DE2411519A1 - PROCESS AND EQUIPMENT FOR MANUFACTURING CERAMIC OBJECTS FROM BETA ALUMINUM - Google Patents

Description

Patent attorneys Dipl. -Ing. F. Wliοϊ:?.: / - νιγ,

Dipl.-Ing. H. ¥ eickmann, Dtpl.-Phys. Dr. K. Fincke Dipl.-Ing. R A / Weickmann, Dipl.-Chem. B. Huber

8 MUNICH 86, POST BOX 860820 TOEHÜ

MÖHLSTRASSE 22, CALL NUMBER 98 39 21/22

■ THE ELECTRICITY COUNCIL

30 Millbank

London, SW1P 4RD, England

Process and device for the production of ceramic objects made of ß-clay

The invention relates to the continuous production of Ceramic objects made of ß-alumina by means of a sintering process, in which material of compressed powder is passed through an oven.

The ceramic mass known as β-alumina is a material with a nominal composition in percentages by weight of 5 % sodium oxide and 95 % aluminum oxide. The amount of sodium oxide can in practice be 5 to 10 % . It can also contain magnesium and / or silicon oxides. The material can be sintered in a temperature range from 1550 to 1900 ⁰ C. It is used in Sodium Plating Cells and other electrochemical devices which require passage of Sodium Cene. The desirable properties for this material in such applications include: high density,

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Impermeability to heliuingas and precise control the composition and properties of the. Inside and especially up to the surface of the material. The material is typically in the form of long thin-walled tubes with a closed or open end or in shape needed by discs.

As described in the applicant's earlier British patent specification 1297373, ceramic articles made of β-alumina can be used in manufactured in such a way that three-dimensional shapes are formed from compressed powder of the required composition and these three-dimensional shapes are moved through a tubular furnace, leaving a short length of material is brought to the sintering temperature. The movement is continuous so that the heated area is gradual is moved along the material to be burned.

At the firing temperature, the volatile component (NapO) evaporates, which leads to differences in the composition and properties of the ceramic within the object. In the usual burning process, the loss of soda can be prevented by buffering, that is, by surrounding the electrolyte with a loose powder of the same composition. However, because of temperature gradients within the furnace that cause differences in the vapor pressure of the soda, the fired ceramic may still have properties that vary between the different parts of the object.

The present invention is based on the object of an improved process for the continuous production of Specify ceramic objects made of ß-alumina that overcomes these disadvantages.

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In accordance with one aspect of the present invention, the method of making one or more ceramic ones comprises Objects made of ß-alumina as process steps are the production of one or more three-dimensional shapes from pressed powder the required composition and the continuous pushing through of the spatial form or a set of these spatial forms through a tubular furnace, at the same time. generation a gas flow through the tubular furnace in the direction of travel of the material at a speed equal to or greater than the speed of movement of the material. In practice the speed of movement of the material is very slow (e.g. 50 mm / min), so that any induced or forced gas flow will in practice flow faster than the material moves. In the following description is therefore, unless specifically stated otherwise, provided that reference is made to a gas flow through the furnace a stream is meant whose speed of movement is greater than that of the material.

Preferably, a three-dimensional shape or a set of three-dimensional shapes is moved through the furnace in such a way that a Short length of the material is heated to the sintering temperature, gradually moving along the heated zone of the material to be burned is moved.

With the method described above, a very short heating and sintering time can be used, as in the one mentioned above Patent 1297373 is described. By suitable selection of the furnace length, the throughput speed and the firing temperature a precise adjustment of the grain size can be achieved, as is also described in the cited document. In the method according to the present invention, the unfired material gives off water vapor and a small amount of

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Sodium oxide fumes off when it enters the oven is heated. The water vapor is in the direction of movement of the material by the gas flow described above transported. The water vapor can therefore not condense on the objects entering the combustion zone and thereby accumulate inside the furnace and one Reach a critical concentration of the unfired material. By evaporating a small amount of Sodium oxide from the incoming objects, a constant sodium oxide-rich atmosphere is generated and maintained in the burn zone. This is in contrast to the conditions that would occur, for example, if the gas flow were directed against the direction of movement of the object. In this case the water vapor would condense again on the cold incoming material. The water concentration would possibly reach a critical level Increase in value and cause the incoming tubes in the furnace to crack. The conditions in the sintering zone are then not uniform and the properties of the Keramilan material deteriorate when the article or a sequence of objects passes through the oven. In the process according to the invention, the gas flow must be sufficient be strong to carry the water vapor through the oven, but not so strong that the sodium oxide gets faster is removed from the burn zone when it can be replaced.

The gas stream, which normally consists of air mixed with the aforementioned water vapor and sodium oxide vapor can be generated by external forces. Most convenient However, there is a convection current that flows through it is generated that the furnace runs obliquely upwards in the direction of movement of the object. The speed of the gas flow depends not only on the inclination of the combustion tube, but also on the temperature, the size of the objects in the furnace and depend on the dimensions of the furnace components. In practice

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however, it is very easy to find the optimal inclination of the oven for objects of a given size and for given Firing conditions to be determined empirically. Dor tilt angle is typically on the order of a few degrees.

The invention further includes a furnace for the production of ceramic articles from β-alumina, comprising a tube open at its ends with a heating device, i.e. an induction coil around the tube and a device for continuous transport of the objects through the tube at a uniform speed. Also has the furnace either a device for generating an air flow through the tube or is provided with a tube that is opposite the horizontal in the direction of movement of the objects rises to the top. ¥ ie described in the above-mentioned patent specification, an induction coil is preferably used used, within which a receiving block, for example a graphite block, is arranged around said tube.

To control the temperature in the oven, a second one, open at its ends, is expediently provided by the receiving block running pipe provided, and a radiation pyroiiieter arranged, to observe a sample element inside the second tube. This sample element can be of any suitable Material exist (for example, recrystallized alumina), so that temperature changes are observed with the pyrometer can be. The temperature of the sample element need not necessarily be the same as that of the ceramic article made of ß-alumina, which is currently being sintered, even if the furnace is preferably built in such a way that the conditions for the Sample element resemble as closely as possible those for the item that is being fired. However, the temperature

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Changes in the sample element correspond to the temperature changes in the ceramic material made of β-alumina, and therefore the measured value from the pyrometer can be used to control the temperature of the furnace. An automatically operating control system is preferably provided for this temperature control.

In an expedient arrangement, the combustion tube through which the ceramic objects made of β-alumina pass and the tube containing the sample element are arranged symmetrically opposite sides to the axis of the induction heating coil. In this arrangement, the tube containing the β-alumina ceramic, hereinafter referred to as the combustion tube, is continuously rotated in order to ensure uniform temperature conditions. It is arranged for this purpose within a second stationary tube of somewhat larger diameter which extends through the receiving block and through the furnace. It has been found that it is possible with this construction to regulate the temperature in the sintering zone without effort with an accuracy of + 5 ⁰ C. Depending on the requirements, the temperature in the sintering zone can be on a. Value between approx. 1550 and 1900 ⁰ C, typically at 1700 ° C; each section of the material to be fired typically remains in the sintering zone for less than 2 minutes.

The following description refers to the accompanying drawings. They represent:

1 shows a schematic section through an embodiment of an induction furnace for sintering ceramics from β-alumina and

Figs. 2 and 3 are graphs showing the relationship

between the material density and the angle of inclination of the furnace for a set of experiments and show that better results are obtained with an increasing 409839/0712

_ 7 -

Slope (as in Fig. 2) can be obtained than with a sloping slope (as in Fig. 3).

In Fig. 1 one recognizes a furnace for sintering ceramic from ß-alumina, with an induction coil 10, which is of a Alternator 11 is fed, typically at 450 kHz. The induction coil 10 is helical, typically only a few inches long and surrounds a receiving block 12 made of graphite. The induction coil 10 and the receiving block 12 are in one a housing 13-forming asbestos card included, the is filled with an insulating material made of alumina containing bubbles, shown schematically at 14. Through the mounting block 12 and parallel to the axis of the induction coil 10 run two bores, which are arranged symmetrically to the coil axis are. The first bore contains an open ended refractory tube and extends through the end walls of the housing 13. The second bore through the receiving block 12 has a larger diameter than the first bore and includes a fixed refractory tube 16 with open ends, within which a combustion tube 17 made of refractory Material, such as alumina for receiving the objects to be sintered is rotatably arranged. The combustion tube 17 is stored in bearings 18 and is continuously rotated by a schematically illustrated drive device 19, in order to achieve a uniform temperature around the firing zone when the furnace is in operation. They are typical turning speeds from 30 to 60 revolutions per minute has been applied. The pipe 15 contains in operation a schematically illustrated, refractory article 20, typically made of recrystallized alumina, as a sample iron to use it for the purpose the temperature control with a pyrometer 21 measuring the total radiation. The reading from the pyrometer can be fed into a control device 22 and / or in an energy control device 23, which the output power of the generator 11 regulates.

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To c obtain cine same physical end sodareiche atmosphere was there, "burner tube 17 may be impervious to Natriumoxiddampf * Ifonn the material dec rotating burner tube 17 with dei <Nat-riuraoxicldarapf reacts, the reaction rate should se low sain that no decrease. Sodium oxide. For example, it was found that sodium oxide did indeed react slowly with the recrystallized alumina of the tube used in the experiments below, converting the alumina to B-alumina. The rate of reaction was rather slow and the burner tube 17 increased in weight by 20 mg for every 40 g of burnt tube. The loss of the respective baked goods was less than 1% of sodium oxide existing _s, however, the conversion of the fuel tube 17 in ß-alumina accompanied by a Volumenänderuiig so that this tube does not remain long impermeable. For this reason, only about 20 m of tube could be burned before the rotating burner tube 17 had to be replaced. However, they can be replaced without cooling the oven.

A continuous flow of air through the combustion tube 17 of the furnace is created during operation by the furnace rising in the direction of movement dor objects. In this way, the use of convection results in a very simple and reliable means of achieving the desired air flow; but of course a forced air flow could be done in other ways, can be obtained without tilting the furnace. In the following examples of the manufacture of ceramic tubes from β-alumina the quality of the sintered products is indicated quantitatively by their density. The ones shown in these examples Differences in density are important in terms of using the material as an electrolyte, though

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the differences can be small in absolute terms. The examples given serve to introduce the invention and are therefore geared towards showing the effect of mitigations in air flow, including in particular the use of an air pressure which is opposite to that required by the method according to the invention is directed. An oven as shown in Figure 1 was used in these examples. In the furnace, the combustion tube 17 had an inside diameter of 19 μm, an outer diameter of 25.5 mm and was 500 mm long. The fixed refractory tube 16 had an inner diameter of 29 mm, an outer diameter of 36 mm and a length of 375 mm. The graphite receiving block 12 was 100 ram long and produced a hot zone of 120 mm in length at (maxiral egg temperature -10 °) ⁰ C. All samples were tubular-frozen samples which had been predried before firing. "

The first of the examples, referred to below as Example 1, relates to a furnace with an ar inward direction of ventilation of the objects "downward slope. The convection caused an air flow in the opposite direction to the direction of movement of the objects. This example is given in order to counteract the effect of the present invention To indicate arrangement. The water vapor was carried upwards and condensed on the incoming material. the Water concentration rose to a critical fourth and caused the incoming pipes in the furnace to crack.

Eg 1 1

The objects ran downwards through the oven in continuous succession at a temperature of 1735 ^° C. and a throughput speed of 50 mm / min. The onset of rupture due to the excess of water occurred so quickly that less

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than 400 min ΚβτβόΙΙζ generated who α an kormton * The density of 40 Dun langcii pieces i;.> t in the table in the order in which the pieces were fired, given and plotted in Fig _.;

Location (now) density ( kg / w ³ )

0-40 3183

40-80 3182

80 »120 3190

120-160 3192

160-200 3177

200-240 3176

240-280 3180

280-320 3170

320-370 3145

In Examples 2, 3 and 4 below, the oven went in Direction of movement of the objects, so that the water vapor and soda vapor are entrained through the sintering zone, to knock down on the burned pipes. The water vapor could not get on the unfired material reach critical value and the soda resulted in a stable sodium oxide atmosphere in the burn zone. Im continuous Operation were generated over 20 m of the electrolyte. The ceramic properties are uniform and constant with time, as in Examples 2, 3 "and below shown.

B is ρ ie 1 2

Continuous, upward run at a temperature of 1745 ^° C. and a throughput speed of 50 mm / min. 2000 mm of ceramics were produced, and it

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an equilibrium was established after 100 ejei kermaik were burned a. During the grate of the experiment, they remained Densities Uniform »These firing conditions produce coarse-grained ceramics.

Pipe no. Location within the Tight Trial (mm) M8 / 1a 0 - 3230 b 51 · 3239 C. 102 - 3250 d 153 - 3255 - ■ 51 - 102 - 153 - 205

M8 / 3 413 «618 - 3249

M8 / 4a 618 - 669 3248

b 669-720 3244

c 720-771 3249

d 771 - 823 3250

MB / 5 823 - 1030 3244

M8 / 8 1441 - 1648 3249

MB / 9 1648 ~ 1848 3249

ο In the case of pipes MB / 1 and MB / 4, the

^ Letters a, b, c and d on four of the respective

<■ *> Samples taken from pipes.

• * a B e 1 game 3

A sample from Example 2 was used to measure uniformity within the wall thickness of the pipe investigate. This examination is carried out by measuring the density, then grinding the outer wall and repeated measurement of the density. The sample material was the same

f.ö ~ ni: l {-. bit L-an outer Ivancl. Sample J-iS / 'Vb (669 ■ - 7PO lm)

Checkout (x 10 ⁵ kg) density

6.642 3243

5.944 x 3252

5.264 3251 4.754

it

The experiment H21 (tubes 21 to 39) was carried out continuously and having an upward slant ^1. 4 m of electrolyte was generated. The temperature was 1720 ⁰ C with a flow rate of 50 MRA / inin, and an angle between the fuel pipe and the horizontal of 4 °.

The mean density for the entire test was 3225 kg / m, with a mean square deviation of 6 kg / m These firing conditions produce fine-grain ceramics.

Rohj r no.

21 22 23 24 25 26 27 28 29 30 31

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Position 5m trial (mm). Density (kg / nP) 0-205 3223 205-408 3224 408-618 3223. 614-820 3214 820-1025 3222 1025-1231 3222 1231-1438 3222 1438-1645 3225 1645-1847 3239 1847-2053 3238 2053-2260 3221

PA?} 2i ^c Ak: J

32 22 bO - 24 & G

33 2466 · ■ - 2663

34 26G0 - 2375 3221

35 2875 ~ 3079 32? 5

36 3079-3284 "3224

37 3284 »3490 3223 39 3695 - 3901 3230

Examples 2, 3 and 4 show v / ie exactly a ro ge development of the Atmosphere achieved by natural convection v / earth ka'uu. For a given shape of the furnace, the convection currents hang from the angle of inclination of the burner tube, and so are the properties of the ceramic through this Angle affects. This is shown with reference to FIG. 2, which in a graphical diagram the relationship between the Shows the density and the angle of inclination with respect to the horizontal for a rising slope. The firing temperature was 1720 ° C with a throughput speed of 50 nra / rain. The curve shows that with the change in the angle, the density has a flat maximum. The rise must be large enough that the convective gas flow moves the water vapor upwards takes with it through the combustion tube. However, the increase should not be so great that the sodium oxide goes out of the water too quickly The burning zone is removed, thereby preventing the build-up of a stable, soda-rich atmosphere.

The advantage of continuous firing is that it is stable Generates atmosphere and improves the ceramic properties can be seen from the following Example 5. Two Sets of pipes were burned under the same conditions except that one was continuous and the other was gradually burned. The result was a marked increase in the absolute value of the density in the continuous Burn. 409839/0712

- old -

ο i r. T) i ο 1 5

At Vcro ^ oh 29 v. ^: iux "'.-? · a number of tubes hai Vfi ⁱ ö ° C ui a pass ^ eoolivrindi ^ ability of 50 Km / min gobramit. They vfurxlen -.- bcr varnished corrLiniiicrlicli. burned. Individual Rohi'O v'urdon π :: ·. "ί- the help of a long cUiuiiön stick (stick or Roar of 3.2 na AusEonäurnb / nssöer-) through the oven gcr.; c] iob £: ii boi a horizontal arrangement of the same The density of the probes 14, 15 and 10 are listed belowϊ

sample

29/14 3195 29/15 3208 29/18 3193

From these examples it can be seen that when natural convection is used in the zonal sintering of Ceramic electrolytes made from ß ~ alumina up the material must be transported through a combustion tube which extends at an angle of 0 ° with respect to the horizontal, preferably with an angle between 4 and 10 °. The quality of a material sintered under these conditions is both uniform over the cross-section of the material as well as as a function of time. Of course, the refers Preferred angles found in this way on a special oven and the special processed Rauia forms. For example, the air flow depends on the ratio of the cross-sections of the objects and the combustion tube. For Given the given conditions, the optimal increase can easily be found empirically.

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- Λ5 »

VJenii also specifically referred to the liorstellunc of Keramikrohron run ß-alumina, so are dr, / ;. Befebr ": o and the device-also-for the Hornellimg Yen Scj) ojbon to vorv / eijden, which pass through the furnace in close succession, in which they form a rod on the dic & e V. ^{r eiso"}

The densities licfrcn considerably low & r than those in that Example 2 achieved) * as Durc.hlaui'gefoch ", vJndi / yieit and Temperature the same V / ert Jiatten, but the samples continuously in upward direction compared to the Horizontal pipe sloping at an angle of 4 ° passed through.

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Claims (1)

Claims Process for the production of one or more ceramic objects from ß-alumina, characterized in that, that one or more three-dimensional shapes are formed from pressed powder of the desired composition, that the three-dimensional shape or a set of these three-dimensional shapes is continuously moved through a tubular furnace and that at the same time a gas flow through the tubular furnace in the direction of movement of the material at one velocity equal to or greater than the speed of movement of the material. 2. The method according to claim 1, characterized in that i c h n e t that as the material moves through the tubular furnace, a short portion of the length of the material is heated to the sintering temperature and that the heated zone gradually extends along the material to be burned moved in the longitudinal direction. 3. The method according to any one of claims 1 or 2, characterized in that the flowing through the furnace A gas stream of air mixed with water and sodium oxide vapor from the pressed powder material consists. 4. The method according to any one of claims 1 to 3, characterized characterized in that the gas flow is generated and maintained by power means. 5. The method according to any one of claims 1 to 3, characterized in that the gas flow is a convection flow is, wherein the furnace rises in the direction of movement of the objects to the convection current produce. 409839/0712 6. The method according to any one of claims 1 to 51, characterized in that the three-dimensional shapes of pressed powder pass through the furnace at a uniform speed ν that the furnace has a temperature distribution with one end "up to the maximum sintering temperature between 1600 and 1900 ⁰ C rising temperature and falling to the other end, so that the length 1 of the sintering zone, which lies within a range of 10 ° C of the maximum furnace temperature, and the speed v at which the furnace is passed through, are chosen so that the time 3rd, during which everyone ν point of the material is in the sintering zone, between 12 seconds and 2 minutes, and that the temperature distribution of the furnace and the rate of flow of the material are chosen so that each point of the material is heated to the sintering temperature and is cooled from the sintering temperature at a rate between 200 ° C / min and 240 ° C / min. 7. The method according to any one of claims 1 to 6, characterized characterized in that the throughput speed is between 25 and 115 mm / min. 8. Furnace for the production of ceramic objects from ß-alumina, characterized in that it is a bilateral open tube (17), a heating device for heating the tube (I7) and a device for continuous transport which has objects through the tube (17) at a uniform speed and that it is further provided with a device is provided for generating an air flow through the tube (17), or that the tube (17) in the direction of movement of the Objects with respect to the horizontal rising upwards is arranged in the oven. 409833/0712 9. Oven according to claim 8, characterized in that that the heating device is arranged around the tube (i7) Has induction coil (10). 10. Oven according to claim 9, characterized in that within the induction coil (10) around the tube (17) a receiving block (12) is arranged. 11. Oven according to claim 10, characterized in that that the receiving block (12) is made of graphite. 12. Oven according to claim 10 or 11, characterized in that that to control the temperature in the oven a second open on both sides, -through the receiving block extending tube (15) is provided and that a radiation pyrometer (21) for observing a is provided within the second tube (15) attached sample element (20). 13. Oven according to claim 12, characterized in that that an automatic control device reacting to the measured values of the radiation pyrometer (21) is provided is to regulate the energy supply to the furnace. 14. Oven according to one of claims 12 or 13 _f, characterized in that the tube (i7) through which the objects of ß-alumina pass, and the tube (15) containing the sample element 20, mutually opposite sides symmetrically to the axis of the the heating causing induction coil (10) are arranged. 15. Oven according to one of claims 8 to 14, characterized in that the ceramic objects made of ß-alumina-containing tube (17) continuously rotatable and within a second, extending through the receiving block (.12) and the stationary tube (16) extending over the furnace A09839 / 0712 is arranged with a slightly larger diameter. 16. The method according to claim 5, characterized in that that the furnace rises upwards at an angle between 4 and 10 ° relative to the horizontal. 17. Oven according to one of claims 8 to 15, characterized in that the tube (17) with a Angle between 4 to 10 ° relative to the horizontal is arranged rising upwards. 409839/0712 30th Blank page