DE4107869C2 - Green ceramic plate for a porous layer - Google Patents

Description

The invention relates to a green ceramic plate, the more appropriate is used as an electrode protective layer for a Manufacture electrochemical element to determine or Determine the concentration of a given ingredient a gas to be measured (hereinafter referred to as measuring gas) is set up. This mixture is made from a mixture that Contains burnout material or pore former, ceramic powder and binder, formed and fired to form a porous ceramic structure.

Porous ceramic structures or layers that have a variety of having interconnected pores are extensive Dimensions have been used for various purposes.

For example, B. the content-related publications DE-OS 38 03 894, DE-OS 38 03 897 and DE-OS 38 03 898 compacts pyrogenic titanium dioxide, aluminum oxide or Zirconium oxide, which consists of fine, corresponding Ceramic powder, binder and urea, sugar and / or Starch as a burnout or pore former. These compacts are as catalysts or as Catalyst carrier provided.

With a sour substance sensor or another electrochemical element for Find the concentration of oxygen or another Ingredient such as that in Chemical Abstracts 105 (22): 193537 y described gas sensor or that in EP-A 0 144 185 described electrochemical element, is generally a gas through casual porous protective layer used to dage an electrode protect conditions that they are a sample gas, for. B. an exhaust gas or Exhaust gas emitted by an internal combustion engine is exposed directly. The porous protective layer is particularly dere provided to the electrode against the harmful one flows of corrosive substances such as B. Lead, Phos to protect phor, silicon and sulfur. If the electrode exposed to these corrosive substances, be there is a tendency that the probe has disadvantages such. B. egg ne change of the control or working point, a reduction of the output signal and deterioration in operation speaking ability occur. The electrode protection layer serves also to prevent evaporation of the components of the Electrode at an elevated operating temperature. To improve the durability of the sensor by protecting the electrode against Corrosion and evaporation it is desirable that the porous Electrode protection layer has a large thickness, so that the Protective layer has long fluid passages or channels through which the sample gas flows in the direction of the electrode.  

The formation of such a porous ceramic layer or ceramic structure, which for Protection of an electrode is used is described in EP-A 0 238 278. It is as one Manufacturing variant common that by Plasma spraying a suitable ceramic material such. B. Spinel on an elec trode, which on a substrate such. B. a solid electrolyte body can be applied, is formed to cover the electrode. Alternatively, will on a solid electrolyte body or another substrate green plate made of a suitable ceramic material laminates around those formed on the substrate Cover electrode, and the green plate is forming fired a porous electrode protection layer, which with the Substrate is connected or forms a whole. The one such green ceramic plate made porous ceramic Layer generally becomes one in one of the following ways given appropriate porosity.

• (1) The green plate becomes like this fired that the sintering of the ceramic powder is insufficient, by placing a ceramic powder in a controlled time Temperature range that is heated between one marked phenomena of expansion and one for one complete sintering is necessary temperature. A such a method is known from DE-OS 30 49 193. In this document also describes that the addition of Binder an additional influence on the pore formation can exercise. About the use of additives, the so-called burn-out substances that are applied when applying heat Pore formation disappears in this prior art nothing mentioned.
• (2) The green plate contains a relatively large bandage medium amount.
• (3) The green plate contains except the bandage medium one or more than one organic additive, d. H. the above-mentioned burnout fuel, which during the Sintering is removed using heat. Such a The way to pore formation is, for example, in DE-OS 38 26 220 described, the pore size generated approximately the grain size of the burnout used.

The porous ceramic formed by the plasma spraying process layer tends to disadvantages, e.g. B. because of a heat ßes or a thermal stress or thermal stress cracks form or jump off or peel off, and to an unung sufficient mechanical strength. Furthermore, it is in general difficult to carry out the plasma spraying process when the desired thickness of the porous ceramic layer exceeds 100 µm tet.

Also the usual method of layering or lami Green plate kidney has some disadvantages or poses problems. For example se shows the porous ceramic layer according to the above mentioned method (1) is produced, an insufficient me mechanical strength of the ceramic network because of the ceramic particles that form the ceramic network are not sufficient are sintered. This disadvantage is particularly serious, though the porous ceramic layer has a high porosity. With others  Words, the above-mentioned method is for forming a porous ceramic layer with a relatively high porosity not suitable, and the same procedure does not allow an exact one Control or adjustment of the pore size.

The method (2), in which an increased amount of binder ver is used also has possible disadvantages. That is, the extent the warping or warping of the fired porous kera microplate increases as the amount of binder increases. Further there is a tendency that the amount of the binder to the upper area of the porous plate is excessively high, resulting in an un sufficient connection of the ceramic particles with those of a plat te or layer with which the porous plate is connected. It also makes the removal of the binder easier reduced, causing bubbles to ver at the connection interface are caused and the layer cohesion deteriorates. As a result, the porous ceramic plate obtained has the after part of the bounce or peeling or similar defects.

The above-mentioned method (3), in which in addition to the pore formers or burnout materials are used, the problem arises low formability of the green ceramic plate, especially if the green plate has a relatively large thickness. In addition, Ris se or other defects occur when the green plate dries is not. While these problems can be increased by increasing the bandage medium can be solved, but this leads to a solution the disadvantages of method (2) discussed above, i. H., deteriorated cohesion between the porous ke ramic plate or layer and a substrate on which the porö se ceramic plate or layer layered or laminated is.

Furthermore, the type of burnout critical. In the case of DE-OS 38 26 220 already mentioned above The ceramic molded body produced is, for example obtained pore structure completely unsuitable for a Protective layer for a gas measuring element serving ceramic plate, because the burnout materials used have a cavernous macroporous structure generate so that the gas passage from one to the other side of the ceramic molding runs much too quickly and thus to the disadvantages mentioned at the beginning for the Guide the sensor.

The invention has for its object a green ceramic plate of the type mentioned for a cover layer of an electrochemical element, such as. B. one To provide oxygen sensors that have a high formability and a ho hen layer cohesion shows and a porous ceramic layer with sufficient mechanical strength.  

The object is achieved in a first embodiment of the invention in that the burnout has a particle diameter D _BET between 0.4 microns and 100 microns, while the ceramic powder has a particle diameter D _BET of 0.02 to 0.5 microns, the The content of the burnout is 20 to 80 vol.%, Based on the total amount of the ceramic powder and the burnout. The particle diameter D _BET corresponds to the diameter of a sphere, which is calculated on the basis of a value of the specific surface measured according to the BET method (Brunauer-Emmett-Teller method).

In a second embodiment of the invention, the object is achieved in that the burnout has a 50% particle size D _{L; 50%} between 0.8 µm and 100 µm, which is determined by a laser scanning method, and that the ceramic powder has the particle diameter D _BET of 0.02 to 0.5 µm, the content of the burnout 20 to 80 vol.%, based on the total amount of the ceramic powder and the burnout.

The green ceramic plate according to the invention preferably has one Porosity or a relative pore volume of at least 5%, so that the green ceramic plate has a high degree of layering has cohesion when focused on a desired structure is brought.  

According to the invention described above, the green ke shows ceramic plate an improved formability and an improved Layer cohesion and is suitably used to a porous ceramic structure with increased mechanical strength ability to manufacture. The green ceramic plate according to the invention enables the production of an electrochemical element, the sen electrode (electrodes) by a sufficiently thick porous Protective layer by burning the green ceramic plate is formed, is protected in an appropriate manner. The Porous protective layer increases the durability of the electrochemical elements to a considerable extent.  

The invention is described below with reference to the beige added drawings explained in more detail.

Fig. 1 is an exploded partial perspective view showing an example of an electrochemical probe made according to the invention.

FIG. 2 is a front view of a section along line 2-2 of FIG. 1.

FIGS. 3 and 4 are graphs showing the results of a gene, the thermal shock test and a corrosion test zei phosphorus, which were carried out samples with the samples of the invention and the comparison.

The mixture for the green ceramic plate according to the invention ent Maintains a ceramic powder as a main component, as described above and a burnout fuel that an elevated temperature, e.g. B. at the sintering temperature of Ceramic powder, disappears. The mixture can also add fabrics such as B. an organic solvent with a high Contain boiling point or other plasticizers. In the In this case, however, the malleability of the green ceramic plate decreases generally with an increase in thickness as above was mentioned. If in the case mentioned above, the amount of the binder is increased to the deterioration of the shape avoidability, the layer cohesion of the green Ceramic plate reduced.

In view of the above, according to the Invention the formability and the layer cohesion the same improved in time, creating a porous ceramic structure with egg ner high mechanical strength is obtained. Ie he Green ceramic plate according to the invention is made from a mixture that forms a ceramic powder and a burnout with certain grain sizes or grain size contains knives.  

The grain size of the ceramic powder and the burnout is expressed as a diameter D _BET , which corresponds to the diameter of a sphere, based on a specific surface area (hereinafter referred to as "specific") measured by the BET method (Brunauer-Emmett-Teller method) BET surface area ") is calculated according to the following equation:

D _BET (µm) = 6 / (ρ · S _BET )

wherein
ρ: true density (g / cm³),
S _BET : specific BET surface area (m² / g).

As a result of extensive studies, the inventors have found that the formability, the layer cohesion and the mechanical strength of a green ceramic plate or a porous ceramic structure obtained therefrom can be significantly improved if a ceramic powder whose particle diameter is in the mixture for the green ceramic plate D _{BET is} in the range of 0.02 to 0.5 µm, and a burnout whose particle diameter D _{BET is} 0.4 µm to 100 µm can be used.

The inventors have also found that when a graphite, carbon or other flaky powder is used as the burnout, the calculated particle diameter D _{BET tends to be} smaller than the actual particle size of the powder. In order to solve this problem, the inventors found that the grain size of such a burnout material is determined by the 50% particle size D _{L; 50%} can be correctly expressed, which is determined by a laser scattering method, ie, using a laser scattering grain size analyzer. The 50% particle size D _{L; 50%} is the largest size of those particles which are no larger than this size, ie, as large or smaller than the 50% particle size, and which in a cumulative particle size chart form 50% of the total number of particles in a powder. The inventors have also found that the object of the invention can be achieved if the 50% particle size D _{L; 50% of} the powder disappearing when heat is applied, e.g. B. Gra phit and carbon powder is 0.8 microns to 100 microns.

Accordingly, according to the invention, the ceramic powder used as a main component of the mixture for the green ceramic plate must have a particle diameter D _BET which is in the range of 0.02 µm to 0.5 µm. If the particle diameter of the ceramic powder is smaller than the lower limit of 0.02 µm, the formability of the green ceramic plate is deteriorated. If the particle diameter of the ceramic powder is larger than the upper limit of 0.5 µm, the sintering of the ceramic particles at their connection interface is not sufficient, resulting in the porous ceramic structure obtained from this green ceramic plate having a reduced mechanical strength. The ceramic powder used in accordance with the invention can essentially consist of aluminum oxide, spinel, cordierite or titanium dioxide or of a solid solution of partially or completely stabilized zirconium dioxide and yttrium (III) oxide or ytter earth, calcium oxide or ytterbi um (III ) oxide or Ytterbinerde exist. Alternatively, the ceramic powder may consist mainly of such a solid solution or a mixture of two or more than two of the above-mentioned types of ceramic powder. The ceramic powder can if necessary one or more than a suitable sintering agent such. B. SiO ₂ , Al ₂ O ₃ , kaolin and clay are added in an amount of 30 mass% or less. In particular, aluminum oxide and zirconium dioxide doped with yttrium (III) oxide or ytterium are preferably used as the ceramic powder. In particular, the ceramic powder preferably consists mainly of partially stabilized zirconium dioxide, the main crystalline phase of which is cubic, or of completely stabilized zirconium dioxide with a cubic crystal phase.

The burnout, the additional to the ceramic powder as another important ingredient is used consists of an organic or inorganic powder that sublimes at a sintering temperature or is thermally decomposed or burned out. Consequently, by Burn the green ceramic plate, which is such a burnout  contains a porous ceramic get plate or structure that a variety of together of connected pores or channels.

In order to ensure a high degree of formability and layer cohesion with a relatively small amount of binder, it is necessary that the burnout have a particle diameter D _BET of at least 0.4 µm and preferably at least 1 µm or a 50% particle size D. _{L; 50%} of at least 0.8 µm and preferably at least 2 µm. It is noted that when the particle diameter D _BET or the 50% particle size D _{L; 50% is} excessively large, the porous ceramic structure obtained has excessively large pores or channels, which leads to the function of the porous ceramic structure as a layer which serves to protect an electrode of an electrochemical element against exposure to corrosive substances , is reduced. Furthermore, an excessively large particle diameter or an excessively large particle size of the powder disappearing when heat is used has the result that the mechanical strength of the porous ceramic structure obtained is reduced. Therefore, the particle diameter D _BET or the 50% particle size D _{L; 50% set} to a maximum of 100 µm. However, it is possible to use a powder which disappears when heat is applied, the particle diameter D _BET or the 50% particle size D _{L; 50%} exceeds 100 µm when the mixture for the green ceramic plate is ground, crushed, crushed, or ground to mix the ingredients to form a slurry to reduce the particle size in the mixture.

The burnout can from or ganic powders such as B. starch, sugar, theobromine and N, N- Diphenylamide or amine and from inorganic powders such as. B. Carbon and graphite can be selected. The mix of green ceramic plate contains 20 to 80 volume%, preferably 30 to 70% by volume of the burnout material, based on the total amount of Kera powder and burnout.  

The binder is in addition to the ceramic powder and the Burnout as another main component of the mixture for the green Kera microplate used. As a binder, a suitable known molding aid can be used. The binder will for example selected from the group to the binder such as B. ethyl cellulose, polyvinyl alcohol, polyvinyl butyral, Polyacrylate and polymethacrylate and other synthetic resin binders belong. Depending on the special binders that can be used, the mixture can also suitable Plastifi ornamental such as B. dioctyl phthalate, dibutyl phthalate, di contain ethylene glycol and dibutyl sebacate. The binder and the plasticizer are generally in a Ge total amount of 5 to 40 parts by mass per 100 parts by mass of the Ge total amount of ceramic powder and burnout added.

If a slurry is used to make the green ceramic plate tion or a slip is generated, the mixture that mainly from the ceramic powder, the burnout material and the binder described above have been described, with a solvent or a solvent mixture depending on the particular whose binders and plasticizers are selected, mixed. Examples of the solvents used for the Slurries used appropriately include: a Alcohol such as B. ethanol, 2-propanol and 1-butanol; an aroma table hydrocarbon such. B. toluene and xylene; a halo hydrogenated hydrocarbon such as B. trichlorethylene and tetra chlorethylene; Acetate; Carbitol (diethylene glycol monoethyl ether); a ketone such as B. acetone and methyl ethyl ketone; Terpineol and Water. The viscosity of the slurry is determined by the amount of the solvent set. It is noted that the Solvent the burnout should not dissolve.

Using the slurry produced in this way mung is by a suitable method such. B. a Rakelmes water or knife application process or a calender roll process  a green ceramic plate is formed, which is a desired one Has thickness. The green ceramic plate formed is at room temperature temperature dried in air or at an elevated temperature forced dried. The green ceramic plate has in general NEN a thickness of 100 to 3000 microns and preferably from 400 to 1500 µm. It is desirable that the porosity or relative Pore volume of the green ceramic plate at least 5% and above is preferably at least 10% so that a high layer is added cohesion is guaranteed. The maximum porosity of the green Ceramic plate is preferably 50% because of the strength the green ceramic plate is too low for suitable handling rig and the mechanical strength of the by burning the green ceramic plate preserved porous ceramic structure inaccurate It is sufficient if the porosity of the green ceramic plate is over 30% steps. The porosity of the green ceramic plate is determined by expressed the following equation:

wherein

V: volume of the green plate
v: volume of each component of the green plate
GD: apparent density of the green plate
Ws: mass of the green plate
Wc: mass of the ceramic powder
We: mass of the burnout
Wb: mass of the binder
Wp: mass of plasticizer
ρc: density of the ceramic powder
ρe: density of the burnout
ρb: density of the binder
ρp: density of plasticizer

In order to produce a porous ceramic structure, the green ceramic plate is fired in the known manner in the atmosphere or in an oxidizing, reducing or inert atmosphere. In the firing process, the ceramic powder contained in the green ceramic plate is sintered, while at the same time the burnout material and the binder (and the plasticizer) are burned out. In this way, a porous ceramic structure is obtained which has a multiplicity of interconnected pores or channels. The porous ceramic structure produced in this way generally has a relative pore volume of open pores of 20 to 80% and preferably 30 to 60%. This relative pore volume of open pores can be easily controlled by changing the amount of the burnout and the particle diameter D _BET or the 50% particle size D _{L; 50% of} the ceramic powder and the burnout can be adjusted.

The porous ceramic structure produced in this way has egg ne improved mechanical strength and finds various Applications. The porous ceramic structure can e.g. B. more suitable as a ceramic filter, as a partition plate or partition or Membrane in a fuel cell and as a porous layer for Protection of an electrode of an oxygen sensor or one other electrochemical element can be used.

Fig. 1 and 2 show an example of the basic structure of such an electrochemical element in the form of an oxygen sensor with a porous electrode protective layer.

In these figures, reference numeral 10 denotes a solid electrolyte body which shows oxygen ion conductivity. On the opposite main surfaces of the solid electrolyte body 10 , a measuring electrode 12 and a comparison electrode 14 are formed ge. On the main surface of the solid electrolyte body 10 on which the comparison electrode 14 is formed, a comparative gas inlet spacer 18 and a sealing layer 20 are formed such that the spacer 18 is arranged between the solid electrolyte body 10 and the sealing layer 20 . The spacer 18 has an elongated, rectangular opening, and it interacts with the solid electrolyte body 10 and with the sealing layer 20 such that a gas inlet passage 16 for the connection of the comparison electrode 14 with a reference gas such as. B. the ambient air is delimited. The main surface of the solid electrolyte body 10 , on which the measuring electrode 12 is formed, is covered by a porous electrode protective layer 22 , which has been produced from a green ceramic plate produced according to the principle of the invention. The oxygen sensor therefore has a laminate or layer structure. The solid electrolyte body 10 , the measuring electrode 12 , the comparative electrode 14 , the spacer 18 and the sealing layer 20 are made of suitable materials known to those skilled in the art.

When operating the oxygen sensor thus constructed, a measuring gas such. B. an exhaust gas or exhaust gas, which flows out of an internal combustion engine, brought into contact with the measuring electrode 12 through the electrode protective layer 22 , while the reference electrode 14 is exposed to the ambient air or another reference gas which has a known oxygen concentration. Between the measuring electrode 12 and the Ver same electrode 14 is according to the principle of a Sauerstoffkon zentrationszelle on the basis of a difference in the sow erstoffkonzentration between the atmospheres, 12 and 14 contact or are connected to them, induced the two electrodes, an electromotive force, the is used as the output signal from the sensor, which reflects the oxygen concentration of the sample gas.

Since the measuring electrode 12 , which is exposed to the measuring gas, is covered by the porous protective layer 22 made from a green ceramic plate according to the invention, the electrode 12 is effectively protected against exposure to the corrosive substances which are present in the measuring gas . In particular, the measuring electrode 12 can be formed with a considerably greater thickness because of the excellent formability and the high lamination quality of the green ceramic plate. Consequently, the length of the small channels formed by the protective layer 22 formed by the green plate can be increased, so that the corrosive substances carried by the measurement gas flowing through the small channels are adsorbed or deposition on the surfaces of the walls defining the small channels are completely prevented from reaching the electrode 12 . The measuring electrode 12 is consequently completely protected against deterioration by such corrosive substances, so that the disadvantages resulting from such a deterioration of a change in the control or operating points of the sensor, a reduction in the output signal of the sensor and a deterioration in the response of the sensor does not occur. The probe consequently has a significantly improved durability or life expectancy.

In the manufacture of an oxygen sensor constructed in the manner described above, an unfired material for the electrodes is applied to an unfired solid electrolyte body ( 10 ) which has a thickness of 100 μm to 1 mm and is formed from a known solid electrolyte material by a suitable known method 12 , 14 applied. The material for the electrodes 12 , 14 is known, for example an electrically conductive metal such as. B. platinum, palladium or rhodium or a cermet-forming mixture of such an electrically conductive metal and a ceramic powder such as. B. alumina and zirconia. The way in which the electrode material is applied is known from the known methods such. B. screen printing, transfer or peeling, spraying or spraying and coating or application selected. The applied electrode material can have a thickness of 3 µm to 30 µm. Then, on one of the opposite main surfaces of the unfired solid electrolyte body ( 10 ), an unfired material for the electrode protective layer 22 , that is, a green ceramic plate according to the invention, is formed. In this embodiment, the green ceramic plate is formed in such a way that it covers at least the unfired measuring electrode ( 12 ). Further, who on the other main surface of the unfired solid electrolyte body ( 10 ) using a known material that is similar to the material of the solid electrolyte body 10 in general, unfired layers for the reference gas inlet spacer 18 and the sealing layer 20 are formed. The unfired layers for the solid electrolyte body 10 , for the electrodes 12 , 14 , for the electrode protective layer 22 , for the spacer 18 and for the sealing layer 20 are baked together, whereby a baked layer structure in the form of the oxygen sensor is produced.

The same oxygen sensor can be made in an alternative manner as described below. That is, the green sheets for the solid electrolyte body 10 and the electrodes 12 , 14 and, if necessary, the green sheets for the spacer 18 and the sealing layer 20 are fired first, thereby obtaining a fired structure. Then, the ceramic green sheet is used for the electrode protection layer 22 so formed as to fired to form the fired measurement electrode 12 is covered, and the electrode protective layer 22nd In a further alternative method for the produc- tion of the oxygen sensor, the unfired layers for the comparing electrode 14 , for the spacer 18 and for the sealing layer 20 are formed on the unfired solid electrolyte body ( 10 ). After the obtained green structure has been fired, the green structure for the measuring electrode 12 and the green ceramic plate for the electrode protection layer 22 are formed on the fired structure, and the green layer for the electrode protection layer ( 12 ) and the green plate for the electrode protection layer ( 22 ) are formed burned.

The oxygen sensor can also be made as follows. Initially, the unfired electrodes ( 12 , 14 ) are formed on the fired solid electrolyte body by 10 , and the green ceramic plate for the electrode protective layer 22 is formed such that it covers the unfired measuring electrode ( 12 ). Furthermore, the unburned spacer ( 18 ) and the unfired sealing layer ( 20 ) are formed on the fired solid electrolyte body 10 . Then the unfired layers ( 12 , 14 , 18 , 20 , 22 ) are fired together to produce the oxygen sensor. In the present method, the sputtering, electroless galvanizing or vacuum evaporation and the methods described above can be used to form the unfired electrodes ( 12 , 14 ) on the fired solid electrolyte body 10 . If the atomization, electroless plating or vacuum evaporation is carried out, the unfired electrodes ( 12 , 14 ) can be formed with a thickness of 0.3 to 5 µm.

To improve the integrity or cohesion between the electrode protective layer 22 and the solid electrolyte body 10 , it is desirable that the thermal expansion coefficient of the electrode protective layer 22 formed in the oxygen sensor according to the invention is substantially the same value as the thermal expansion coefficient of the solid electrolyte body 10 , which acts as the substrate for the layer 22 serves, or has a value close to it. For this purpose, the ceramic powder for the green ceramic plate, which is used to produce the protective electrode layer 22 , is preferably the same as the material for the solid electrolyte body 10 . The electrode protective layer 22 is formed in particular from a completely stabilized zirconium dioxide, which mainly consists of a cubic crystal phase, while the solid electrolyte body 10 is formed from a partially stabilized zirconium dioxide. In this case, the heat resistance of the electrode protective layer 22 and the strength of the solid electrolyte body 10 are compatible and both are improved.

While relatively large pores or channels are formed in the porous electrode protective layer 22 or through the use of the burnout material, which disappears at an elevated temperature, it is advantageous to add smaller pores or channels to such relatively large pores or channels form by forming the green ceramic plate for the electrode protective layer 22 from a mixture which has a lower degree of sinterability than the material of the solid electrolytic body 10 . In this case, the relatively large pores and the relatively small pores are formed by jointly burning the unfired electrode protective layer ( 22 ) and the unfired solid electrolyte body ( 10 ).

The electrode protective layer 22 may consist of at least two or a plurality of sublayers instead of a single layer. Furthermore, a second porous layer can be arranged between the electrode protection layer 22 and the measuring electrode 12 , the porosity and pore size of which are different from those of the electrode protection layer 22 . The porosity of the second electrode protection layer may be higher or lower than the porosity of the electrode protection layer 22 of the invention.

The invention is true for the case of its application to one Oxygen sensor with a typical basic structural Arrangement has been described as an electrochemical element, each however, it is understood that the principle of the invention is alike on the other types of known oxygen sensors with under different structural arrangements and other electro chemical elements can be applied.

The electrochemical element for the manufacture of which the inventor green ceramic plate is used, for example with an appropriate heater to maintain an optimal operating temperature of the element his. In this case, the heater can either in the electrochemical element installed or housed or a separate heating element that is connected to the electrochemical ele ment is appropriate.

While the oxygen sensor illustrated in Figs. 1 and 2 has a generally elongated, planar structure, the invention can be applied to an electrochemical element having a different structure or shape, such as a tubular or a cylindrical shape as shown are known in the art. The illustrated oxygen sensor has only a single electrochemical cell, which consists of the solid electrolyte body 10 and the two electrodes 12 , 14 . However, the invention is applicable to an electrochemical sensor which has more than one electrochemical cell, for example an electrochemical oxygen pump cell and an electrochemical oxygen measuring cell, as described in EP-A1 01 44 185, which relates to JP-OS 60-1 08 745 of the unexamined JP patent application 58-2 18 399 correspondingly disclosed.

To further explain the invention, ei Some examples of the invention described.

example 1

In a pot mill in which zirconium dioxide balls (diameter: 10 mm) were used, 75 parts by weight of partially stabilized zirconium dioxide powder (ZrO ₂ powder), 25 parts by weight of theobromine powder, 10 parts by weight of polyvinyl butyral resin, 6 parts by weight of dibutyl phthalate and 90 parts by weight of a toluene solvent mixture 2-propanol introduced. The introduced materials were mixed in the mill for 10 hours, and the mixture was passed through a sieve, thereby producing a slurry having a viscosity of 20.0 Pa · s. The zirconium dioxide powder had a particle diameter D _BET of 0.08 μm and consisted of 93 mol% ZrO ₂ , 7 mol% Y ₂ O ₃ and 0.5 mass% clay. The zirconia powder had a density ρc of 6 g / cm ³ . The theobromine powder had a particle diameter D _BET of 4 μm and a density ρe of 1.4 g / cm ³ . The polyvinyl butyral resin and the dibutyl phthalate had densities ρb, ρp of about 1 g / cm ³ . The solvent mixture consisted of 1 part by weight of toluene and 1 part by weight of 2-propanol.

Using the resulting slurry, a green ZrO ₂ plate (green ceramic plate) according to the invention was formed by a doctor knife or knife application method. Then the green plate was dried at 100 ° C for 2 hours; its thickness was 600 µm after drying. The green zirconia plate formed had an apparent density GD (green density) of 2.12 g / cm ³ and a porosity of 15% calculated according to the above equation.

The dried green ZrO ₂ plate was fired at 1400 ° C in the atmosphere for 2 hours, thereby producing a porous ceramic plate. The ceramic plate had a relative pore volume of 55% open pores and showed high mechanical strength, and the amount of warpage caused by the firing was small in the ceramic plate.

Example 2

In a pot mill using aluminum oxide balls (diameter: 10 mm), 80 parts by weight of aluminum oxide powder (Al ₂ O ₃ powder) (density ρc = 4 g / cm ³ ), 20 parts by weight of starch powder (density ρe = 1.4 g / cm ³ ), 12 parts by weight of polybutyl methacrylate (density ρb = about 1 g / cm ³ ), 4 parts by weight of dioctyl phthalate (density ρp = about 1 g / cm ³ ) and 70 parts by weight of toluene. The introduced materials were mixed in the mill for 10 hours and the mixture was passed through a sieve, thereby producing a slurry having a viscosity of 10.0 Pa · s. The alumina powder had a particle diameter D _BET of 0.1 µm and a purity of at least 99.9%. The starch powder had a particle diameter D _BET of 3 µm.

Using the obtained slurry, a green alumina plate (green ceramic plate) of the present invention was formed by a doctor knife method. Then the green plate was dried at 100 ° C for 3 hours; its thickness after drying was 1 mm. The green alumina plate formed had an apparent density GD of 1.96 g / cm ³ and a porosity of 15%.

The dried green ceramic plate was at 1400 ° C for 2 hours burned in the atmosphere, creating a porous ceramic plate was made. The ceramic plate had a relative one Pore volume of open pores of 40% and showed a high mecha strength, and the extent of the ceramic plate warping caused by the burning was slight.

Example 3

In the same pot mill used in Example 1, 70 parts by weight of fully stabilized zirconium dioxide powder (ZrO ₂ powder) (density ρc = 6 g / cm ³ ), 30 parts by weight of graphite powder (density ρe = 2.2 g / cm ³ ), 13 parts by weight of polyvinyl tyral resin (density ρb = 1 g / cm ³ ), 6 parts by weight of dibutyl phthalate (density ρp = 1 g / cm ³ ) and 110 parts by weight of a solvent mixture of toluene and ethanol. The introduced materials were mixed in the mill for 10 hours and the mixture was passed through a sieve, thereby producing a slurry with a viscosity of 5.0 Pa · s. The zirconia powder had a particle diameter D _BET of 0.04 μm and consisted of 92 mol% ZrO ₂ , 8 mol% Y ₂ O ₃ and 1 mass% kaolin. The graphite powder had a 50% particle size D _{L; 50%} of 2 µm.

Using the obtained slurry, a green zirconia plate (green ceramic plate) according to the invention was formed by a doctor knife or knife application method. Then the green plate was dried at 100 ° C for 1 hour; their thickness after drying was 400 µm. The green zirconia plate formed had an apparent density GD of 1.95 g / cm ³ and a porosity calculated according to the equation given above of 27%.

The dried green ceramic plate was at 1400 ° C for 2 hours burned in the atmosphere, creating a porous ceramic plate was made. The ceramic plate had a relative one Pore volume of open pores of 50% and showed a high mecha strength, and the extent of the ceramic plate warping caused by the burning was slight.

Example 4

Green ceramic plates 500 µm thick were prepared as Sample Nos. 1 to 7 in the same manner as in Example 1 using the following ingredients: a zirconia powder (ZrO ₂ powder) (ρc = 6 g / cm ³ ) containing 7 mol% of Y ₂ O ₃ as ceramic powder; Theobromine (ρe = 1.4 g / cm ³ ) as burnout; Polybutyl methacrylate (ρb = about 1 g / cm ³ ) as a binder and dioctyl phthalate (ρp = about 1 g / cm ³ ) as a plasticizer. Table 1 shows the particle diameter D _{BET of} the ceramic powder per sample and the content (parts by mass) of the constituents of each sample.

The formability, apparent density (GD) and porosity (%) of the samples produced were assessed or measured. The results of the evaluation and the measurement results are shown in Table 2. The green sample plates were each fired with green ZrO ₂ solid electrolyte plates in the atmosphere at 1350 ° C. for 2 hours, and in the porous plates obtained, the layer cohesion with the solid electrolyte plate, the occurrence of warpage after sintering, the relative pore volume became more open Pores and mechanical strength assessed or measured. The results of the evaluation and the measurement results are also shown in Table 2.

Table 1

Table 2

Tables 1 and 2 show that Samples Nos. 1 to 3, in which ceramic powder whose particle diameter D _{BET was} in the range from 0.02 to 0.5 μm, and burnout material whose particle diameter D _{BET was at} least 0, 4 µm, were used, with respect to the formability of the green ceramic plates, the layer cohesion between the fired porous plate and the fired solid electrolyte plate and the absence of warping after sintering and the mechanical strength of the porous plate showed excellent results. On the other hand, Sample No. 4, in which the burnout particle diameter D _{BET was} less than 0.4 µm, showed poor moldability, and Sample No. 5, in which larger amounts of binder and plasticizer were used, to improve moldability improve, showed poor layer cohesion between the fired porous plate and the fired solid electrolyte plate. In addition, Sample No. 6, in which the burnout was not used and in which excessive amounts of binder and plasticizer were used, showed a significant amount of warpage after sintering, reduced mechanical strength and poor layer hold together. Sample No. 7, using a ceramic powder whose particle diameter D _{BET was} larger than 0.5 µm, showed low porosity of the fired porous plate and low mechanical strength.

Example 5

A green ZrO ₂ plate with a thickness of 500 microns was made from a mixture consisting of 100 parts by weight of ceramic powder (96 mol% ZrO ₂ , 4 mol% Y ₂ O ₃ ), 3 parts by weight of clay as a sintering aid, 12 parts by weight of polyvinyl butyral resin as a binder and 5 parts by weight of dibutyl phthalate as a plasticizer.

The green plate thus prepared was used as an unburned solid electrolyte body ( 10 ) to manufacture an oxygen sensor as shown in FIGS . 1 and 2. On the opposite main surfaces of the unfired solid electrolyte body ( 10 ) two unfired electrodes ( 12 , 14 ) were formed by applying an electrically conductive paste by screen printing, the powder consisting of 80 parts by weight of platinum and 20 parts by weight of a 4 mol% Y ₂ O ₃ ZrO ₂ powder existed. The applied paste was dried at 100 ° C for 20 minutes.

On one of the main surfaces of the unfired solid electrolyte body ( 10 ) on which the unfired electrode ( 12 ) had been formed, a green ceramic plate, as was produced in example 1, was formed to produce the electrode protection layer 22 . Furthermore, green ZrO ₂ plates similar to the unfired solid electrolyte body ( 10 ) were formed on the other surface of the unfired solid electrolyte body to produce the reference gas inlet spacer 18 and the sealing layer 20 . The unfired layers thus stacked or laminated were pressed together under pressure and heat, whereby an unfired layer structure was obtained. This layer structure was then fired at 1400 ° C in the atmosphere for 2 hours. In this way, the oxygen sensor (electrochemical element) of FIGS. 1 and 2 with the electrode protective layer 22 , which showed high adhesion to the solid electrolyte body 10 , was produced. The electrode protective layer 22 had a thickness of approximately 500 μm and a porosity of 55%. An examination showed that the manufactured oxygen sensor showed high thermal shock resistance, high mechanical strength and excellent corrosion resistance.

Example 6

A green ZrO ₂ plate with a thickness of 500 microns was made from a mixture consisting of 100 parts by weight of ceramic powder (96 mol% ZrO ₂ , 4 mol% Y ₂ O ₃ ), 14 parts by weight of polybutyl methacrylic lat and 3 parts by weight of dioctyl phthalate.

The green zirconia plate thus prepared was used as an unfired solid electrolyte body ( 10 ) on which unfired electrodes ( 12 , 14 ) were formed in the same manner as in Example 5 and on which using a green alumina plate as prepared in Example 2 , an unfired electrode protective layer ( 22 ) was formed. Furthermore, an unfired spacer ( 18 ) and an unfired sealing layer ( 20 ) were formed using the same green zirconia plates as for the unfired solid electrolyte body ( 10 ). The unfired layers so laminated or laminated were pressed together under pressure and heat, whereby an unfired layer structure was obtained, which was then fired at 1450 ° C. in the atmosphere for 2 hours. In this way, an oxygen sensor with a porous aluminum oxide electrode protection layer 22 was made with a thickness of about 800 microns. This electrode protective layer 22 showed high adhesion to the solid electrolyte body 10 and had a porosity of 40%. A test showed excellent heat shock resistance, mechanical strength and corrosion resistance.

Example 7

As in Example 5, an oxygen sensor, as shown in FIGS. 1 and 2, was produced, which had a porous ZrO ₂ electro-protective layer 22 with a thickness of about 500 microns and a porosity of 55%. The electrode protective layer 22 was produced from a green ZrO ₂ plate with a thickness of 600 μm in accordance with Example 1. The oxygen sensor produced is referred to as "he oxygen sensor according to the invention".

A comparative oxygen sensor "A" was made which had a porous Spi nell electrode protective layer 22 formed by a plasma spraying process. The electrode protective layer 22 had a thickness of 100 μm and a porosity of 35%. Another comparative oxygen sensor "B" was manufactured which had a porous ZrO ₂ electrode protective layer 22 with a thickness of approximately 120 μm and a porosity of 50%. The ZrO ₂ electrode protective layer 22 was produced from a green zirconium dioxide plate with a thickness of 150 μm, which contained theobromine as the burnout substance, the particle diameter D _{BET of which was} 0.3 μm. The green zirconia plate was fired with an unfired solid electrolyte body, whereby the comparative oxygen sensor "B" was produced.

Another comparative oxygen sensor was made using a green ZrO ₂ plate having the same composition as the green ZrO ₂ plate used for the comparative oxygen sensor "B", but the thickness of which was 600 µm, thereby making a porous zirconia Electrode protection layer 22 with a thickness of approximately 500 μm and a porosity of 50% was obtained. However, this comparative oxygen sensor had the disadvantage that a separation occurred between the measuring electrode 12 and the electrode protective layer 22 .

With the oxygen sensor according to the invention and with the Ver Equal oxygen sensors "A" and "B" became the following Tests carried out:  

(1) thermal shock test

The samples were subjected to repeated heating and cooling cycles using a burner. The samples were heated to 1000 ° C in each cycle and cooled to 200 ° C. After the heating and cooling cycles, the samples were examined to determine whether cracks had formed in the electrode protection layer 22 and whether a separation had occurred between the electrode protection layer 22 and the measuring electrode 12 , and they were used for reliability of a Weibull distribution diagram.

(2) Phosphorus corrosion test

The samples were exposed to a corrosive gas containing phosphorus. Specifically, phosphorus pentoxide powder (P ₂ O ₅ powder) was heated to 300 ° C in order to produce corrosive phosphorus gas (P gas). This phosphorus gas was supplied to the samples at a fixed temperature (250 ° C) using a carrier gas stream (air; 150 ml / min) flowing through a quartz glass tube. The samples were kept in contact with the phosphorus gas for a predetermined time. The excess air ratio (λ) of the measurement gas was measured before and after this test with the oxygen sensors used as samples. In particular, the excess air ratio of the measurement gas was changed very slowly (e.g. from 0.95 to 1.20) to measure the value of the excess air ratio at the time when the electromotive force caused by the sensors is generated, suddenly changes. The amount of change in the excess air ratio measured before and after each sample was exposed to the corrosive gas was obtained. The corrosion resistance of the samples increases as the amount of change in the measured excess air ratio decreases.

The result of the thermal shock test is shown in Fig. 3, while the result of the phosphorus corrosion test is shown in Fig. 4 ge. It can be seen that the comparative oxygen sensor "A" has a very low thermal shock resistance and that the comparative oxygen sensor "B" has a very low corrosion resistance to phosphorus (corrosive gas). The oxygen sensor of the present invention has both significantly improved thermal shock resistance and corrosion resistance compared to the comparative oxygen sensors "A" and "B".

Claims (11)

1. Green ceramic plate, which is fired to form a porous ceramic structure and is formed from a mixture which contains burnout material, ceramic powder and binder, characterized in that the mixture mainly consists of a burnout material which has a particle diameter D _{BET of} between 0.4 µm and 100 microns, the particle diameter D _BET corresponds to the diameter of a sphere, which is calculated on the basis of a value of the specific surface area measured by the BET method (Brunauer-Emmett-Teller method), a ceramic powder which has a particle diameter D _{BET has} from 0.02 to 0.5 μm, and the binder consists, the content of the burnout being 20 to 80% by volume, based on the total amount of the ceramic powder and the burnout. 2. Green ceramic plate which is fired to form a porous ceramic structure and is formed from a mixture containing burnout material, ceramic powder and binder, characterized in that the mixture consists mainly of a burnout material which has a 50% particle size D _{L; 50%} between 0.8 microns and 100 microns, the 50% particle size D _{L; 50% is} determined by a laser scanning method, a ceramic powder which has a particle diameter D _BET of 0.02 to 0.5 μm, the particle diameter D _BET corresponding to the diameter of a sphere which is based on a method according to the BET method (Brunauer -Emmett-Teller method) measured value of the specific surface area, and the binder exists, the content of the burnout being 20 to 80% by volume, based on the total amount of the ceramic powder and the burnout. 3. Green ceramic plate according to claim 1 or 2, characterized in that they have a relative pore volume of at least 5%. 4. Green ceramic plate according to one of claims 1 to 3, characterized in that the ceramic powder mainly partially or completely stabilized zirconium dioxide, Alumina, spinel, cordierite or titanium dioxide. 5. Green ceramic plate according to one of claims 1 and 3 to 4, characterized in that the particle diameter D _{BET of} the burnout material is at least 1 µm. 6. Green ceramic plate according to one of claims 2 to 4, characterized in that the 50% particle size D _{L; 50% of} the burnout is at least 2 µm. 7. Green ceramic plate according to one of claims 1 to 6, characterized in that the burnout from the group selected from starch, sugar, theobromine, N, N-diphenylamide, There is carbon and graphite. 8. Green ceramic plate according to one of claims 1 to 7, characterized in that the mixture 30 to 70 volume%  of the burnout, based on the total amount of Ceramic powder and the burnout contains. 9. Green ceramic plate according to one of claims 1 to 8, characterized in that it also contains a plasticizer contains and that the binder and the plasticizer in a total amount of 5 to 40 parts by mass are. 10. Green ceramic plate according to one of claims 1 to 9, characterized in that they have a thickness of 10 to 300 microns Has. 11. Green ceramic plate according to one of claims 1 to 10, characterized in that it has a relative pore volume of Has 5 to 50%.