JPH02124787A - Method for metallizing oxidic ceramics - Google Patents

Abstract

PURPOSE:To improve vacuum sealing property and brazing characteristic by further forming a secondary layer by means of a paste containing Ni oxide, etc., on a primary layer prepared by applying a paste containing powdered W, powdered glass, etc., of the prescribed conditions and then burning the above under the prescribed conditions. CONSTITUTION:A paste in which a powder mixture consisting of, by weight, 70-95% W powder of <=10mum average grain size and 30-5% glass powder of 1100-1300 deg.C melting point and <=10mum average grain size is kneaded with an organic vehicle is prepared. This paste is applied to the surface of an oxidic ceramic sintered compact to form a primary layer. Subsequently, a paste containing one or more kinds among the powders, etc., of Ni, Ni oxides, and Ni salts of <=10mum average grain size is applied to the above layer to form a secondary layer. Then, the above sintered compact is burnt at 1200-1400 deg.C in a mixed atmosphere in which the ratio of H2 to H2O is regulated to 1-100000.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for metallizing ceramics, and more particularly to a method for metallizing oxide-based ceramics having excellent vacuum sealing properties and brazing properties.

[Prior Art] Examples of devices that utilize the bonding of metal and oxide ceramics include electron tubes such as magnetron insulators for microwave ovens and envelopes for vacuum switches.

When using metal-ceramic bonding for these purposes, the bonding interface must maintain vacuum-sealing properties, and bonding methods that meet this condition include: - For boat fishing, metallizing ceramics; A method is used in which the ceramic is brazed to a mating metal after metallization.

Conventionally, high melting point metal methods have been widely used as a metallization method. The high melting point metal method involves mixing high melting point metal powder such as molybdenum or tungsten with manganese powder, applying a paste made by mixing this with an organic solvent to the surface of oxide ceramics, and firing it to form a molybdenum layer on the ceramic surface. A tungsten layer is formed, then nickel plating is applied to coat the surface with nickel, and then the target metal is brazed.

Without this nickel coating, the flowability and heat resistance of the brazing filler metal will be poor. Therefore, in the refractory metal method, it is essential to coat the surface with nickel.

However, it is very difficult to perform nickel plating on a metallized surface using the high melting point metal method, and various treatments are required. In other words, since the metallized surface obtained by the high melting point metal method is partially covered with glass, it is necessary to remove the glass, and to modify the surface, it is necessary to perform acid treatment, activation treatment with palladium chloride, etc. be. As described above, the high melting point metal method is a metallization method with excellent vacuum sealability, but it has the disadvantage that the process is long and the manufacturing cost required for this process is extremely high.

The present inventors investigated a method of omitting the latter of two major steps, the metallization step and the Ni adhesion step.

This is because while the former is the essential process of metallization, the latter is an additional process called Ni plating, but it requires a lot of equipment for the preliminary treatment process, plating equipment, water treatment equipment, etc. It is. Hereinafter, the metallized main body made of molybdenum or tungsten will be referred to as the lower metallized layer, and the layer above it, which is mainly composed of Ni, will be referred to as the Ni upper layer. Reducing the process of forming the Ni upper layer is very advantageous in reducing the cost of the metallization process. From this viewpoint, the present inventors investigated a method of omitting all the various steps for forming the Ni upper layer.

To omit various steps for forming the Ni upper layer,
A two-time firing method can be easily considered, in which a metallized layer is formed by a high melting point metal method, a paste containing Ni mixed with an organic solvent is printed, and the paste is fired again to form a Ni layer. However, this method requires firing twice,
Even if the plating equipment could be omitted, the firing process would still be twice as necessary, and the cost would not be reduced. Therefore, the present inventors focused on forming the metallized lower layer and the Ni upper layer at the same time. That is, they devised a method in which a paste made by mixing a mixed powder of molybdenum and manganese in an organic solvent was applied to the ceramic surface, and then a nickel paste was printed on the top surface and fired at the same time. but,
In actual experiments, the melting points and sintering speeds of these two metal layers are significantly different, so if they were fired at the same time, the Ni layer would melt, and the lower high-melting point metal layer would have a low strength value. It turned out to be a failure. In this way, it has been found that a sound metallized layer cannot be formed by simply applying two coats of high melting point metal paste and Ni paste. Also,
The process of two coats and simultaneous firing is often used in the metallization field, but all of them are related to improving the formation of the lower metallized layer as defined above, and the lower metallized layer and the upper Ni layer are formed in one firing. This is different from the purpose of the present invention to do so. For example, in the method described in Japanese Patent Publication No. 36-6542, a mixed powder of a high melting point metal and ceramics is provided in the 1st N, and a metal layer made of a high melting point metal is provided in the 2nd N. This invention relates to improving the formation of the metallized lower layer, and does not omit various steps for forming the Ni upper layer, and its gist is completely different from the present invention. Also, regarding the method, the metal of the first layer and the metal of the second layer are the same Mo, and in the case of such a simultaneous firing method of the same metal, there are essentially no differences from the firing conditions for IN only. do not have. It is extremely difficult to sinter different metals such as W and Ni at the same time. Furthermore, JP-A-58-213688 discloses a method of reducing the resistance of the metallized portion by using an oxide powder of a high-melting point metal in the first layer and using a high-melting point metal in the second layer. However, like the invention described in Japanese Patent Publication No. 36-6542, this also relates to the formation of a metallized lower layer, and is completely different in spirit from the present invention, which includes plating.

That is, the metallized surface obtained by the method described in JP-A-58-213688 is not made of Ni, and various steps are required to form a Ni upper layer when brazing.

The present invention relates to an invention in which the optimum conditions for a two-coat method, which aims at a fundamental improvement in simultaneously forming a lower metallized layer and an upper Ni layer, are a constituent feature.

[Problems to be Solved by the Invention] As mentioned above, in order to metallize oxide ceramics and obtain brazing properties, the surface must be coated with nickel, and when used as an electron tube material, Must have excellent vacuum sealability. However, the conventional two-step process of forming the metallized lower layer and subsequent various steps for forming the Ni upper layer has the drawback of high manufacturing costs.

Therefore, it has been desired to develop a method for metallizing ceramics that reduces manufacturing costs, has comparable vacuum sealing properties, and has good brazing properties.

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides 70 to 95% by weight of tungsten powder having an average particle size of 1oIlrn or less and a melting point of 1100% on the surface of an oxide-based ceramic sintered body. ~
At 1300°C, a paste made by kneading a mixed powder of 30 to 5% by weight of glass powder with an average particle size of 1101I or less in an organic vehicle is applied and then dried to form a first layer,
Next, on the first M, nickel and nickel oxide having an average particle size of 10tIrn or less are placed.

A paste made by kneading powder of one or more of the nickel salts in an organic vehicle is applied and dried to form a second layer, and then 12
In the temperature range of 00 to 1400℃, 1<82/II□
The gist of the present invention is a method for metallizing oxide-based ceramics with excellent vacuum sealing properties and brazing properties, which is characterized by firing in a mixed atmosphere adjusted to 0 < 100,000.

[Function] The present inventors have realized that the manufacturing cost is lower than that of the conventional high melting point metal method.
Consideration of a metallization method for alumina ceramics that has significantly lower manufacturing costs and that allows the finished product to maintain vacuum sealing and brazing properties equal to or better than those produced using the refractory metal method. I've been repeating it.

The steps of the refractory metal method, which has been conventionally used in various fields, are shown below. First, a high-melting point metal such as molybdenum or tungsten and manganese are made into a paste and applied to the top surface of the alumina ceramic. Next, it is kept in a furnace in a controlled atmosphere where manganese is oxidized but molybdenum is not oxidized. A specific example is N2 :H,=
9: 1°, dew point of 40°C, and a temperature of about 1450°C.

The molybdenum sinteres at this temperature, forming a metallic network and forming the skeleton of the metallized layer.

However, plating is hardly possible immediately after the formation of this molybdenum layer. For this purpose, degreasing, deglass treatment, acid treatment 2 activation treatment, etc. are performed. The reason why such a treatment is carried out is because the metallized surface is considered to be coated with glass or the high melting point metal is thinly oxidized.

The only drawback of the refractory metal method is that after the formation of the molybdenum layer, there are long steps such as pretreatment, plating, etc.
It requires a step to form a layer on the surface.

Therefore, the inventors of the present invention undercoated molybdenum, overcoated the top surface with nickel, and then fired at the same time.
The experimental research focused on the fact that the process only needed to be completed once. However, in this case, nickel diffuses into molybdenum, and molybdenum and nickel form a eutectic and melt or oversinter, making it impossible to obtain a sound metallized layer.

This is due to the fact that nickel was dissolved in molybdenum during firing, and the self-diffusion coefficient of molybdenum increased. That is, in order to coat nickel in one firing, the first N
The selection of metal is a very important item. Therefore, the inventors of the present invention applied a paste such as molybdenum or tungsten as an undercoat, and then overcoated the undercoat metal with a metal having an activation energy of 0.2 to 0.6 times the self-diffusion of that metal. It was found that there is a possibility of good metallization. For example, if the base coat metal is tungsten,
Nickel is good for the topcoat, and if the undercoat is molybdenum, silver is good for the topcoat. As mentioned above, since the metallized surface in the present invention is required to be Ni, it has been found that tungsten is preferable as the undercoat metal. Therefore, we investigated the possibility of metallization in this combination of metals.

After further research, it was discovered that tungsten was the best metal to use in the first layer when coating the top with nickel. It has been found that the above-mentioned problems can be solved.

The present invention is characterized in that the long steps after forming the molybdenum layer in the refractory metal method are omitted without deteriorating its properties.

The metallization mechanism in the metallization method of the present invention is as follows.

First, as shown in FIG. 2(a), a mixed powder consisting of tungsten powder and glass powder is kneaded with an organic vehicle on the surface of an alumina ceramic sintered body 2, which is one of the oxide ceramics. After applying the paste, it is dried to form the first layer 301. The tungsten powder and the glass powder are each adjusted to have an average particle size (unless otherwise specified, this average particle size will be simply referred to as particle size hereinafter) of 10 μm or less.

The properties of glass vary greatly depending on its composition, and as will be described later, a healthy metallized structure cannot be formed unless the melting point is in the range of 1100 to 1300°C. As an example, the AhO+-MnO-5iO□ system can be used. Further, as an example of the organic vehicle, a mixture of an organic binder such as ethyl cellulose and an organic solvent such as terpineol to obtain an appropriate viscosity can be used. Next, a paste made by kneading one or more of nickel, nickel oxide, and nickel salt in the organic vehicle is applied to the upper surface of the first layer 301, and then dried. A second layer 302 is formed. The particle size of nickel, nickel oxide, and nickel salt is 10 μm.
You can use any one as long as it is adjusted as follows.

FIG. 2(b) schematically shows these.

After forming the first layer 301 and the second layer 302 in this manner, the ceramic sintered body 2 is fired at a temperature in the range of 1200 to 1400°C under atmospheric conditions described below. During this firing, the tungsten powder of the first layer 301 is sintered to some extent, as shown in the schematic diagram of FIG. On the other hand, the glass powder in the paste melts, and a part of it flows onto the surface of the ceramic sintered body 2 and reacts with the alumina. The remainder of the glass fills the voids in the tungsten without any gaps.
Form a tungsten and glass mixed layer. On the other hand, nickel, nickel oxide, and nickel salt in the second layer 302 are reduced to nickel metal in the firing atmosphere and are sufficiently sintered to cover the tungsten layer and form a nickel layer by themselves.

FIG. 1 shows a cross section of the metallized layer 3 thus generated on the surface of the ceramic sintered body 2. The above-mentioned tungsten-glass mixed layer 31 is also present on the surface of the ceramic sintered body 2. A nickel layer 32 is formed on the upper surface of the mixed layer 31 and constitutes the metallized layer 3. In this case, tungsten can be dissolved in the nickel layer 32 up to its solid solubility limit (approximately 40% by weight), but this does not affect the brazing properties. In this way, the ceramic sintered body 2
A strongly bonded metallized layer 3 is formed on top.

After cooling, alumina and glass become a continuous phase, and tungsten and glass are strongly bonded due to the anchor effect. Thus, alumina and tungsten can be used as a medium through glass.
Tungsten and nickel are bonded to each other by mutual diffusion, and the metallized alumina body 1 is completed.

Glass powder is limited to those having a melting point of 1100 to 1300°C. Such glass is A7z03-MnO
3iO□ type is preferable, but it is not limited to this as long as it has the above-mentioned melting point.

That is, if the melting point is less than 1100° C., the strength of the glass itself will be low, and the glass will flow too much, resulting in a significant decrease in strength after metallization. On the other hand, if the melting point exceeds 1300° C., the fluidity of the glass deteriorates, and the voids in the tungsten are not completely filled, making it impossible to obtain sufficient vacuum sealing properties.

Further, the tungsten grains are limited to those having a grain size of 10 μm or less. Outside this range, the difference in distance between tungsten grains becomes too large, and glass moves to areas with narrower tungsten grains due to capillary action, making it impossible for glass to exist in areas with wider grains, resulting in vacuum sealing. No stopping properties are obtained. The particle size of the glass is also limited to 10 μm or less for the same reason as the above-mentioned reason for limiting the tungsten particle size. That is, when glass having a particle size of more than 10 μm is melted, the glass is absorbed into the area where the tungsten particle size is small, and the area where the glass was present becomes a void.

The mixing ratio of tungsten powder and glass powder is limited to 70 to 95% by weight of tungsten and 30 to 5% by weight of glass. If this ratio deviates from the side with more tungsten, the absolute amount of glass becomes too small, and the glass cannot fill the tungsten network, making it impossible to guarantee vacuum sealability. On the other hand, if the amount of tungsten is shifted to the side where there is less tungsten, the tungsten will not be able to form a network and sufficient strength will not be obtained.

Nickel sources: nickel, nickel oxide.

The particle size of nickel salt is also limited to 10 μm or less. If it is outside this range, sintering will not progress sufficiently and the surface of the mixed layer 31 (tungsten-glass layer) will not be completely covered, resulting in poor brazing properties.Also, the number of contact sites between tungsten and nickel will decrease, resulting in a decrease in strength. Further, when nickel, nickel oxide, and nickel salt are fired in the atmosphere described below, they are all reduced to nickel metal, and no significant difference is observed among these types.

The firing temperature is limited to 1200-1400°C. 120
At temperatures below 0°C, the amount of reaction between alumina and glass is small, and the fluidity of the glass is also poor, making it impossible to obtain sufficient strength and vacuum sealability. When the temperature exceeds 1400"C, tungsten is oversintered and there are fewer voids to be filled with glass, which reduces the anchoring effect between glass and tungsten, resulting in a decrease in strength. Regarding the firing atmosphere, the ratio of hydrogen to water vapor ( H, /H20) is greater than 1 and less than 100000 (1<
H□/1lzO<100000). If it becomes less than 1, tungsten is oxidized and becomes 100,000
In the above case, the glass is reduced and metal is precipitated, and the alumina becomes more discolored.

In this way, if the ratio of hydrogen and water vapor is within the specified range,
An inert gas such as a tie can be mixed into NZ+8r. For example, dew point 20℃ 1Hz: Nt=
A gas flow of 10=90 is preferred.

The characteristics of the Mi weave after metallization under the conditions of the present invention will be described below. The present invention is particularly characterized by the Ni layer. The first feature is that since the Ni paste is applied by screen printing or the like, the thickness of the Ni layer in the product is 10 μm or more, which is advantageous in terms of heat resistance, etc., compared to the plating method. That is, Ni is originally a heat-resistant metal, and the thicker the Ni layer, the better the heat resistance of the metallized surface. However, in the conventional plating method, increasing the thickness of the Ni layer is very costly, and there is also the problem that the strength decreases as the thickness increases, so the thickness must usually be 2 to 4 μm. This is very disadvantageous from the viewpoint of heat resistance. That is,
In an air retention test at 650°C, it was oxidized in a few minutes, losing its vacuum sealing properties and reducing its strength to the initial level of about 173.

On the other hand, the product of the present invention has vacuum sealability even after being held for 60 minutes.

There was no change in strength.

As described above, in the present invention, since the Ni layer is applied and sintered, it has excellent heat resistance and is very strong despite its thickness, and the cost increase with respect to the thickness is small.

Next, in the case of the method of the present invention, due to simultaneous firing, interdiffusion of metal occurs between the first layer and the second layer, resulting in solid solution of up to 40% by weight of tungsten in the Ni layer. It is. In the examples of the present invention, the amount of solid solution of tungsten is usually about 20 to 40% by weight. On the other hand, conventional Mo-Mn
In the metallization methods of metallization and plating, the amount of solid solution metal in N1Jl is 5% by weight or less, and a clear difference in structure appears.

The solid solution of tungsten increases the strength of the Ni layer, which is extremely advantageous in that it is less prone to scratches during handling of parts.

As described above, the present invention has significant characteristics not only in the various characteristic values but also in the structure, and this can be easily determined from EPMA or SEM photographs of the cross section.

The ceramics used in the above experimental examples are aluminum-based (/
'jz(h), but we also conducted various experimental studies on other types of ceramics. As a result, mullite (
2A7z (h・2SiO□), steatite (MgO・
5in2) + forsterite (2MgO・5iO2
) l cordierite (2MgO・2 y 20 y・5
Similar strength and vacuum sealability were obtained with SiO□).

This shows that the metallization method of the present invention is particularly useful for ceramic systems containing components that easily react with the glass component in the metallization lower layer forming paste. That is, the present invention is particularly effective for ceramics containing oxides. Among the above-mentioned ceramics, steatite and forsterite have a lower heat resistance temperature than Al2O2, and it has been difficult to form a sound metallized layer using the conventional metallization method of firing at around 1450°C. However, according to the present invention, although the firing temperature is as low as 1200 to 1400'C, even these low heat-resistant temperature ceramics can be sufficiently metalized.

Tungsten for the first layer and Ni for the second layer usually have a purity of about 99%.

[Example] Example 1 Inner diameter 12 cylinder φ x outer diameter 16 mm φ shown in the perspective view of Fig. 4
Cylindrical body 20 which is a ceramic sintered body with a height of io+nm
The present invention was carried out when joining copper pipes 6 as shown in FIG.

The cylindrical body 20 is made of alumina containing 92% lV2O, and at the upper end 20a of this cylinder 20, lV2O3:MnO;
Qz=26:34:40 weight ratio IVzOt-
MnO5iO□ based glass powder and tungsten powder,
Mixed powders mixed in the proportions shown in Table 1 were kneaded using an organic vehicle, and the resulting paste was applied by screen printing. After applying the paste, put it in the oven for 2
A drying process was performed at 00° C. for 120 minutes to form a first layer 311 as shown in FIG. Next, this first layer 311
Nickel oxide powder adjusted to a particle size of 5 μm or less was kneaded using the above-mentioned organic vehicle, and the resulting paste was applied to the surface 311a by screen printing. After coating, a drying process was performed in an oven at 200°C for 120 minutes to form No. 21321. Thereafter, the mixture was heated to the temperature shown in Table 1 at a heating rate of 10°C/min in a mixed gas flow of 90% by volume of nitrogen and 10% by volume of hydrogen with a dew point of 20°C.
It was held for a minute. Next, it was cooled at a rate of 10° C./min to form a metallized layer 30, thereby obtaining an alumina body 10.

A copper tube 6 was bonded to this alumina body 10 as shown in FIG. 5, and the vacuum sealability and bonding strength were measured.

The connection with the copper tube 6 is made using a Nilow material 5 (in this example, JIS Z 3261 BAg) between the metallized N30 and the copper tube 6.
-8) was interposed and brazed in a hydrogen atmosphere to obtain a joined body. After obtaining the bonded body, an iron jig was attached to the bottom surface 20b of the alumina body 10 using an adhesive that maintains sufficient sealing properties.

The vacuum sealability was determined by measuring the amount of leakage from the tip of the copper tube 6 using a helium leak detector. The evaluation criteria is I
Those exhibiting a sealing property of 10-10 Torr-1/sec or less were passed. The bonding strength was measured by fixing an iron jig and pulling the copper tube 6 upward. Table 1 shows an example of the results of an investigation using the mixing ratio of the glass powder and tungsten powder and the firing temperature as parameters. As long as the firing temperature was 1200 to 1400°C and the weight ratio of tungsten to glass was in the range of 70:30 to 95:5, a bonded body with good properties was obtained.

Next, the body tube joint product shown in Fig. 5 was heated at 50°C to 600°C.
After heating to 50℃ and maintaining that state for 10 minutes,
A thermal cycle test was conducted to cool down to The heating rate is 3
The temperature drop rate was 4°C/mln + 37°/min, and the above-mentioned changes in vacuum sealability and tensile strength were investigated by repeating this heating and cooling process.As a result, it was found that the conventional molybdenum-manganese method After 10 thermal cycles, the vacuum sealability deteriorated and the tensile strength also decreased to below 100 kgf.

On the other hand, the device based on the present invention has vacuum sealability I x 10-” Torr-j2/
sec or less, and the tensile strength is 350 kgf.
That is all (the copper pipe 6 was destroyed).

Example 2 Next, in order to investigate the influence of the melting point of glass, the mixing ratio of tungsten powder and glass powder was kept constant at 80:20 in Example 1, the melting point of the glass powder was adjusted, and other Metallized 1i30 was formed under the same conditions as in Example 1, and the vacuum sealability and bonding strength were measured.

Table 2 shows an example of the results of the investigation. In order to obtain good characteristics, the firing temperature should be 1200 to 1400°C.

It was confirmed that it was necessary to control the melting point of the glass within the range of 1100 to 1300'C.

Example 3 Tungsten and glass were prepared in the same manner as in Example 1.

The influence of particle size of nickel, etc. was investigated.

In the same manner as in Example 2, the effect of the particle size of tungsten was determined by keeping the particle sizes of glass and nickel constant at 5 μm, and the effect of the particle size of glass was determined by changing the particle size of tungsten and nickel to 5 μm.
In order to examine the influence of the particle size of nickel, the particle sizes of tungsten and glass were fixed at 51, respectively. The glass had the same composition as in Example 1 above. Table 3 shows an example of the survey results. Tungsten for good properties.

It was found that it was necessary to adjust the thickness of both glass and nickel powder to 10 μm or less.

Table 3 × : Vacuum sealability is the criterion value (IXIO visit 0 Torr゛ ffi/5ec) or less.

Δ: Vacuum sealability satisfies the above criteria, but tensile strength is 1
Less than 50 kgf.

○: Vacuum sealability satisfies the above criteria and tensile strength is 1
Range from 50 to 350 kgf.

◎: Vacuum sealability satisfies the above criteria and tensile strength is 3
Over 50 kgf.

[Effects of the Invention] As explained above, according to the present invention, the equipment cost when metalizing an alumina ceramic sintered body is reduced by 50% compared to the conventional high melting point metal method, and the cost is also reduced. Despite the reduction, it has become possible to manufacture products with properties that are not inferior to those obtained by conventional methods. Furthermore, the present invention can also be applied to ceramics with a low melting point, which has been difficult to produce using the Mo--Mn method.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing a metallized layer formed according to the present invention, and Fig. 2 shows a state in which paste is applied to the surface of an alumina ceramic sintered body. Fig. 2 (a)
is a sectional view, and FIG. 2(b) is a schematic diagram. Fig. 3 is a schematic diagram showing the state in which the paste applied to the surface of the alumina ceramic sintered body is baked, and Fig. 4 shows a specific embodiment of the present invention. A perspective view, FIG. 5 is a sectional view showing a state in which a steel pipe is joined to the alumina body of FIG. 4, and FIG. 6 is a sectional view showing a metallized layer in the alumina body of FIG. 5. 1.10... Alumina body, 2.20... Alumina ceramic sintered body, 3,30... Metallized layer, 3
1... Tungsten-glass mixed layer, 32... Nickel layer, 301, 311... First layer, 302°32
1... 2nd N, 5... Brazing metal, 6... Copper tube. Figure 2 (b) Figure 3 Figure 4

Claims (1)

[Claims] On the surface of the oxide ceramic sintered body, an average grain size of 1
A mixed powder consisting of 70 to 95% by weight of tungsten powder of 0 μm or less and 30 to 5% of glass powder with a melting point of 1100 to 1300°C and an average particle size of 10 μm or less is kneaded in an organic vehicle and then dried. first
layer, and then on said first layer, particles having an average particle size of 10μ
After applying a paste made by kneading powder of any one or a combination of two or more of nickel, nickel oxide, and nickel salt with an organic vehicle and drying it to form a second layer, Later, 1200-1400℃
and 1<H_2/H_2O<10000
A method for metallizing oxide ceramics having excellent vacuum sealing properties and brazing properties, characterized by firing in a mixed atmosphere adjusted to zero.