A three-dimensional heat flux sensor for differential scanning calorimetry and its preparation process

Abstract

The present invention discloses a three-dimensional heat flux sensor for differential scanning calorimetry and its preparation process. In the sensor body of the present invention, platinum-platinum rhodium thermocouple materials are used to print surface electrodes on the upper and lower surfaces of a ceramic substrate, and axial electrodes are inkjet printed on the side walls of the sensor to connect the surface electrodes, forming a three-dimensional thermopile structure. A differential line is printed on the sensor base of the present invention for differentially connecting the potential difference signals of the sample-end heat flux sensor and the reference-end heat flux sensor, and a thin-film platinum resistor for measuring the temperature of the sensor base is also printed. The heat flux sensor utilizes inkjet printing of thin-film electrodes on the side walls, which can increase the printing density of the thermopile on the upper and lower surfaces of the heat flux sensor and improve the heat flux measurement sensitivity of the sensor. The heat flux sensor body and the base are supported by a ceramic fine cylinder, separating a small gap between the two, and a solder paste is uniformly filled in the gap, and reliable connection between the two is achieved through soldering.

Description

Technical Field

[0001] The present invention belongs to the field of thin film electronic printing, and particularly relates to a three-dimensional heat flux sensor for differential scanning calorimetry and a preparation process thereof. Background Art

[0002] As a typical analytical test instrument, a differential scanning calorimeter (DSC) is widely used in thermodynamic research, material composition analysis, and thermal property testing in related fields such as materials, food, medicine, and chemical engineering. In recent years, with the booming development of new material research, the differential scanning calorimeter has become an indispensable scientific instrument in popular research such as phase change energy storage materials, green chemical thermodynamics, polymer glass transition and crystallization mechanism, lithium battery materials, biomolecules, thermal stability of energetic materials, and thermal properties of aviation materials.

[0003] According to different measurement methods, DSC can be divided into power compensation type and heat flux type. The power compensation type DSC measures the relationship between the power difference required at both ends of the sample and the reference sample to meet the condition that the temperature at the sample end and the reference end remains the same over time. The heat flux type DSC measures the relationship between the heat flux difference between the sample end and the reference end and temperature or time under a programmed temperature environment and a constant flow atmosphere environment provided for the substance. Compared with the power compensation type DSC, the heat flux type DSC has the advantages of stable baseline and high sensitivity. Therefore, the market share of the heat flux type DSC is much larger than that of the power compensation type DSC at present.

[0004] Currently, the leading international manufacturers of heat flux type DSCs include TA Instruments in the United States, Mettler Instruments in Switzerland, Netzsch Instruments in Germany, Setaram Instruments in France, etc. The heat flux detection sensitivity is one of the core indicators for evaluating the performance of a differential scanning calorimeter, and the heat flux sensor is the key component that determines the heat flux detection sensitivity. Among them, the disk type heat flux sensor of Mettler Instruments in Switzerland prints a high-density gold-gold palladium thermopile on a ceramic substrate, making the minimum heat flux detection sensitivity reach 0.02 μW [1] . However, this heat flux sensor is limited by the disk type structure and cannot solve the crosstalk problem between the sample end and the reference end. The tower type heat flux sensor of TA Instruments in the United States is formed by machining a whole piece of constantan. There is a nickel-chromium disk under each of the sample end and the reference end, and a thermocouple is integrated on each disk to measure the temperature difference between the sample end and the reference end [2]Since there is only a pair of thermocouples on this sensor, TA improves the thermal flux detection sensitivity by increasing the thermal resistance of the sensor, that is, minimizing the sum of the thickness of the constantan substrate and the thickness of the side wall of the tower-shaped boss as much as possible. However, this places very high requirements on the mechanical manufacturing process and will greatly increase the production cost of a single sensor. Moreover, increasing the thermal resistance of the sensor will reduce the resolution of the thermal flux measurement, that is, thermal accumulation will form on the thermal flux sensor, which is not conducive to the experimental data analysis of DSC.

[0005] In recent years, Hangzhou Yangyi Technology Co., Ltd. has proposed a thermopile thermal flux sensor for a tower-shaped differential scanning calorimeter. By means of 3D structure design, the number of nodes of the thermopile is increased, and the sensitivity of the thermal flux sensor to measure the thermal flux transmitted along the axis is improved. [3] However, during the preparation process of this sensor, a large number of through holes need to be drilled on the ceramic substrate by laser, which will consume a lot of production time and cost. Especially when a ceramic substrate with a high thermal conductivity is selected, the production cost will increase exponentially. Among the sensors currently used to measure the axial thermal flux on DSC, due to the problems of low structural accuracy of printed thin-film sensors and the difficulty of patterning the sensor structure on the micro-circular surface, the side wall space of the thermal flux sensor substrate has not been fully utilized. This limits the improvement of the printing density of the thermopile on the surface of the thermal flux sensor and is also not conducive to the improvement of the sensitivity of the DSC tower-shaped thermal flux sensor.

[0006] In summary, aiming at the limitation that the thermal flux detection sensitivity and resolution of the current tower-shaped thermal flux type DSC cannot be improved simultaneously, the present invention proposes a high-sensitivity three-dimensional thermopile thermal flux sensor for a tower-shaped differential scanning calorimeter. Aiming at the difficulties in patterning the micro-circular surface of the existing thin-film sensors, the present invention proposes a preparation process for printing the axial thin-film electrodes on the side walls of the tower-shaped thermal flux sensor.

[0007] References

[0008] [1] Thomas Hüttl, Bernd Dammal, Urs Niedermann. Thermal analysis sensor and method for manufacturing a thermal analysis sensor [P]. Chinese Patent: 200410087946, 2005-5-4.

[0009] [2] Robert L. Danley. Differential scanning calorimeter sensor and method [P]. US Patent: 747057B2, 2008-12-30.

[0010] [3] Wu Yuxin, Fang Weizhen. A thermopile thermal flux sensor for a tower-shaped differential scanning calorimeter [P]. Chinese Patent: 201911094994, 2021-01-29. Summary of the Invention

[0011] Aiming at the deficiencies of the prior art, the present invention provides a three-dimensional heat flux sensor for a tower-type differential scanning calorimeter and a preparation process thereof.

[0012] Based on the basic structure of the DSC tower heat flux sensor, the present invention realizes the measurement of the axial heat flux of the heat flux sensor by inkjet printing axial thin-film electrodes on the side wall, increases the printing density of the thermopile, and improves the sensitivity of heat flux detection. A ceramic material with a high thermal conductivity is selected as the sensor substrate material, which can effectively reduce the thermal hysteresis generated on the heat flux sensor and improve the resolution of heat flux measurement of the heat flux sensor. By integrating a high-precision rotary stepping motor, a lead screw, a gear transmission mechanism and a high-definition camera on a traditional inkjet printing workbench, high-precision printing of the axial thin-film electrodes of the heat flux sensor is achieved at a relatively low production cost and production time.

[0013] The present invention includes a heat flux sensor body, which is composed of a sample-end heat flux sensor and a reference-end heat flux sensor. The structures of the two are exactly the same, in a cylindrical shape, and are fixed at positions equidistant from the center of the sensor base.

[0014] An axial through hole is provided in the center of the cylinder for positioning the sensor during inkjet printing; a circular groove slightly larger in diameter than the axial through hole is also provided in the center of the cylinder for mating with the ceramic fine cylinder on the heat flux sensor base to fix the installation position of the heat flux sensor on the base.

[0015] Equal amounts of platinum-platinum rhodium 13% surface electrodes are screen-printed on the upper and lower surfaces of the substrate. Thin-film electrodes with corresponding polarities are inkjet printed on the side wall of the cylinder to connect the electrodes on the upper and lower surfaces. These electrodes are connected end to end to form a three-dimensional thermopile structure. The thermopile is used to measure the temperature difference between the upper and lower surfaces of the heat flux sensor, and the heat flux difference transmitted along the axis of the heat flux sensor body can be measured according to the thermal resistance on the sensor substrate. The sensitivity of heat flux detection of the heat flux sensor is improved by integrating a high-density thermopile on the heat flux sensor body.

[0016] The present invention also includes a heat flux sensor base. Differential lines are printed on the base for differentially connecting the thermal electromotive forces caused by the temperature differences between the upper and lower surfaces of the sample-end heat flux sensor and the reference-end heat flux sensor; a circular thin-film platinum resistor for measuring the temperature of the sensor base is printed, making the measurement accuracy of the temperature of the sensor base higher. In addition, there are 2 positioning through holes on the base for filling and supporting the fine cylinders; 1 straight through hole for leading out 4 leads of the thin-film platinum resistor; 1 through hole for leading out the common lead on the differential line; 2 through holes for leading out the tail electrode lead-out wires of the sample-end heat flux sensor and the reference-end heat flux sensor; 2 internal threaded holes for assembling with the DSC furnace body structure.

[0017] The support between the heat flux sensor body and the base by a fine ceramic cylinder separates the two with a tiny gap, and the gap is uniformly filled with solder paste. Then, reliable connection between the two is achieved through soldering, and the thermal resistance between the heat flux sensor body and the base is small and controllable, which significantly helps improve the resolution of heat flux detection.

[0018] The beneficial effects of the present invention are as follows:

[0019] 1. The three-dimensional thermopile heat flux sensor designed for the tower-type heat flux differential scanning calorimeter in the present invention greatly reduces the heat flux crosstalk between the sample end and the reference end compared with the disk-type heat flux sensor, and can effectively reduce the thermal resistance on the sensor, improving the resolution of heat flux detection. Compared with the heat flux sensor formed by welding a single pair of thermocouples using the thermocouple substrate as the base material; the heat flux sensor designed in the present invention uses a ceramic material as the base, which has a higher thermal conductivity and can effectively reduce the heat accumulation on the heat flux sensor, further improving the resolution of heat flux detection.

[0020] 2. By inkjet printing thin-film thermocouple electrodes on the side wall of the heat flux sensor base to cascade the thermopiles on the upper and lower surfaces, a three-dimensional thermopile structure is formed to realize multi-point temperature measurement on the upper and lower surfaces of the heat flux sensor. The above three-dimensional structure can eliminate the occupation of the surface space of the heat flux sensor base by the connecting electrodes, making it possible to increase the density of the thermopile nodes printed on the base surface. By increasing the density of the thermopile nodes on the base surface, the sensitivity of heat flux detection is improved. Compared with the tower-type heat flux sensor formed by welding a single pair of thermocouples, without reducing the resolution of heat flux detection, the tiny temperature difference between the sample end and the reference end can be quickly measured, effectively improving the sensitivity of heat flux detection, up to 0.05 μW.

[0021] 3. The sensor axial electrode inkjet printing process involved in the present invention, compared with the process of axially drilling holes in the base and pouring thermocouple slurry, while ensuring high-precision printing, can greatly compress the production time and production cost of the heat flux sensor under mass production. Description of the Drawings

[0022] Figure 1 Overall perspective view of the heat flux sensor;

[0023] Figure 2 Front view of the sample end heat flux sensor structure;

[0024] Figure 3 Side view of the sample end heat flux sensor structure;

[0025] Figure 4 Front perspective view of the heat flux sensor base structure;

[0026] Figure 5Overall sectional view of the heat flux sensor;

[0027] Figure 6 Schematic diagram of the sample end heat flux sensor with 2 thermopile junctions on both the upper and lower surfaces;

[0028] Figure 7 Schematic diagram of the differential connection of the thermopile of the sample end heat flux sensor with 2 thermopile junctions on both the upper and lower surfaces and the reference end heat flux sensor;

[0029] Figure 8 Schematic diagram of the principle of the lead wire of the heat flux sensor.

[0030] Legend:

[0031] Figure 1 , 1.1 Sample end heat flux sensor; 1.2 Reference end heat flux sensor; 1.3 Sensor base.

[0032] Figure 2 , 2.1 Positioning through hole; 2.2 Surface platinum-rhodium 13% electrode; 2.3 Surface platinum electrode; 2.4 Thermopile junction; 2.5 Axial platinum-rhodium 13% electrode; 2.6 Axial platinum electrode; 2.7 Head electrode of the thermopile of the heat flux sensor.

[0033] Figure 3 , 3.1 Cylindrical assembly positioning groove 3.2 Tail electrode of the thermopile of the heat flux sensor.

[0034] Figure 4 , 4.1 Differential line; 4.2 Starting point of the differential line; 4.3 End point of the differential line; 4.4 Welding pin of the differential line; 4.5 Through hole for the differential line to lead out; 4.6 Platinum resistance thermometer of the sensor base; 4.7 Welding pin for the lead wire of the platinum resistance; 4.8 Straight slot hole for the lead wire of the platinum resistance to lead out; 4.9 Through hole for the tail lead wire of the thermopile of the heat flux sensor; 4.10 Fixed through hole for the thin cylindrical support; 4.11 Internal threaded hole.

[0035] Figure 5 , 5.1 Sensor body; 5.2 Sensor base; 5.3 Through hole for the tail lead wire of the sensor thermopile; 5.4 Fixed through hole for the thin cylindrical support; 5.5 Thin cylindrical support; 5.6 Solder paste. Specific implementation mode

[0036] To make the steps, technical solutions and advantages of the examples of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be described more clearly, in detail and completely below with reference to the accompanying drawings in the examples of the embodiments of the present invention.

[0037] The design solution adopted by the present invention is:

[0038] The overall perspective view of the three-dimensional thermopile heat flux sensor for a tower-type differential scanning calorimeter described in the present invention is asFigure 1 As shown. In the heat flux sensor body, the sample-end heat flux sensor 1.1 and the reference-end heat flux sensor 1.2 are evenly distributed on both sides of the symmetry axis of the sensor base 1.3, and are respectively brazed to the sensor base into one body with a brazing material of Bi2O3 and B2O3 with a molar ratio of 50:50 under the condition of 600 - 700 °C, realizing a reliable connection between the heat flux sensor body and the sensor base.

[0039] In the structural design of the sensor body: the sample-end heat flux sensor and the reference-end heat flux sensor have exactly the same structure and are cylindrical. Figure 2 is the front view of the structure of the sample-end heat flux sensor, Figure 3 is the side view of the structure of the sample-end heat flux sensor. There is a positioning through-hole 2.1 with a diameter of 0.8 mm in the center of the heat flux sensor base, which is used to sequentially and neatly connect the sensor semi-finished products with the upper and lower surface electrodes printed by screen printing in series on the rotating shaft of the inkjet printing device, facilitating the inkjet printing of the electrodes on the side wall. A cylindrical groove 3.1 with a diameter of 1.5 mm and a depth of 1 mm is opened on the lower surface of the heat flux sensor, which is used to wedge in a thin cylindrical body for support and assemble with the sensor base. The surface platinum-rhodium 13% electrode 2.2 (the mass fraction of rhodium is 13% and the mass fraction of platinum is 87%) and the surface platinum electrode 2.3 on the surface of the sensor body base are cuboid, with a length of 1.5 mm, a width of 0.1 mm, and a screen-printed film thickness of 10 μm. The angle formed by each surface electrode and the straight line connecting the center point of each electrode to the center of the sensor base is 5 degrees, and the angle formed by the extension lines of two adjacent like-pole electrodes is 24.5 degrees. The surface electrodes of the two materials intersect to form thermopile junctions 2.4, and the intersection angle of the two electrodes forming each thermopile junction is 15 degrees. In the present invention, the number of thermopile junctions on the upper and lower surfaces of a sample-end or reference-end heat flux sensor is equal, both being 14, and these nodes are all on the circumference at a distance of 1.5 mm from the center of the base. One end of each surface electrode forms a thermopile junction with the adjacent unlike-pole electrode, and the other end is correspondingly connected to the axial like-pole electrode on the side wall. The axial platinum-rhodium 13% electrode 2.5 and the axial platinum electrode 2.6 printed on the side wall of the sensor body are both cuboid, with a length of 2.5 mm, and the width and thickness are equal to the width and thickness of the upper and lower surface electrodes, and are inkjet-printed on the side wall of the sensor base. The different polarities of the axial electrodes are arranged alternately and are all parallel to the central axis of the base, so that the thermopiles on the upper and lower surfaces of the sensor can be connected in series. By printing a high-density thermopile on the heat flux sensor body, the sensitivity of heat flux detection can be improved. The head electrode 2.7 of the entire thermopile is located on the side wall of the sensor and forms a reliable contact with the differential line printed on the sensor base. The tail electrode 3.2 is located on the lower surface of the sensor, is appropriately widened, and is welded to the like-pole lead-out wire by laser spot welding. The lead-out wire passes through the through-hole reserved on the sensor base and is finally connected to the heat flux measurement circuit.

[0040] In the sensor body, the sample-end heat flux sensor and the reference-end sensor have a base diameter of 6.0 mm and a thickness of 2.5 mm. The base material of the sensor body can be ceramics with a higher thermal conductivity than alloys, such as alumina, zirconia, aluminum nitride, etc. Among them, aluminum nitride ceramic is preferably used, which has a higher thermal conductivity, can reduce the heat accumulation on the sensor, and improve the resolution of the heat flux measurement of the heat flux sensor.

[0041] In the structural design of the sensor base: The sensor base is in the shape of a flat cylinder. Figure 4 Figure 4.1 is a front perspective view of the sensor base structure. A differential line 4.1 is printed on the sensor base, with a width of 0.5 mm and a screen-printed film thickness of 0.1 mm. The starting point 4.2 of the differential line is located at the intersection of the head of the thermopile on the side wall of the sample-end heat flux sensor and the base; the ending point 4.3 of the differential line is located at the intersection of the head of the thermopile on the side wall of the reference-end heat flux sensor and the base. The starting and ending points of the differential line are reliably connected to the electrodes on the side wall of the sensor through electrode widening treatment. The overall trend of the differential line is zigzag, and the middle of the line has welding pins 4.4 for facilitating the welding of the same-pole leads. The external lead is connected to the measurement circuit through the differential line lead-out hole 4.5. In particular, an external wire for measuring the temperature difference between the upper and lower surfaces of the sample-end sensor and an external wire for measuring the temperature difference between the upper and lower surfaces of the reference-end sensor are in a shared relationship. The temperature-measuring platinum resistance 4.6 of the sensor base is in a circular ring shape, with a screen-printed film thickness of 10 μm, a pole width of 0.1 mm, an inner radius of the circular ring of 1.55 mm, and an outer radius of 1.65 mm. According to the four-wire measurement resistance principle of the platinum resistance, four welding pins 4.7 are led out at the head and tail of the arc for welding the platinum electrode leads, which is convenient for static calibration of the platinum resistance. The circular thin-film platinum resistance can more truly reflect the temperature on the sensor base compared with the thermocouple with a single measurement node, has a higher temperature measurement accuracy, and is easy for static calibration. The sensor base includes straight slot holes 4.8 for leading out the leads of the temperature-measuring platinum resistance of the sensor base; through holes 4.9a for leading out the tail leads of the thermopile of the sample-end heat flux sensor and through holes 4.9b for leading out the tail leads of the thermopile of the reference-end heat sensor; through holes 4.10a and 4.10b for filling and supporting the thin cylinder; and internal thread holes 4.11a and 4.11b for assembling with the DSC furnace body structure.

[0042] In the sensor base described above, the base diameter is 24.0 mm and the thickness is 2.5 mm. The base material of the sensor base is the same as that of the sensor body.

[0043] For a DSC differential heat flow sensor, the consistency of the sensor is a particularly critical indicator. In the sensor designed in the present invention, if the heat flow sensor body and the sensor base are closely attached in a conventional manner and the brazing paste is directly applied around the sensor body, such a fixing method will make the contact thermal resistance between the sensor body and the sensor base uncontrollable, and this thermal resistance is directly affected by the surface roughness of the lower surface of the sensor body and the upper surface of the sensor base. To overcome this problem, the present invention solves it by separating the sensor body from the sensor base by a small gap through the support of a short cylinder and filling the uniformly mixed solder paste. This solution can not only reduce the thermal resistance between the sensor body and the base to a controllable range, but also achieve a reliable connection between the sensor body and the base. The overall cross-sectional view of the sensor is as shown in Figure 5 shown below. This design solution will be described with the aid of this figure. 5.1 is the sensor body, and 5.2 is the sensor base. The through holes 5.3 for the tail leads of the thermopile and the through holes 5.4 for fixing the thin cylindrical support are pre-opened on the sensor base. Subsequently, the thin cylindrical support 5.5 for support is fixed on the sensor base. Then, the solder paste is evenly applied at the contact position between the sensor body and the base, and the specific application position is as shown in 5.6. Then, the sensor body with the leads welded is fixed to the base, and the excess solder paste is scraped off so that the solder paste is evenly distributed within the cylinder covered by the sensor body. Compared with the conventional solution of directly brazing the sensor body and the base, the thermal resistance between the sensor body and the sensor base is greatly reduced, and the magnitude repeatability is controllable, which helps to improve the resolution of heat flow detection.

[0044] The small gap in the above-described solution is 1 mm. The diameter of the thin cylindrical support for support is 1.5 mm, the height is 4.5 mm, and the material is the same as that of the sensor base.

[0045] The following content is the derivation of the heat flow measurement principle of the heat flow sensor described in the present invention and the principle of improving the heat flow measurement sensitivity.

[0046] According to the working principle of DSC, the DSC heat flow sensor needs to measure the temperature difference

[0047] ΔT = T S - T R (1)

[0048] where T S is the temperature of the heat flow sensor at the sample end of DSC, and T R is the temperature of the heat flow sensor at the reference end of DSC. Since the sample amount required for the DSC experiment is in the milligram level, ΔT is a small quantity.

[0049] In the present invention, the thermoelectromotive force output by the sample-end heat flux sensor is actually formed by the temperature difference between the thermopile nodes on the upper surface and the lower surface of the sample-end heat flux sensor. Similarly, the thermoelectromotive force output by the reference-end heat flux sensor is actually formed by the temperature difference between the thermopile nodes on the upper surface and the lower surface of the reference-end heat flux sensor.

[0050] Let the temperature of the node on the upper surface of the sample-end heat flux sensor be T Sn , and the temperature of the node on the lower surface be T bn ; the temperature of the node on the upper surface of the reference-end heat flux sensor be T Rn , and the temperature of the node on the lower surface be T bn ’, n represents the number of nodes of the thermopile on the upper surface. Let T Sn >T bn , T Rn >T bn . Since DSC sample experiments are often carried out under conditions of linear heating or cooling, and the object of linear temperature control is the temperature of the furnace body. Considering that there is a good heat conduction path between the DSC furnace body and the sensor, the heat flux sensor is in a quasi-steady state condition during the experiment. Therefore, it can be considered that the temperature of the node on the lower surface of the sample-end heat flux sensor is equal to the temperature of the node on the lower surface of the reference-end heat flux sensor, that is, T bn =T bn ’. Therefore, the expression for the temperature difference ΔT n between the sample-end heat flux sensor and the reference-end heat flux sensor, both with n thermopile nodes on the upper and lower surfaces, is

[0051]

[0052] To obtain the temperature difference ΔT n between the sample-end heat flux sensor and the reference-end heat flux sensor, both with n thermopile nodes on the upper and lower surfaces, the following formula derivation is carried out. Taking the sample-end heat flux sensor as an example, the temperature difference T Sn -T bn between the nodes on the upper and lower surfaces of the sample-end heat flux sensor in the present invention is obtained by reversely connecting in series the thermopiles with n thermopile nodes on both the upper and lower surfaces. Figure 6 Fig. is a schematic diagram of the sample-end heat flux sensor with 2 thermopile nodes on both the upper and lower surfaces. The thick solid line represents the positive electrode material of the thermocouple, and the thin solid line represents the negative electrode material of the thermocouple.

[0053] Let Tc be the ambient temperature where the thermopile lead-out wire is located, and S e be the Seebeck coefficient of the thermocouple. Then the output thermoelectromotive force E1 generated by the temperature difference between the head and tail of the thermopile of the sample-end heat flux sensor is:

[0054]

[0055] Similarly, the output thermoelectric potential generated by the temperature difference between the first and the last ends of the thermopile of the reference end heat flux sensor can be calculated.

[0056] In order to obtain the temperature difference between the sample end and the reference end, the present invention cascades the sample end thermopile and the reference end again. Figure 7 The figure is a schematic diagram of the differential connection between the sample end heat flux sensor thermopile and the reference end heat flux sensor, which have two thermopile nodes on the upper and lower surfaces. According to the calculation formula of the thermopile output potential difference, the output potential difference E2 generated by the differential connection between the sample end heat flux sensor thermopile and the reference end heat flux sensor, which have two thermopile nodes on the upper and lower surfaces, is:

[0057]

[0058] The above theory can be extended to the case where there are n thermocouple nodes on the upper and lower surfaces of the sample end heat flux sensor and the reference end heat flux sensor, and the ΔT in formula (2) is n It can be deduced from the above theory. According to the known number of thermopile nodes n on the upper and lower surfaces of the sensor and the Seebeck coefficient S of the platinum-platinum-rhodium 13% thermocouple at the sensor base temperature T0, e (T0), the output potential difference V between the sample end heat flux sensor and the reference end heat flux sensor with n thermopile nodes on the upper and lower surfaces can be obtained. EMF The calculation formula is:

[0059] V EMF =S e (T0) ΔT n (5)

[0060] According to Fourier's law, when the thermal conductivity, thickness and heat transfer area of the thermal resistance layer material are known, the heat flux conducted in the thermal resistance layer can be calculated by combining the temperature difference measured by the heat flux sensor. In the present invention, the thermal resistance layer is the ceramic substrate of the sensor. Assume that the thermal conductivity of the thermal resistance layer is λ, the thickness is d, and the heat transfer area is A. Then the output heat flux difference Φ between the sample end heat flux sensor and the reference end heat flux sensor is expressed as:

[0061]

[0062] The negative sign in the formula indicates that the direction of heat flow transfer is opposite to the direction of temperature gradient. Combining formula (5) and formula (6), we can get:

[0063]

[0064] In the formula, S q is the response sensitivity of the heat flow sensor.

[0065] According to formula (7), the present invention effectively improves the heat flux detection sensitivity of the heat flux sensor by increasing the number of thermopile nodes n on the upper and lower surfaces of the heat flux sensor substrate, selecting aluminum nitride ceramic with a large thermal conductivity as the material of the sensor body and the sensor base, selecting a platinum-platinum rhodium 13% thermocouple combination with a larger Seebeck coefficient instead of the traditional gold-platinum thermocouple combination, and minimizing the thickness d of the heat flux sensor while considering the mechanical strength and processability of the substrate.

[0066] As described above, since both the heat flux sensor and the furnace body are in a quasi-steady state condition during the DSC experiment, it can be considered that the temperature measured on the heat flux sensor base is equal to the DSC furnace body temperature. Therefore, the temperature at the sample end of the heat flux sensor of the present invention can be obtained by adding the temperature difference between the upper and lower surfaces measured by the heat flux sensor at the sample end and n times the temperature of the sensor base, and then dividing the sum by n (n is the number of thermopile nodes on the upper surface of the heat flux sensor at the sample end). Similarly, the temperature at the reference end of the heat flux sensor can be obtained by adding the temperature difference between the upper and lower surfaces measured by the reference heat flux sensor and n times the temperature of the sensor base, and then dividing the sum by n.

[0067] Design the lead wires of the heat flux sensor according to the above relationships. Figure 8 It is a schematic diagram of the principle of the lead wires of the heat flux sensor. The combination of lead wire 1 and lead wire 2 connected to the measurement circuit can measure the temperature difference between the upper and lower surfaces measured by the heat flux sensor at the sample end; the combination of lead wires 3, 4, 5, and 6 connected to the measurement circuit is used to measure the temperature of the sensor base; the combination of lead wire 2 and lead wire 7 connected to the measurement circuit can measure the temperature difference between the upper and lower surfaces measured by the reference heat flux sensor at the reference end; the combination of lead wire 1 and lead wire 7 connected to the measurement circuit can measure the thermal electromotive force value caused by the temperature difference obtained by subtracting the temperature difference between the upper and lower surfaces of the heat flux sensor at the sample end and the temperature difference between the upper and lower surfaces of the reference heat flux sensor at the reference end. Based on the above known quantities, basic parameters such as the temperature of the sensor base, the temperature at the sample end of the sensor, the temperature at the reference end of the sensor, and the potential difference between the sample end and the reference end of the sensor can be obtained through calculation. Combining the heat capacity and thermal resistance of the sensor at the sample end and the heat capacity and thermal resistance of the sensor at the reference end obtained from the heat flux calibration experiment, the data thermal hysteresis correction of the DSC experiment can be carried out.

[0068] In the preparation of the sensor: The present invention mainly uses a thin-film electronic printing process to prepare the thermopile on the heat flux sensor. The surface electrodes in the thermopile are printed by a screen printing process. Compared with the traditional evaporation coating and magnetron sputtering preparation processes, the prepared thin-film electrodes by electronic screen printing have good repeatability, low cost, and greatly improved production efficiency. In view of the difficulties in printing the circumferential surface patterns of current printed thin-film sensors, the present invention proposes an improved inkjet printing process. This process integrates a high-precision rotary stepping motor, a lead screw, a gear transmission mechanism, and a high-definition camera on a traditional inkjet printing workbench, and uses an industrial control computer to coordinate and control these working devices, and applies them to printing the axial electrodes of the heat flux sensor. It overcomes the problems of difficult patterning and poor precision of the thin-film sensor structure on the micro circumferential surface, can achieve high-precision printing of the axial thin-film electrodes of the heat flux sensor on the premise of consuming less production cost and production time, and the printed electrodes have good repeatability. The connection between the heat flux sensor body and the base is connected by brazing, and the solder paste is filled through the small gap formed by the assembly of the cylindrical end support columns, so that the contact thermal resistance between the two is small and controllable.

[0069] The main production process is as follows:

[0070] Preparation of the sensor body

[0071] 1. Substrate cleaning: Precision grinding, polishing, and drilling through holes (grooves) are carried out on the ceramic substrate, ultrasonic cleaning is performed using alcohol and deionized water, then it is put into the organic solvent ethyl ether butyrate to remove oil stains, and finally dried with dry nitrogen.

[0072] 2. Slurry preparation: A certain proportion of organic solvents such as acetone and alcohol are added to the platinum and platinum-rhodium 13% slurries to increase the adhesion between electrodes and between the electrode and the substrate.

[0073] 3. Screen printing plate preparation: It mainly includes steps such as stretching the screen, cleaning the screen mesh, coating and emulsifying, and drying.

[0074] 4. Printing of the upper and lower surface electrodes: Move the squeegee to scrape and press the platinum-rhodium 13% slurry. After the scraping and pressing is completed, the platinum-rhodium 13% slurry is leveled for 10 minutes, then the sensor is dried at 150 °C for 10 minutes, sintered in a muffle furnace at 1900 °C, and kept warm for 40 minutes, and finally naturally cooled to room temperature to complete the screen printing of the platinum-rhodium 13% electrode on the upper surface of the sensor. Move the squeegee to scrape and press the platinum electrode slurry. After the scraping and pressing is completed, the platinum electrode slurry is leveled for 10 minutes, then the sensor is dried at 150 °C for 10 minutes to volatilize the organic components in the film and enhance the adhesion between the film layer and the substrate. After sintering at 1790 °C in a muffle furnace for 10 minutes, it is naturally cooled to room temperature to complete the screen printing of the platinum electrode on the upper surface of the sensor. The screen printing of the lower surface electrode is the same.

[0075] 5. Sidewall Electrode Printing: Align and clamp a batch of heat flux sensors completed by screen printing axially, arrange them in sequence to form a long string, and fix them on the rotating shaft of the lead screw in the inkjet printing workbench. Control the stepping motor to drive the lead screw to rotate through the industrial control computer, and rotate the long string of heat flux sensors to the initial printing position with the positioning of the high-definition camera. Then control the micro inkjet head to first fill and spray the Pt-PtRh13% paste, and then control the stepping motor to rotate to the next spraying position of the Pt-PtRh13% paste to continue spraying, and so on until the Pt-PtRh13% paste at the entire circumferential position is sprayed, and check the spraying quality. After spraying one kind of paste, remove the sensor from the inkjet printing workbench, put it into a muffle furnace, dry it at 150 °C for 10 min first, then sinter it at a high temperature state, and keep it warm for 40 min, and then naturally cool it to room temperature. Then spray the platinum electrode paste according to the same process.

[0076] 6. Lead Wire Welding: Weld the PtRh13% couple wire with a diameter of 0.3 mm and a length of about 500 mm to the tail position of the sensor thermopile through a laser spot welder, and check the reliability after welding.

[0077] Preparation of Sensor Base

[0078] 1. Substrate Cleaning: The steps are the same as step 1 in the production of the sensor body.

[0079] 2. Screen Printing Plate Preparation: The steps are the same as step 3 in the production of the sensor body, the difference is that different pattern mask plates are required for the screen printing of the sensor base and the screen printing of the sensor body.

[0080] 3. Paste Printing: The steps are the same as step 4 in the production of the sensor body.

[0081] 4. Lead Wire Welding: First pass the PtRh13% couple wire with a diameter of 0.3 mm and a length of about 500 mm through the lead wire through hole from bottom to top, and then weld it to the differential line through a laser spot welder. The welding steps for the four platinum electrode lead wires are the same as above, and check the welding reliability after welding.

[0082] Connection and Molding of Sensor Body and Base

[0083] 1. Brazing Filler Metal Preparation: First weigh the Bi2O3-B2O3 raw material powder according to a molar ratio of 50:50, and mix it evenly. Prepare small particle glass brazing filler metal by the water quenching method, and then put the brazing filler metal into a planetary ball mill and grind it at 500 r / min for 3 - 4 hours to obtain glass brazing filler metal powder with a smaller diameter. In order to facilitate the coating of the brazing filler metal, a certain amount of terpineol is dropped into the glass brazing filler metal powder and stirred evenly to obtain brazing filler metal paste with a certain viscosity.

[0084] 2. Overall assembly: First, fill 2 thin cylinders for support into the through holes reserved on the sensor base, ensure that the lower surfaces of the thin cylinders and the sensor base are on the same plane, and bond the connection between the thin cylinders and the sensor base with high-temperature resistant glue. Then, evenly apply a layer of solder paste on the sensor base at the position where the sensor body is fixed. Place the sample-end heat flux sensor and the reference-end heat flux sensor on the base of the heat flux sensor, and ensure that the heads and tails of the differential lines on the sensor base form reliable connections with the differential line lead-out ends of the two heat flux sensors. If there is poor contact at the connection between the thermopile on the sensor body and the sensor base, conductive silver glue needs to be applied for bonding. If there is solder paste overflow, it needs to be cleaned up with tools. Use a fixture to properly fix the whole sensor to ensure good contact between the base material and the solder and prevent the base material from slipping.

[0085] 3. Overall welding: Put the fixed base material into a muffle furnace. First, heat it up to 300°C at a heating rate of 5°C / min and keep it at a constant temperature for 15 min to remove the terpineol in the solder. Subsequently, continue to heat it up to the soldering temperature of 700°C at a heating rate of 5°C / min, keep it at a constant temperature for 15 min, then cool it down to 400°C at a cooling rate of 5°C / min, and then turn off the muffle furnace and let it cool naturally to room temperature.

[0086] 4. Insulating encapsulation: The SiO2-B2O3 series glass glaze is selected in the present invention, and 5%-15% of Al2O3 and MgO are doped to improve the mechanical strength and chemical stability of the glass glaze. Evenly apply the prepared glass glaze on the upper surface of the sensor, then put it into a muffle furnace, heat it up to 800°C, keep it at a constant temperature for 30 min until the glass glaze is cured, and then cool it to room temperature.

[0087] Through the above steps, the preparation of the three-dimensional thermopile heat flux sensor for a tower-type differential scanning calorimeter described in the present invention can be completed.

[0088] In summary, the sensor of the present invention forms a three-dimensional thermopile through inkjet printing of axial electrodes and electrodes on the upper and lower surfaces, realizes the measurement of the axial heat flux of the sensor, and improves the sensitivity of the sensor's heat flux detection by increasing the node density of the thermopile. Compared with other tower-type heat flux sensors on the market, it can ensure a relatively high heat flux detection sensitivity without reducing the heat flux detection resolution, and overcomes the problem of large heat flux crosstalk between the sample end and the reference end of the traditional disk-type DSC heat flux sensor; moreover, the manufacturing process is novel, the production time is less, the production cost is relatively low, and the quality is controllable. Therefore, the present invention has great significance for the research of sensors for heat flux-type differential scanning calorimeters.

Claims

1. A three-dimensional heat flux sensor for differential scanning calorimetry, characterized in that: It includes a heat flux sensor body and a heat flux sensor base; The heat flux sensor body consists of a sample - end heat flux sensor and a reference - end heat flux sensor. The two have exactly the same structure and a cylindrical base; Equal amounts of platinum - platinum rhodium surface electrodes are screen - printed on the upper and lower surfaces of the base. There are thin - film electrodes with corresponding polarities on the side wall of the base to connect the electrodes on the upper and lower surfaces. These electrodes are connected end - to - end to form a three - dimensional thermopile structure. The three - dimensional thermopile is used to measure the temperature difference between the upper and lower surfaces of the heat flux sensor, and the heat flux difference transmitted along the axial direction of the heat flux sensor body is obtained according to the thermal resistance on the sensor base. The thin - film electrodes on the side wall of the base are in the form of inkjet printing; There is a differential line on the heat flux sensor base, which is used for differential connection of the thermal electromotive forces caused by the temperature difference between the upper and lower surfaces of the sample - end heat flux sensor and the reference - end heat flux sensor. There is a circular thin - film platinum resistor for measuring the temperature of the sensor base, making the measurement accuracy of the sensor base temperature higher; The heat flux sensor body and the heat flux sensor base are supported by a ceramic fine cylinder, separating a small gap between the two. And the gap is evenly filled with solder paste, and reliable connection between the two is achieved through soldering, and the thermal resistance between the body and the base of the heat flux sensor is small and controllable.

2. The three-dimensional heat flux sensor for differential scanning calorimetry according to claim 1, characterized in that: The sample - end heat flux sensor and the reference - end heat flux sensor are fixed at positions equidistant from the center of the sensor base.

3. The three-dimensional heat flux sensor for differential scanning calorimetry according to claim 1, wherein: The center of the base has an axial through - hole for positioning the sensor during inkjet printing.

4. A three-dimensional heat flux sensor for differential scanning calorimetry according to any one of claims 1 to 3, characterized in that: The center of the base also has a circular groove larger in diameter than the axial through - hole, which is used to cooperate with the ceramic fine cylinder on the heat flux sensor base to fix the installation position of the heat flux sensor on the base.

5. The three-dimensional heat flux sensor for differential scanning calorimetry according to claim 1, characterized in that: The base also has two positioning through - holes for filling and supporting the ceramic fine cylinder, a straight - groove through - hole for leading out four leads of the thin - film platinum resistor, a through - hole for leading out the common lead on the differential line; two through - holes for leading out the tail - end electrode leads of the sample - end heat flux sensor and the reference - end heat flux sensor; two internal threaded holes for assembling with the DSC furnace body structure.

6. The three-dimensional heat flux sensor for differential scanning calorimetry according to claim 1, wherein: The material of the base is aluminum nitride.

7. A three-dimensional heat flux sensor for differential scanning calorimetry according to claim 1, characterized in that: The composition of the solder paste is Bi2O3 and B2O3, and the molar ratio of the two is 1:1.

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