GB2442614A

GB2442614A - Brazing leads to a ceramic heater assembly

Info

Publication number: GB2442614A
Application number: GB0720568A
Authority: GB
Inventors: Satoshi Tanaka; Yuuki Fujino
Original assignee: Kyocera Corp
Current assignee: Kyocera Corp
Priority date: 2004-05-27
Filing date: 2005-05-26
Publication date: 2008-04-09
Anticipated expiration: 2025-05-26
Also published as: GB0720568D0; GB2442614B

Abstract

The ceramic heater comprises a ceramic base, a heat generating resistive member embedded in the ceramic base 22, a lead-out electrode 27, electrically connected to the heat generating resistive member, provided on the surface of the ceramic base, and a lead member 24 brazed onto the external electrode via a opening in the base, the brazing material 35 consisting of at least three layers. In order from the lead-out electrode the layers comprise a layer 35a of NiWCu having a main component of Ni, a layer 35b of NiCu having a main component if Ni and a layer 35c of CuNi having a main component of Cu. A eutectic layer 35d is provided. The metal layers are formed with a mean thickness in the range from 2 to 30 microns.

Description

CERAMIC HEATER, AND OXYGEN SENSOR AND HAIR IRON USING THE

CERAMIC HEATER

TECHNICAL FIELD

[0001) The present invention relates to a ceramic heater which is used as the heater for fuel-to-air ratio sensor or carburetor heater for automobile or soldering iron heater, and to an oxygen sensor and a hair iron that use the hair iron.

BACKGROUND ART

2] Ceramic heaters have been widely used for heating the fuel-to-air ratio sensor used in automobiles. The ceramic heater has such a constitution as, for example, a heat generating resistive member formed from a metal having high melting point such as W, Re or Mo is incorporated in a ceramic base which is constituted, for example, from alumina as the main component, and metal terminal (lead member) is bonded via external electrode to the heat generating resistive member (refer to Japanese Unexamined Patent Publication (Kokai) No. 5-34313 and Japanese Unexamined Patent Publication (Kokai) No. 5-161955) [0003) a The ceramic heater is manufactured by, for example, preparing a core ceramic member and a ceramic sheet, printing a paste of metal having high melting point such as W, Re or Mo on one side of the ceramic sheet thereby forming the heat generating resistive member and electrode lead-out section, winding the ceramic sheet with the surface having the heat generating resistive member and the electrode lead-out section formed thereon facing inside around the core ceramic member and firing so as to consolidate the assembly (refer to Japanese Unexamined Patent Publication (Kokai) No. 5-34313).

4] More specifically, the ceramic sheet has the heat generating resistive member and the electrode lead-out section connected to the former formed on the top surface thereof, and the external electrode formed on the back surface. The electrode lead-out section of the ceramic sheet is connected to the external electrode via through hole. The through hole is filled with an electrically conductive paste as required.

5] The ceramic heater shown in Fig. 8A and Fig. 8B is the ceramic heater 51 described in Japanese Unexamined Patent Publication (Kokai) No. 6-196253. In the ceramic heater shown in Fig. 8, lead-out electrodes 57 are connected to both ends of the heat generating resistive member 53, and the lead-out electrodes 57 are exposed in opening 58 formed in the ceramic base 52 and lead members 54 are bonded by means of a brazing material such as solder.

6] The opening 58 where the lead-out electrode 57 is exposed is for defining a region wherein the lead-out electrode 57 and the lead member 54 are brazed, and is formed at the end of the ceramic base 52 by punching through a hole in advance in a ceramic green sheet which would become the ceramic base 52.

7] In the ceramic heater described in Japanese Unexamined Patent Publication (Kokai) No. 6-196253, the opening 58 has a recess 56, having a size corresponding to the diameter of the lead member 54, formed on the side wall thereof. When brazing the heat generating resistive member 53 and the lead member 54 in the opening 58, the lead member 54 is inserted into the recess 56 so that the lead member 54 can be accurately positioned at the center of the heat generating resistive member 53 and thereby the lead member 54 is brazed very firmly on the heat generating resistive member 53.

DISCLOSURE OF THE INVENTION

[0008) The ceramic heater of the prior art has such a problem that bonded portions deteriorate and durability decreases significantly when used under such a condition as the electrode is subjected to repetitive changes in temperature.

In recent years, under the strict regulation on the emission from automobiles, the oxygen sensor used for controlling the fuel to air ratio is required to start up faster and accordingly the ceramic heater used therein is required to be capable of quick startup. The problems described above have become urgent with such a background.

The ceramic heater used in a device which is required to have the capability of quick startup is used under demanding conditions, and there is a tendency toward higher temperature around the lead-out electrode. This leads to stress concentration in the brazed portion due to the difference in thermal expansion between the brazing material and the ceramic base, thus resulting in the requirement for higher durability. The ceramic heater used in automobile, in particular, is required to have high reliability and therefore must have very high durability.

9] In the case of a ceramic heater which has a large heat generating area and is held as a whole between holding members, such as a hair iron, high durability is required in the brazed portion since the lead-out electrode is heated quickly immediately when power is supplied.

Accordingly, first object of the present invention is to provide a ceramic heater having high durability.

Second object of the present invention is to provide an oxygen sensor having high durability.

Third object of the present invention is to provide a hair iron having high durability.

[00101 In order to achieve the objects described above, a first ceramic heater of the present invention comprises a ceramic base, a heat generating resistive member embedded in the ceramic base, an external electrode measuring 5 to 200 pm in thickness which is electrically connected to the heat generating resistive member and is provided on the surface of the ceramic base, and a lead member brazed onto the external electrode.

[00111 A second ceramic heater of the present invention comprises a ceramic base, a heat generating resistive member embedded in the ceramic base, an external electrode measuring 5 to 50 pm in thickness which is electrically connected to the heat generating resistive member, includes 1 to 10% by weight of an additive consisting of the same component as the main component of the ceramic base and is provided on the surface of the ceramic base, and a lead member brazed onto the external electrode.

2] The oxygen sensor of the present invention comprises the first or second ceramic heater of the present invention.

3] A third ceramic heater of the present invention comprises a ceramic base, a heat generating resistive member incorporated in the ceramic base and a lead-out electrode electrically connected to the heat generating resistive member and exposed in an opening which is formed in the ceramic base, wherein at least a part of corner of wall surface of the opening and/or at least a part of top end of outer periphery of the opening is at least one selected from a group consisting of C surface having chamfering size of 0.05 mm or more and R surface having radius of 0.05 mm or more.

In the present invention, the C surface refers to a surface which is chamfered so as to form a sloped surface at the intersect of two surfaces, and the R surface refers to a surface which is chamfered so as to form a curved surface at the intersect of two surfaces.

4] A forth ceramic heater of the present invention comprises a ceramic base, a heat generating resistive member incorporated in the ceramic base, lead-out electrode electrically connected to the heat generating resistive member and exposed in an opening which is formed in the ceramic base and lead member brazed onto the surface of the lead-out electrode, wherein the brazing material has layered structure consisting of three or more layers.

5] The hair iron of the present invention is characterized in that one of the first through fourth ceramic heaters of the present invention is used as heating means.

6] In the first ceramic heater of the present invention having the constitution described above, since the external electrode to which the lead member is brazed has a thickness in a range from 5 to 200 pm, it is made possible to improve the durability of the external electrode and of the area surrounding it and improve the bonding strength of the lead member.

[00 171 In the second ceramic heater of the present invention, since the external electrode to which the lead member is brazed has a thickness in a range from 5 to 50 pm and the external electrode includes 1 to 10% by weight of the additive consisting of the same component as the main component of the ceramic base, it is made possible to improve the durability of the external electrode and of the area surrounding it and improve the bonding strength of the lead member.

8] In the third ceramic heater of the present invention, since at least a part of the corner of the wall surface of the opening and/or at least a part of top end of the outer periphery of the opening is at least one selected from a group consisting of C surface having chamfering size of 0.05 mm or more and R surface having radius of 0.05 mm or more, it is made possible to mitigate the stress concentration in the top end of the outer periphery due to difference in thermal expansion between the brazing material and the ceramic base, and prevent cracks from occurring in the top end of the outer periphery.

9] In the fourth ceramic heater of the present invention, since the brazing material has layered structure consisting of three or more layers, the lead-out electrode and the lead member can be firmly bonded by the brazing material.

Thus the first through fourth ceramic heaters of the present invention have high durability.

0] The present invention also provides an oxygen sensor having high durability, since is has the first or second ceramic heater of the present invention.

1] Also the hair iron of the present invention employs one of the first through fourth ceramic heaters of the present invention used as heating means, and therefore has high durability.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A is a perspective view that is partially cut away for explaining the constitution of the ceramic heater according to the first embodiment of the present invention.

Fig. lB shows the ceramic base 2 of the ceramic heater according to the first embodiment before being assembled.

Fig. 2 is an enlarged partial sectional view of a junction in the ceramic heater according to the first embodiment.

Fig. 3A is a perspective view showing the constitution of the ceramic heater according to the second embodiment of the present invention.

Fig. 3B is a plan view of a ceramic sheet 22a used in the manufacture of the ceramic heater according to the second embodiment.

Fig. 3C is a plan view of a ceramic sheet 22b used in the manufacture of the ceramic heater according to the second embodiment.

Fig. 4 is an enlarged plan view of lead-out electrode in the ceramic heater according to the second embodiment.

Fig. 5A is a sectional view (1) of the lead-out electrode of the ceramic heater according to the second embodiment.

Fig. 5B is a sectional view (2) of the lead-out electrode of the ceramic heater according to the second embodiment.

Fig. 5C is a sectional view (3) of the lead-out electrode of the ceramic heater according to the second embodiment.

Fig. 6 is an enlarged sectional view of brazed portion in the ceramic heater according to the third embodiment.

Fig. 7 is a perspective view showing an example of hair iron which uses the ceramic heater of the present invention.

Fig. 8A is a plan view of a ceramic heater of the prior art.

Fig. 83 is an enlarged perspective view of lead-out

electrode in the ceramic heater of the prior art.

(Description of Reference Numerals)

3] 1, 21: ceramic heater 2, 22: ceramic base 3, 23: heat generating resistive member 3a: electrode lead-out section 4: external electrode 5: plating layer 6, 25: brazing material 7, 24: lead member 8, 22a, 22b: ceramic sheet 9: through hole 10: core ceramic member 20: paste 21c: C surface 22s: wall surface 26: recess 26e: Corner of wall surface in recess 27: lead-out electrode 28: opening 28e: Corner of wall surface in opening 29: plating layer 30e: top end of outer periphery

BEST MODE FOR CARRYING OUT THE INVENTION

[00241 The present invention will now be described by way of embodiments thereof with reference to the accompanying drawings.

First Embodiment Fig. 1A is a perspective view that is partially cut away showing the constitution of the ceramic heater according to the first embodiment of the present invention, and Fig. lB shows a portion of the ceramic base 2 before being assembled.

5] The ceramic heater 1 of the first embodiment has the heat generating resistive member 3 incorporated in the S ceramic base 2 as shown in Fig. 1A. The ceramic heater 1 of the first embodiment also has the external electrode 4 which supplies electric power to the heat generating resistive member 3 formed on the surface of the ceramic base 2, while a plating layer 5 is formed on the external electrode 4 which is connected via the brazing material to the lead member 7 that is metal terminal. The first ceramic heater 1 of the first embodiment is characterized in that the external electrode 4 has a thickness in a range from 5 to 200 urn.

6] The first ceramic heater 1 of the first embodiment is manufactured as follows.

First, a core ceramic member 10 and a ceramic sheet 8 are prepared, and a paste of metal having high melting point such as W, Re or Mo is printed on one side of the ceramic sheet 8 thereby forming the heat generating resistive member 3 and electrode lead-out section 3a.

Then the ceramic sheet 8 is wound around the core ceramic member 10, with the surface having the heat generating resistive member 3 and the electrode lead-out section 3a formed thereon facing inside, and the assembly is fired so as to consolidate.

The ceramic base 2 having the heat generating resistive member 3 incorporated therein is manufactured by putting the ceramic sheet 8 into contact the core ceramic member 10, with the surface having the heat generating resistive member 3 formed thereon facing inside, and firing the assembly as described above.

7] Various ceramic materials may be used to form the ceramic base 2, such as alumina ceramics, silicon nitride ceramics, aluminum nitride ceramics, silicon carbide ceramics or the like. Among these, ceramic material which includes alumina or silicon nitride as the main component is preferably used, which enables it to make a ceramic heater having capability of quick heating and high durability. ib the case of alumina ceramics, for example, it preferably has such a composition that includes 88 to 95% by weight of A1203, 2 to 7% by weight of Si02, 0.5 to 3% by weight of CaO, 0.5 to 3% by weight of MgO and 1 to 3% by weight of Zr02. In addition to the components described above, trace of impurities may also be included. When the concentration of Al203 is less than 88% by weight, higher content of glass component may cause significant migration when electric power is supplied. When the concentration of Al203 exceeds 95% by weight, on the other hand, durability of the ceramic heater 1 may become lower due to insufficient content of glass component which diffuses into the metal layer of the heat generating resistive member 3 which is incorporated in the ceramic base 2. In the case of silicon nitride ceramics, it is preferable to add oxide of rare earth element, as a sintering additive, in proportion of 3 to 12% by weight and Al203, in 0.5 to 3% by weight in proportion to the main component of silicon nitride, and Si02 in such a quantity as the concentration of Si02 in the sintered material becomes 1.5 to 5% by weight. The concentration of Si02 here means the sum of S102 formed from impurity oxygen included in the stock material of silicon nitride, Si02 included in other additives as impurity and Si02 that is intentionally added.

Durability of the heat generating resistive member 3 can be improved by dispersing MoSi2 or WSi2 in silicon nitride of the base material, thereby making the thermal expansion coefficient of the base material proximate to the thermal expansion coefficient of the heat generating resistive member 3.

8] When aluminum nitride is used, it is preferable to add oxide of rare earth element such as Y203 and/or CaO as a sintering additive in concentration of 2 to 8% by weight in proportion to aluminum nitride.

9] In the first embodiment, the ceramic base 2 consisting of the core ceramic member 10 and the ceramic sheet 8 has a shape of cylinder or tube measuring 2 to 20 mm in outer diameter and 40 to 200 mm in length. When used for heating fuel-to-air ratio sensor for automobile, the ceramic base 2 preferably has a shape of cylinder or tube measuring 2 to 4 mm in outer diameter and 40 to 65 mm in length. While tubular shape is employed in the first embodiment, the present invention is not limited to this and plate shape may also be employed.

0] The heat generating resistive member 3 and the electrode lead-out section 3a formed to connect to the heat generating resistive member 3 are formed from a material including a metal having high melting point such as W, Re or Mo as main component, and the electrode lead-out section 3a is bonded via a through hole shown in Fig. 2 to the external electrode 4.

[00311 The external electrode 4 is formed around the through hole 9 on the surface of the ceramic base 2 as shown in Fig. 2 and consists of a metallized layer including a metal having high melting point such as W, Re or Mo as main component. It is particularly preferable that the main component is W or a compound of W, since W is a metal having a high melting point and high resistance to acid and allows it to sinter while maintaining the shape of the external electrode. It is important that thickness D of the external electrode 4 is in a range from 5 to 200 pm. It is necessary that the thickness D is in the range of values described above in terms of the average of entire thickness of the external electrode 4.

Setting the thickness of the external electrode 4 in this range makes it possible to mitigate the stress caused by the difference in thermal expansion between the ceramic base 2 and the brazing material 6 which is a metal, and maintain the strength and durability of the junction at sufficient levels, even when the terminal is subjected to repetitive thermal cycles. When the thickness is less than 5 pm, bonding strength of the lead member 7 decreases significantly due to the difference in thermal expansion as the thermal cycles are repeated. When the thickness is larger than 200 pm, bonding strength in the direction of thickness of the external electrode decreases, resulting in the deterioration of bonding strength of the lead member 7 due to peel-off from the inside of the external electrode caused by the thermal cycles.

2] When thickness D is controlled in a range from 5 to 50 pm, in particular, durability can be improved more effectively. The external electrode 4 can be formed on the principal surface of the ceramic sheet 8 opposite to the surface where the electrode lead-out section 3a is formed, by printing or other transferring technique similarly to the formation of the heat generating resistive member 3 and the electrode lead-out section 3a.

3] The external electrode 4 can be formed with a thickness larger than in the prior art by increasing the aperture ratio of the mesh of a printing plate used in the printing process.

However, there occurs a problem in the smoothness of the surfaces of the external electrode 4 which is formed, in case the thickness is made too large. After studying including other parameters, it was found that the thickness can be increased further by increasing the moving speed of the squeegee used in printing, in addition to the investigation of the aperture ratio of the printing plate. Moreover, the thickness can be increased further by increasing the pressure applied to the squeegee downward during printing.

Configuration of the contact area between the squeegee and the printing plate is also of importance, and the thickness can be increased by making the contact area in round shape.

It is made easier to form the external electrode thicker by slanting the squeegee toward the moving direction with an angle of 90 degrees or less. While the external electrode can also be formed thicker by increasing the viscosity of the paste to form the external electrode before printing, releasability of the printing plate must also be fully taken into consideration. It is also effective to use the printing plate of larger thickness.

4] The inventors of the present application found out optimum conditions which enable it to form the external electrode 4 with larger thickness, by taking into consideration the aperture ratio, moving speed, pressure, shape and inclination of the squeegee, viscosity of the paste to form the external electrode, releasability of the printing plate and the balance between the thickness of the printing plate and the entire configuration.

5] Strength can be stabilized by setting the width Hi of the external electrode 4 formed on the surface of the ceramic base 2 larger than the width H of the lead member 7 to be described later so that the brazing material 6 flows smoothly from the lead member 7 to the end of the external electrode such that meniscus of the brazing material, thereby stabilizing the strength. While the strength can be maintained by keeping the width Hi of the external electrode 4 from becoming smaller than the width H of the lead member 7, it is preferable to make Hi 1.1 times electrode lead-out section or larger which enables it to increase the bonding strength.

6] By forming the external electrode 4 including an additive (not shown) consisting of the main component of the ceramic base 2, bonding strength of the external electrode 4 and the ceramic base 2 is increased as the additive diffuses into the ceramic base 2 and the material of the ceramic base 2 itself diffuses into the external electrode 4 and vice versa. Proportion of the additive (not shown) consisting of the main component of the ceramic base 2 in the external electrode 4 is preferably in a range from 1 to 30% by weight, and more preferably from 1 to 10% by weight, which enables it to further improve the bonding strength of the external electrode through mutual diffusion.

[0037) The ceramic heater having high strength and high durability can be made by controlling the thickness D of the external electrode 4 in a range from 5 to 50 pm and controlling the proportion of the an additive consisting of the main component of the ceramic base 2 in the external electrode in a range from 1 to 10% by weight.

8] The external electrode 4 may have a plating layer 5 formed on the surface thereof as shown in Fig. 2. By forming the plating layer 5 on the external electrode 4, flow of the brazing material 6 can be made smoother and the brazing strength can be increased. The plating layer 5 is formed from Ni, Cr or a composite material which includes these metals as the main component with a thickness from 1 to 5 pm.

9] Provided on the external electrode 4 is the lead member 7, made of an alloy based on Ni or Fe-Ni which has satisfactory heat resistance for a metal terminal, brazed by using the brazing material 6. The brazing material 6 includes Ag-Cu, Au-Cu, Ag, Cu, Au or the like as the main component, with a resin as binder and/or a metal such as Ti, Mo, V or the like as an active metal added thereto as required, and is formed by hardening in reducing atmosphere which includes water vapor.

0] A method of manufacturing the ceramic heater of the first embodiment will now be described.

First, the ceramic sheet 8 is prepared by molding a ceramic slurry based on alumina as the main component and including Si02, CaO, MgO and Zr02 with concentration of 4 to l2 by weight in total as the sintering additive.

The heat generating resistive member 3 and the electrode lead-out section 3a are formed on one of the principal surfaces of the ceramic sheet 8 by printing or other transferring technique, and the external electrode 4 is formed on the other principal surface of the ceramic sheet 8, opposite to the surface where the electrode lead-out section 3a is formed, similarly by printing or other transferring technique.

1] The through hole 9 is formed between the electrode lead-out section 3a and the external electrode 4, and the through hole 9 is filled with an electrically conductive material including at least one of W, Mo and Re as the main component or the electrically conductive material is applied to the inner surface of the through hole 9, so as to electrically connect the electrode lead-out section 3a and the external electrode 4 with each other.

Then the heat generating resistive member 3 and the electrode lead-out section 3a are coated with a material having substantially the same composition as the material of the ceramic sheet 8, and the ceramic sheet 8 is wound around the core ceramic member 10 in close contact therewith, thereby forming a green compact having tubular shape. The green compact is fired at a temperature from 1500 to 1650 C in reducing atmosphere, thereby making the ceramic base 2.

Then the plating layer 5 is formed on the external electrode 4 from a metal such as Ni, Cr or the like by electroplating or electroless plating method.

Then the external electrode 4 and the lead member 7 are bonded with each other by means of a brazing material including Au-Cu as the main component in reducing atmosphere which includes water vapor.

[00421 Second Embodiment The ceramic heater according to the second embodiment of the present invention will now be described with reference to Figs. 3A through 3C.

3] The ceramic heater 21 of the second embodiment is a ceramic heater having the shape of flat plate comprising a ceramic base 22 which incorporates a heat generating resistive member 23 inside thereof, where the lead member 24 is connected to the lead-out electrode 27 which is exposed in an opening 28 formed in the ceramic base 22 by means of a brazing material.

4] The ceramic heater 21 of the second embodiment can be manufactured by forming the heat generating resistive member 23 and the lead-out electrode 27 which is connected to the former on the ceramic sheet 22a as shown in Fig. 3B, putting another ceramic sheet 22b having the opening 28 and the recess 26 formed therein on the ceramic sheet 22a in close contact therewith as shown in Fig. 3C, and firing the assembly at a temperature from 1500 to 1650 C in reducing atmosphere.

5] The ceramic heater of the second embodiment is characterized in that at least a part of the corner of the wall surface of the opening and/or at least a part of top end of the outer periphery of the opening is at least one selected from a group consisting of C surface having chamfering size of 0.05 mm or more and R surface having radius of 0.05 mm or more.

In the case of the ceramic heater shown in Fig. 4, an R surface having radius of 0.05 mm or more is formed at the corner 28e of the wall surface of the opening 28, thereby improving the durability of the electrode. An R surface having radius of 0.05 mm or more is formed also at the corner 26e of the wall surface of the recess 26.

When the size of the C surface or the R surface is less than 0.05 mm, stress due to the difference in thermal expansion between the brazing material and the ceramics becomes concentrated in the corner 28e, thus making it difficult to effectively improve the durability of the electrode. In order to further improve the durability of the electrode, it is preferable that the size of the C surface or the R surface is 0.1 mm or larger, and more preferably 0.2 mm or larger.

It is further preferable to form a C surface 2lc (refer 8 I to Fig. 5C) having chamfering size of 0.05 mm or larger also on top end of outer periphery 30e (refer to Fig. SB) at the border between the wall surface 22s of the opening 28 (or the recess 26) and the top surface of the ceramic base 2. An R surface having radius of 0.05 mm or more may also be formed at the top end of the outer periphery 30e. While the C surface or Rsurface formed at the top end 30e of the outer periphery is preferably formed over the entire circumference of the opening 28 and the recess 26, the C surface or R surface may also be formed in a part of the top end of outer periphery 30e where the stress due to the difference in thermal expansion between the brazing material and the ceramics is likely to be concentrated.

[0046) When the C surface 21c (or R surface) is formed on top end of the outer periphery 30e of the wall surface 22s of the opening 28 as shown in Fig. SC, the lead member 24 can be prevented from being damaged when placing the lead member 24.

The C surface 21c (or R surface) is useful in improving the durability, since damage on the lead member 24 can become a cause of corrosion which occurs during the use of the ceramic heater 1.

[00471 Since forming the C surface or the R surface makes it possible to suppress the generation of waste chips during

S

machining for forming the opening in the ceramic sheet 22b, such problems can be prevented from occurring as the chips are caught between the ceramic sheets 22a, 22b, and the sheets can be prevented from making close contact with each other, thus resulting in lower durability of the heat generating resistive member 23.

[00481 It is preferable that 5O or more of the periphery of the lead-out electrode 27 which is exposed in the opening 28 is embedded in the ceramic base 22 as shown in Fig. 4. In case the lead member 24 is brazed onto the lead-out electrode 27, because of the difference in thermal expansion coefficient from the ceramic base 22, when the brazing material flows to the periphery of the lead-out electrode 27, stress is concentrated in the periphery due to the difference in thermal expansion coefficient. Therefore, in case 5O or more of the periphery of the lead-out electrode 27 is exposed without being embedded in the ceramics, cracks tend to occur in the exposed periphery due to the thermal cycles during use.

9] For the reasons described above, 50% or more of the periphery of the lead-out electrode 27 is embedded in the ceramic base 22, which makes it possible to prevent cracks from occurring in the periphery and prevent the durability from decreasing. It is more preferable that 75% or more of the periphery of the lead-out electrode 27 is embedded in the ceramic base 22, which makes it possible to more effectively prevent cracks from occurring. It is also preferable that the angle e between the wall surface 22s of the opening 28 and the lead-out electrode 27 is in a range from 60 to 1100.

The angle e between the wall surface 22s and the lead-out electrode 27 is the angle formed by the top surface of the portion of the lead-out electrode 27 embedded in the ceramic base 22 and the wall surface 22s.

0] When the angle U exceeds 110 , end of the brazing material 25 is subjected to stress generated by the expansion and shrinkage of the brazing material 25 formed in an area near up to the wall surface 22s, and cracks tend to occur in the ceramic base 22 at the end of the brazing material 25.

1] When the angle U is less than 60 , pressure cannot be properly applied to the interface between the lead-out electrode 27 at the opening 28 and the ceramic sheet 22a thus resulting in a gap due to insufficient contact when the ceramic sheet 22b is placed on the ceramic sheet 22a, thus making it impossible to prevent the lead-out electrode 27 from peeling off.

2] The angle U is more preferably in a range from 60 to 90 .

3] When the angle 9 between the wall surface 22s and the lead-out electrode 27 is controlled within 1100, more preferably 90 , in the vicinity (for example, within 0.2 mm from the border) of the interface between the lead-out electrode 27 and the wall surface 22s, stress can be prevented from being concentrated during expansion and shrinkage of the brazing material 5 at the end of the brazing material 25, thus making it possible to prevent cracks from occurring in the ceramic base 22.

[00541 Also as shown in Fig. 5B, a paste 20 of the same nature as the ceramic base 22 may be placed in the interface of bonding with the lead-out electrode 27 of the opening 28 and fired, so that the brazing material 25 does not flow to the wall surface 22s of the opening 28.

[00551 By keeping the brazing material 25 from flowing to the wall surface 22s of the opening 28, it is made possible to prevent cracks from occurring in the end of the lead-out electrode 27 in the opening 28 due to the stress that presses the wall surface 22s upward by the thermal expansion of the brazing material 25.

6] It also becomes possible to prevent chipping from occurring in the vicinity of the C surface 21c (or R surface) when a metal plate is placed on the ceramic heater 21.

7] Thickness of the lead-out electrode 27 is preferably 10 pm or more. When the thickness of the lead-out electrode 27 is less than 10 pm, bonding strength between the lead-out electrode 27 and the ceramic base 22 becomes lower, thus resulting in lower durability of tensile strength of the lead member 24 under thermal cycles during use.

8] The thickness is more preferably 15 pm or more, and ideally 20 pm or more.

9] Thickness of the lead-out electrode 27 affects the tensile strength of the lead member 24 by a mechanism as described below. In the lead-out electrode 27, glass component existing in the grain boundary diffuses from the ceramic base 22 into the pores of the sintered metal having high melting point such as W, Mo, Re or the like, and this anchor effect increases the strength. As a result, tensile strength of the lead member 24 becomes higher as the thickness of the lead-out electrode 27 increases.

0] The heat generating resistive member 23 may be formed from a metal element such as W, Mo or Re, an alloy of these metals or a compound such as silicate or carbide of metal including TiN and WC.

1] When a metal as described above having high melting point is used as the material of the heat generating resistive member 23, durability is improved since sintering of the metal does not proceed during use.

2] Bonding strength of the lead-out electrode 24 can be improved by interposing the periphery of the lead-out electrode 27 between the ceramic base 22 as shown in Fig. 5A.

3] Fluidity of the brazing material 25 during brazing of the lead member 24 can be improved by forming a primary plating layer 29 on the surface of the lead-out electrode 27 as shown in Fig. 5B. At this time, residual stress remaining after brazing can be reduced by setting the brazing temperature to 100OC or lower for the brazing material 25 used to fasten the lead member 24.

4] In case the ceramic heater 21 is used in an atmosphere of high humidity1 it is preferable to use the brazing material 25 based on Au or Cu since it makes migration less likely to occur. The brazing material 25 made of a material based on Au, Cu, Ag-Cu, Au-Ni or Ag-Cu is used. The Au-Cu brazing material includes 25 to 95% by weight of Au, and Au- Ni brazing material includes 50 to 95% by weight of Au. Ag-Cu brazing material includes 60 to 90% by weight, more preferably 70 to 75% by weight of Ag, which results in composition of eutectic point which enables it to prevent the formation of alloy having different composition when the temperature is raised and lowered during the brazing process, thus reducing the residual stress after brazing.

[00651 In case the ceramic heater is used in an atmosphere of high humidity, it is preferable to use the brazing material based on Au or Cu since it makes migration less likely to occur.

[00661 It is preferable to form a secondary plating layer which is usually formed from Ni on the surface of the brazing material 25, in order to improve the durability at high temperatures and protect the brazing material 25 from corrosion.

[0067) It is effective to control the grain size of the crystal that constitutes the secondary plating layer to 5 1.im or smaller, for improving the durability. When the grain size is larger than 5 pm, the secondary plating layer becomes lower in strength and brittle, and cracks are likely to occur when left to stand at a high temperature.

8] It is considered that smaller grain size of the crystal that constitutes the secondary plating layer makes it possible to prevent microscopic defects from being generated because the plating layer has higher density, although the reason is not known, and it is preferable to use electroless Ni plating method based on boron for the secondary plating layer.

9] The electroless plating method may be of a type based on phosphorus as well as boron. In case there is a possibility that the ceramic heater be used at high temperatures, however, electroless plating method based on boron is usually applied.

The grain size of the secondary plating layer can be controlled by changing the temperature of heat treatment after applying the secondary plating.

[00701 The lead member 24 is preferably formed from an alloy based on Ni or Fe-Ni which has high heat resistance. This is because temperature of the lead member 24 may rise during use and deteriorate due to the heat transferred from the heat generating resistive member 23.

[00711 In case Ni or Fe-Ni alloy is used as the material of the lead member 24, it is preferable to control the mean crystal grain size to 400 pm or smaller. When the mean crystal grain size exceeds 400 pm, the lead member 24 fatigues in the vicinity of the brazed portion due to vibration and thermal cycles during use, thus causing cracks.

2] With respect to other materials, too, it is not desirable that grain size of the lead member 24 be larger than the thickness of the lead member 24, which causes stress to be concentrated in the grain boundary in the vicinity of the boundary between the brazing material 25 and the lead member 24, thus causing crack to occur.

3] In order to keep variability among samples small, it is necessary to carry out the heat treatment applied during brazing process at a high temperature with a sufficient margin above the melting point of the brazing material 25.

In order to control the mean crystal grain size of the lead member 24 to 400 pm or smaller, however, it is preferable to keep the temperature as low as possible during brazing and make the duration of treatment shorter.

[00741 In case the ceramic heater 21 is made of alumina, it preferably has such a composition that includes 88 to 95% by weight of A1203, 2 to 7% by weight of Si02, 0.5 to 3% by weight of CaO, 0.5 to 3 by weight of MgO and 1 to 3 by weight of Zr02. While alumina ceramics are used in the example described above, the present invention is not limited to alumina ceramics and can be applied to silicon nitride ceramics, aluminum nitride ceramics, silicon carbide ceramics and the like, and also to any applications of Au-based brazing, besides the ceramic heater 1.

Third Embodiment [0075] The ceramic heater according to the third embodiment of the present invention will now be described with reference to the accompanying drawings.

[00761 The ceramic heater of the third embodiment has a constitution similar to that of the second embodiment, except f or using a different brazing material 35 for brazing the lead-out electrode 27 and the lead member 24.

[00771 The third embodiment is characterized by the structure of the brazed portion 35 where the lead-out electrode 27 and the lead member 24 are brazed together. Au-Cu brazing material used as the brazing material for the ceramic heater 1 of the third embodiment is most commonly used as the material for holding the lead member 24.

8] The brazed portion 35 between the lead-out electrode 27 and the lead member 24 has such a structure as three layers of a first layer 35a, a second layer 35b and a third layer 35c are formed in order from the side of the lead-out electrode 27, and a eutectic layer 35d is placed thereon as shown in Fig. 6.

Such a structure is formed by forming a plating layer on the surface of the lead-out electrode 27, and brazing the lead member 24 using a brazing material such as Au-Cu brazing material (BAg-8). Melting temperature of the brazing material (brazing temperature) and the duration of melting (temperature holding time) are controlled to predetermined values in accordance to the materials used in brazing and plating, so that the electrically conductive material included in the lead-out electrode 27 and the component of the brazing material diffuse into the plating layer. This results in the three layers of the first layer 35a, the second layer 35b and the third layer 35c formed between the lead-out electrode 27 and the eutectic layer 35d.

9] The lead member 24 is formed from Ni or an alloy based on Fe-Ni, and preferably from Fe-Ni-Co alloy.

0] A metal having high melting point such as W, Re or Mo or an alloy thereof is preferably used as the electrically conductive material (hereafter denoted as Me) of the lead-out electrode 27.

1] The first layer 35a located nearest to the lead-out electrode 27 is formed by causing the electrically conductive material Me to diffuse from the lead-out electrode 27 into the Ni plating layer formed on the lead-out electrode 27, and causing Cu to diffuse from the brazing material, and is described as Ni(Me)Cu layer with Ni being the main component.

In the third embodiment, bonding strength between the lead-out electrode 27 and the brazing material is improved by the presence of the Ni(Me)Cu layer. The first layer 35a is preferably NiWCu layer including Ni as the main component, and bonding strength between the lead-out electrode 27 and the brazing material can be improved further by the presence of the NiWCu layer. The first layer 35a constituted from NiWCu can be formed by forming the lead-out electrode 27 from W, causing W to diffuse from the lead-out electrode 27 into the Ni plating layer formed on the lead-out electrode 27, and causing Cu to diffuse from the brazing material.

2] The second layer 35b formed on the first layer 35a is a NiCu layer including Ni as the main component. The second layer 35b includes the highest content of Ni. The second layer 35b which is Ni-rich is constituted from Ni of the plating layer which is formed on the lead-out electrode 27 before brazing and Cu of the brazing material 35. The second layer 35b serves as a protective layer for the first layer 35a where W forms solid solution.

3] The third layer 35c formed on the second layer 35b is CuNi layer including Cu as the main component. The third layer 35c includes the highest content of Cu. The third layer 35c may include Ag. The third layer 35c serves as a stress relieving layer which mitigates the stress generated by the difference in thermal expansion between the eutectic phase 35d of the Ag-Cu brazing material and the lead-out electrode 27.

The second layer 35b and the third layer 35c have different compositions as described above, and therefore can be distinguished by the difference in color on a photograph taken with SEM (scanning electron microscope).

4] In the ceramic heater of the third embodiment constituted as described above, tensile strength of the lead member 24 can be increased and durability can be improved by forming the first layer 35a, the second layer 35b and the third layer 35c between the eutectic layer 35d and the lead-out electrode 27.

5] The first layer 35a, the second layer 35b and the third layer 35c are formed with mean thickness preferably in a range from 2 to 30 pm, more preferably from 2 to 20 pm, and furthermore preferably from 2 to 12 pm.

6] When thickness is less than 2 pm, tensile strength of the lead member 24 cannot be effectively improved. When thickness exceeds 30 pm, difference in the properties between the layers becomes manifest to make the structure brittle, thus decreasing the tensile strength over a long period of use.

7] The thickness of the second layer 35b is affected by the thickness of the Ni plating layer formed on the lead-out electrode 27, and thickness of the Ni plating layer is preferably from 2 to 30 pm.

8] The third layer 35c is formed between the eutectic layer of Ag-Cu brazing material and the Ni. plating layer as an intermediate layer through reaction of these layers.

Thicknesses of the first layer 35a, the second layer 35b and the third layer 35c are affected by the melting temperature of the brazing material (brazing temperature) and the duration of melting (temperature holding time). There are no limitations on the brazing temperature and the temperature holding time which may be determined in accordance to the composition of the brazing material and the material that constitutes the plating layer. In case brazing is carried out at a temperature from 800 to 900 C using BAg-B (JIS standard) as the Ag-cu brazing material, for example, the holding time is controlled within a range from 0.5 to 5 hours, preferably from 1 to 5 hours and more from 1 to 2 hours.

9] In the ceramic heaters of the second and third embodiments, oxide ceramics such as alumina, mullite or forsterite or non-oxide ceramics such as silicon nitride or aluminum nitride may be used to form the ceramic base 22, while oxide ceramics is preferably used.

0] Fig. 7 is a perspective view showing an example of hair iron which uses the ceramic heater of the second or third embodiment of the present invention.

1] The hair iron is used to dress hair by placing the hair between arms 42 and holding handles 41 so as to apply heat and pressure to the hair. The arms 42 have ceramic heaters 46 incorporated therein, and have metal plates 43 such as stainless steel provided in portions which make contact with the hair.

2] The arms 42 also have a cover made of heat resistant plastics provided on the outside thereof so as to prevent human skin from being burned.

3] The ceramic heater of the present invention has been described. It should be understood, however, that the present invention is not limited to the embodiments described above, and various modifications may be made without departing from the scope of the present invention.

Examples

Example 1

In Example 1, tests described below were conducted on samples made for the purpose of verifying the usefulness of the present invention according to the first embodiment.

Samples of the ceramic heater shown in Fig. 1 were made as follows. The heat generating resistive member 3 made of W-Re and the electrode lead-out section 3a made of W were printed on the ceramic sheet 8 constituted from A1203 as the main component and Si02, CaO, MgO and Zr02 with concentration of 10% by weight or less in total for the ceramic base 2.

The external electrode 4 was printed on the back surface of the ceramic sheet 8.

5] A through hole was formed at the end of the electrode lead-out section 3a made of W, and the through hole was filled with a paste so as to establish electrical continuity between the external electrode 4 and the electrode lead-out section 3a. Position of the through hole was set so as to lie within the brazed portion when brazing is applied.

6] The surface of the heat generating resistive member 3 was coated with a material having substantially the same composition as the ceramic sheet 8. After sufficiently drying the coating layer, a contact liquid including ceramics having substantially the same composition as the ceramic sheet 8 dispersed therein was applied. The ceramic sheet 8 prepared in this way was put into close contact around the core ceramic member 10, and was fired at a temperature from 1500 to 1600 C.

7] The plating layer 5 was formed from Ni on the surface of the external electrode 4, and was subjected to heat treatment at a temperature from 700 to 800 C in reducing atmosphere.

Then the lead member 7 made of Ni having diameter of 0.8 mm was brazed at 830 C in reducing atmosphere using a brazing material 6 made of Au-Cu, and an Ni plating layer was formed on the surface thereof at the end, followed by heat treatment applied at 700 C.

8] Samples were made by varying the thickness of the external electrode 4 and the proportions of the additives for the ceramic heater described above.

[0099J Resistance of the sample of the ceramic heater was measured with a digital multimeter, to check the stability of the resistance.

0] Then with the ceramic heater held horizontally with a holding fixture, the lead member was pulled in a direction perpendicular to the brazing surface of the lead member, and the initial bonding strength of the lead member 7 was measured with a digital force gage.

[0101) Durability of the electrode of the sample of the ceramic heater at a high temperature was evaluated. The ceramic heater was placed in a high temperature durability furnace, where the sample was left at 400 C for 3 minutes and was then cooled down below 100 C in 3 minutes. This thermal cycle was repeated 3000 times, before measuring the tensile strength of the lead member. Results of the test are shown in Table 1.

(0102]

Table 1

Thickness Bonding Proportion Initial of strength Stability Sample of an bonding after thermal of external No. additive strength cycle test resistance electrode (%) (N) _______ (pm) __________ __________ (N) __________ *1 0 46 5 A *2 5 77 14 A 3 0 72 51 A 4 1 115 115 A 5 119 113 A 6 10 121 114 A 7 30 108 89 A 8 ___________ 40 109 79 B 9 0 76 55 A 5 117 114 A 11 0 81 58 A 12 5 107 109 A 13 0 85 61 A 14 5 115 114 A 0 78 54 A 16 1 114 112 A 17 5 122 114 A 18 10 117 107 A 19 30 106 89 A ___________ 40 105 94 B 21 0 76 53 A 22 5 103 89 A 23 0 80 53 A 24 1 106 87 A 5 107 92 A 26 10 109 83 A 27 30 105 90 A 28 40 104 79 B *29 0 77 23 A *30 5 119 27 A [01031 As shown in Table 1, samples Nos. 3 through 28 which are ceramic heaters of the present invention having external electrode measuring 5 to 200 pm in thickness showed initial bonding strength of 70 N or more, indicating that sufficient strength was ensured. With regards to the bonding strength of the lead member 7 after thermal cycle test, too, practically sufficient strength of 50 N or more is maintained.

4] Samples Nos. 4 through 8, 10, 12, 14, 16 through 20, 22 and 24 through 28 where the external electrode included an additive showed initial bonding strength of 100 N or more.

With regards to the bonding strength after thermal cycle test, higher strength of 70 N or more is maintained.

5] Samples Nos. 4 through 7, 10, 12, 14, 16 through 19, 22 and 24 through 27 in which proportion of the an additive was in a range from 1 to 30 by weight showed sufficient levels of initial bonding strength and the bonding strength after thermal cycle test and stable values of resistance, thus ensuring stable properties of the product.

6] Samples Nos. 4 through 6, 10, 12, 14 and 16 through 18 having external electrode measuring S to 50 pm in thickness and containing 1 to 10% by weight of an additive showed bonding strength of the lead member after thermal cycle test of 100 N or higher, substantially similar to the initial bonding strength, indicating particularly good properties.

7] Then samples were made by varying the width Hi of the external electrode 4 of the ceramic heater thus obtained for each thickness of the external electrode.

Initial bonding strength and the bonding strength of the lead member 7 after 3,000 cycles of thermal cycle test of the samples of the ceramic heater were measured similarly to the above, with the results shown in Table 2.

Table 2

Thickness Width Hi of Bonding of external Initial ---. strenqth after external eiectroae Ratio nonaing No. . thermal cycle electrode to width H of strength (N) test (N) _______ (.im) lead member) ____________ _______________ 31 0.9H 70 53 32 l.OH 109 82 33 5 l.1H 115 109 34 1.511 117 101 2.OH 114 104 36 0.9H 79 60 37 l.OH 110 79 38 30 i.1H 115 108 39 l.5H 115 109 2.011 116 105 41 0.9H 79 59 42 l.OH 112 91 43 100 i.1H 114 110 44 l.5H 117 106 2.OH 116 106 [0109] As shown in Table 2, samples Nos. 32 through 35, 37 through 40 and 42 through 45 in which the width Hi of the external electrode was larger than the width H of the lead member showed initial bonding strength of 100 N or more and bonding strength of the lead member after thermal cycle test of 70 N or more, indicating that sufficient strength was maintained.

0] Above all, samples Nos. 33 through 35, 38 through 40 and 43 through 45 in which the width Hi of the external electrode was 1.1 times as large as the width H of the lead member showed bonding strength of the lead member 7 after thermal cycle test of 100 N or higher, substantially similar to the initial bonding strength, indicating particularly good properties.

Example 2

1] Examples 2 through 5 described below are Examples related to the second embodiment of the present invention.

A paste made of W was printed on a ceramic sheet 22a constituted from A1203 as the main component and S102, CaO, MgO and Zr02 with concentration of 10% by weight or less in total as shown in Fig. 3B, thereby forming the heat generating resistive member 23 and the lead-out electrode 27.

2] Another ceramic sheet 22b was placed on the ceramic sheet 22a in close contact therewith, with the opening 28 and the recess 26 being formed in varying shapes in the ceramic sheet 22b, and the assembly was fired at a temperature of 1600 C in reducing atmosphere, thereby making 20 ceramic heaters 1 measuring 100 mm in length, 10 mm in width and 1.2 mm in thickness.

3] At this time, shapes of the opening 28 and the recess 26 were varied such that, with respect to the corner 28e related to the 4 sides of the opening 28 of rectangular shape, shape of the die used in punching through the opening 28 was changed so as to change the size of the C surface or the R surface as 0.01 mm, 0.03 mm, 0.05 mm, 0.10 mm, 0. 20 mm, 0.30 mm and 0.50 mm.

4] Then electroless Ni plating was applied to the surface of the lead-out electrode 27 exposed in the opening 28, followed by brazing of Ni wire 0.6 mm in diameter by means of Ag-Cu brazing material (BAg-B).

5] The ceramic heater 21 thus prepared was covered over the entire surface on both sides thereof with aluminum plates measuring 110 mm in length, 12 mm in width and 5 mm in thickness, which were secured at the center of the ceramic heater 21. This was subjected to acceleration test in which such a voltage was applied for 5 minutes as the highest temperature of the ceramic heater 21 reached 300 C, followed by forced air cooling of blowing air for 5 minutes so as to cool down the entire ceramic heater to 40 C or lower, repeating this thermal cycle 3000 times. Then the change in tensile strength of the lead member 24 of the ceramic heater 1 was measured.

6] Mean value of tensile strength taken over N=10 is shown

in Table 3.

Table 3

Chamfering Initial Strength after No. . . . Remark size (mm) strength (N) durability test (N) _____________ 46* None 120 22 C surface - 47* 0.01 120 27 C surface 48* 0.03 120 29 C surface 49 0.05 120 46 C surface 0.10 120 48 C surface 51 0.20 120 62 C surface 52 0.30 120 58 C surface 53 0.50 120 60 C surface 54* 0.03 120 30 R surface 0.10 120 61 R surface 56 0.50 120 64 R surface Sample marked with * is out of the scope of the present invention.

8] As can be seen from Table 3, tensile strength after durability test was 30 N or less in sample No. 46 in which C surface was not formed, and in samples Nos. 47, 48 in which C surface size was less than 0.05 mm.

9] Samples Nos. 49 through 53 in which C surface size was 0.05 mm or larger showed strength of 40 N or higher.

0] Samples Nos. 51 through 53 in which C surface size was 0.2 mm or larger showed strength of about 60 N. Samples Nos. 55, 56 in which R surface was formed instead of C surface showed similar results, although sample No. 54 showed low tensile strength after durability test of 30 N or lower.

Example 3 [0121] The ceramic heaters 21 were made by varying the proportion of

the periphery of the lead-out electrode 27 embedded in the ceramic base 22 at the opening 28 as 30%, 50%, 70% and 90%. The ceramic heater 21 in bare state was put into a thermostat which was controlled to 400 C and was left to reach equilibrium for 10 minutes, then taken out of the thermostat and was subjected to blowing air for 5 minutes so as to cool down to 40 C or lower. This thermal cycle was repeated 2000 times, and the tensile strength of the lead member 24 was measured.

2] Pull-off test was conducted in which the end of the lead member 24 was pulled in a direction perpendicular to the surface of the ceramic heater 21, and the peel-off strength was measured.

3] The lead member 24 was provided by brazing of Ni wire 0.6 mm in diameter by using Ag-cu brazing material (BAg-8) as the brazing material 25.

4] Mean value of tensile strength taken over N=l0 on samples made similarly to Example 2 is shown in Table 4.

Table 4

Sample Embedding ratio Initial strength Strength after No. (%) (N) durability test (N) 57 30 120 20 120 50 - 59 70 120 65 L 60 90 120 75 [0126] As can be seen from Table 4, peel-off strength of the lead member 24 after the durability test was 20 N in samples No. 57 in which the proportion of the periphery of the lead-out electrode 27 embedded in the ceramic base 22 was 30%, while satisfactory tensile strength of 50 N or higher was obtained in samples Nos. 58 through 60 in which the proportion was 50% or higher.

Example 4

7] The angle 0 between the wall surface 22s of the opening 28 and the lead-out electrode 27 of the ceramic heaters 21

SO

and the tensile strength of the lead member 24 after the thermal cycle durability test were measured.

[01281 Samples were made by varying the angle e as 500, 600, 80 , 90 , 100 , 1100 and 120 .

9] Durability test was evaluated similarly to Example 3, and mean value of tensile strength taken over n=l0 is shown

in Table 5.

Table 5

No Angle Initial strength Strength after durability _______ (0) (N) test (N) 61 50 120 45 62 60 120 60 63 80 120 64 64 90 120 66 100 120 65 66 110 120 65 67 120 120 46 [0131] As can be seen from Table 5, sample No. 61 in which the angle e was 50 showed tensile strength after durability test of 50 N or less, and sample No. 67 in which the angle e was 120 showed tensile strength of 50 N or less, while samples Nos. 62 through 66 in which the angle 8 was from 60 to 110 showed high tensile strength of 60 N or higher.

Example 5

2] Relation between the thickness of the lead-out electrode 27 and tensile strength after durability test was investigated.

3] Durability test was conducted on 20 samples made for each value of thickness of the lead-out electrode 27 varied as 5 pm, 10 pm, 20 pm, 40 pm, 60 pm, 80 pm and 100 pm, and was evaluated similarly to Example 3, with the results shown

in Table 6.

Table 6

Sample Thickness of the lead-Initial Strength after No. out electrode (pm) strength (N) durability test (N) 68 5 100 30 69 10 120 55 20 120 60 71 40 120 62 72 60 120 65 73 80 120 65 74 100 120 66 [01351 As can be seen from Table 6, sample No. 68 in which thickness of the lead- out electrode 27 was set to 5 pm showed low tensile strength after durability test of 30 N, while samples Nos. 69 through 74 in which the thickness was in a range from 10 to 100 pm showed satisfactory durability.

6] Samples Nos. 70 through 74 in which the thickness was set to 20 I.lm or larger, in particular, showed strength of 60 N or higher.

Example 6

7] Example 6 of the present invention is an example related to the third embodiment.

A paste 10 made of w was printed on a ceramic sheet 22a constituted from A1203 as the main component and S102, CaO, MgO and Zr02 with concentration of l0 by weight or less in total as shown in Fig. 3B, thereby forming the heat generating resistive member 23 and the lead-out electrode 27.

8] Another ceramic sheet 22b was placed on the ceramic sheet 22a in close contact therewith, with the opening 28 and the recess 26 formed in the ceramic sheet 22b, and the assembly was fired at a temperature of 1600 C in reducing atmosphere, thereby making 20 ceramic heaters 21 measuring mm in length, 10 mm in width and 1.2 mm in thickness.

[0139) Then electroless Ni plating was applied with thickness of 5 im to the surface of the lead-out electrode 27 exposed in the opening 28, followed by brazing of Ni wire 0.6 mm in diameter by means of Ag-Cu brazing material (EAg-8).

[0140) Samples made by applying electroless Cr plating instead of electroless Ni plating were also evaluated.

1] Conditions of brazing were varied as 800 C, 850 C and 900 C for temperature and 0.5 hours, 1 hour, 2 hours and 5 hours for the holding time.

2] Initial tensile strength and the tensile strength after supplying power to heat to 400 C continuously for 800 hours were measured in order to evaluate the durability in continuous operation. Tensile test was conducted in which the end of the lead member 24 was pulled in a direction perpendicular to the principal surface of the ceramic heater 21, and the peel-off strength was measured.

[0143) Cross sections of two samples taken from each lot were observed under an electron microscope, to study the structure around the interface between the lead-out electrode 27 and the brazing material.

4] Ni wire 1.0 mm in diameter was used as the lead member 24.

5] The results are shown in Table 7 (Table 7-1, Table 7-2) [0146]

Table 7

Table 7-1

No. First layer Second layer Third layer Top layer 75* Ni-N --Eutectic 76 Ni-N Ni-Cu Cu-Ni Eutectic 77 Ni-N Ni-Cu Cu-Ni Eutectic 78 Ni-N Ni-Cu Cu-Ni Eutectic 79* Ni-N Ni-Cu -Eutectic Ni-N Ni-Cu Cu-Ni Eutectic 81 Ni-N Ni-Cu Cu-Ni Eutectic - 82 Ni-N Ni-Cu Cu-Ni Eutectic 83 Ni-N Ni-Cu Cu-Ni Eutectic 84 Ni-N Ni-Cu Cu-Ni Eutectic Ni-N Ni-Cu Cu-Ni Eutectic 86 Ni-N Ni-Cu Cu-Ni Eutectic 87 Cr-N Cr-Cu Cu-Cr Eutectic Sample marked with * is out of the scope of the present invention.

Table 7-2

Brazing Holding Initial Tensile strength No. temperature time bonding after durability ( C) (hours) strength (N) test (N) 75* 800 0.5 340 178 76 800 1 340 245 77 800 2 340 258 78 800 5 340 280 79* 850 0.5 340 195 850 1 340 264 81 850 2 340 266 82 850 5 340 285 83 900 0.5 340 232 84 900 1 340 264 900 2 340 267 86 900 5 340 273 87 900 [ 5 340 273 Sample marked with * is out of the scope of the present invention.

[01471 As can be seen from Table 7, samples Nos. 75, 79 in which the three-layer structure as shown in Fig. 6 was not observed around the interface between the lead-out electrode 27 and the brazing material showed low tensile strength after durability test of 200 N or less, while samples Nos. 76, 77, 78 and 80 through 87 in which the three-layer structure was observed around the interface showed high tensile strength of N or higher.

Example 7

8] Example 7 of the present invention is also an example related to the third embodiment, in which thickness of the plating layer was changed as 1, 2, 4, 8 and 12 I.lm and the influence thereof was studied by durability test.

9] Brazing was carried out by using Ag-Cu brazing material (BAg-B) with heat treatment applied at a temperature of 900 C for 1 hour. Samples shown in Table 8 were made similarly to Example 6, except for that described above.

0] The ceramic heater 21 thus prepared was covered over the entire surface thereof on both sides with aluminum plates measuring 110 mm in length, 12 mm in width and 5 mm in thickness, which were secured at the center of the ceramic heater 21. This was subjected to acceleration test in which such a voltage was applied for 5 minutes as the highest temperature of the ceramic heater 21 reached 3000C, followed by forced air cooling of blowing air for 5 minutes so as to cool down the entire ceramic heater to 40 C or lower, repeating this thermal cycle 3,000 times. Then the change in resistance of the ceramic heater 21 was measured.

1] Samples were made similarly to Example 6.

2] The results are shown in Table 8.

Table 8

Thickness Thickness Thickness Thickness Initial Tensile No of Ni of first of second of third tensile strength after plating layer layer layer strength durability (i.im) (pm) (pm) (pm) (N) test (N) 88 1 0.4 0.8 2.6 340 72 89 2 1.2 1.4 2.8 340 152 4 1.8 3.]. 2.6 340 158 91 8 2.6 6.3 2.9 340 165 92 12 2.8 11.5 2.8 340 181 93 20 3.1 18.8 2.9 340 171 94 30 3.3 28.9 3.2 340 143 40 3.2 39.8 3.0 340 98 96 12 3.8 13.8 28.8 340 138 [01541 As can be seen from Table 8, sample No. 88 in which thickness of the plating layer was 1 pm showed tensile strength of only 100 N or less after the durability test, while samples Nos. 89 through 92 in which thickness of the plating layer was 2 to 12 pm showed high tensile strength of N or higher after the durability test.

Claims

1. A ceramic heater comprising: a ceramic base; a heat generating resistive member incorporated in the ceramic base; a lead-out electrode electrically connected to the heat generating resistive member and exposed in an opening which is formed in the ceramic base; and a lead member brazed onto the surface of the lead-out electrode, wherein the brazing material has layered structure consisting of three or more metal layers.

2. The ceramic heater according to claim 1, wherein the metal layers comprise a first metal layer, a second metal layer and a third metal layer in order from the lead-out electrode, the first metal layer being NiWCu layer a main component of which is Ni, a second metal layer being NiCu layer a main component of which is Ni, the third metal layer being CuNi layer a main component of which is Cu.

3. The ceramic heater according to claims 1 or 2, wherein the first metal layer, the second metal layer and the third metal layer are formed with mean thickness in a range from 2 to 30 pm.

4. An oxygen sensor comprising the ceramic heater as in one of claims 1 to 3.

5. The hair iron having the ceramic heater as in one of the claims 1 to 3 as heating means.