CN113091939B - Preparation method of high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction - Google Patents

Abstract

The invention discloses a preparation method of a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction, belongs to the field of temperature sensors, and aims to solve the problem that the sensitivity and response speed of the existing temperature sensor are low. The preparation method comprises the following steps: 1. preparing barium strontium titanate ceramic by adopting a solid phase sintering method; 2. cutting graphene loaded on a copper sheet, and spin-coating polymethyl methacrylate on the graphene; 3. putting the spin-coated graphene into copper etching liquid, and fishing the graphene from the copper etching liquid; 4. transferring graphene onto a polished surface of barium strontium titanate ceramic, and drying; 5. putting the dried barium strontium titanate ceramic into acetone to dissolve polymethyl methacrylate; 6. and (5) placing the graphene-barium strontium titanate heterojunction into an oven for drying. When the temperature is increased to the vicinity of the phase transition temperature of the barium strontium titanate, the current respectively increases at a higher rate than the room temperature, and has a higher current change rate in a narrower phase transition temperature region, thereby having higher detection sensitivity.

Description

Preparation method of high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction

Technical Field

The invention belongs to the field of temperature sensors, and in particular relates to a method for preparing a high-sensitivity graphene/barium strontium titanate heterostructure temperature sensor.

Background technique

A temperature sensor is a device that can convert a temperature signal into an electrical signal and output it. Among them, converting it into a resistance signal is the most common and easy-to-design method. Commonly used devices for measuring temperature include thermocouples and thermistors. Among them, the semiconductor-related thermistors are based on the principle of converting temperature increase signals into resistance decrease signals. Although thermistors have been widely studied in the field of temperature sensors, there are still many key issues to be solved in the key materials and device structures of temperature sensors with high sensitivity and high response speed requirements. For example, the problem of large current causing thermistor self-heating is urgently needed to be solved. Therefore, exploring new high-sensitivity and high-response speed temperature sensor materials and device structures is still a new challenge.

The emergence of graphene has opened up a new research idea for the study of high-speed response devices. An ideal single-layer graphene has a completely symmetrical conical band structure. The bonds in the molecular structure are conjugated to form large π bonds, which also enable carriers to move at high speed. The theoretical carrier mobility is about 200,000 cm ² /Vs, far exceeding the indium antimonide (about 77,000 cm ² /Vs) that was previously considered to have the highest carrier mobility. However, the type of substrate can affect the carrier mobility of graphene. Specifically, the self-polarization electric field of the ferroelectric dipole in the ferroelectric material has a significant effect on the graphene carrier characteristics and resistivity in the graphene/ferroelectric film heterostructure. These ferroelectric dipoles will disappear during the phase transition, and the Curie temperature can be adjusted by ion doping using dopants such as BaTiO ₃ , PbZrTiO ₃ and (K,Na)NbO ₃ .

Summary of the invention

The purpose of the present invention is to solve the problem of low sensitivity and response speed of existing temperature sensors and to provide a method for preparing a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction.

The preparation method of the high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction of the present invention is implemented by the following steps:

1. Prepare barium strontium titanate ceramics by solid phase sintering method according to the stoichiometric ratio of Ba _x Sr _1-x TiO ₃ and polish;

2. cutting the graphene loaded on the copper sheet, and spin coating a layer of polymethyl methacrylate on the graphene to obtain spin-coated graphene;

3. placing the spin-coated graphene into a copper etching solution, scooping out the graphene from the copper etching solution, and washing the graphene in deionized water to obtain cleaned graphene;

4. Using a wet transfer method to transfer the cleaned graphene to the polished surface of the barium strontium titanate ceramic, and then putting the barium strontium titanate ceramic into an oven for drying to obtain a dried barium strontium titanate ceramic;

5. Put the dried barium strontium titanate ceramic into acetone to dissolve the polymethyl methacrylate on the surface of graphene, and then put it into deionized water to clean it, so as to obtain a graphene-barium strontium titanate heterojunction;

6. Place the graphene-barium strontium titanate heterojunction in an oven and dry it to obtain a high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction.

The general chemical formula of the graphene/barium strontium titanate heterojunction of the present invention is graphene/Ba _x Sr _{1- x} TiO ₃ (0.1≤x≤0.4).

The method for preparing a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction of the present invention has the following beneficial effects:

The number of positive and negative charges adsorbed by the ferroelectric dipoles in different directions inside barium strontium titanate has an adsorption and repulsion effect on the large π bonds in graphene. This behavior affects the carrier mobility and resistivity of graphene. Specifically, the change in the physical state of the large π bonds has a significant effect on the charge transport properties. Near the Curie temperature of the barium strontium titanate heterostructure, the electric dipoles will disappear as the temperature increases. At the same time, the positive and negative charges adsorbed on the surface of barium strontium titanate will also disappear, resulting in a change in the physical state of the large π bonds in graphene, and then the carrier mobility of graphene will also change. Therefore, when a small temperature adjustment is made around the Curie temperature of barium strontium titanate, the current of the graphene/barium strontium titanate heterostructure changes significantly under a constant applied voltage.

The graphene/barium strontium titanate heterostructure provided by the present invention has a phase transition temperature of barium strontium titanate of 0 to 90° C. Near the phase transition temperature of barium strontium titanate, the current of the heterojunction will change greatly under certain constant voltage conditions. When the temperature rises to near the phase transition temperature, the current rises at a higher rate than at room temperature, and has a very high current change rate in a narrow phase transition temperature range, thereby having a higher detection sensitivity.

The ceramic material prepared by the solid phase sintering method of the present invention has simple processes and equipment, is easy to obtain raw materials, has low cost, is easy to integrate devices, is suitable for industrial production, and provides a guarantee for the application of graphene/ferroelectric material heterojunction in the field of temperature sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of the preparation of a high-sensitivity temperature sensor of a graphene/barium strontium titanate heterojunction according to the present invention;

FIG2 is a curve of the dielectric constant of Ba _0.7 Sr _0.3 TiO ₃ obtained in Example 3 as a function of temperature, and the frequencies along the arrow direction are 1 kHz, 10 kHz, 100 kHz and 1 MHz, respectively;

FIG3 is a graph showing the variation of the G peak of graphene with temperature;

FIG4 is a graph showing the change of 2D peak of graphene with temperature;

FIG5 is a graph showing the change of current versus temperature of the graphene/Ba _0.7 Sr _0.3 TiO ₃ heterojunction obtained in Example 3 under constant voltage conditions;

FIG6 is a graph showing the change in resistance of the graphene/Ba _0.7 Sr _0.3 TiO ₃ heterojunction obtained in Example 3 under constant voltage conditions as a function of temperature.

Detailed ways

Specific implementation method 1: The preparation method of the high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction in this implementation method is implemented according to the following steps:

1. Prepare barium strontium titanate ceramics by solid phase sintering method according to the stoichiometric ratio of Ba _x Sr _1-x TiO ₃ and polish;

2. cutting the graphene loaded on the copper sheet, and spin coating a layer of polymethyl methacrylate on the graphene to obtain spin-coated graphene;

3. placing the spin-coated graphene into a copper etching solution, scooping out the graphene from the copper etching solution, and washing the graphene in deionized water to obtain clean graphene;

4. Using a wet transfer method to transfer the cleaned graphene to the polished surface of the barium strontium titanate ceramic, and then putting the barium strontium titanate ceramic into an oven for drying to obtain a dried barium strontium titanate ceramic;

5. Put the dried barium strontium titanate ceramic into acetone to dissolve the polymethyl methacrylate on the surface of graphene, and then put it into deionized water to clean it to obtain a graphene-barium strontium titanate heterojunction;

6. Place the graphene-barium strontium titanate heterojunction in an oven and dry it to obtain a high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction.

This embodiment constructs a temperature sensor using graphene and ferroelectric materials. The sensor can operate under different temperature conditions based on the graphene/ferroelectric heterostructure.

Specific implementation method 2: This implementation method is different from specific implementation method 1 in that in step 1, in the chemical formula Ba _x Sr _{1- x} TiO ₃ , the value of x is 0.1≤x≤0.4.

Specific implementation method three: The difference between this implementation method and specific implementation method one or two is that the solid phase sintering method in step one is sintering at 1200-1450° C. for 6-12 hours.

Specific implementation method 4: The difference between this implementation method and specific implementation method 3 is that the solid phase sintering method in step 1 is sintering at 1300° C. for 8 hours.

Specific implementation method 5: The difference between this implementation method and specific implementation methods 1 to 4 is that the size of the copper sheet cut in step 2 is (4-8)×(2-5) mm.

Specific implementation example 6: This implementation example is different from specific implementation examples 1 to 5 in that the graphene after spin coating in step 3 is placed in a copper etching solution for 10 to 50 minutes.

The purpose of etching in this embodiment is to release graphene from the copper support so as to transfer it to the polished surface of barium strontium titanate.

Specific implementation method seven: The difference between this implementation method and specific implementation methods one to six is that in step three, the graphene is placed in deionized water for cleaning for 5 to 30 minutes, and the cleaning is repeated 3 to 5 times.

In this embodiment, the etching solution remaining on the graphene is removed by a deionized water cleaning process to prevent the etching solution from affecting the heterogeneous structure.

Specific implementation eight: This implementation differs from any one of specific implementations one to seven in that the drying temperature in step four is 50 to 80° C. and the drying time is 30 to 90 minutes.

Specific embodiment 9: The difference between this embodiment and specific embodiments 1 to 8 is that in step 5, the dried barium strontium titanate ceramic is dissolved in acetone for 5 to 30 minutes.

The dried barium strontium titanate ceramic of this embodiment can be placed in acetone and repeatedly treated for 3 to 5 times. The purpose of using acetone is to completely dissolve polymethyl methacrylate, remove its influence on the heterostructure, and allow electrodes to be evaporated on graphene and barium strontium titanate at the same time.

Specific embodiment ten: This embodiment differs from specific embodiments one to nine in that the drying temperature in step six is 50 to 80° C. and the drying time is 30 to 90 minutes.

Specific embodiment eleven: The difference between this embodiment and specific embodiments one to ten is that in step six, a gold electrode is deposited on the graphene surface of the high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction by vacuum evaporation.

Example 1: The preparation method of the high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction (Graphene/Ba _0.9 Sr _0.1 TiO ₃ ) in this example is implemented according to the following steps:

1. BaCO ₃ , TiO ₂ and SrCO ₃ are weighed according to the stoichiometric ratio of Ba _0.9 Sr _0.1 TiO 3, and Ba _0.9 Sr _0.1 TiO _{3 ceramics} are prepared by solid phase sintering at 1250°C for 8 hours and polished;

2. Cutting the graphene loaded on the copper sheet, and spin-coating a layer of polymethyl methacrylate on the graphene to obtain the spin-coated graphene, so as to play a supporting role when transferring the graphene;

3. Put the spin-coated graphene into a copper etching solution for 30 minutes, use a copper mesh to pick up the graphene from the copper etching solution, put the graphene into deionized water for 20 minutes to remove the etching solution remaining on the graphene, and obtain cleaned graphene;

4. Using a wet transfer method to transfer the cleaned graphene to the polished surface of the barium strontium titanate ceramic, and then drying the barium strontium titanate ceramic in an oven at 60° C. for 60 minutes to obtain the dried barium strontium titanate ceramic;

5. Put the dried barium strontium titanate ceramic into acetone for 20 minutes to dissolve the polymethyl methacrylate on the surface of graphene, repeat putting into acetone for 3 times to completely remove the polymethyl methacrylate, and then put into deionized water to clean, to obtain a graphene-barium strontium titanate heterojunction;

6. The graphene-barium strontium titanate heterojunction was placed in an oven and dried at 60° C. for 60 minutes to obtain a high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction.

In this embodiment, two gold electrodes were deposited on the surface of the graphene/barium strontium titanate heterojunction by vacuum evaporation before testing.

Example 2: The preparation method of the high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction (Graphene/Ba _0.8 Sr _0.2 TiO ₃ ) in this example is implemented according to the following steps:

1. Raw materials BaCO ₃ , TiO ₂ and SrCO ₃ were weighed according to the stoichiometric ratio of Ba _0.8 Sr _0.2 TiO 3, and Ba _0.8 Sr _0.2 TiO _{3 ceramics} were prepared by solid phase sintering at 1300°C for 8 hours;

2. Cutting the graphene loaded on the copper sheet, and spin-coating a layer of polymethyl methacrylate on the graphene to obtain the spin-coated graphene, so as to play a supporting role when transferring the graphene;

3. Put the spin-coated graphene into a copper etching solution for 35 minutes, use a copper mesh to pick up the graphene from the copper etching solution, put the graphene into deionized water for 20 minutes to remove the etching solution remaining on the graphene, and obtain cleaned graphene;

4. Using a wet transfer method, the cleaned graphene is transferred to the polished surface of the barium strontium titanate ceramic, and then the barium strontium titanate ceramic is placed in an oven and dried at 60° C. for 50 minutes to obtain the dried barium strontium titanate ceramic;

5. Put the dried barium strontium titanate ceramic into acetone for 20 minutes to dissolve the polymethyl methacrylate on the surface of graphene, repeat putting into acetone for 3 times to completely remove the polymethyl methacrylate, and then put into deionized water to clean, to obtain a graphene-barium strontium titanate heterojunction;

6. Place the graphene-barium strontium titanate heterojunction in an oven and dry it at 60° C. for 50 minutes to obtain a high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction.

Example 3: This example is different from Example 1 in that in step 1, raw materials BaCO ₃ , TiO ₂ and SrCO ₃ are weighed according to the stoichiometric ratio of Ba _0.7 Sr _0.3 TiO 3, and _{the Ba 0.7 Sr 0.3 TiO 3 ceramics are prepared} by sintering at 1350° C. for 10 hours using a solid phase sintering method.

Example 4: This example differs from Example 1 in that in step 1, raw materials BaCO ₃ , TiO ₂ and SrCO ₃ are weighed according to the stoichiometric ratio of Ba _0.6 Sr _0.4 TiO ₃ , and the Ba _0.6 Sr _0.4 TiO ₃ ceramics are prepared by sintering at 1350° C. for 10 hours using a solid phase sintering method.

FIG1 is a flow chart of the preparation of a graphene/barium strontium titanate heterostructure, which corresponds to Examples 1 to 4. The preparation process of the graphene/barium strontium titanate heterostructure can be seen from the figure. The graphene loaded on the copper sheet is cut into a size of (4 to 8) × (2 to 5) mm, a layer of polymethyl methacrylate is spin-coated on the graphene, and the spin-coated graphene is placed in a copper etching solution for 10 to 50 minutes. After cleaning, the cleaned graphene is transferred to the polished surface of the barium strontium titanate by a wet transfer method to initially form a graphene/barium strontium titanate heterostructure. After drying, it is placed in acetone to completely dissolve the polymethyl methacrylate. After cleaning and drying again, a certain area of gold electrode is deposited by vacuum evaporation to prepare for electrical performance testing.

FIG2 is a curve of the dielectric constant of Ba _0.7 Sr _0.3 TiO ₃ changing with temperature. This figure corresponds to Example 3. The dielectric temperature spectra of different frequencies show that the dielectric constant at different temperatures initially increases with the increase of the test frequency and then decreases. A single dielectric peak appears at about 30°C, which is attributed to the transition from the ferroelectric phase to the paraelectric phase. This shows that the phase transition temperature of Ba _0.7 Sr _0.3 TiO ₃ is ～30°C (wherein the phase transition temperatures of the barium strontium titanate ceramics obtained in Examples 1, 2 and 4 are ～90°C, ～60°C and ～0°C, respectively). Around 30°C is the region where the electric dipole exists and disappears. At the same time, the adsorbed charge will also disappear, causing the physical state of the large π bond of graphene to change, thereby further affecting the internal carrier mobility of graphene, which is manifested in the macroscopic changes of heterojunction current and resistance.

Fig. 3 and Fig. 4 are graphene G peak and 2D peak with temperature variation spectrum, this figure corresponds to embodiment 1～4, it can be seen from the figure that the law of G peak and 2D peak with temperature variation is more obvious, this is because before the phase change of barium strontium titanate, graphene and barium strontium titanate are in a state of mutual adsorption, and the lateral vibration of graphene is affected during the current-voltage test process, after the phase change, the adsorption state of graphene and barium strontium titanate disappears, and during the current-voltage test process, the lateral contraction of graphene is not restricted, so the G peak area of variable temperature Raman shows an increasing trend near the phase change temperature, and continues to increase the temperature and tends to be stable. On the other hand, before the phase change, the adsorption effect of the positive charge on the surface of barium strontium titanate on the large π bond of graphene causes the longitudinal vibration of graphene to be larger, and after the phase change, the adsorption state disappears, and the longitudinal vibration decreases, so the 2D peak area of variable temperature Raman shows a decreasing trend near the phase change temperature, and continues to increase the temperature and tends to be stable.

Figure 5 shows the graphene/Ba _0.7 Sr _0.3 TiO ₃ heterojunction current versus temperature curve under constant voltage conditions; this figure corresponds to Example 3. At room temperature, the current is 16.33 μA, which initially increases slightly with increasing temperature. When the ambient temperature crosses the phase transition temperature of barium strontium titanate, the current begins to increase significantly, from 16.38 μA to 16.99 μA, an increase of 0.3% and 4%, respectively. These results show that the relative change in the heterostructure current is high near the phase transition temperature of barium strontium titanate.

Figure 6 shows the graphene/Ba _0.7 Sr _0.3 TiO ₃ heterojunction resistance versus temperature under constant voltage conditions. This figure corresponds to Example 3. At temperatures between 27 and 31°C, the relative change percentages of resistance are 0.3% and 3.9%, respectively, and the average change ratio is 0.9%/°C. However, at temperatures below the phase transition temperature, the average change ratio is only 0.06%/°C, which is higher than 0.38%/°C at the phase transition temperature. Therefore, the resistance of the heterostructure decreases the most near the phase transition temperature of barium strontium titanate.

Claims (11)

1. A method for preparing a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction, characterized in that the preparation method is implemented according to the following steps: 1. Prepare barium strontium titanate ceramics by solid phase sintering method according to the stoichiometric ratio of Ba _x Sr _1-x TiO ₃ and polish; 2. cutting the graphene loaded on the copper sheet, and spin coating a layer of polymethyl methacrylate on the graphene to obtain spin-coated graphene; 3. placing the spin-coated graphene into a copper etching solution, scooping out the graphene from the copper etching solution, and washing the graphene in deionized water to obtain cleaned graphene; 4. Using a wet transfer method to transfer the cleaned graphene to the polished surface of the barium strontium titanate ceramic, and then putting the barium strontium titanate ceramic into an oven for drying to obtain a dried barium strontium titanate ceramic; 5. Put the dried barium strontium titanate ceramic into acetone to dissolve the polymethyl methacrylate on the surface of graphene, and then put it into deionized water to clean it, so as to obtain a graphene-barium strontium titanate heterojunction; 6. The graphene-barium strontium titanate heterojunction is placed in an oven for drying to obtain a high-sensitivity temperature sensor based on the graphene/barium strontium titanate heterojunction; when the temperature is adjusted around the Curie temperature of barium strontium titanate, the current of the graphene/barium strontium titanate heterostructure changes significantly under a constant applied voltage. 2. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 1, characterized in that in step 1, the value of x in the chemical formula Ba _x Sr _1-x TiO ₃ is 0.1≤x≤0.4. 3. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 1 is characterized in that the solid-phase sintering method in step 1 is sintering at 1200-1450° C. for 6-12 hours. 4. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 3 is characterized in that the solid-phase sintering method in step 1 is sintering at 1300° C. for 8 hours. 5. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 1 is characterized in that the size of the copper sheet cut in step 2 is (4-8)×(2-5) mm. 6. The method for preparing a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction according to claim 1, characterized in that the graphene after spin coating in step 3 is placed in a copper etching solution for 10 to 50 minutes. 7. The method for preparing a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction according to claim 1 is characterized in that in step three, the graphene is placed in deionized water and washed for 5 to 30 minutes, and the washing is repeated 3 to 5 times. 8. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 1, characterized in that the drying temperature in step 4 is 50-80°C and the drying time is 30-90 minutes. 9. The method for preparing a high-sensitivity temperature sensor based on a graphene/barium strontium titanate heterojunction according to claim 1, characterized in that in step 5, the dried barium strontium titanate ceramic is placed in acetone for a dissolution treatment time of 5 to 30 minutes. 10. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 1, characterized in that the drying temperature in step 6 is 50-80°C and the drying time is 30-90 minutes. 11. The method for preparing a high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction according to claim 1, characterized in that in step six, a gold electrode is deposited on the graphene surface of the high-sensitivity temperature sensor based on graphene/barium strontium titanate heterojunction by vacuum evaporation.