EP4237245A1

EP4237245A1 - Method for sintering ceramic materials

Info

Publication number: EP4237245A1
Application number: EP21777987.5A
Authority: EP
Inventors: Apurv DASH; Robert Mücke; Olivier Guillon
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2020-10-30
Filing date: 2021-09-13
Publication date: 2023-09-06
Also published as: US20230382807A1; WO2022089823A1; DE102020213680A1

Abstract

The invention relates to a method for producing a compacted component and to an article which comprises a compacted component. In a method for producing a compacted component (10), a starting material (20) at a temperature (T) below 800°C is exposed to an electric field (12). The starting material (20) comprises a first material (14) from the group of cuprates. The method is technically undemanding because compacting is possible without the starting material being heated.

Description

Process for sintering ceramic materials

description

The invention relates to a method for producing a compressed component and an object that includes a compressed component.

Sintering is a process for processing materials. A material is heated and, if necessary, subjected to increased pressure so that the material is compressed. Sintering takes place at high temperatures, but below the melting temperature of the main components, so that the shape of the workpiece is retained during sintering. Shrinkage typically occurs as the source material is compacted. A solid workpiece is produced by sintering, whereby properties such as hardness, compressive strength and thermal conductivity can be influenced by suitable process parameters.

The sintering of ceramic materials is used for the production of ceramic components, which in terms of their properties such. B. Hardness, compressive strength, wear resistance, high temperature resistance, thermal conductivity and electrical conductivity find multiple applications. In some processes, shaping takes place, in which a green compact is produced from mostly powdery starting materials, if necessary drying and then compaction. The starting material is exposed to a temperature between 800°C and 2500°C. In this step, grain growth occurs. Due to the high temperatures, this process is technically complex and energy-intensive. Strong grain growth can lead to undesirable properties of the manufactured component. In addition, the long duration of the process, which is between a few hours and days, is a major disadvantage.

To shorten the sintering process, field-assisted sintering was developed, in which heating is carried out using an electric current. In this process, which is also known as field-activated sintering, "Field-assisted sintering technology" (FAST) or "Spark Plasma Sintering" (SPS), a direct electric current is passed through the powder to be sintered, which leads to further heating leads to the Joule effect. In addition, a pressure of 50 MPa to 400 MPa is built up and sintering takes place under protective gas or vacuum. Compared to conventional sintering, the Heating is accelerated and can take place, for example, at several 100 K/min, so that significantly shorter process times are possible. This process is also technically complex due to the high temperatures well above 800°C and the high pressures. In addition, it can only be used with materials that have sufficient electrical conductivity at room temperature, which is usually not the case with ceramic materials. Flash sintering, which is based on a current flow through the ceramic body in combination with external heating, was developed to eliminate this disadvantage. First of all, external heating is used and when a specific temperature is exceeded at which the sample becomes sufficiently conductive, a current flow is realized across the sample cross-section. This method is also technically complex. Other methods rely on heating in an oven to increase conductivity before an electric field is applied.

The publication "Oxide Superconductors" by Robert J. Cava from the "Journal of the American Ceramic Society" [1] 5 - 28, 2000, describes the development of ceramic superconductors with a focus on copper oxide superconductors from the cuprate group.

The object of the invention is to provide an improved method for producing a compacted component and an improved object.

The object is achieved by the method according to claim 1 and the subject matter according to the independent claim. Refinements are specified in the dependent claims.

A method for producing a compacted component is used to solve the task. A starting material comprises a first material from the group of cuprates. The starting material is exposed to an electric field at a temperature T below 800°C. In this way, a compacted component is produced from the starting material.

In contrast to many other ceramic materials, ceramic materials from the cuprate group exhibit electrical conductivity even at temperatures below 800°C. This electrical conductivity enables compaction by a electric field. The process requires little technical effort, since the starting material does not have to be heated for compaction. In particular, the method does not include any heating of the starting material in an oven and/or with an external heating device. In addition, the materials used are typically oxides and for this reason are not prone to oxidation. Compared to metals, alloys and other oxidizable materials, this opens up additional areas of application.

The process is used to produce a compacted component. It can also be referred to as sintering. Since processes that are carried out at high temperatures are usually referred to as sintering, the process is referred to here as compaction.

The electric field is generated in particular by arranging electrodes on different, for example opposite, sides of the starting material and by applying an electric voltage to the electrodes or realizing an electric current through the electrodes and the starting material.

The electric field can be generated by a power supply device. This can be designed as a DC voltage or direct current source or as an AC voltage or alternating current source.

In particular, the electric field strength is steadily increased from zero to a target value. Due to the electrical conductivity at a certain field strength, there can be a sudden increase in the electrical current flowing through the starting material. In particular, a switch is made from field-based control to current-based control after a sudden increase and/or spike in electrical current flowing through the feedstock. The electric current is thus limited. This prevents melting or destruction of the starting material to be compacted. Typically, the energy supply device is set up for such a switchover.

The starting material can be in the form of a green body, ie an object preformed from the starting material. The method may include shaping to produce the green body. This is used to produce the green compact, in particular of powdered material. A packing density that is as homogeneous as possible, i.e. a uniform mass distribution, can be aimed for in the entire green compact. The shaping takes place in particular by pressing, casting and/or by plastic shaping. In this way, geometrically complex components can be manufactured.

Alternatively or additionally, the starting material can be in powder form. This enables a particularly simple and quick method. The starting material can thus be introduced into a mold as a powder and exposed to the electric field in this form.

Cuprates are ceramic superconductors that are known as high-temperature superconductors due to their comparatively high critical temperature. The first material is in particular a ceramic superconductor. Chemical compounds containing a copper-containing anion can be referred to as cuprates. These can be salt-like cuprates, which contain oxygen in addition to copper. In particular, however, oxides are meant. Typically, the first material is a substance that has a transition temperature above -196°C, the boiling temperature of liquid nitrogen. In particular, the material from the cuprate group has an electrical conductivity at room temperature that is between that of a good conductor and that of an insulator. Said electrical conductivity of the material can be between 10 ⁴ S/m and 10 ⁷ S/m, in particular between 5*10 ⁴ S/m and 3*10 ⁶ S/m, preferably between 8*10 ⁴ S/m and 1 2*10 ⁵ S/m.

The group of cuprates includes, among others, LaBaCuO, LaSrCuo, YBaCuO, BiSrCaCuO, BiSrCuOCO, TIBaCaCuO, HgBaCaCuO, HgTIBaCaCuO, BaCaCuO, BaCaCuCO, SrKCuOCI. Only the elements contained are given here, but not the correct stoichiometric ratios.

The group of cuprates includes, among others, La4BaCusOi3, La ₂ - _x Ba _x CuO, La 1.8 Sr _0.2 CuO ₄ _, YBa ₂ Cu ₃ O ₇ , Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ , Bi ₂ Sr ₂ CaCu ₂ O ₈ , Bi ₂ Sr ₂ CuO ₆ , HgBa ₂ Ca ₂ Cu ₃ O ₈ , HgBa ₂ Ca ₂ Cu ₃ O ₉ , Y ₂ Ba ₄ Cu ₇ O ₁₅ , Hg _0.8 TI _0.2 Ba ₂ Ca ₂ Cu ₃ O _8.33 ,

Hg ₁₂ Tl ₃ Ba ₃₀ Ca ₃₀ Cu ₄₅ O ₁₂₇ , HgBa ₂ CaCu ₂ O ₆ , Tl ₂ Ba ₂ Ca ₂ Cu ₃ O ₁₀ , Pb ₂ Sr ₂ YCu ₃ O ₈ , Nd ₂ CuO ₄ , Cao,84Sro,i6CuO2 , TIBa ₂ (Eu,Ce)2Cu20g, GaSr ₂ (Y,Ca)Cu ₂ O ₇ , Pb2Sr ₂ Y _1-x Ca _x Cu ₃ O ₈ , Sr _3-x K _x Cu ₂ O ₄ Cl ₂ , Sr _{2 -x} K _x CuO ₂ Cl ₂ , NbSr ₂ (Nd,Ce) ₂ Cu ₂ O ₁₀ , (Sr,Ca)CuO ₂ , YBa ₂ Cu ₃ O ₇ , YBa ₂ Cu ₃ O _7-x , YBa ₂ Cu ₃ O _7-δ , for example with δ between 0.05 and 0.65, too referred to as YBCO, YBaCuO, Y-123, 123 oxide or "123 compound". In one embodiment, the first material is yttrium barium copper oxide. This has a high transition temperature and good availability.

The temperature T means the temperature at the beginning of the action of the electric field on the starting material. In particular, the starting material is not heated before it is exposed to the electric field. Nevertheless, it is possible that the temperature will rise to values above 500°C, 400°C, 300°C, 200°C, or 100°C at certain points due to the effect of the electric field. In particular, however, this temperature is well below 800°C. The temperature T can e.g. B. describe an average temperature inside the starting material.

In particular, the temperature T is less than 700°C, less than 600°C, less than 500°C, less than 400°C, less than 300°C, less than 200°C or less than 150°C. Surprisingly, it has been shown that compaction similar to a conventional sintering process is possible at these temperatures.

The temperature T is higher than the transition temperature of the starting material and/or the first material. At the critical temperature, the electrical resistance jumps towards zero. At lower temperatures, due to the lack of electrical resistance, a short circuit occurs and little or no compression takes place. In particular, the temperature is higher than -150°C, for example higher than -125°C, preferably higher than -100°C, in one embodiment higher than -75°C, for example higher than -50°C, in particular higher than -25 °C and in one example higher than 0°C.

In one configuration, the temperature T is less than 100°C, in particular less than 50°C. In one embodiment, the temperature is less than 80°C, less than 70°C, less than 60°C, less than 40°C, less than 30°C, or less than 25°C. It can correspond to room temperature or be lower than room temperature.

In one embodiment, the starting material is exposed to the electric field for a period of less than 10 minutes, preferably less than 1 minute. Experiments have shown that the method according to the invention allows complete compaction within the short period of time mentioned. With regard to the essential parameters such as shrinkage, change in porosity, increase in density and/or strength, compaction is comparable to conventional processes, which are associated with orders of magnitude more time. A particularly fast, resource-saving and cost-effective method is thus provided.

In one embodiment, the starting material is exposed to the electric field under atmospheric pressure.

In other words, the method is carried out without applying any pressure. No additional pressure to atmospheric pressure is built up. By gravity, feedstock below is subjected to pressure from feedstock above, in addition to atmospheric pressure. However, this should be neglected here. In one embodiment, the maximum pressure in the starting material at the start or shortly before the start of the effect of the electric field is less than 1.6 bar, preferably less than 1.4 bar and particularly preferably less than 1.2 bar or less than 1.1 bar.

In one embodiment, the starting material is mechanically compacted, in particular by pressing, prior to the action of the electric field. The starting material can be pressed uniaxially or isostatically. For example, a preform or green compact can be produced. In this way, more complex compacted components can be produced. The arrangement of electrodes for applying an electrical voltage and/or an electrical current is also possible as a result more easily and in a particularly reproducible manner. In addition, a particularly uniform distribution of the grains and/or pores is ensured, which makes particularly homogeneous compacted components possible.

In a further configuration, the electric field has an electric field strength greater than 50 V/cm. In particular, the electric field strength is between 100 V/cm and 5 kV/cm. In particular, this means the average field strength that acts on the starting material. When sintering conventional ceramics, the required field strength is lower at higher temperatures and vice versa. Due to the electrical conductivity of the materials from the group of cuprates, this is not the case with the method according to the invention. The required electric field strength increases with increasing temperature. A starting material can be compacted at room temperature with 300 V/cm, while only a field of 50 V/cm is required for the compaction of the same starting material at a temperature of -100°C. Here it is important to weigh up the advantages and disadvantages of the very low temperatures. Although energy savings can be achieved at low temperatures, the cost of any necessary cooling is higher.

In one embodiment, the starting material has a mass fraction of the first material from the cuprate group of between 50% and 100%.

In one configuration, the starting material has a mass fraction of a second material between 0% and 50%.

The second material is different from the first material. In particular, it does not contain cuprate. It has been found that densification at low temperatures is also possible when only a portion of the starting material is a cuprate. This is possible from a cuprate content of around 50%. In this way, there are many possibilities for adapting the properties of the compacted component to the respective requirements by means of suitable admixtures.

In a further configuration, the second material is an electrically insulating material, in particular a ceramic material.

Up to now, such materials could not be compacted at low temperatures. Up to now, heating to high temperatures of, for example, 1400° C. to 1600° C. has been necessary. The addition of the first material now allows compaction at low temperatures for the first time, which is also very fast. The power consumption of the starting material is ensured by the first material. Different mixtures can thus be used to produce and compact a wide variety of material compositions with the respectively desired properties. For example, the second material is an aluminum oxide. Aluminum oxide is a widely used technical ceramic with a wide range of possible applications. Up to now, aluminum oxide has been difficult to sinter because it has electrically insulating properties even at high temperatures. According to the invention, an "indirect" sintering of aluminum oxide is made possible via the combination with cuprate.

In one embodiment, the starting material contains a mass fraction between 0% and 99% of a second material and a mass fraction between 0% and 99% of a third material, optionally a mass fraction between 0% and 99% of a fourth material and optionally a mass fraction between 0% and 99% of a fifth material. The mass fractions of the second, third, fourth and/or fifth material can be below 40%. They can be below 20%. They can be below 10%. They can be below 5%. The second, third, fourth and/or fifth material can be an insulating ceramic material.

In one configuration, at least a first region, in particular at least a first layer, of the starting material consists essentially of the first material. At least a second region, in particular at least a second layer, of the starting material essentially consists of the second material.

In other words, an arrangement of the two materials in regions, for example in layers, is possible. In one embodiment, the starting material consists of a first layer and a second layer, which is arranged in particular directly adjacent. In one embodiment, the starting material comprises three layers, with a second layer being arranged between two first layers. In particular, the three layers are each arranged immediately adjacent. The starting material can consist of the three layers.

In one embodiment, a portion of the first material is surrounded by second material. Accordingly, a region of the first material lies between the second material along at least one viewing direction. The second material can be arranged as a coating of the first material. This can influence the properties of the compacted component. In a further configuration, at least a third region of the starting material comprises a mixture of the first material and the second material. Preferably the mixture is essentially homogeneous.

Thus, at least one region contains a substantially uniform mixture of two different types of particles, at least one type of particle being cuprate particles. The third region can consist of the mixture of the first material and the second material. Homogeneous means in particular a uniform mixture of the different particles. In this way, a homogeneous compacted component with the desired properties can be produced.

In a further configuration, an average grain size of the compacted component is larger by a factor F than an average grain size of the starting material. The following applies: F<5, in particular F<2, preferably F<1.25.

The mean grain size means in particular the mean value or median of the grain diameter. The compacted components produced by the method described can be recognized in particular by the fact that, in contrast to compacted components produced using conventional methods, their grain size is not significantly increased due to the low temperature and the short compaction time. With conventional methods, the factor F can be between 50 and 100 or higher. Accordingly, the grain size after compaction according to the invention essentially corresponds to the grain size before compaction. With the same grain sizes, the grain size in components produced using the method according to the invention is therefore significantly smaller. Components with very small grain sizes can be produced with the method according to the invention. The grain sizes can be adjusted according to the respective requirements.

An average particle size of the starting material and/or the compacted component can be between 0.1 μm and 100 μm, preferably between 0.5 μm and 50 μm, particularly preferably between 0.8 μm and 25 μm and for example between 1 μm and 10 μm. The average grain size can be determined, for example, by scanning electron microscopy and image data analysis.

Another aspect of the invention is an article that includes a compacted component. The compacted component was produced in that a Starting material was exposed to an electric field at a temperature below 800°C. The starting material comprises a first material from the group of cuprates.

In particular, the compacted component was produced using the method according to the invention. All features, embodiments and effects of the method described above also apply accordingly to the object. Conversely, all features, embodiments and effects listed here also apply to the method.

The object can be the component, for example a ceramic superconductor. The object can include other elements in addition to the component.

The compacted component comprises a material from the cuprate group. It typically has an average grain size of the compacted component which is greater by a factor F than an average grain size of the starting material, where: F<5, in particular F<2, preferably F<1.25. It is typically recognizable as a compacted or sintered component based on its properties. In one configuration, the component has an average grain size of between 0.8 μm and 20 μm, preferably between 1 μm and 10 μm.

In one configuration, the component is a ceramic superconductor. A ceramic superconductor is a ceramic material whose electrical resistance abruptly approaches zero when it reaches or falls below the transition temperature. In particular, the component is a high-temperature superconductor. It has been shown that superconductors can be produced particularly easily and with little technical effort in the manner described.

In a further embodiment, the object is an electromagnet and the component is arranged as a coil winding of the electromagnet. Due to the extremely low resistance-related losses of a superconductor, the electromagnet can generate high magnetic fields with little energy input. In particular, the electromagnet is a DC magnet.

In a further embodiment, the object is a device for generating light. The object is designed so that the component for the purpose of emission Light can be subjected to an electrical voltage and/or an electrical field. The object may include an energy supply device for subjecting the component to an electrical voltage and/or an electrical field. The object can be set up in such a way that the component has a temperature below 0° C. when light is generated.

A further aspect is the use of a compacted component produced with the method according to the invention or an object according to the invention for generating light, in which the component is exposed to an electrical voltage and/or an electric field so that the component emits light. In particular, this takes place at a temperature below 0°C. All features, embodiments and effects of the method and the object described above also apply accordingly to the use.

Exemplary embodiments of the invention are also explained in more detail below with reference to figures. Features of the exemplary embodiments can be combined with the claimed subject matter individually or in a plurality, unless stated otherwise. The claimed scopes of protection are not limited to the exemplary embodiments.

Show it:

Figure 1: a method for producing a compacted component,

Figure 2: a first embodiment of a component,

Figure 3: a second embodiment of a component, and

FIG. 4: a third embodiment of a component.

FIG. 1 schematically shows a method for producing a compacted component 10. A starting material 20 is exposed to an electric field 12. FIG. A temperature T below 800° C. acts on the starting material 20, particularly at the beginning of the effect of the electric field 12. The temperature T typically corresponds to room temperature. In other words, the component is not heated either before or during the effect of the electric field 12 . The electric field 12 acts for a period of time t of about 30 seconds. No pressure is applied, so that the starting material 20 is exposed to the electric field 12 under the prevailing atmospheric pressure P. The electric field strength is continuously increased, starting from 0 and/or up to a maximum value of 1 kV/cm. FIG. 2 schematically shows a first embodiment of a starting material 20 for producing a compacted component. The starting material 20 includes a third region 28 which includes a homogeneous mixture of a first material 14 from the group of cuprates and a second material 16 . In particular, the starting material 20 consists of the third region 28. The second material 16 differs from the first material 14 and in particular contains no cuprate. Suitable admixtures of the second material 16 can be used to adjust properties of the compacted component in a targeted manner. At the same time, compaction is possible with the method according to the invention.

FIG. 3 shows a second embodiment of a starting material 20. The starting material 20 comprises a first region 24 and a second region 26, which are designed in the form of layers and directly adjoin one another. The first region 24 is made from the first material 14 from the group of cuprates. The second region 26 is made from the second material 16, which differs from the first material 14 and, in particular, does not contain any cuprate. The first region 24 can be coated with the second material 16 in such a way that properties of the compacted component are influenced in a desired manner. For example, a color temperature of the emitted light can be adjusted in a desired manner.

FIG. 4 shows a third embodiment of a starting material 20. The starting material 20 comprises a second region 26, which is arranged in the manner of a sandwich between two first regions 24, which in particular are configured in the same way. In particular, the starting material 20 consists of the areas 24 and 26 mentioned. Here, too, the first area 24 is made of the first material 14 from the group of cuprates and the second area 26 is made of the second material 16, which differs from the first material 14 differs and in particular contains no cuprate. The first regions 24 and the second region 26 are arranged in superimposed layers.

The layer thicknesses shown in the diagrammatic FIGS. 2 to 4 and their ratios are not to scale. The areas 14 and 16 can have the same or different layer thicknesses. The layer thickness of the first area 14 can be greater, less than or equal to the layer thickness of the second area 16 . reference list

Component 10 starting material 20

First material 14

Second material 16

First area 24

Second area 26 Third area 28

Temperature T

length of time t

Atmospheric pressure P

Claims

Expectations

1. A method for producing a compacted component (10), in which a starting material (20), comprising a first material (14) from the group of cuprates, is subjected to an electric field (12) at a temperature (T) below 800° C. is suspended.

2. Method according to the preceding claim, characterized in that the temperature (T) is less than 100°C, in particular less than 50°C.

3. The method according to any one of the preceding claims, characterized in that the starting material (20) is exposed to the electric field (12) for a period of time (t) of less than 10 minutes, preferably less than 1 minute.

4. The method according to any one of the preceding claims, characterized in that the starting material (20) is exposed to the electric field (12) under atmospheric pressure (P).

5. The method as claimed in one of the preceding claims, characterized in that the electric field (12) has an electric field strength greater than 50 V/cm, the electric field strength being in particular between 100 V/cm and 5 kV/cm.

6. The method according to any one of the preceding claims, characterized in that the starting material (20) has a mass fraction between 0% and 50% of a second material (16).

7. The method according to the preceding claim, characterized in that the second material (16) is an electrically insulating material, in particular a ceramic material such as aluminum oxide.

8. The method according to one of the two preceding claims, characterized in that at least a first region (24), in particular at least a first layer, of the starting material (20) essentially consists of the first material (14) and at least a second region (26 ), in particular at least one second layer of the starting material (20) consists essentially of the second material (16).

9. The method according to any one of the three preceding claims, characterized in that at least a third region (28) of the starting material (20) comprises a preferably substantially homogeneous mixture of the first material (14) and the second material (16).

10. The method according to any one of the preceding claims, characterized in that an average grain size of the compacted component (10) is greater by a factor F than an average grain size of the starting material (20), where the following applies: F<5, in particular F<2, preferably F<1.25.

11. An object comprising a compacted component (10) which was produced in that a starting material (20) comprising a first material (14) from the group of cuprates, at a temperature (T) below 800 ° C an electrical Field (12) has been suspended.

12. Object according to the preceding claim, characterized in that the component (10) is a ceramic superconductor.

13. Object according to one of the two preceding claims, characterized in that the object is an electromagnet and the component (10) is arranged as a coil winding of the electromagnet.

14. Object according to one of the three preceding claims, characterized in that the component (10) has an average grain size between 0.8 μm and 20 μm, preferably between 1 μm and 10 μm.

15. Object according to one of the four preceding claims, characterized in that the object is a device for generating light, the object being designed such that the component (10) for the purpose of emitting light an electrical voltage and / or an electric field can be suspended.