Method of manufacturing ceramic sheet and method of manufacturing gas sensing element

Methods of manufacturing a ceramic sheet and a gas sensing element are disclosed. At least ceramic powder, a binder and a plasticizer are blended and mixed in slurry. The slurry is formed into unfired green sheets, on which paste is printed. Each of the unfired green sheets has porosity greater than 5%. In the manufacturing methods, the unfired green sheets are pressurized with a pressure of 10 MPa at a temperature above 60° C., after which the paste is printed on surfaces of the unfired green sheets. In the method of manufacturing the gas sensing element, a shielding layer, a porous diffusion resistance layer and the unfired green sheets for a sensing layer, a reference gas airspace forming layer and a heating layer are stacked to form a stacked ceramic body, whish is fired to obtain the gas sensing element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to Japanese Patent Application Nos. 2006-292593, 2007-138969 and 2007-240881, filed on Oct. 27, 2006, May 25, 2007 and Sep. 18, 2007, respectively, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to methods of manufacturing a ceramic sheet and, more particularly, to a method of manufacturing a ceramic sheet for use in a gas sensing element or the like.

2. Description of the Related Art

In the related art, an attempt has heretofore been made to provide

An attempt has heretofore been made to provide a method of manufacturing a stacked type gas sensing element for detecting a specified gas concentration in measuring gases in a manner as shown inFIG. 10Atypically showing the related art disclosed in, for instance, Japanese Patent Application Publication No. 2002-286680.

As shown inFIG. 10A, raw materials are prepared using a ceramic powder, a binder912, and a plasticizer913and mixed. The resulting mixture is then formed into unfired green sheets90. Subsequently, an electrically conductive paste is printed on a surface900of each unfired green sheet90in one areas forming an electrode pattern and a heating pattern or the like. In addition, a ceramic insulating paste is printed on the surface900of the unfired green sheet90in the other areas thereof with no formation of the electrode pattern and the heating pattern in a reversed pattern opposite to the electrically conductive pattern. Then, a plurality of unfired green sheets90, subjected to the printing processes mentioned above, are stacked to form a stacked ceramic body. Thereafter, the stacked ceramic body is fired, thereby obtaining a gas sensing element composed of the ceramic sheets in a stacked structure.

In recent years, the stacked type gas sensing eminent has been formed in a complicated structure. In a manufacturing process of such a stacked type gas sensing element, the number of times for the paste to be printed on the surface900of the unfired green sheet90has been increasing with an increase in the number of unfired green sheets90to be stacked. Therefore, the number of times for the unfired green sheet90to be dried after the paste has been printed has been increasing with the resultant consequence of a progressive increase in deformation of the unfired green sheet90during drying stages. That is, as the paste is printed on the unfired green sheet90, the wetting and contraction occur on the unfired green sheet90, causing a change to occur in dimension of the unfired green sheet90in a dried state.

Such a tendency seems to occur because of the reasons listed below.

That is, when the paste is printed on the unfired green sheet90, the solvent present in the paste penetrates the unfired green sheet90. This causes the solvent to dissolve the binder912contained in the unfired green sheet90. Thus, ceramic particles911present in the ceramic powder move with respect to each other in a rearranged state, as shown inFIG. 10B, under which the unfired green sheet90is dried. As a result, the dimension of the unfired green sheet90is conceived to change after the drying step as shown inFIG. 10B.

To address such an issue, a measure has been taken in the related art to execute a method of selecting a solvent that is hard to cause the wetting and contraction of the unfired green sheet90.

However, with such a method, there is a fear of a drop occurring in a thermal compression force between the unfired green sheets90with the resultant degradation in a bonding capability.

SUMMARY OF THE INVENTION

The present invention has been completed with a view to addressing the above issues and has an object to provide a method of manufacturing a ceramic sheet that can minimize a dimensional change with no need for selecting a solvent.

To achieve the above object, a first aspect of the present invention provides a method of manufacturing a ceramic sheet, comprising the steps of: blending at least a ceramic powder, a binder and a plasticizer to prepare a mixture of raw materials; mixing the raw materials; forming an unfired green sheet; and printing a paste on a surface of the unfired green sheet; wherein the unfired green sheet has a plurality of pores with a porosity greater than 5%.

With the method mentioned above, the unfired green sheet has the pores with porosity greater than 5%, enabling a reduction in change of the unfired green sheet in a dimension thereof. This seems to be based on two reasons as described below. That is, first, with the unfired green sheet having porosity greater than 5%, a solvent of the paste can rapidly escape to the outside through the pores formed inside the unfired green sheet, providing an ease of drying even if the solvent penetrates the inside of the unfired green sheet. This minimizes the binder and the plasticizer from dissolving due to the presence of the solvent. This results in a difficulty of causing a rearrangement of ceramic particles in the ceramic powder.

Second, there are conditions, under which the unfired green sheet has pores with porosity greater than 5%, which are considered to include three cases including: (1) a first case in which the ceramic particles have large particle diameter; (2) a second case in which the ceramic particles agglutinate; and (3) a third case in which the ceramic particles have complicated configurations. Under such conditions, in usual practice, the ceramic particles are arranged in a status with the particles agglutinating in a distance narrowed to some extent on a stage in which the unfired green sheet is formed. Therefore, even if the rearrangement occurs in the ceramic particles, the distance between the particles is hard to be narrowed. Thus, even if the binder or the like are dissolved due to the solvent, no dimensional change occurs in the unfired green sheet.

However, in a fourth case, the ceramic particles have a small average particle diameter with well-regulated shapes (in the form of, for instance, a contoured shape closer to spheres). In this case, the porosity becomes small on a stage of the unfired green sheet with the resultant ease of causing the rearrangement of the ceramic particles during the dissolving of the binder or the like. This results in a consequence of the unfired green sheet being dried and the rearrangement occurs in the ceramic particles, causing an increase in dimensional change.

As set forth above, with the unfired green sheet set to have a large porosity, that is, a porosity exceeding a value of 5%, no need arises for choosing a solvent, while making it possible to eliminate the occurrence of dimensional change in the unfired green sheet.

As set forth above, the present embodiment can provide a method of manufacturing a ceramic sheet with less variation in dimension with no need for preliminarily selecting a solvent.

A second aspect of the present invention provides a method of manufacturing a gas sensing element for detecting a specified gas concentration in measuring gases, comprising the steps of: preparing first to third unfired green sheets for a sensing layer, a reference gas airspace forming layer and a heating layer, respectively; pressuring the first to third unfired green sheets with a pressure of 10 MPa at a temperature above 60° C. such that each of the first to third unfired green sheets has a plurality of pores with a porosity greater than 5%; preparing the sensing layer by forming a measuring gas side electrode on one surface of the first unfired green sheet and forming a reference gas side electrode on the other surface of the first unfired green sheet; preparing the reference gas airspace forming layer using the second unfired green sheet; preparing the heating layer by forming a heating section on the third unfired green sheet at one surface thereof in face with the sensing layer; stacking a shielding layer, a porous diffusion resistance layer, the sensing layer, the reference gas airspace forming layer and the heating layer to form a stacked ceramic body; and firing the stacked ceramic body for thereby obtaining the gas sensing element.

With the method mentioned above, the unfired green sheet has the pores with porosity greater than 5%, enabling a reduction in change of the unfired green sheet in a dimension thereof. This seems to be based on two reasons as described below. That is, first, with the unfired green sheet having porosity greater than 5%, a solvent of the paste can rapidly escape to the outside through the pores formed inside the unfired green sheet, providing an ease of drying even if the solvent penetrates the inside of the unfired green sheet. This minimizes the binder and the plasticizer from dissolving due to the presence of the solvent. This results in a difficulty of causing a rearrangement of ceramic particles in the ceramic powder. Thus, the gas sensing element using such unfired green sheets can be manufactured in a structure with a lessened dimensional change.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Now, a method of manufacturing a ceramic sheet and a method of manufacturing a gas sensing element of first and second aspects of the present invention will be described below in detail with reference to various embodiments shown in the accompanying drawings. However, the present invention is construed not to be limited to such embodiments described below and technical concepts of the present invention may be implemented in combination with other known technologies or the other technology having functions equivalent to such known technologies.

(First Aspect of the Invention)

A method of manufacturing a ceramic sheet of a first aspect of the present invention comprises the steps of: blending at least a ceramic powder, a binder and a plasticizer to prepare a mixture of raw materials; mixing the raw materials; forming an unfired green sheet; and printing a paste on a surface of the unfired green sheet, wherein the unfired green sheet has a plurality of pores with a porosity greater than 5%.

In carrying out the method of the first aspect of the present invention, the ceramic material is prepared. Examples of the ceramic material include at least one element selected from the group consisting of, for instance, alumina, zirconia and titania or the like.

Although the binder may include various compounds with no particular limitation provided that these compounds can afford formability to the ceramic powder, a lipophilic binder may be preferably used. Examples of the lipophilic binder include, for instance, polyvinyl butyral resin and acryl resin or the like. Among these, polyvinyl butyral resin has a high chemical adsorbability to the ceramic powder with an increased sheet binding ability and increased thermally applicable range with a less incursion of impurities and, so, is preferably employed.

Further, the plasticizer may include various compounds with no particular limitation provided that these compounds can afford plasticity to the binder. In this case, examples of the plasticizer include, for instance, butyl benzyl phthalate (BBP), dibutyl phthalate (DBP), dioctylphthalate (DOP) and dibutyl sebacate (DBS), etc. Among these, especially, DBP and DOP have high vapor pressures with low volatilities and high boiling points and are preferably employed. In addition, the plasticizer may preferably contain 30 to 80% of the binder by mass thereof.

Moreover, in pressurizing the unfired green sheet, the unfired green sheet may preferably have a porosity greater than 5% on a stage before the pressurization step being conducted.

Further, the unfired green sheet may preferably have the porosity greater than 10%.

In this case, the unfired green sheet is able to eject the solvent to the outside at a further high speed to be easily dried. In addition, even if rearrangement occurs in ceramic grains, the grains can be kept in a distance that further becomes hard to thicken. This further prevents the occurrence of a dimensional change of the unfired green sheet.

Moreover, the unfired green sheet may preferably have the porosity greater than 15%.

In this case, the unfired green sheet is able to eject the solvent to the outside at a further high speed to be easily dried. In addition, even if rearrangement occurs in ceramic grains, the grains can be kept in a distance that further becomes hard to thicken. This further prevents the occurrence of a dimensional change of the unfired green sheet.

Further, the unfired green sheet may be preferably pressurized with a pressure greater than 10 MPa at a temperature higher than 60° C. after which the paste is printed on the surface of the unfired green sheet.

In this case, the unfired green sheet becomes hard in structure with an increased density. This makes it possible to further suppress the dimensional change of the ceramic grains due to rearrangement thereof. In addition, this results in an effect of suppressing the solvent in paste from penetrating the unfired green sheet.

Meanwhile, when the unfired green sheet is pressurized with a pressure less than 10 MPa at a temperature lower than 60° C., there is a fear of a difficulty arising in adequately obtaining an advantageous effect of the present invention.

Furthermore, the unfired green sheet may be preferably pressurized with a pressure greater than 10 MPa at a temperature higher than 60° C. after which the unfired green sheet has the porosity less than 7.5%.

In this case, the occurrence of a dimensional change of the unfired green sheet can be further minimized, making it possible to obtain a ceramic sheet with a further increased dimensional precision.

Meanwhile, if the unfired green sheet, subjected to the pressurizing step, has the porosity exceeding 7.5%, a firing shrinkage factor increases with a resultant drop in dimensional precision of the ceramic sheet, causing a fear to occur with a difficulty of fabricating a dense ceramic sheet.

Further, the ceramic sheet may preferably form a part of a gas sensing element for detecting a specified gas concentration in measuring gases.

In this case, the gas sensing element includes the ceramic sheet with a high dimensional precision, making it possible to provide a gas sensing element with increased reliability.

Furthermore, the unfired green sheet may preferably have a dimensional change less than that of an unfired green sheet, whose porosity lies at a value less than 5%, on a stage before and after the paste is repeatedly printed on the surface and then dried multiple times.

In this case, even if the paste is repeatedly printed on the surface and then dried multiple times, the dimensional change of the unfired green sheet can be adequately minimized, resulting in a capability of obtaining a ceramic sheet with further increased dimensional precision.

Embodiment

A method of manufacturing the ceramic sheet of the first aspect of the present invention will be described below with reference to Example shown inFIGS. 1 and 2of the accompanying drawings.

As shown inFIG. 1, in carrying out the method of manufacturing the ceramic sheet of the first aspect of the present invention, a mixture of raw materials is prepared by blending a ceramic powder composed of ceramic particles11, a binder12and a plasticizer13in given blending ratios. The resulting raw materials are mixed in slurry. The resulting slurry is then formed in an unfired green sheet10. Subsequently, a paste is printed on a surface100of the unfired green sheet10. In addition, the unfired green sheet10may preferably contain dispersant or the like.

The unfired green sheet10has pores14with porosity of a value of 5%.

The porosity of the unfired green sheet10is calculated in a manner described below. That is, first, a sum of a volume of organic matters, such as a binder or the like, and a volume of ceramic powder is obtained. Then, dividing the sum of these volumes by an entire volume of the unfired green sheet10allows a related proportion to be obtained representing a substantial volume ratio. Thereafter, the substantial volume ratio is subtracted from a value of 100%, thereby deriving porosity.

Accordingly, with the substantial volume ratio being less than 95%, porosity becomes more than 5%.

With the present example, further, the unfired green sheet10has a structure that has a dimensional change made less than that of an unfired green sheet, formed with pores with porosity less than 5%, before and after the paste is repeatedly printed on the surface100of the unfired green sheet10and dried multiple times.

With the present example, further, ceramic sheets1, manufactured in the manufacturing method set forth above, are used for forming a part of a gas sensing element3for detecting a specified gas concentration in measuring gases as shown inFIG. 2.

Now, a method of manufacturing the ceramic sheet1and the gas sensing element3employing the ceramic sheet1will be described below.

The gas sensing element3, manufactured according to the present invention, may operate as, for instance, an A/F sensing element, an O2sensing element and a NOx sensing element.

As shown inFIG. 2, the gas sensing element3includes a sensing layer330that is comprised of a solid electrolyte body33having oxygen ion conductivity, a measuring gas side electrode34formed on one surface of the solid electrolyte body33, and a reference gas side electrode35formed on the other surface of the solid electrolyte body33.

As shown inFIG. 2, the measuring gas side electrode34has one end formed with a longitudinally extending lead portion341, which has a distal end formed with a terminal portion342at a position in opposition to the measuring gas side electrode34for outputting an output current to the outside.

Further, like the measuring gas side electrode34, the reference gas side electrode35has one end formed with a longitudinally extending lead portion351, which has a distal end formed with a laterally extending terminal portion352at a position in opposition to the reference gas side electrode35. The terminal portion352has a lateral distal end352aelectrically connected to the terminal portion353, formed on the one surface of the solid electrolyte body33in a position adjacent to the terminal portion342, via an electrically conductive element (not shown) extending through a hole33aformed in the solid electrolyte body33.

As shown inFIG. 2, further, a reference gas airspace forming layer36is stacked on the solid electrolyte body33at the other surface thereof so as to cover the reference gas side electrode35. The reference gas airspace forming layer36has one surface, facing the other surface of the solid electrolyte body33, which is formed with a recessed portion360. The recessed portion360longitudinally extends for defining a reference gas airspace under a status surrounded with the reference gas airspace forming layer36and the solid electrolyte body33. This allows the reference gas side electrode35to be exposed to the reference gas airspace, thereby enabling atmospheric air as reference gas to be introduced to the reference gas airspace.

As shown inFIG. 2, furthermore, the reference gas airspace forming layer36has the other surface, in opposition to the surface facing the reference gas side electrode35, on which a heating layer380is laminated. The heating layer380is comprised of a heating section37operative to develop a heat when applied with electric power, a pair of lead portions371for supplying electric power to the heating section37, a pair of terminal portions372connected to the lead portions371, respectively, and a heater substrate38for supporting these component parts.

As shown inFIG. 2, moreover, the heater substrate38has a bottom surface38a, placed in opposition to the heating section37and the pair of lead portions371, which is formed with a pair of laterally spaced terminal portions373. The terminal portions373are electrically connected to the pair of terminal portions372, provided on the one surface of the heater substrate38, via a plurality of electrically conductive materials vertically extending through a plurality of holes38bformed in the heater substrate38.

In addition, as shown inFIG. 2, a porous diffusion resistance layer32is stacked on the one surface of the solid electrolyte body33so as to cover the measuring gas side electrode34. Moreover, a shielding layer31is stacked on the porous diffusion resistance layer32so as to cover the same.

The porous diffusion resistance layer32is made of porous material with gas permeability. In addition, as shown inFIG. 2, the porous diffusion resistance layer32has a sidewall320configured in a structure to introduce measuring gases to the measuring gas side electrode34through the sidewall320.

(Second Aspect of the Invention)

Now, a method of manufacturing the gas sensing element3of a second aspect of the present invention will be described below in detail with reference toFIG. 2.

With the method of manufacturing the gas sensing element3of the second aspect of the present invention, first, as shown inFIG. 2, the unfired sheets10are fabricated for forming thereon the porous diffusion resistance layer32and the reference gas airspace forming layer36, respectively. Subsequently, the associated component parts, described above, are stacked on the unfired sheets10to form a stacked ceramic body with the unfired sheets10left unfired. Thereafter, the stacked ceramic body is fired, thereby obtaining the gas sensing element3.

First, description is made of how the heating layer380is formed.

For instance, 12 g of butyral resin serving as the binder12, 9 g of butyl benzyl phthalate serving as the plasticizer13, 2 g of sorbitan trioleate as the dispersant, and a mixed solvent composed of a given amount of ethanol, 2-butanol and isoamyl acetate are added to 100 g of an alumina powder serving as the ceramic powder. Then, the resulting mixture is wet blended, thereby preparing a slurry. By using this slurry, the unfired green sheet10is fabricated for the heater substrate38by, for instance, a doctor blade method. In addition, the unfired green sheet10is formed in a structure with porosity greater than 5%. Also, the unfired green sheet10may be preferably structured to have porosity greater than 10% and, more preferably, 15%.

Next, the unfired green sheet10is pressurized under a pressure greater than 10 MPa at a temperature above 60° C.

The pressurization may be performed using, for instance, a WIP (Warm Isostatic Press) device. In an alternative, the unfired green sheet10may be placed in a die, after which the unfired green sheet10is pressed with a pressing machine.

Then, the unfired green sheet10for the heater substrate38has porosity less than 7.5% when subjected to the pressurization performed under the condition set forth above.

Thereafter, electrically conductive paste layers are printed on the surfaces100of the unfired green sheet10for the heater substrate38to form the heating section37, the lead portions371and the terminal portions372,373, respectively.

Further, an insulating paste layer is formed on the one surface100of the unfired green sheet10for the heater substrate38by printing a reverse pattern in an area where no conductive paste layer for the heating section37is present. This results in capability of eliminating a difference in level on the one surface100of the unfired green sheet10with respect to the conductive paste layer for the heating section37. Subsequently, the unfired green sheet10is dried.

Furthermore, examples of electrically conductive paste forming the heating section37may include a first raw material containing, for instance, 1.8 g of alumina powder and 15 g of platinum and a given amount of a second raw material containing a binder and a solvent or the like. The resulting raw materials are then mixed in a paste.

Moreover, examples of electrically conductive paste forming the terminal portions372,373may include a raw material containing, for instance, 1 g of alumina powder and 15 g of platinum, and a given amount of a second raw material containing a binder and a solvent or the like. The resulting raw materials are then mixed in a paste.

Next, a method of forming the reference gas airspace forming layer36will be described below.

A plurality of unfired green sheets10for the reference gas airspace forming layer36are prepared in the same method and raw materials as those used for preparing the unfired green sheet10for the heater substrate38. In this case, the plurality of unfired green sheets10are stacked into a stacked body to form the reference gas airspace forming layer36. In an alternative, the reference gas airspace forming layer36may be structured using a single sheet of unfired green sheet10with a large thickness.

Next, a method of preparing the sensing layer330will be described below.

In preparing the solid electrolyte body33, a slurry may be employed by preparing 100 g of a zirconium powder added admixed with a blend of 7 g of butyral resin serving as the binder12, 5 g of butyl benzyl phthalate serving as the plasticizer13and a given amount of a mixed solvent containing ethanol, 2-butanol and isoamyl acetate.

Like the unfired green sheet10for the heater substrate38, the unfired green sheet10for the solid electrolyte body33is adjusted to have pores with porosity greater than 5%. In addition, the unfired green sheet10may be preferably structures to have porosity greater than 10% and, more preferably, 15%.

To this end, the unfired green sheet10for the solid electrolyte body33is pressurized with a pressure greater than 10 MPa at a temperature above 60° C. in the same method in which the unfired green sheet10for the heater substrate38is pressurized.

The unfired green sheet10for the solid electrolyte body33is subjected to the pressurization under the condition set forth above with porosity less than 7.5%.

Subsequently, an electrically conductive paste is printed on the surface100of the unfired green sheet10for the solid electrolyte body33in areas for the measuring gas side electrode34, the reference gas side electrode35, the lead portions341,351and the terminal portions342,352,353, respectively.

Further, an insulating paste is formed on the one surface100of the unfired green sheet10for the solid electrolyte body33in other areas, where no conductive paste layer formed for the measuring gas side electrode34is present, by printing a reverse pattern with the same thickness as that of the conductive paste layer for the measuring gas side electrode34. This eliminates a difference in level on the one surface100of the unfired green sheet10with respect to the conductive paste layer for the measuring gas side electrode34. Subsequently, the resulting unfired green sheet10is dried.

Furthermore, examples of electrically conductive paste for the measuring gas side electrode34and the reference gas side electrode35may include a raw material, containing, for instance, 2.9 g of zirconium powder and 20 g of platinum, and a given amount of a mixed solvent, containing a binder and a solvent or the like, which is mixed to the raw material.

Moreover, examples of electrically conductive paste for the lead portions341,351and the terminal portions342,352,353may include a raw material, containing, for instance, 1.6 g of zirconium powder and 20 g of platinum, and a given amount of a mixed solvent, containing a binder and a solvent or the like, which is mixed to the raw material.

Next, a description is made of a method of manufacturing the porous diffusion resistance layer32.

The porous diffusion resistance layer32is fabricated using a ceramic powder. For the ceramic powder, one alumina powder in the form of the ceramic particles11with an average particle diameter of 0.3 μm and a tap density of 1.4 g/cc and another alumina powder with an average particle diameter of 0.4 μm and a tap density of 0.81 g/cc are blended and mixed in a blending ratio of 1:9 to provide 100 g of mixed alumina powder. Then, 22 g of butyral resin serving as the binder12, 8 g of butyl benzyl phthalate serving as the plasticizer13, 2 g of sorbitan trioleate as the dispersant, and a given amount of mixed solvent containing ethanol, 2-butanol and isoamyl acetate are added to 100 g of mixed alumina powder and wet blended, thereby preparing a slurry. By using this slurry, the unfired green sheet10for the porous diffusion resistance layer32is fabricated for the heater substrate38by, for instance, a doctor blade method.

Further, the unfired green sheet10for the shielding layer31can be manufactured in the same method as that in which the unfired green sheet10for the heater substrate38is manufactured.

As set forth above, with the respective layers being completely manufactured, these layers are stacked in a manner described below.

First, the heating layer380and the reference gas airspace forming layer36are unitized by thermal compression bonding, thereby providing a first unitized body.

Further, the sensing layer330, the porous diffusion resistance layer32and the shielding layer31are unitized by thermal compression bonding, thereby providing a second unitized body.

Then, the resulting first unitized body, composed of the heating layer380and the reference gas airspace forming layer36, the sensing layer330, and the resulting second unitized body, composed of the sensing layer330, the porous diffusion resistance layer32and the shielding layer31, are stacked, upon which the first and second unitized bodies are bonded to each other by adhesive.

With such a sequence mentioned above, a stacked ceramic body is obtained including the shielding layer31, the porous diffusion resistance layer32, the sensing layer330, the reference gas airspace forming layer36and the heating layer380.

Finally, the stacked ceramic body is fired at a maximal temperature ranging from 1400 to 1550° C., thereby obtaining the gas sensing element3shown inFIG. 2.

Although the present invention has been described with reference to an exemplary case of the gas sensing element3and the related manufacturing method, particular arrangements disclosed are meant to be illustrative only and not limiting the scope of the invention.

Next, various advantageous effects of the present embodiment will be described below.

The unfired green sheet10, used in the present embodiment, is formed with porosity higher than 5%, making it possible to minimize a change in dimension of the unfired green sheet10. This mechanism seems to come from two reasons. That is, first, with the unfired green sheet10having the pores with large porosity as high as 5%, even if the solvent of the paste penetrates the unfired green sheet10, the solvent can escape to the outside through pores14present in the unfired green sheet10. This provides an ease of drying the unfired green sheet10. It is thus conceived that such a phenomenon precludes the binder12or the plasticizer13from dissolving for thereby precluding the rearrangement of the ceramic particles11in the ceramic powder.

Second, there are conditions under which the unfired green sheet10has the pores with porosity greater than 5%. These conditions seem to be involved in the following cases including: (1) a first case in which the ceramic particles11have a large particle diameter; (2) a second case in which the ceramic particles11agglutinate; and (3) a third case in which the ceramic particles11have complicated configurations.

Under such conditions, in usual practice, the ceramic particles11are arranged in a status with the particles agglutinating in a distance narrowed to some extent on a stage in which the unfired green sheet10is formed. Therefore, even if the rearrangement occurs in the ceramic particles11, the distance between the particles is hard to be narrowed. Thus, even if the binder12or the like are dissolved due to the solvent, no dimensional change occurs in the unfired green sheet10.

However, in a fourth case, the ceramic particles11have a small average particle diameter with well-regulated shapes (in the form of, for instance, a contoured shape closer to spheres). In this case, the porosity becomes small on a stage of the unfired green sheet10. This causes the rearrangement of the ceramic particles11to easily occur during the dissolving of the binder12or the like. This results in a consequence of the unfired green sheet10being dried with the rearrangement caused in the ceramic particles11, causing an increase in dimensional change.

As set forth above, with the unfired green sheet10having the large porosity, that is, a porosity exceeding a value of 5%, no need arises for choosing a solvent, while making it possible to eliminate the occurrence of a change in dimension of the unfired green sheet10.

Also, it will be appreciated that the illustrative view ofFIG. 1, showing a status of the unfired green sheet10, represents one exemplary case representing a status of the second case (2) described above.

Further, the unfired green sheet10is pressurized at a temperature of 60° C. under the pressure greater than a value of 10 MPa after which the paste is printed on the surface100of the unfired green sheet10, enabling the unfired green sheet10to be hardened with increased density. This adequately prevents a solvent in the paste from permeating into the unfired green sheet10.

Furthermore, with the unfired green sheet10pressurized under the condition mentioned above, the unfired green sheet10has porosity less than 7.5%. This enables the unfired green sheet10to be less liable to suffer from dimensional variation, making it possible to obtain the ceramic sheet1with further increased dimensional precision. In addition, the ceramic sheet1forms a part of the gas sensing element3for detecting a specified gas concentration of measuring gases. In this case, the gas sensing element3can employ the ceramic sheet with an increased precision in dimension, making it possible to obtain the gas sensing element3with high reliability.

Moreover, the unfired green sheet10has a structure in that the unfired green sheet10has a less dimensional change, even if the paste is repeatedly printed on the surface100and dried multiple times, than that of the unfired green sheet10rendered to have the porosity less than 5%. This enables the ceramic sheet1to be obtained with a further increase in dimensional precision.

As set forth above, with the present embodiment, it becomes possible to provide a method of manufacturing a ceramic sheet with less variation in dimension and no need for preliminarily selecting a solvent.

Tests were conducted on the unfired green sheets10to check dimensional change rates of the unfired green sheets10with substantial volume ratios varied in different parameters.

That is, a paste was prepared containing an alumina powder serving as a ceramic powder, terpineol serving as a solvent, and a binder. Then, the paste was printed on the surfaces100of the unfired green sheets10with the substantial volume ratios varied in different parameters. Thereafter, the unfired green sheets10were dried, thereby obtaining specimens. Subsequently, the unfired green sheets10of the specimens, after the printing step, were measured to check the dimensional change rates of the unfired green sheets10in contrast to those of the unfired green sheets10before the paste was printed.

Further, as used herein, the term “substantial volume ratio” refers to a value representing a ratio obtained by dividing a sum of a volume of an organic matter such as the binder or the like and a volume of the ceramic powder by an entire volume of the unfired green sheets10. Then, the porosity can be calculated based on the resulting volume ratios using a formula (1) expressed below.
(Porosity)=100(%)−(Substantial Volume Ratio)  (1)

Further, it will be appreciated that like reference characters used in the present tests designate like or corresponding parts used in the embodiment shown inFIG. 1.

In measuring the specimens during the present tests, first, the unfired green sheet10was prepared with four corners formed with pinholes4, respectively, as shown inFIG. 3. Then, distances w1to w4between centers of the pinholes4for upper, lower, left and right sides of the unfired green sheet10were measured. Next, an electrically conductive paste was printed on the surface100of the unfired green sheet10in one area. Thereafter, an insulating paste is formed on the surface100of the unfired green sheet10in the other area, in which no conductive paste was present, by printing a reverse pattern with the same thickness as that of the conductive paste. The insulating paste was printed three times. Printing such insulting paste on the surface100of the unfired green sheet10in the other area thereof minimized an uneven stepped portion caused by the electrically conductive paste printed in advance.

Subsequently, a whole of the unfired green sheet10was dried at a temperature of 55° C. for 20 minutes. Thereafter, the center distances w1to w4between the pinholes4were measured again, thereby measuring the dimensional change of the unfired green sheet10after the printing of the paste.

The dimensional change rate of the unfired green sheet10for each of the upper, lower, left and right sides of the unfired green sheet10was calculated based on the dimensions between the pinholes4before the paste was printed and the dimensions between the pinholes4after the paste was printed.

FIG. 4shows variation in dimensional change rate (%) in terms of the substantially volume ratio (%) with the dimensional change rate plotted in values obtained upon calculating the dimensional change rates of the unfired green sheet10on each of the respective sides thereof and subsequently calculating an average value of the dimensional change rates on each side.

The test results are shown inFIG. 4.

As shown by a curve C1inFIG. 4, as the unfired green sheet10has a relative density exceeding a value of 95% (that is, with porosity less than 5%), the dimensional change rate of the unfired green sheet10after the printing of the paste lies at a substantially low level less than approximately 0.2%.

Meanwhile, as will be apparent fromFIG. 4, as the unfired green sheet10has the relative density greater than 95%, that is, with the porosity less than 5%, the dimensional change rate of the unfired green sheet10rapidly increases.

As will be apparent from the foregoing, with the unfired green sheet10having the relative density less than 95%, that is, with the porosity greater than 5%, the dimensional change of the unfired green sheet10after the printing of the paste can be adequately suppressed.

Second tests were conducted on the unfired green sheets10to check the dimensional change rates of the unfired green sheets10that were formed upon varying the temperature and pressure under which the unfired green sheets10were pressurized. That is, with the present tests, the temperatures were varied in value from 0 to 100° C. with the pressures varied in value from 0 to 49 MPa. The unfired green sheets10were placed and pressurized in the WIP device.

FIG. 5shows the representation of the dimensional change rate (%) in terms of the temperature (° C.). InFIG. 5, empty circles “∘” represent the dimensional change rate with the pressurization executed at a pressure of 0 MPa. An empty box “□” represents the dimensional change rate with the pressurization executed at a pressure of 2 MPa. Empty boxes “⋄” represent the dimensional change rate with the pressurization executed at a pressure of 8 MPa. Filled boxes “♦” represent the dimensional change rate with the pressurization executed at pressures ranging from 10 to 49 MPa.FIG. 5shows a graph representing the dimensional change rates (%) of the unfired green sheets10plotted in terms of the respective temperature conditions.

The present tests were conducted on the same other conditions as those of the unfired green sheet10on which the first tests were conducted.

Further, it will be appreciated that like reference characters used in the present tests designate like or corresponding parts used in the embodiment shown inFIG. 1.

The test results are shown inFIG. 5.

As will be apparent from the graph ofFIG. 5, with the unfired green sheet10pressurized with the pressure greater than 10 MPa at the temperatures above 60° C., the unfired green sheets10had the dimensional change rate that was sufficiently less than 0.2% with a further decrease in dimensional change.

Meanwhile, as will be apparent from the graph ofFIG. 5, further, with the unfired green sheet10pressurized with the pressure lower than 10 MPa at the temperatures below 60° C., the unfired green sheet10had the dimensional change rate that was greater than 0.2% with the resultant difficulty of achieving an adequate reduction in dimensional change.

As will be turned out from the foregoing, the unfired green sheet10is preferable to be pressurized with the pressure above 10 MPa at the temperatures above 60° C.

Third tests were conducted on the unfired green sheets10to check variations in dimensional change rates of the unfired green sheets10with the paste printed multiple times in one case (hereinafter referred to as “in the presence of WIP”) where the unfired green sheet10was pressurized with the pressure of 50 MPa at the temperature of 850° C. and in the other case (hereinafter referred to as “in the absence of WIP”) where the unfired green sheet10was not pressurized.

InFIG. 6, a curve C2is plotted with empty boxes “⋄” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w1(on the upper side of the unfired green sheet10) before and after the paste is printed in the presence of WIP. A curve C3is plotted with empty boxes “□” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w2(on the lower side of the unfired green sheet10) before and after the paste is printed in the presence of WIP. A curve C4is plotted with empty triangles “Δ” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w3(on the left side of the unfired green sheet10) before and after the paste is printed in the presence of WIP. A curve C5is plotted with empty circles “∘” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w4(on the right side of the unfired green sheet10) before and after the paste is printed in the presence of WIP.

InFIG. 7, a curve C6is plotted with empty boxes “⋄” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w1(on the upper side of the unfired green sheet10) before and after the paste is printed in the absence of WIP. A curve C7is plotted with empty boxes “□” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w2(on the lower side of the unfired green sheet10) before and after the paste is printed in the absence of WIP. A curve C8is plotted with empty triangles “Δ” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w3(on the left side of the unfired green sheet10) before and after the paste is printed in the absence of WIP. A curve C9is plotted with empty circles “∘” representing variation in dimensional change rate (%) of the unfired green sheet10calculated on the parameter of the dimension w4(on the right side of the unfired green sheet10) before and after the paste is printed in the absence of WIP.

The present tests were conducted under the same other conditions as those of the unfired green sheets10subjected to the second tests mentioned above.

The test results are plotted inFIGS. 6 and 7.

As will be apparent from the graph ofFIG. 6, with the one case in the presence of WIP, the unfired green sheet10had the dimensional change rate of ±0.1% even when the paste was printed multiple times. Thus, the dimensional change was adequately suppressed.

On the contrary, as will be understood fromFIG. 7, with the other case in the absence of WIP, the unfired green sheet10had the dimensional change rate having an absolute value increasing with an increase in the number of times the paste was printed. That is, with the other case in the absence of WIP, it will be turned out that if the paste is printed multiple times, then, a difficulty is encountered in adequately maintaining the unfired green sheet10in a given dimension.

(Third Aspect of the Invention)

A method of manufacturing an unfired green sheet with porosity greater than 5% of a third aspect of the present invention will be described below.

With the third aspect of the present invention, the method of manufacturing an unfired green sheet includes the steps of preparing a raw material containing a ceramic powder, a binder, a plasticizer and a solvent, mixing the raw material, and forming the resulting mixture in an unfired green sheet.

More particularly, in carrying out the method of manufacturing the unfired green sheet, a content of 7 to 9 wt % of the binder is weighed relative to the ceramic powder. The content of 7 to 9 wt % of the binder lies at a small value equal to about three-fourth of, for instance, a binder contained in a raw material prepared for fabricating an unfired green sheet of the related art.

With the third aspect of the present invention, further, a given amount of the binder, the plasticizer and the solvent are added to the ceramic powder to provide a mixture. The resulting mixture is then mixed for 4 to 8 hours. The mixing time interval of 4 to 8 hours lies at a lessened value equal to a value of, for instance, about one-sixth to one-tenth of a time interval for which a blend of a ceramic powder, a binder, a plasticizer and a solvent is mixed in fabricating an unfired green sheet of the related art.

Although adjusting the content of the binder and adjusting the mixing time interval enables an unfired green sheet to be easily formed with porosity greater than 5%, even performing one of these two adjustments enables the unfired green sheet to be easily formed with porosity greater than 5%.

If the mixing time interval lies at a value less than 4 hours or if the binder has the content less than 7 wt %, then, there is a fear of a difficulty arising in adequately mixing the ceramic powder and the binder. This results in a fear of cracking occurring in a ceramic sheet.

Further, if the mixing time interval exceeds 8 hours or the binder has the content greater than 9 wt %, then, there is a fear of a difficulty arising in forming an unfired green sheet with an adequate amount of pores. This results in a fear of a difficulty arising in forming the unfired green sheet with an adequate increase in porosity.

Tests were conducted on the unfired green sheets10to check dimensional change rates of the unfired green sheets10on a stage before and after the paste has been printed with porosity of the unfired green sheets10varied in different parameters.

In measuring the dimensional change rates of the unfired green sheets10, first, square-shaped unfired green sheets10were fabricated with four corners of each unfired green sheet10being clamped by pins41, respectively. Each unfired green sheet10had a surface10ain which a print object area101is partially defined within an area formed in the square shape with apexes on the four pins41while having eight measuring points102set in positions as shown inFIG. 8. Among these measuring points102, measuring points102in a pair are connected to each other with lengths of six line segments, extending parallel to sides of each unfired green sheet10, being measured, respectively. That is, distances D1to D6were measured as lengths of the six line segments, respectively, as shown inFIG. 8.

Then, electrically conductive paste was printed on the surface10ain an area surrounded with the pins41.

Thereafter, a reverse pattern printing was conducted to form insulating paste on the surface of each unfired green sheet10in an area, where no electrically conductive paste was printed, so as to have the same film thickness as that of the electrically conductive paste. This minimized an uneven stepped portion caused by the electrically conductive paste printed in advance. In addition, the electrically conductive paste and the insulating paste were printed four times, respectively.

Subsequently, a whole of each unfired green sheet10was dried at a temperature of 55° C. for 20 minutes.

Thereafter, the lengths (D1to D6) of the six line segments were measured again, thereby checking the dimensional change in the unfired green sheets10on a stage subsequent to the paste printing.

Then, a dimensional change rate of each unfired green sheet10in the six measuring points were calculated based on the distance prior to the printing with paste and the distance subsequent to the printing with paste.FIG. 9shows the dimensional change rate representing a value obtained by a process in which the dimensional change rates on lengths (D1to D6) of the six line segments of each unfired green sheet10were calculated after which an average value on the dimensional changes rates on all the distances for the lengths (D1to D6) was calculated.

Measured results are indicated inFIG. 9. The porosity, plotted on the abscissa ofFIG. 9, represents porosity of each unfired green sheet10on a stage prior to the pressurization being conducted.

As will be apparent fromFIG. 9, with the unfired green sheet10having porosity greater than 10%, the unfired green sheet10exhibited the dimensional change rate laying at a value of nearly 0% on a stage before and after the printing with paste, enabling the dimensional change rate to be reduced to an adequately minimized level. In addition, it is turned out that with the unfired green sheet10having porosity greater than 15%, the dimensional change rate can be decreased to a further minimized extent.