DE102016010974A1 - Material conversion arrangement and method - Google Patents

Abstract

The invention relates to a material conversion arrangement (2), in particular for growing crystals, comprising: a furnace (4) having a furnace chamber (9); a receiving vessel (10) arranged in the furnace chamber (9) for receiving a material (12) or a starting material; a heating device (6, 26) for heating the furnace chamber (9), wherein the heating power of the heating device is adjustable by means of a power control signal; a temperature sensor (14) for measuring the temperature of the receptacle (10) and / or the material (12) or the starting material and outputting a temperature signal corresponding to the temperature; and a control unit (22) for adjusting the heating power of the heater (6, 26) and having an input for receiving the temperature signal. The control unit (22) is designed to: - determine from the temperature signal a temperature anomaly or change caused by a phase transition of the material (12) or the starting material, and - the power control signal for adjusting the heating power as a function of the detected temperature anomaly or temperature change of the temperature signal adjust. Furthermore, the invention relates to a method for converting a material or starting material.

Description

The invention relates to a material conversion arrangement, in particular for growing crystals, and to a method for material conversion or synthesis.

In conventional crystal growing systems, the temperature of the crystal material is generally increased far beyond the phase transition temperature, and then, starting from the very high temperature, the furnace temperature is lowered very slowly. This avoids uncertainties regarding the actual phase transition temperature. Slow cooling is time and energy intensive.

In known crystal materials with a known degree of contamination, phase transition temperatures are measured and published, which are determined, for example, by means of DTA or DSC methods.

It is an object of the invention to provide a material conversion assembly and method in which the conversion efficiency or crystal growth efficiency is increased.

This object is achieved with the features of claim 1 and 10, respectively. Advantageous embodiments are the subject of the dependent claims.

According to claim 1, a material synthesis arrangement is provided, in particular an arrangement for growing crystals. The assembly comprises: a furnace having a furnace space; a receptacle arranged in the furnace chamber for receiving a material or a raw material; a heater for heating the oven space, wherein the heating power of the heater is adjustable by means of a power control signal; a temperature sensor for measuring the temperature of the receptacle and / or the material and the output material and outputting a temperature signal corresponding to the temperature; and a control unit for adjusting the heating power of the heater. The control unit has an input for the temperature signal and is designed to: determine from the temperature signal a temperature anomaly or change caused by a phase transition of the material or the starting material; and adjust the power control signal for adjusting the heating power depending on the detected temperature anomaly or temperature change of the temperature signal.

This is generally under 'material conversion' application-dependent and the 'material synthesis' to understand.

Most preferably, the 'material' is a material having a phase transition between the material and the starting material. In an embodiment, the material and the starting material may be the same material, which differ only in that the solid or crystalline phase has changed structurally or z. B. the starting material is amorphous, while the (end) material is crystalline. In one embodiment, one or more starting materials are present as the starting material, and the (end) material is formed by (thermal) synthesis through the thermal treatment by means of the material conversion arrangement or by means of the material conversion process. During the thermal treatment, the phase state of the starting material and / or (end) material changes, in particular the starting material and / or the material is solid (especially crystalline) and the material is liquid (or less) in an intermediate state of the material during the thermal treatment preferably vice versa). In the synthesis, intermediates that are not included in the material can escape from the starting material (eg, hydrogen).

Advantageously, the temperature sensor is either directly in contact with the material or the starting material (consequently also with the material of the intermediate state) or in the best possible thermal contact. For example, the temperature sensor does not contact the material / starting material (in order to avoid reactions with the temperature sensor material and / or not to disturb a crystallization process) and is arranged on or in a recess of the receptacle as close as possible to the vicinity of the material.

Preferably, the phase transition of the material (or the starting material) causes a temperature change or a temperature anomaly. Preferably, the material and / or the starting material at the phase transition to a so-called phase transition temperature at the z. B. due to the transition necessary or released latent heat, a temperature anomaly occurs. The temperature anomaly occurring during the phase transition is preferably a temperature change or deviation from an expected temperature profile without the presence of the phase transition. Specifically, the temperature anomaly is a non-change of the temperature despite the change of the heating power or the oven temperature.

Example of a temperature anomaly: At a certain heat output, an equilibrium sets in and thus a constant temperature. If the heating power is increased at a temperature at the phase transition, the temperature of the material (or of the starting material) initially does not increase ( Temperature increase of the material is delayed), since the supplied energy is dissipated into the transition enthalpy (increase of the latent heat). Conversely, if the heating power is lowered at a temperature at the phase transition, the temperature of the material initially does not decrease (the temperature decrease of the material is delayed), since the energy released to the environment is taken from the transition enthalpy or latent heat.

In an advantageous embodiment, the furnace chamber is designed to be flooded during the thermal treatment with a gas (eg inert gas or inert gas) or to be evacuated.

According to one embodiment, the control unit (or a computing unit arranged or integrated therein) advantageously has an analog-to-digital converter (ADC) in order to convert the normally analogue temperature signal into a digital signal for further digital evaluation.

In an advantageous embodiment, the receptacle is a crucible and / or the receptacle has a bore or depression in which the temperature sensor is inserted. The recess may be a groove or bore or a longer channel in or through the crucible material. In this case, the sensor, in particular a resistance sensor, can be arranged as long as possible in the vicinity of the material or in contact with the material. Preferably, the bore or recess is such that there is no (direct physical) contact between the sensor and the material, but in such a way that the thermal coupling is as good as possible. For example, a recess (eg, groove) may (at least partially) extend around the crucible, e.g. B. circular or spiral along the outer wall of the receptacle along. Preferably, the distance of the temperature sensor installed in the crucible is at least partially smaller than 0.5 mm.

In an embodiment, the crucible or the receptacle is electrically insulating and / or thermally well conductive.

Preferably, the temperature sensor is glued with an adhesive in the bore or recess of the receptacle. More preferably, the adhesive is high temperature resistant and / or thermally well conductive and / or electrically insulating.

In one embodiment, the heating device has power electronics and at least one heating element, wherein the power electronics emits the electric power supplied to the heating element as a function of the power control signal. The power electronics, for example, an inverter, a power supply with phase control gate, a power supply with pulse width control or the like. The heating device can also be or have a combustion heater, for example a gas burner in which the gas supply can be set by means of the power control signal.

Preferably, the (average) output power of the power electronics is proportional to the power control signal.

Advantageously, the arrangement has a signal processing unit with an input for supplying the temperature signal and / or with an output for outputting the processed temperature signal, wherein the input of the control unit, the processed temperature signal is supplied.

According to one embodiment, the signal processing unit performs a phase-sensitive signal amplification and / or has a lock-in amplifier or is a lock-in amplifier. For example, the temperature sensor is supplied with a signal (eg current / voltage at resistance temperature sensor) which is modulated, so that the phase-sensitive signal amplification (noise suppression) can be carried out on the basis of the modulated signal. By means of the phase-sensitive signal amplification, for example, interference voltages or currents z. B. suppressed by the furnace heating.

Advantageously, the arrangement has a reference material arranged in the furnace chamber or a reference measuring point arranged in the furnace chamber and a reference temperature sensor. This records the temperature of the reference material or the reference measuring point.

The reference temperature sensor outputs a reference temperature signal corresponding to the reference temperature. Further advantageously, the control unit has an input for the reference temperature signal and determines a caused by a phase transition of the material or the starting material temperature change as a function of the temperature signal of the material or starting material and the reference temperature signal. Preferably, the reference temperature signal is also processed by one or the signal processing unit (see above). In an embodiment, the reference measuring point can be arranged at a location at which a furnace temperature sensor is arranged and / or the reference temperature sensor is or an oven temperature sensor.

By means of the reference temperature signal, a differential thermal signal is determined, for example, from which then the onset or the current state of the phase transition is determined. One such differential thermal signal is known from differential thermal analysis (DTA) - cf. DIN 53765 ,

The temperature sensor and / or the reference temperature sensor are preferably a resistance sensor, a measuring wire, an NTC sensor or a thermocouple. This is particularly preferably a resistance sensor. If a resistance sensor (also measuring wire, NTC) is used, the length is preferably kept short and extends only a few millimeters out of the receptacle (eg crucible).

Preferably, the ends of the temperature sensor (eg, resistance sensor) are connected to low-resistance conductors (in terms of ohms per unit length), so that the change in resistance of the sensor with leads is essentially determined by the resistance sensor itself.

In an advantageous embodiment, the material or the starting material is in the solid state and during the thermal heating of the phase transition to the liquid state of the (intermediate) material takes place. In a further embodiment, the solid state of the (final) material is crystalline or the material has crystals. In an embodiment, the starting material is solid and may have (compared to the (end) material) smaller crystallites or a polycrystalline structure.

According to claim 10, a method for controlling the heating power in a material conversion arrangement is provided, in particular in an arrangement for growing crystals and / or in an arrangement as described above and below. Advantageously, the material conversion arrangement comprises: a furnace having a furnace space; a receptacle arranged in the furnace chamber for receiving a material or a raw material; a heater for heating the oven space, wherein the heating power of the heater is adjustable by means of a power control signal; a temperature sensor for measuring the temperature of the receptacle and / or the material and the output material and outputting a temperature signal corresponding to the temperature; and a control unit for adjusting the heating power of the heater. Advantageously, the method comprises the steps of: changing the temperature in the furnace chamber; Detecting the temperature of the material; Determining a temperature anomaly or deviation; and adjusting the heating power as a function of the determined temperature anomaly or change of the temperature signal.

• – Einstellen eines zeitlichen Temperaturverlaufs mittels eines Temperaturprofils, das in Abhängigkeit der ermittelten Temperaturanomalie oder -änderung ausgewählt ist,
• – in einem vorgegebenen Temperaturbereich um die Temperatur der Temperaturanomalie oder -änderung erfolgt eine sehr langsame Änderung der Ofentemperatur,
• – außerhalb eines oder des vorgegebenen Temperaturbereichs um die Temperatur der Temperaturanomalie oder -änderung erfolgt eine schnelle Änderung der Ofentemperatur, insbesondere oberhalb und/oder unterhalb des vorgegebenen Temperaturbereichs,
• – in einem oder dem vorgegebenen Temperaturbereich um die Temperatur der Temperaturanomalie oder -änderung erfolgt eine zyklische Temperaturerhöhung und -erniedrigung, wobei insbesondere die zyklische Temperaturerhöhung oder -erniedrigung im Mittel zu einer sehr langsamen Änderung der Ofentemperatur führt.

In an embodiment of the method, the heating power is set as a function of the determined temperature anomaly or change according to one or more of the following steps:

• Setting a temporal temperature profile by means of a temperature profile selected as a function of the determined temperature anomaly or change,
• In a predetermined temperature range around the temperature of the temperature anomaly or change takes place a very slow change in the furnace temperature,
• Outside of or the predetermined temperature range around the temperature of the temperature anomaly or change takes place a rapid change of the oven temperature, in particular above and / or below the predetermined temperature range,
• - In one or the predetermined temperature range around the temperature of the temperature anomaly or change occurs a cyclic temperature increase and decrease, in particular, the cyclic temperature increase or decrease on average leads to a very slow change in the furnace temperature.

• – nach unten durch Tph –1%, –2%, –3%, –5%, –8%, –10%, –15% oder –20% (Beispiel: Tph –10% bei Tph = 1000°C ist ein unterer Wert von 900°C), und/oder
• – nach oben durch Tph +1%, +2%, +3%, +5%, +8%, +10%, +15% oder +20%.

The temperature change may be a temperature increase or preferably a decrease. Advantageously, the predetermined temperature range includes or includes the temperature Tph at which the temperature anomaly or change occurs. Further advantageously, the temperature range is limited

• Down through Tph -1%, -2%, -3%, -5%, -8%, -10%, -15% or -20% (Example: Tph -10% at Tph = 1000 ° C a lower value of 900 ° C), and / or
• - upwards by Tph + 1%, + 2%, + 3%, + 5%, + 8%, + 10%, + 15% or + 20%.

A cyclical increase and decrease in temperature preferably takes place within a corridor about an average, which may be constant (on average without temperature change), may fall, or may rise (on average a temperature increase).

Herein the terms 'very slow' and 'very fast' are related.

Advantageously, the change in the temperature of the furnace is over a range including the temperature Tph of the expected phase transition; and / or changing the temperature of the furnace, passing through the temperature range with the temperature increase phase transition once, twice, three times or more than three times, and / or changing the temperature of the furnace, passing through the temperature range with the temperature reduction phase transition once, twice, three times or more than three times.

The determination of the temperature Tph of the phase transition can be carried out by or comprises a function of the one-time, two-time, determining the temperature of the phase transition three times or more during a temperature increase and / or during a temperature reduction.

The above-disclosed features of the invention can be combined with each other in any combination or sub-combination, in particular, process features with device features (or vice versa) in any combination or sub-combination can be combined. Moreover, the features or combinations of features disclosed above may be combined with any feature of the detailed or basic description described below, individually or in any sub-combination.

With reference to figures, an embodiment of the invention will be explained in more detail. The figures show:

1 an arrangement of a crystal growing system in a schematic representation,

2 a detailed view of the attachment of a resistance wire to an electrode,

3a a side cross-sectional view of a in the arrangement of 1 inserted crucible in the lower area,

3b the view of the bottom of the crucible from below,

4 a temporal temperature profile during the heating process with averaged gradients,

5a a method for temperature control in crystal growth, and

5b an illustration of a modification of the method of 5a ,

The schematic representation of 1 shows in block diagram the control components of a crystal growing arrangement or plant 2 and their high temperature oven 4 in cross section. The oven 4 has a furnace chamber 8th who have a stove room 9 surrounds. The oven is heated by a heating element 6 heated, for example, one around the chamber 8th Spiral heating coil or a variety of around the chamber 8th arranged heating rods are. When displayed in 1 insulating elements of the furnace are not shown and the chamber 8th may for example be composed of wall elements such as firebricks.

In the oven room 9 is a crucible 10 arranged, which is a predetermined amount of a starting material 12 receives. The crucible material is preferably inert to the starting material and does not react with it. The starting material 12 For example, it may be an amorphous phase or crystallites or a powder or mixture of the material to be produced by the thermal process (may also be referred to as a 'sample'). The starting material and the material may be identical, but differ for example in crystallization (eg multicrystalline / monocrystalline) and / or in purity and / or in homogeneity. Preferably, the arrangement for the production of single crystals and / or for the modification of single crystals is used.

At the crucible 10 is a temperature sensor 14 which is preferably a resistance sensor in which the resistance changes in a known manner as a function of the temperature of the sensor. Further preferred is the sensor 14 a resistance wire with high resistance (compared to the leads). The temperature sensor is preferred 14 in a recess 44 (see below) of the crucible used and arranged as close to the inner wall of the crucible. According to one embodiment and if the material of the sensor 14 not with the material 12 or reacting raw material and / or influences its crystallization, the sensor 14 also directly with the material 12 or starting material are brought into contact.

In the example shown, the ends of the temperature sensor 14 each with a ladder 16 connected, which has a much lower resistance than the sensor 14 have temperature resistant and the electrical conduction of the sensor signal to the area outside the chamber 8th provide or provide. The ends of the ladder 16 outside the chamber 8th are each with a line 18 connected, the lines 18 the sensor signal a signal amplifier 20 respectively. The signal amplifier 20 is, for example, a phase-sensitive amplifier, preferably a lock-in amplifier. The signal amplifier 20 amplifies that from the sensor 14 incoming temperature signal and / or suppresses or filters the signal superimposed on the signal noise.

The output signal of the signal amplifier 20 becomes a control unit 22 the arrangement 2 supplied and in this the signal is preferably a processor unit 24 fed. The control unit may have analog elements, so that in the presence of the signal from the signal amplifier 20 in analog form, the signal is processed and evaluated analogously and the power control signal (see below) is 'calculated' in an analogous manner. Alternatively, the signal from the signal amplifier 20 converted into a digital signal by means of an analog-to-digital converter (ADC), which is then stored in the control unit 20 or in the processor unit 24 digital is processed. The A / D conversion of the signal may be in the processor unit 24 , in the control unit 22 or in the signal amplifier 20 respectively. In an embodiment, the signal amplifier 20 in the control unit 22 be integrated. Hybrid processing of the temperature signal, in which the signal in the control unit or in the processor unit is partially processed and evaluated in analog form and partly in digital form, is also possible.

As a result of the signal processing and evaluation, a power control signal is generated by the control unit 22 to a controllable power supply 26 is issued. In the example shown, the power supply is disconnected from the grid 30 (eg AC 230 V) and there is power over the line 28 to the heating element 6 which is controlled according to the power control signal.

The control by the control unit 22 can serve in certain phases of the conversion of the starting material to the material (and possibly the material to the starting material) as a control loop, a setpoint for the temperature is predetermined and regulated in the form of a feedback between the setpoint temperature and the actual temperature, the temperature to the target temperature becomes. In the phase of the control in which, for example, a phase transformation between starting material and material takes place, it is preferable to control the heating power and / or a control using the temperature signal from the furnace sensor 13 made, since in the presence of a phase transition of the energy input through the heater does not directly lead to a change in the temperature of the material or starting material (see below).

The temperature of the furnace 4 comes with a temperature sensor 13 which is a thermocouple or also a resistance temperature sensor. The temperature value of the sensor 13 can be used as oven temperature and / or reference temperature with respect to the sensor 14 measured material temperature Tsens in the control of the control unit 22 (or processor unit 24 ) are used (see below).

2 shows a detailed view of an exemplary connection between one end of the sensor 14 and one of the leaders 16 , The end of the sensor, which is a resistance wire here, gets around the conductor 16 wound several times (eg in helix form 34 ) and with an electrically conductive and temperature-resistant adhesive 36 or another bonding agent (eg silver solder or brazing solder) fixed to the conductor.

3a shows the lower part of the crucible 10 in cross-section and 3b shows an outside view of the floor 42 from underneath. The crucible is made of sidewall 40 and soil 42 educated. In the ground 42 is on the bottom of a depression or groove 44 educated. In the groove 44 is the temperature sensor 14 taken in and with a glue 46 attached. The adhesive is preferably an electrically insulating, temperature-resistant adhesive and / or a good thermal conductivity adhesive. For example, ceramic adhesive with high thermal conductivity can be used.

4 shows a temperature-time diagram of the temperature profile during heating of a starting material (or material), in which around the time c around the relevant phase transition, for. B. from the solid to the liquid phase of a preferably crystalline material in the solid state starts. In this case, the heating element 6 from the power supply 26 under the control of the control unit 22 supplied with a constant electric power, which would lead to a straight-line temperature profile in a material without phase transition, as indicated by the dotted (ideal) line Tid. The curve Tid corresponds to the linear temperature increase in the furnace chamber 8th by the constant power supply (here it is assumed that at these temperatures and due to the heating rate, no balance between supplied power and heat loss has occurred). Due to the phase transition solid or crystalline to liquid at the time c (or by the temperature Tph +) occurs during the temperature curve Tsens of the starting material, the temperature increase until a time delay, since the material 12 supplied heat is converted into latent heat.

In 4 is indicated by the dotted curve Thy that when the furnace is cooled 4 with linearly falling furnace temperature (reverse time course) during the phase transition (at the temperature Tph-) from liquid to solid or crystalline the temperature of the material (the liquid 'material' is converted back to the crystalline 'material') during this phase transition only falls off with a time delay because the latent heat is released here. In addition, hysteresis can occur between the heating and cooling during the course of the temperature. The phase transition temperature during heating and cooling can be different due to the material and / or the characteristic of the temperature profiles during heating and cooling (for example, not mirror-symmetrical to the curve Tid) can be different. Or, due to metrology or heating technology, different characteristic curves or phase transition temperatures may result. Hereinafter, the phase transition temperature during heating is designated Tph + and that on cooling is Tph-, where in 4 is drawn that they are at different temperatures.

For crystal growth, the liquidus phase transition is very particularly preferred relevant phase transition, which is referred to as Tph-, since at this temperature transition Tph- use the nucleation and growth of the desired phase. Hereinafter, the phase transition temperature during heating is designated Tph + and that on cooling is Tph-, where in 4 is drawn that they are at different temperatures.

Further shows 4 on the basis of the broad arrows the gradient of the temperature at the times a, b, c and d. The gradients are determined on the basis of average values of the temperature, whereby any fluctuations (noise) of the measured temperature signal occurring are averaged out or suppressed. As indicated by the slope of the arrows, the lowest gradients during the temperature increase or decrease are at the temperatures Tph + and Tph-, respectively, so that during a temperature increase / decrease operation by the determination of the lowest gradients the temperature Tph + and the Temperature Tph- the phase transition and the temperature anomaly can be determined.

With regard to the metrological determination of the phase anomaly on the basis of the deviation between the measured temperature curve Tsens and the ideal temperature curve Tid, it should be noted that the ideal course, for example, results from a reference measurement of the oven temperature (sensor 13 ) is known or can be estimated. Or the characteristic of the temperature profile of the furnace during heating and / or cooling is known per se, so that the anomaly can be determined on the basis of the estimation of the ideal temperature profile. The ideal temperature profile can be linear, as in 4 represented or curved (nonlinear increase or decrease). In any case, these temperature changes are small compared to the phase transition deviations.

5a FIG. 12 shows a flow chart for a crystal growth process, preferably using the above-described crystal growth assembly 2 , In the method, the phase transition temperature is first determined, and based on the determined phase transition temperature or temperature Tph, an optimized crystallization process is performed.

In step S2, the oven 4 heated at a constant rate, for example, by constant power. By way of example, such heating is associated with 4 described. The phase temperature for the material and / or under the apparatus conditions of the arrangement 2 are not exactly known, but the material is usually an expectation Tph ~ the phase temperature, so that the heating for determining the phase temperature, for example up to a furnace temperature T = Tph ~ + 10% (or + 15%) takes place. The measured during heating (S2) material temperature Tsens is evaluated in step S4 and from a first phase temperature Tph + (1) of the heating determined. In step S6, the furnace is cooled at a constant rate. For cooling, the heating element 6 be shut off or it is by means of the control or regulation by the control unit 22 Cooled with a predetermined temperature and / or heating power profile. Preferably, the cooling to a temperature below the previously determined first phase temperature T = Tph + (1) - 10% (or - 15%).

The material temperature Thy recorded during the cooling (S6) is evaluated in step S8 and the phase temperature Tph- of the cooling is determined therefrom. After step S8, in step S10, the material is reheated to a temperature just above the first phase temperature Tph + (1), for example, to a temperature T = Tph + (1) + 5%. In step S12, a second phase temperature Tph + (2) of the heating is determined from the measured material temperature Tsens.

In step S14, a 'computational' phase temperature Tph is determined which is one of the determined temperatures Tph + (1), Tph + (2) or Tph-, a function of two of the determined temperatures (eg, average of Tph + (1) and Tph + (2)), or a function of the three temperatures determined may be Tph = f (Tph + (1), Tph + (2), Tph-).

Based on the calculated phase temperature Tph then the furnace is cooled extremely slowly to a temperature of z. T = Tph - 50%, so that crystal formation is optimized during this extremely slow cooling. Since the extremely slow cooling process begins just above the phase temperature Tph and ends below the phase temperature Tph, it is not necessary to go through a wider temperature range under extremely slow cooling due to the now well known phase temperature, so that the crystal growth process is considerably shortened and on the concentrated in the relevant temperature range.

Since (depending on the properties of the material) the crystal growth can continue until well below the phase temperature Tph, in the embodiment the extremely slow cooling takes place for example up to Tph - 50%, Tph - 70%, Tph - 40% or Tph - 30%. For material systems where the temperature transition range is smaller until complete solidification, the temperature at which the temperature is cooled extremely slowly can be Tph-20%, Tph-10% or Tph-5%. When completely frozen the nucleation and consequently the germ selection are finished.

In step S18, a rapid cooling takes place after the temperature has reached below Tph at S16. Instead of fast cooling, the crucible can also be used 10 from the oven 4 and the solution is poured off or centrifuged so that the crystal is easier to remove. Similarly to the liquidus temperature, the solidus temperature, Ts, where all the material is in a solid state (or some other characteristic signature signaling the end of growth of the desired phase) can also be determined. This allows subsequent reheating to a temperature just above Ts (eg, after melting the sample under inert gas in a container suitable for centrifugation) followed by centrifuging the melt solution.

Since, as mentioned above, for the selective selection of the temperature range in which the very slow cooling takes place, the liquidus phase transition upon cooling is Tph-critical, it can be provided in an embodiment that this phase transition is achieved by repeated rapid cooling from a range T> Tph-. after T <Tph-, while the value Tph- (for Tph) is determined. For example, the value can be determined twice, three times or five times or two to five times and then averaged.

Here too, the advantage of the method is to be emphasized, since the temperature data for the phase transition (solidus / liquidus), which are often known from DTA or DSC measurements, are normally measured during the heating process. Due to hypothermia, however, the phase transition temperatures Tph can be up to 20% below the transition temperatures Tph +.

5b shows a variation of step S16, wherein in a narrow range to Tph cyclic heating and cooling takes place, but on average still continues to cool. Under the control of the control unit, the temperature is cooled extremely slowly from just above Tph (eg from Tph + 5%) to a temperature immediately before Tph (rate 1). For example, the temperature immediately before Tph is a temperature of Tph + 2%. Thereafter, it is heated cyclically and with very low temperature strokes for a short time and cooled, the temperature strokes are in a temperature corridor whose mean corresponds to a cooling. Preferably, the average cooling rate (rate 2) during these cycles is still lower than during the first cooling phase (ie rate 2 <rate 1). This cyclical cooling takes place up to a temperature immediately below Tph (for example up to Tph - 2% as shown, or until Tph - 5%). Thereafter, the cooling is again carried out with an (ideally) linear course up to the temperature just below Tph (eg T = Tph - 50%, see above). Subsequently, the rapid cooling takes place as described at step S18 and in 5b shown schematically.

The cyclic cooling promotes the formation of a single crystallite and the impurity diffusion away from growing crystallites.

Basics and other arrangement and process details

The following describes the principles of the temperature control according to the invention and the material conversion and / or synthesis arrangement. The features disclosed below may be combined individually or in any sub-combination or combination with the features of the arrangement or method disclosed above in the introduction to the description and / or the detailed description. Conversely, the features disclosed below can be combined with any of the features disclosed above in the description introduction and / or the detailed description or sub-combination or combination of features.

Basics of temperature control

The temperature control in conventional furnaces is done by measuring the temperature, comparing it with a setpoint and the corresponding adjustment of the heating power. The state of the heated material itself and its possible reaction to a temperature change are not taken into account. If a phase transition, for example, the solidification of a melt during slow cooling, targeted and well-controlled, the corresponding transition temperature from previous investigations must already be known and set the temperature profile of the furnace accordingly. The determination of such (phase) transition temperatures as a function of the composition of a substance mixture (hereinafter referred to as material) is carried out by standard means of differential thermal analysis (DTA) or differential scanning calorimetry (DSC). While both processes-determination of transition temperature and actual material conversion - have so far been strictly separated, they can in the inventive arrangement 2 from oven 4 and control 20 - 26 be carried out simultaneously. The control of the process parameters, in particular the heating power of the furnace 4 , is thus coupled to the reaction (phase transition) of the material. The resulting possibilities and advantages over the conventional Procedures are further explained herein, particularly with respect to single crystal growth from melt solutions. However, since phase transitions are relevant in almost all heating processes / anneals, the described invention is applicable to a variety of other techniques.

State of the art

• – die Temperatur am Ort des Ofen-Thermometers (vgl. 13) kann deutlich von der Temperatur am Ort des Materials abweichen,
• – das Ofen-Thermometer (z. B. 13) unterliegt Alterungseffekten, die zu signifikanten Abweichungen zwischen nomineller und tatsächlicher Temperatur führen können,
• – die Phasenübergangstemperatur kann signifikant von Verunreinigungen beeinflusst sein, deren Konzentration sich deutlich von der während der in einem separaten Verfahren und Messapparatur durchgeführten DTA/DSC-Messung vorliegenden unterscheiden kann,
• – die Unterkühlung und deren Abhängigkeit von Verunreinigungen beeinflusst die Phasenbildung beim Abkühlen (Kristallisation aus der Schmelze),
• – für eine Vielzahl von mehr-komponentigen Verbindungen liegt kein Phasendiagramm vor oder beinhaltet die relevante Zusammensetzung nicht.

Phase transition temperatures can be determined by DTA / DSC with high accuracy, eg. For example, the liquidus and solidus temperatures of alloys (see above). Small-volume crucibles (eg approx. 5 mm ³ ) are preferably used in order to ensure the most homogeneous possible temperature over the entire material volume. In addition to the crucible containing the material, a second (reference) crucible is used in conventional DTA / DSC. The detection of phase transitions is always based on the temperature difference (or the suppression of these by an energy input to be measured) between the two crucibles. Information obtained in this way on phase formation is compiled in the form of phase diagrams and published for a large number of binary alloys ( TB Massalski, "Binary Alloy Phase Diagrams," American Society for Metals, Materials Park, Ohio (1990) ). The transfer of the known from DTA / DSC measurements transition temperatures on the crystal growth in a conventional oven, however, is limited:

• - the temperature at the location of the furnace thermometer (see 13) may differ significantly from the temperature at the location of the material,
• The oven thermometer (eg 13) is subject to aging effects which can lead to significant deviations between nominal and actual temperature,
• The phase transition temperature may be significantly influenced by impurities whose concentration may differ significantly from that present during the DTA / DSC measurement carried out in a separate process and measuring apparatus,
• The subcooling and its dependence on impurities influences the phase formation during cooling (crystallization from the melt),
• For a plurality of multi-component compounds, there is no phase diagram or contain the relevant composition.

The condition of a material is thus not clearly characterized by the temperature indicated on the oven thermometer. Should z. For example, when a single crystal is grown from a melt solution, the liquidus temperature must first be significantly exceeded and the melt subsequently cooled slowly. The aforementioned uncertainties make it necessary to start the (slow) cooling process usually already 100 ° C above the nominal liquidus temperature.

Taking into account a possible supercooling of the melt by up to 20%, it may take several days for the crystallization process to start at a cooling rate of 2 ° C./h. However, even during nucleation, an even lower cooling rate would be desirable to prevent coalescence of the crystallites and to obtain as few, large single crystals as possible. Time and energy consumption as well as a possible, slow attack of the crucible material by the melt solution limit the cooling rate accordingly.

The adaptation of common DTA / DSC equipment to the needs of material conversion, and particularly single crystal growth, would be inherently difficult. However, the coupling to the furnace control has not been taken into consideration and technically unworkable in the known apparatuses. In addition, the conventional design requires immersion of the thermocouple in the melt, which can lead to undesirable reactions and adversely affect nucleation and crystal growth. In general, the reference crucible required for DTA / DSC requires more than double the size of the furnace space and inevitably leads to an asymmetric temperature gradient.

Operation and structure of the invention

In addition to the above description is to be further carried out: The detection of phase transitions in the arrangement described above 2 preferably does not take place via a differential measurement (between material and reference crucible), but based on anomalies in the time course of the material temperature as described above. The temperature is proportional to the electrical resistance of a thin wire (measuring wire 14 ), in the immediate vicinity of the material 12 is appropriate. For this purpose, the crucible bottom 42 with a notch 44 provided and the wire with a high-temperature adhesive 46 attached, which has good thermal conductivity but high electrical resistance. The distance from wire 14 to material 12 (= Interior of the crucible 10 ) is preferably less than 0.5 millimeters. Thus, an excellent thermal coupling without direct contact between wire and material is given.

In an embodiment it can be provided that also in the inventive arrangement 2 for further signal improvement, a reference signal of the temperature by means of a second temperature sensor (eg 13 measuring the oven temperature) and whose measurement is not subject to phase transition, is used for normalization or signal enhancement. Preferably, and as mentioned, however, the phase temperature determination in the invention is performed without a reference temperature measurement, so that only one (and only one) temperature sensor 14 is provided.

The size of the anomaly caused by a phase transition in the crystal growth process over the course of time of the temperature is usually only a fraction of the respective absolute value (about 10 ^-2 to 10 ^-4 ). The measurement of such small changes is possible, for example, by phase-sensitive measurement of the electrical resistance by means of lock-in technology - see the lock-in amplifier in the example above 20 , The voltage across the measuring wire (proportional to the electrical resistance) is thereby measured with a relative resolution of <10 ^-5 at a sampling rate of about 25 points per second and is available in real time for analyzing and controlling the heating power (by the control unit) 22 ) ready. Optional also allows one on the bottom of the crucible 42 attached thermocouple (not shown), a calibration of the voltage drop across the measuring wire with the (absolute) temperature.

For contacting the measuring wire 14 become temperature-resistant, metallic rods 16 (Tungsten, diameter ~ 3 mm) into the oven chamber 8th guided and with a special, further explained below, bonding technology with the thin measuring wire (tungsten, diameter ~ 20 microns) connected. The electrical resistance is proportional to the cross section of the conductor and thus through the measuring wire 14 dominates (99.9%), about half of its length with the crucible 10 thermally connected. The metallic bars 16 can be used as an integral part of the oven 4 considered and after detachment of the measuring wire 14 be reused. The oven chamber 8th can be evacuated and flooded with inert gas.

As technically challenging, the stable contacting of the measuring wire has 14 with the metallic bars 16 proved that at high temperatures (previously up to 1000 ° C) are claimed without causing disturbing anomalies in electrical resistance. For this purpose, measuring wire and rod were using conductive, silver-based paint 36 bonded. After drying, a thin layer of corundum adhesive was applied over the paint.

Elements of the invention (innovations)

Finds a phase transition during a synthesis or conversion process in the conversion array 2 instead, this can be directly detected and used without loss of time to adapt the process parameters, in particular the heating power.

The condition of a material 12 is thus not only due to the nominal temperature in the furnace chamber 8th , but also determined by the onset or absence of a characteristic signature (eg Tph-). This is realized by the application of phase-sensitive resistance measurement and the coupling to the heating power of the furnace to this measurement or the control of the heating power as a function of the temperature signal.

Advantages and improvements

The in situ detection of nucleation during single crystal growth from a melt solution allows unprecedented control of the temperature profile during crystal growth. On the one hand, the temperature can be lowered very slowly after the onset of crystallization. This allows optimal diffusion of solute towards the growth front of the single crystal. Similarly, the accumulation of impurities on the growth front is suppressed. On the other hand, the temperature can be driven cyclically or oscillating around the liquidus temperature (cf. 5b ), which leads to the re-dissolution of smaller and undisturbed growth of larger crystallites (seed selection). The single crystals thus obtained are characterized by size, lower defect density and fewer adhesions.

Undesirable reactions and phase transitions due to excessive or too low temperatures can be detected, suppressed in the initial stage and, if reversible, reversed by rapidly increasing / decreasing the heating power. Even in the non-reversible case, the information obtained serves to characterize the material and to optimize subsequent experiments.

• – Übliche Thermoelemente zeigen eine Auflösung von 40 μV/Kelvin (die Thermospannung ist dabei eine Materialkonstante). Über den Messdraht 14 im Rückkopplungsofen werden bis zu 300 μV/Kelvin erreicht, wobei eine weitere Optimierung durch Anpassen von Stromstärke und Drahtgeometrie möglich ist.
• – Die über den Messdraht 14 abfallende Spannung kann phasen-sensitiv ausgelesen werden, was ein hohes Signal-zu-Rausch-Verhältnis ermöglicht. Vor allem wird die Spannungsmessung nicht durch die elektrischen Felder der Heizstäbe 6 des Ofens 4 verfälscht. Eine phasensensitive Messung der Spannung von Thermoelementen ist dagegen prinzipiell nicht möglich (da diese auf Diffusionsprozessen der Ladungsträger basiert und nicht extern angeregt wird).
• – Da eine gute thermische Ankopplung der Messstelle erforderlich ist, müssen Draht 14 bzw. Thermoelement sorgfältig an den Tiegel 10 geklebt werden und sind somit nur einmal verwendbar. Dies führt im Falle des Thermoelementes zu einem erheblichen Kostenfaktor (zweistelliger Euro-Betrag), während der Preis für den Messdraht im Cent-Bereich liegt.
• – Durch geeignete Konstruktion des Messdrahtes 14 kann dieser neben seiner Funktion als Thermometer auch als Kältefinger genutzt werden. Die Kristallisation (z. B. aus Schmelzlösungen) und die damit verbundene Freisetzung latenter Wärme setzt somit bevorzugt in unmittelbarer Nähe der Messstelle ein, was die Detektion der entsprechenden Anomalie im Temperaturverlauf erleichtert.

In addition to the in situ applicability, there are further advantages compared to the conventional thermometric methods used in the analysis of phase transitions:

• - Typical thermocouples show a resolution of 40 μV / Kelvin (the thermoelectric voltage is a material constant). Over the measuring wire 14 in the feedback furnace up to 300 μV / Kelvin can be achieved, with further optimization by adjusting the current and wire geometry is possible.
• - The over the measuring wire 14 decaying voltage can be read phase-sensitive, which allows a high signal-to-noise ratio. Above all, the voltage measurement is not due to the electric fields of the heating elements 6 of the oven 4 falsified. A phase-sensitive measurement of the voltage of thermocouples is in contrast, not possible in principle (since this is based on diffusion processes of the charge carriers and is not excited externally).
• - Since a good thermal coupling of the measuring point is required, must wire 14 or thermocouple carefully to the crucible 10 are glued and are therefore only usable once. In the case of the thermocouple, this leads to a considerable cost factor (two-digit euro amount), while the price for the measuring wire is in the cent range.
• - By suitable construction of the measuring wire 14 This can be used in addition to its function as a thermometer as a cold finger. The crystallization (eg from melt solutions) and the associated release of latent heat thus preferably takes place in the immediate vicinity of the measuring point, which facilitates the detection of the corresponding anomaly in the course of the temperature.

state of development

The functionality of the feedback furnace was based on a first experimental setup, consisting of furnace chamber 8th , Heater 6 , Lock-in amplifier 20 and corresponding wiring. Detection of nucleation has been achieved in various alloy systems (Bi-Ni, Bi-Pd, Bi-Mn, In-Pd, In-Au, In-Sb), with the molar concentration of the second component being 5-10%. For the binary alloy Ni-Bi, the single crystal growth of Bi ₃ Ni was optimized on the basis of the detected liquid-state signal Tph +. The resulting single crystals proved to be significantly larger, less fused and more clearly faceted than those obtained by monotonic cooling.

LIST OF REFERENCE NUMBERS

2

Material Conversion Arrangement (Crystal Growing Plant)

4

High-temperature furnace

6

Heating element (heating rods)

8th

furnace chamber

9

furnace

10

crucible

12

Crystal / melt

13

Temperature sensor (oven temperature)

14

Resistance wire (temperature sensor)

16

ladder

18

cables

20

Lock-in amplifier (signal amplifier or phase-sensitive amplifier)

22

control unit

24

processor unit

26

power adapter

28

supply

30

mains connection

34

winding

36

conductive adhesive

40

Side wall

42

ground

44

groove

46

Glue

tph

Phase transition temperature (averaged)

tph +

Phase transition temperature during heating

TPH

Phase transition temperature on cooling

Tsens

Temperature course during heating

Thy

Temperature course during cooling

Tid

linear temperature profile

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• DIN 53765 [0023]
• TB Massalski, "Binary Alloy Phase Diagrams", American Society for Metals, Materials Park, Ohio (1990) [0071]

Claims (13)

Material conversion arrangement ( 2 ), in particular for growing crystals, comprising: a furnace ( 4 ), which has a furnace room ( 9 ), one in the furnace room ( 9 ) receiving vessel ( 10 ) for recording a material ( 12 ) or a starting material, a heating device ( 6 . 26 ) for heating the oven space ( 9 ), wherein the heating power of the heating device is adjustable by means of a power control signal, a temperature sensor ( 14 ) for measuring the temperature of the receptacle ( 10 ) and / or the material ( 12 ) and the output material and for outputting a temperature signal corresponding to the temperature, and a control unit ( 22 ) for adjusting the heating power of the heating device ( 6 . 26 ), the control unit ( 22 ) has an input for receiving the temperature signal and is designed to: - from the temperature signal by a phase transition of the material ( 12 ) or the starting material to determine temperature anomaly or change, and - to adjust the power control signal for adjusting the heating power as a function of the determined temperature anomaly or temperature change of the temperature signal. Arrangement according to claim 1, wherein the receptacle ( 10 ) is a crucible, and / or wherein the receptacle ( 10 ) a bore or depression ( 44 ) into or into which the temperature sensor ( 14 ) is used. Arrangement according to claim 1 or 2, wherein the heating device ( 6 . 26 ) a power electronics ( 26 ) and at least one heating element ( 6 ), wherein the power electronics ( 26 ) the heating element ( 6 ) delivers supplied electric power in response to the power control signal. Arrangement according to Claim 1, 2 or 3, with a signal processing unit ( 20 ) having an input for supplying the temperature signal and having an output for outputting the processed temperature signal, wherein the input of the control unit ( 22 ) the processed temperature signal is supplied. Arrangement according to claim 4, wherein the signal processing unit ( 20 ) is designed to perform a phase-sensitive signal amplification, and / or has a lock-in amplifier or is a lock-in amplifier. Arrangement according to one of the preceding claims, with one in the furnace chamber ( 9 ) arranged reference material or one in the furnace chamber ( 9 ) and a reference temperature sensor ( 13 ) for detecting the temperature of the reference material or the reference measuring point and outputting a reference temperature signal corresponding to the reference temperature, wherein the control unit ( 22 ) has an input for the reference temperature signal and determines a change in temperature caused by a phase transition of the material or the starting material as a function of the temperature signal of the material or starting material and the reference temperature signal. Arrangement according to one of the preceding claims, wherein the temperature sensor ( 14 ) and / or the reference temperature sensor ( 13 ) is a resistance sensor, a measuring wire, an NTC sensor or a thermocouple. Arrangement according to one of the preceding claims, wherein the material ( 12 ) or the starting material in the solid state and the phase transition is a phase transition from the solid state to the liquid state of the material or starting material. Arrangement according to one of the preceding claims, wherein the material ( 12 ) is crystalline or has crystals, preferably is a single crystal, and the starting material has a multiplicity of crystallites, is polycrystalline or amorphous, and / or the material ( 12 ) is the synthesis product from the starting material, wherein the starting material comprises at least the starting materials of the material preferably in the stoichiometry of the synthesized material. Method for controlling the heat output in a material conversion arrangement ( 2 ), in particular arrangement for growing crystals and / or arrangement according to one of the preceding claims, wherein the material conversion arrangement comprises: a furnace ( 4 ), which has a furnace room ( 9 ), one in the furnace room ( 9 ) arranged receiving vessel ( 10 ) for recording a material ( 12 ) or a starting material, a heating device ( 6 . 26 ) for heating the oven space ( 9 ), wherein the heating power of the heater is adjustable by means of a power control signal, a temperature sensor ( 14 ) for measuring the temperature of the receptacle ( 10 ) and / or the material ( 12 ) and the output material and for outputting a temperature signal corresponding to the temperature, and a control unit ( 22 ) for adjusting the heating power of the heating device ( 6 . 26 ), the method comprising: changing the temperature in the furnace chamber ( 9 ) Detecting the temperature of the material, determining a temperature anomaly or deviation, and adjusting the heating power in response to the determined temperature anomaly or change in the temperature signal. The method of claim 10, wherein the adjusting of the heating power in dependence on the determined temperature anomaly or change according to one or more of the following steps: Setting a temporal temperature profile by means of a temperature profile selected as a function of the determined temperature anomaly or change, In a predetermined temperature range around the temperature of the temperature anomaly or change takes place a very slow change in the furnace temperature, - outside of or the predetermined temperature range around the temperature of the temperature anomaly or change takes place a rapid change in the furnace temperature, in particular above and / or below the predetermined temperature range, and - In one or the predetermined temperature range around the temperature of the temperature anomaly or change occurs a cyclic temperature increase and decrease, in particular, the cyclic temperature increase or decrease on average leads to a very slow change in the furnace temperature. Method according to claim 10 or 11, wherein the change in temperature of the furnace is over a range including the temperature of the expected phase transition, and / or wherein the change in the temperature of the furnace includes passing through the temperature range with the temperature-elevation phase transition once, twice, thrice or more than three times, and / or wherein the change in the temperature of the furnace includes passing through the temperature region having the temperature reduction phase transition once, twice, three times or more than three times, The method of claim 10, 11 or 12, wherein the determination of the temperature of the phase transition is a function of the one, two, three or more times determining the temperature of the phase transition during a temperature increase and / or during a temperature decrease.

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