High-energy ball mill, and method for producing thermoelectric materials using a high-energy ball mill

Abstract

The invention relates to a high-energy ball mill (1) comprising at least one mill container (3) for receiving milling material and grinding balls; a drive device (5); at least one container mounting (11) for receiving the at least one mill container; at least one holding device (9) which is provided so as to be rotatable about a rotational axis (13) and / or capable of oscillating about an oscillation axis, for supporting and entraining the container mounting (11) during operation, wherein the drive device drives the holding device (9); and a control device for controlling the drive device (5). The invention is characterized by a movement detection device (17) for detecting a movement characteristic of the holding device (9), said control device controlling the rotational speed of the drive device (5) on the basis of the detected movement characteristic.

Description

[0001] High-energy ball mill and method for producing thermoelectric materials using a high-energy ball mill

[0002] The present invention relates to a high-energy ball mill and a method for producing thermoelectric materials using a high-energy ball mill.

[0003] Components based on thermoelectric (TE) materials enable the direct conversion of heat and electrical energy due to solid-state physics effects. For example, thermogenerators can convert heat directly into electrical current and therefore have a wide range of application potential (cars, the steel and aluminum industry, waste heat utilization in aircraft, autonomous sensor technology). Due to their fast and tunable controllability, thermoelectric heat pumps are also used for temperature control (heating or cooling) in areas such as optoelectronics (semiconductor lasers) and medical technology (PCR testers). The use of alloys made of bismuth, antimony, and tellurium as (thermoelectric) functional materials is technologically and commercially established. Alternatives are being sought, particularly those that can be produced while consistently avoiding or reducing the use of elements that are classified as critical raw materials (e.g.Tellurium and some rare earths) are classified as rare earths. Materials that exhibit good functional properties in the temperature range from -50 to +300 °C have particularly high application potential, as they could replace bismuth telluride in existing products. For such materials, low-temperature process routes are often necessary or advantageous. This is the case, for example, when the starting elements have very different melting points and vapor pressures and a melting route therefore leads to the evaporation of a reactant and thus to a change in the actual material composition. Since the functional properties of thermoelectric materials are determined by point defects, changes in composition in the sub-% range can lead to a dramatic deterioration in their performance. Therefore, thermoelectric materials are often produced by mechanical alloying using ball milling (close to room temperature).It has been found that when carrying out a manufacturing process using the same or an identical ball mill, deviations in the quality of the thermoelectric materials produced can occur.

[0004] It is therefore an object of the present invention to provide a ball mill and a method for producing a thermoelectric material, wherein the quality of the thermoelectric material produced is substantially consistently high.

[0005] The invention relates to a high-energy ball mill with at least one grinding vessel for receiving material to be ground and grinding balls, with a drive device, with at least one vessel holder for receiving the at least one grinding vessel, with at least one holding device arranged so as to be rotatable about a rotation axis and / or oscillatable about an oscillation axis for holding and guiding the vessel holder during operation, wherein the drive device drives the holding device, and with a control device for controlling the drive device. The high-energy ball mill according to the invention is characterized by a movement detection device for detecting a movement characteristic of the holding device, wherein the control device controls the speed of the drive device depending on the detected movement characteristic.

[0006] In an additional or alternative embodiment of the invention, a high-energy ball mill is provided with at least one grinding vessel for receiving material to be ground and grinding balls, with a drive device, with at least one vessel holder for receiving the at least one grinding vessel, with at least one holding device arranged so as to be rotatable about a rotation axis and / or oscillatable about an oscillation axis for holding and guiding the vessel holder during operation, wherein the drive device drives the holding device, and with a control device for controlling the drive device. This embodiment of the invention is characterized by a temperature determination device for determining a temperature in the grinding vessel, wherein the control device controls the speed of the drive device as a function of the determined temperature in the grinding vessel.

[0007] In principle, it is also possible for the high-energy ball mill according to the invention to have a movement detection device for detecting a movement characteristic of the holding device and a temperature determination device for determining a temperature in the at least one grinding vessel, wherein the control device controls the rotational speed of the drive device as a function of the detected movement characteristic and the determined temperature in the grinding vessel.

[0008] It has been shown that, particularly in the production of thermoelectric materials, in addition to common parameters such as composition, grinding time, etc., the temperature prevailing in a grinding vessel during the grinding process and / or the movement characteristic of the holding device that carries the grinding vessel have a decisive influence on the quality of the material. With conventional ball mills, at best, a rough, usually manual adjustment of the speed of the drive device is possible, so adjustments to improve the quality of the material being ground are not possible with these.

[0009] The present invention now advantageously enables the speed of the ball mill to be controlled as a function of the temperature in the grinding vessel and / or a movement characteristic of the holding device to which the grinding vessel is attached. Thus, the ball mill can be controlled very precisely with regard to the temperature prevailing in the grinding vessel and / or the movement directly acting on the material to be ground.

[0010] The holding device of the high-energy ball mill according to the invention can, for example, be rotatable about a rotational axis, with the motion detection device detecting a rotational speed of the holding device as a motion index. The drive device can be connected to the holding device, for example, via a gear, via which the rotational movement is transmitted from the drive device to the holding device. The gear can, for example, be a belt drive. Because the motion index, for example the rotational speed, is determined directly at the holding device, it is independent of any wear on the gear, thus enabling very precise detection of the motion index.As a result, the high-energy ball mill according to the invention can be controlled in a particularly advantageous manner, since the movement acting directly on the grinding vessels and thus the material to be ground is detected and sources of error such as wear on a gear can be disregarded.

[0011] The holding device can, for example, comprise a flywheel, wherein the rotational speed of the flywheel is detected.

[0012] The motion detection device can, for example, comprise an optical sensor that interacts with a reflector attached to the holding device, for example, to the flywheel. In this way, the rotational speed of the holding device can be advantageously determined.

[0013] In principle, the holding device can also be designed to oscillate about an oscillation axis, and the motion detection device can record the oscillation frequency of the holding device as a motion index. The oscillation frequency of the holding device can also be measured optically, for example, using an optical sensor and a reflector attached to the holding device.

[0014] The use of an optical sensor and a reflector attached to the holding device makes it particularly easy to determine the movement index. The optical sensor can be an optical tachometer, for example.

[0015] It can be provided that the temperature-determining device determines the temperature in the grinding vessel optically. For example, the temperature can be determined directly, with a sensor arranged in a wall of the grinding vessel. The temperature can be determined optically or in another way, for example, by tactile temperature measurement. The temperature can also be determined indirectly, for example, by determining the temperature on an outside of the grinding vessel, which can be used to determine the temperature in the grinding vessel.

[0016] The invention further provides a method for synthesizing thermoelectric materials, comprising the following steps:

[0017] - Provision of raw materials

[0018] - Filling the or part of the starting materials with grinding balls into a grinding vessel of a high-energy ball mill according to the invention

[0019] - Ball milling of the charged starting materials over a predetermined period of time for mechanically alloying the starting materials, wherein during the ball milling a movement characteristic of the holding device for holding and guiding the vessel holder of the grinding vessel is determined and a speed of the drive device is controlled as a function of the determined movement characteristic.

[0020] Alternatively or additionally, it can be provided that during ball milling the temperature in the grinding vessel is determined and a rotational speed of the drive device is controlled as a function of the determined temperature.

[0021] Of course, it is possible that the temperature and the movement characteristic are determined and the speed of the drive device is controlled depending on both determined values.

[0022] By means of the methods according to the invention, the synthesis of thermoelectric materials can be carried out in a particularly advantageous manner, since the temperature in the grinding vessel and / or the movement of the grinding vessel can be monitored during ball milling and thus the speed of the drive device of the high-energy ball mill can be controlled accordingly.

[0023] For example, it can be provided that the speed of the drive device is reduced when a predetermined maximum threshold temperature is reached. Since the temperature in the grinding vessel increases at higher speeds, it can be ensured that a threshold temperature is not exceeded during the synthesis of thermoelectric material, for example.

[0024] It can also be provided, for example, that when a predetermined minimum threshold temperature is reached, the speed of the drive device is increased so that a minimum temperature is maintained in the grinding vessel. This makes it possible, for example, to maintain a temperature window in the grinding vessel.

[0025] The method according to the invention can also provide for the speed of the drive device to be reduced or kept constant upon reaching a predetermined maximum movement index and / or for the speed of the drive device to be increased or kept constant upon reaching a predetermined minimum movement index. Thus, the movement index can also be maintained, for example, within a range that is particularly advantageous for the synthesis of thermoelectric material.

[0026] The thermoelectric properties of the thermoelectric material can be partially expressed by the thermoelectric figure of merit. For example, it has been shown that even a slight reduction in speed of about 2%, for example below a specified minimum speed, can lead to a significant reduction in the thermoelectric figure of merit of 10% or more.

[0027] Mg, Ag, and Sb, for example, can be used as starting materials, so that the produced thermoelectric material is MgAgSb. This compound (as a thermodynamic o-phase) is an advantageous alternative to previous thermoelectric materials such as bismuth telluride, as tellurium can be omitted and good functional properties are present in the temperature range between 0 °C and 300 °C. During the synthesis of some thermoelectric materials, such as MgAgSb, unwanted phase transformations can occur at high synthesis temperatures, i.e., the material retains its nominal composition but changes its crystal structure and thus its functional properties. Therefore, the method according to the invention and the high-energy ball mill according to the invention are particularly advantageous because the desired temperature range can be maintained.

[0028] The production of thermoelectric materials also poses the fundamental problem of the creation of unwanted secondary phases that are temperature-stable. Therefore, controlling the speed of the drive device as a function of a specific temperature is advantageous.

[0029] In the synthesis of thermoelectric materials according to the method of the invention, the steps of charging the starting materials and ball milling the starting materials can also be repeated, so that, for example, a portion of the starting materials can first be charged and ground, followed by another starting material or several additional starting materials being charged and ground with the already ground starting materials. For the starting materials Mg, Ag, and Sb, for example, Mg and Ag can be ground first, followed by Sb being added.

[0030] The invention is explained in more detail below with reference to the following figure.

[0031] The single figure schematically shows a high-energy ball mill 1 according to the invention. The high-energy ball mill 1 has two grinding vessels 3 for holding the material to be ground and grinding balls (not shown). A drive device 5 is connected to a holding device 9 via a belt drive 7 and drives the holding device 9. The holding device 9 holds vessel holders 11, to which the grinding vessels 3 are attached, and guides them during operation, thereby causing the grinding movement of the grinding vessels 3. In the embodiment shown in Figure 1, the holding device 9 is rotatable about a rotational axis 13 and has a flywheel 15. By carrying the grinding vessels 3 and executing the grinding movement, the grinding balls are moved back and forth in the grinding vessel 3 and grind the material to be ground.The high-energy ball mill 1 according to the invention can be used, for example, for the synthesis of thermoelectric materials, whereby starting materials are mechanically alloyed. The drive device 5 can be controlled via a control device (not shown), so that the speed of the drive device 5 can be varied.

[0032] The high-energy ball mill 1 according to the invention has a motion detection device 17, via which a movement characteristic of the holding device 9 is detected. For example, the motion detection device 17 can have an optical sensor 19, which interacts with a reflector 21 attached to the flywheel 15 and detects the rotational speed of the flywheel 15 and thus of the holding device 9 as a movement characteristic. The high-energy ball mill 1 according to the invention can, for example, be controlled such that a predetermined rotational speed is not undershot and / or that a predetermined rotational speed is not exceeded.

[0033] Additionally or alternatively, the high-energy ball mill 1 according to the invention can also have a temperature-determining device 23 for determining a temperature in the grinding vessel 3. The control device can also control the rotational speed of the drive device 5 depending on the determined temperature in the grinding vessel 3. In principle, the control can also be carried out depending on both the determined temperature and the determined movement characteristic.

[0034] The high-energy ball mill 1 according to the invention can advantageously be used to produce, for example, the thermoelectric material MgAgSb. For this purpose, Mg, Ag, and Sb are first provided as starting materials. These are then filled into the grinding vessels 3 of the high-energy ball mill 1 according to the invention together with grinding balls. The drive device 5 is then driven, so that the starting materials are ball-milled, resulting in a mechanical alloying of the starting materials. During ball milling, the rotational speed of the holding device 9 on the flywheel 15 is measured via an optical sensor 19 of the motion detection device 17. The drive device 5 is controlled depending on the detected rotational speed, so that a desired rotational speed of the holding device 9 is maintained or the rotational speed of the holding device 9 remains within a predetermined rotational speed window.In this way, the synthesis of the thermoelectric material is advantageously possible, since during the synthesis the holding device 9 is driven at a speed which partially avoids a reduction of the thermoelectric figure of merit.

[0035] List of reference symbols

[0036] High-energy ball mill

[0037] grinding vessel

[0038] drive device

[0039] Belt drive

[0040] Holding device

[0041] vessel holder

[0042] axis of rotation

[0043] flywheel

[0044] Motion detection device optical sensor

[0045] reflector

[0046] Temperature determination device

Claims

Patent claims 1. High-energy ball mill (1) with at least one grinding vessel (3) for holding material to be ground and grinding balls, with a drive device (5), with at least one vessel holder (11) for holding the at least one grinding vessel, with at least one holding device (9) arranged so as to be rotatable about an axis of rotation (13) and / or so as to be oscillatable about an oscillation axis for holding and carrying the vessel holder (11) during operation, wherein the drive device drives the holding device (9), and with a control device for controlling the drive device (5), characterized by a movement detection device (17) for detecting a movement characteristic of the holding device (9) during operation, wherein the control device controls the speed of the drive device (5) as a function of the detected movement characteristic.

2. High-energy ball mill according to claim 1, characterized in that the holding device (9) is rotatable about the rotational axis (13) and the movement detection device (17) detects a rotational speed of the holding device (9) as a movement characteristic.

3. High-energy ball mill according to claim 2, characterized in that the holding device has a flywheel (15), wherein the rotational speed of the flywheel (15) is detected.

4. High-energy ball mill according to claim 2 or 3, characterized in that the movement detection device (17) has an optical sensor (19) which cooperates with a reflector (21) fastened to the holding device (9).

5. High-energy ball mill according to claim 1, characterized in that the holding device (9) is capable of oscillating about an oscillation axis and the movement detection device (17) detects a frequency of the oscillation of the holding device (9) as a movement characteristic number.

6. High-energy ball mill (1) according to the preamble of claim 1 or according to one of claims 1 to 5, characterized by a temperature determining device (23) for determining a temperature in the at least one grinding vessel (3) during operation, wherein the control device controls the speed of the drive device (5) as a function of the determined temperature in the at least one grinding vessel (3).

7. High-energy ball mill according to claim 6, characterized in that the temperature determining device (23) optically determines the temperature in the at least one grinding vessel (3).

8. High-energy ball mill according to claim 6 or 7, characterized in that the temperature determining device (23) determines the temperature in the at least one grinding vessel (3) directly, wherein a sensor is arranged in a wall of the grinding vessel (3) or that the temperature determining device (23) determines the temperature in the at least one grinding vessel (3) indirectly via a measurement of the temperature on an outer side of a wall of the at least one grinding vessel (3).

9. A process for the synthesis of thermoelectric materials, comprising the following steps: - Provision of raw materials - filling the or part of the starting materials and grinding balls into at least one grinding vessel (3) of a high-energy ball mill (1) according to one of claims 1 to 8, - ball milling of the charged starting materials over a predetermined period of time for mechanically alloying the starting materials, wherein during the ball milling a movement characteristic of the holding device (9) for holding and carrying the vessel holder (11) of the grinding vessel (3) is determined and a speed of the drive device (5) is controlled as a function of the determined movement characteristic.

10. Method according to claim 9, characterized in that the rotational speed of the drive device (5) is reduced or kept constant when a predetermined maximum movement characteristic is reached.

11. Method according to claim 9 or 10, characterized in that the rotational speed of the drive device (5) is increased or kept constant when a predetermined minimum movement characteristic is reached.

12. A process for the synthesis of thermoelectric materials, comprising the following steps: - Provision of raw materials - filling the or part of the starting materials and grinding balls into a grinding vessel of a high-energy ball mill (1) according to one of claims 6 to 8, - ball milling of the introduced starting materials over a predetermined period of time for mechanically alloying the starting materials, wherein during the ball milling the temperature in the at least one grinding vessel (3) is determined and a rotational speed of the drive device (5) is controlled as a function of the determined temperature.

13. Method according to claim 12, characterized in that the rotational speed of the drive device (5) is reduced or kept constant when a predetermined maximum threshold temperature is reached.

14. Method according to claim 12 or 13, characterized in that the rotational speed of the drive device (5) is increased or kept constant when a predetermined minimum threshold temperature is reached.

15. Process according to one of claims 9 to 14, characterized in that the starting materials are Mg, Ag, and Sb.

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