrate with the resistance paste of the present invention and baking it, it is possible to reliably produce a resistor which is lower priced than any conventional resistor. In addition, the increase in the resistance value of the resistor thus produced of the present invention is gentle, and the reproducibility of the resistor of the present invention with such gentle increase in the resistance value is good. Specifically, according to the present invention, it is possible to obtain a resistance material having a composition of La x Sr 1-x CoO 3 (where x is from 0.40 to 0.60) and having a variable specific resistivity value on a level of 10 -4 Ω•cm by suitably selecting the value x within the defined range and by suitably varying the temperature at which the components constituting the material are heat-treated within a range between 800°C and 1150°C. In addition, it is also possible to reliably produce a resistor having a resistance value variable within a broad range by employing the resistance paste of the present invention which comprises the resistance material and a glass frit at a suitably variable ratio. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof as defined by the claims.

Claims: A resistance paste comprising an organic vehicle and a solid component comprising from 60 to 95 % by weight of a resistance material having a composition of the formula: La x Sr 1-x CoO 3 wherein x is from 0.40 to 0.60 moles and from 5 to 40 % by weight of glass frit. The resistance paste as claimed in claim 1, wherein the glass frit is (a) a lead zinc borosilicate glass frit comprising from 15 to 25 mol% of B 2 O 3 , from 40 to 50 mol% of SiO 2 , from 15 to 25 mol% of PbO and from 7 to 13 mol% of ZnO, or (b) a calcium barium borosilicate glass frit comprising from 3 to 10 mol% of B 2 O 3 , from 35 to 45 mol% of SiO 2 , from 25 to 35 mol% of CaO and from 15 to 20 mol% of BaO. A method of producing a resistance paste comprising an organic vehicle and a solid component comprising from 60 to 95 % by weight of a resistance material having a composition of the formula: La x Sr 1-x CoO 3 wherein x is from 0.40 to 0.60 moles and from 5 to 40 % by weight of glass frit, whereby said resistance material is produced by sintering a raw material mixture of La, Sr and Co containing raw material substances at a temperature between 800°C and 1150°C. The method as claimed in claim 3, wherein the glass frit is (a) a lead zinc borosilicate glass frit comprising from 15 to 25 mol% of B 2 O 3 , from 40 to 50 mol% of SiO 2 , from 15 to 25 mol% of PbO and from 7 to 13 mol% of ZnO, or (b) a calcium barium borosilicate glass frit comprising from 3 to 10 mol% of B 2 O 3 , from 35 to 45 mol% of SiO 2 , from 25 to 35 mol% of CaO and from 15 to 20 mol% of BaO. A resistor comprising a substrate having a resistance paste of claim 1 or 2 baked thereon. A method of producing a resistor wherein a resistance paste is produced according to claim 3 or 4 and wherein said resistance paste is baked on a substrate.

Inventors: NAGATA KEISUKE, TANI HIROJI, NAGATA, KEISUKE, TANI, HIROJI, Nagata, Keisuke, c/o Murata Manufacturing Co., Ltd, TANI, HIROJI, C/O MURATA MANUFACTURING CO., LTD
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1995
Main classification: H01C17
IPC codes: H01C17, H01C7, C04B35