Preparation method for regulating ferroelectric texture ceramic grain size by phase field method

Abstract

The application discloses a preparation method for regulating and controlling ferroelectric texture ceramic grain size by using a phase field method simulation technology, and comprises the following steps: preparing a series of microcrystalline powders with different size distributions; setting an initial phase field structure meeting the particle size distribution; establishing a phase field model for grain orientation growth; simulating and analoging the grain orientation growth process; preparing a film; preparing a green body with microcrystalline parallel distribution; and preparing ferroelectric texture ceramics with target grain size distribution. The application fully utilizes the computer simulation intelligent prediction guidance technology, does not need repeated trial and error experiments, saves time and effort, has good product reproducibility and stability, easily realizes accurate control of the ferroelectric texture ceramic grain size distribution, and is easy to be industrialized; the application effectively controls the occurrence of grain abnormal growth, maintains high grain orientation, and promotes the densification of the ceramic; and the application can not only prepare high-texture-density ferroelectric ceramics, but also regulate and control the grain size.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method for controlling the grain size of ferroelectric texture ceramics by using a phase field method simulation technique, and belongs to the fields of computational materials science and ferroelectric ceramics. BACKGROUND

[0002] Since the template grain growth method was proposed, a large number of ferroelectric texture ceramics have been synthesized. In the process, the size advantage of the template particles is utilized to swallow the surrounding small matrix particles through abnormal growth to complete the oriented growth and form the texture morphology. Due to the occurrence of abnormal grain growth, the density of the ferroelectric texture ceramics is difficult to guarantee, which further limits the development and application of the electric properties of the ferroelectric texture ceramics. In order to limit the abnormal growth of the ceramic template grains, scholars have proposed a new preparation method for ferroelectric texture ceramics [Liu Liangliang, et al. A preparation method for layered and dense potassium strontium niobate lead-free piezoelectric ceramics, Chinese invention patent, ZL202010348847.4.], that is, the template and the matrix are respectively tape cast into a film, then stacked according to a certain proportion, and finally sintered to obtain high-texture ceramics. Due to the significant difference in shrinkage rate of the template layer and the matrix layer during high-temperature process, a large number of microcracks and even cracks inevitably occur in the ceramics. In 2014, scholars completely used acicular microcrystals as raw materials to successfully solve the cracking problem of ferroelectric texture ceramics [Gao Feng, et al. Potassium strontium niobate lead-free dense texture ceramic and its preparation method, Chinese invention patent, ZL 201310337876.0]. However, in order to meet the requirements of high density and texture degree of the ferroelectric ceramics, the particle size distribution of the initial raw materials must be strictly controlled. This is because under the current tape casting process conditions, only particles with a large aspect ratio can be arranged in the film. If the particle size is large, it will seriously hinder the densification process of the ceramics. Therefore, in order to prepare high-texture and dense ferroelectric ceramics, the particle size distribution of the initial raw materials must be strictly controlled.

[0003] It is well known that the grain size of ferroelectric ceramic materials is directly related to their electric properties, and controlling the particle size distribution of the initial raw materials is one of the most effective means to adjust the grain size of ferroelectric ceramic materials. In order to explore the influence of the grain size of ferroelectric texture ceramics on their electric properties, and further optimize the process parameters to prepare ceramic materials with target grain size, it is crucial to invent a method that can arrange short acicular particles in the film. In addition, the density of the texture ceramic with grain orientation distribution is more sensitive to the initial particle size distribution than the grain free orientation ceramic. Therefore, using conventional exploratory test methods is not only time-consuming and laborious, but also difficult to obtain high-texture and dense ferroelectric ceramics with various grain sizes.

[0004] With the development of computational materials science, phase field simulation has gradually become an important means of grain growth research. This technology is mainly applied in the field of metal materials, while the research in the field of ceramic materials is relatively less. This is mainly because the initial structure setting of the current phase field method is too idealized (assuming some circles), which cannot be used to simulate the growth process of the initial raw material needle-shaped particles, not to mention adjusting the size distribution of the particles for research. Another reason is that the grain boundary formed between the needle-shaped particles has strong anisotropy. The previous phase field model is based on the condition that the grain boundary energy is isotropic, so it is not suitable for simulating the grain orientation growth process of textured ceramics. Therefore, setting the initial structure close to the real situation and introducing the grain boundary energy anisotropy function are two key factors for the phase field method to study the grain growth behavior of textured ceramics, which can accurately guide the experiment and provide a reliable basis for the regulation of the grain size of ferroelectric textured ceramics. It has important scientific significance and engineering application value to study the preparation of ferroelectric textured ceramics with different grain sizes by using phase field simulation technology. SUMMARY

[0005] In order to solve the problem that the short needle-shaped particles of ferroelectric ceramic raw materials cannot be arranged in a directional manner during the forming process, while ensuring that the ferroelectric ceramic has high texture and density, and regulating different grain sizes, the present application proposes a preparation method for regulating the grain size of ferroelectric textured ceramics by using phase field simulation technology.

[0006] The preparation method for regulating the grain size of ferroelectric textured ceramics by using phase field simulation technology proposed in the present application applies the phase field method to the grain growth process of textured ceramics. It takes potassium strontium niobate ferroelectric material as the research object, uses needle-shaped potassium strontium niobate microcrystalline powder synthesized by the molten salt method as the raw material, obtains a series of microcrystalline powder samples with different size distributions through the ball milling process, and constructs the grain boundary energy anisotropy function according to the growth rate of different crystal faces of potassium strontium niobate microcrystals. Then, according to the size distribution curve of the microcrystalline powder, the initial structure of the phase field method is set, the grain boundary energy anisotropy function is introduced into the model, and the phase field method is used for simulation and analysis. The growth kinetics curve and the microstructure of the ceramic grains are analyzed to determine whether the normal grain growth behavior is met. Finally, the potassium strontium niobate microcrystalline powder sample corresponding to the normal grain growth is used as the raw material, mixed with a sintering aid, an organic solvent and a binder to obtain a slurry with a certain viscosity, and then the slurry is coated and flow-casted into a film, and after rolling and pressing, different grain size ferroelectric textured ceramics are obtained through a two-step sintering technology. Compared with the traditional ferroelectric textured ceramic forming process, the preparation method of the present application has obvious advantages. It is difficult to achieve directional arrangement of needle-shaped particles in the two-dimensional film plane by the traditional method, especially when short particles are used as raw materials, making it more difficult to achieve parallel directional arrangement. In addition, it is difficult to achieve stable preparation of high-quality textured ferroelectric ceramics by relying solely on experience, and it is even more impossible to achieve the regulation of grain size.

[0007] The application provides a preparation method for regulating and controlling ferroelectric texture ceramic grain size by using a phase field method simulation technology, and comprises the following steps:

[0008] Step one: preparing a series of microcrystal powders with different size distributions, and the specific process is as follows:

[0009] (1) mixing analytical pure strontium carbonate and niobium pentoxide powders, then adding potassium chloride powders to obtain a raw material mixture of strontium potassium niobate microcrystals; the molar ratio of strontium carbonate to niobium pentoxide is 4:5; the potassium chloride is 2 times the weight of strontium carbonate and niobium pentoxide;

[0010] (2) placing the raw material mixture of strontium potassium niobate microcrystals in a polytetrafluoroethylene ball mill tank, adding anhydrous ethanol, and ball milling on a ball mill for 12 hours to obtain strontium potassium niobate microcrystal wet material; the weight ratio of the anhydrous ethanol to the raw material mixture of strontium potassium niobate microcrystals is 1:1. The strontium potassium niobate microcrystal wet material after ball milling is placed in an oven and dried at 70 DEG C. The dried powder is ground. The ground powder is placed in a corundum crucible, heated to 1100 DEG C to 1300 DEG C at a heating rate of 5 DEG C / min, and heat preserved for 2 hours to calcine the ground powder. The calcined powder is cooled to room temperature in the furnace to obtain the calcined powder.

[0011] (3) repeatedly washing and filtering the calcined powder in 100 DEG C distilled water until no Cl - is detected in the filtrate. The washed powder is dried at 70 DEG C to obtain needle-like strontium potassium niobate microcrystal powder.

[0012] (4) placing the obtained needle-like strontium potassium niobate microcrystal powder in an alumina ball mill tank, adding anhydrous ethanol, and ball milling on a ball mill for 2h to 20h to obtain a series of strontium potassium niobate microcrystal powder samples with different size distributions after drying.

[0013] Step two: setting an initial phase field structure meeting the particle size distribution;

[0014] The specific process is as follows: according to the size distribution (similar to a standard sample) of the obtained strontium potassium niobate microcrystal powder sample, an initial phase field structure meeting the particle size distribution is set; firstly, a two-dimensional (N1xN2) grid space system is opened, needle-like particles meeting the size distribution are added to the system, and the needle-like particles are parallel and do not intersect with each other; the size distribution function of the needle-like particles is f(x), and 4 grid points are equal to 1 micrometer; then the regions in the system are valued, and a group of order parameters η i (r,t)(i=1,2,3…p) representing grain orientation are used, and only one order parameter is 1 and the rest are 0 in all the particles.

[0015] Step three: Establishing the phase field model of grain-oriented growth: According to the characteristics of the grain boundary formed between each crystal face of potassium strontium niobate crystal, the anisotropy function of grain boundary energy is constructed and introduced into the phase field model, and the phase field model of grain-oriented growth is obtained, so as to realize the grain-oriented growth process;

[0016] The specific process is: through the gradient energy density coefficient k i , the anisotropy coefficient C of grain boundary energy is introduced, k i is discretized into k x and k z , and their relationship can be defined as:

[0017] k x =Ck z

[0018] According to the XRD pattern of potassium strontium niobate microcrystalline powder, the side of the particle is mainly (410), (320) and (420), so C is a variable related to the orientation of the crystal face. Since k x is much larger than k z , and the side of the particle is mainly (410), in order to simplify the phase field model, we assume that C is a constant. Keeping the modulus unchanged, the discretized k x and k z satisfy the following equation:

[0019]

[0020] According to the experimental data of the growth rate of each crystal face of potassium strontium niobate crystal, the value range of C can be inferred. The total free energy function can be expressed as:

[0021]

[0022] In the formula, f is the local free energy density function, k i is the gradient energy density related to the grain boundary energy. Among them, η i (r,t)(i=1,2,3…p) is called a crystal direction field variable that distinguishes different grains, p represents the number of possible crystal directions, and α, β and γ are phenomenological coefficients. When α=β=1 and γ=0.5, f0 has 2p minima. In this model, each of the 2p represents a specific crystal direction of the grain. Determine the phase field parameters and boundary conditions, and program to calculate the phase field equation:

[0023]

[0024] In the formula, L i and M are kinetic coefficients related to interface mobility and thermodynamic diffusion coefficient, and ▽ is Hamilton operator. Solving the phase field equation can continuously update the value of all order parameters of each grid point in the system. In order to display the microstructure evolution process, the function The visualization is realized in the form as follows:

[0025]

[0026] When When equal to 1, the microstructure is white, indicating a grain; When between 0 and 1, the microstructure is gray, indicating a grain boundary.

[0027] Step four: simulation of the grain oriented growth process;

[0028] The specific process is: first, select one of the generated initial structure, then assign values to the phase field parameters and perform simulation; the simulation system is set to 200x400 grid points, the order parameter number p is selected to be 400. Other parameters are set as follows: a, β, γ and L are all taken as 1.0, k i Take 2.0, the space step Δx is taken as 2 grid points, the time step (dimensionless) is taken as Δt is 0.1, the grain boundary energy anisotropy coefficient C is taken as 20. By statistical analysis of the change trend of the grain size with time step, whether it meets the normal growth curve is studied. The initial particle size distribution corresponding to the normal growth curve is recorded as the experimental guidance basis; then another initial structure is selected for simulation, the change trend of the grain size is counted, and whether it meets the normal growth curve is studied; the operation is repeated until all the initial structures are studied, and the potassium strontium niobate microcrystalline powder sample corresponding to the normal growth of the grain is obtained.

[0029] Step five: preparation of the film;

[0030] The specific process is:

[0031] (1) Put the sintering aid, binder and organic solvent into a polytetrafluoroethylene ball mill jar in turn, ball mill for 12 hours, then add the potassium strontium niobate microcrystalline powder sample obtained by simulation guidance and continue to ball mill for 2 hours to obtain a slurry. The sintering aid is bismuth trioxide, which is 4% of the weight of the microcrystalline powder; the organic solvent is prepared by mixing toluene and anhydrous ethanol in a weight ratio of 2:1, which is 2 times the weight of the microcrystalline powder; the binder is prepared by mixing anhydrous ethanol, toluene, isopropyl alcohol, dibutyl phthalate and polyvinyl butyral, which is 45% of the weight of the microcrystalline powder; the weight ratio of anhydrous ethanol to toluene in the binder is 1:1; isopropyl alcohol is 30% of the weight of anhydrous ethanol and toluene; dibutyl phthalate is 20% of the weight of anhydrous ethanol and toluene; polyvinyl butyral is 25% of the weight of anhydrous ethanol and toluene

[0032] (2) The filtered slurry was cast on a stainless steel plate by a conventional brushing method using a nylon brush with a width of 1.5 cm to obtain a film. The casting process parameters were as follows: brushing rate 2 cm / s, thickness 20-40 μm, and drying temperature 70 °C.

[0033] Step six: preparing a green body with microcrystalline phases distributed in parallel;

[0034] The specific process is as follows:

[0035] (1) The obtained microcrystalline film was cut into a rectangular strip with a length of 100 mm and a width of 30 mm. The width direction was parallel to the brushing direction, and the length direction was perpendicular to the brushing direction.

[0036] (2) The obtained rectangular strip was rolled along the width direction at 50 °C until it was completely rolled up, and then it was unrolled and rolled along the width direction of the other side of the strip. This process was repeated 20-50 times. The strip roll was pressed through the vertical side of a tablet press; the pressure was 50-200 MPa, and the pressure holding time was 10-30 min.

[0037] (3) The pressed strip roll was cut into a green body with a desired size by a cutting machine;

[0038] Step seven: preparing a ferroelectric texture ceramic with a target grain size distribution;

[0039] The specific process is as follows:

[0040] (1) The green body was heated from room temperature to 100 °C for 4 h and then to 200 °C for 2 h, and then heated to 600 °C for 18 h and kept at this temperature for 4 h. After the heat preservation was completed, the sample was cooled to room temperature in the furnace, and a degummed sample was obtained.

[0041] (2) The degummed sample was placed in a high-temperature box furnace and pre-fired at 1100-1200 °C for 0.5-4 h. After pre-firing, the temperature was decreased to 500 °C at a rate of 2 °C / min, and then the furnace was cooled to room temperature. When the pre-fired sample was cooled to room temperature, the high-temperature box furnace was heated to 1300 °C at a rate of 5 °C / min, and the degummed sample was sintered for 1-4 h to obtain a sintered sample. After polishing, a ferroelectric texture ceramic with a target grain size distribution was obtained.

[0042] Compared with the traditional ferroelectric texture ceramic forming process, the preparation method has obvious advantages. Firstly, the directional arrangement of the needle-shaped particles is realized, which is difficult to achieve in the traditional method, especially when the short particles are used as raw materials, it is more difficult to arrange them in parallel. Secondly, the high-quality stable product is prepared by the method, and it is difficult to realize the stable preparation of high-quality texture ferroelectric ceramics by experience in the prior art, and it is impossible to control the grain size.

[0043] The beneficial effects of the present application are:

[0044] (1) The present application has simple steps, and fully utilizes the computer simulation and intelligent prediction guidance technology, so that repeated trial and error experiments are not needed, time and labor are saved, the product has good reproducibility and stability, the accurate control of the grain size distribution of the ferroelectric texture ceramic is easily realized, and the industrialization is easy; in addition, the existing sheet electronic component production equipment in the factory can be used to complete the preparation of the ferroelectric texture ceramic without the need to purchase new equipment;

[0045] (2) The preparation method of the present application does not require that the microcrystalline particles have a large aspect ratio, and the brushing process enables part of the particles to be arranged in parallel, and under the repeated operation of the curling process, the particles that are not arranged in parallel are driven to be parallel to each other under the action of shearing force; due to the use of short particles, and the parallel distribution of these particles in the green body, in the high-temperature sintering process, the microcrystals compete with each other for growth, effectively controlling the occurrence of abnormal grain growth, maintaining the high orientation of the grains, and promoting the densification of the ceramic. The present application not only can prepare high-texture-density ferroelectric ceramics, but also can control the grain size. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 Fig. 1 is an SEM photograph of the 1-micron-diameter and 3-micron-diameter potassium strontium niobate microcrystalline powder prepared in Example 1 (left) and Example 2 (right).

[0047] Figure 2 Fig. 2 is a microcrystalline particle size distribution graph of the 2.5-micron-long and 3.75-micron-long microcrystalline particles prepared in Example 1 and Example 2.

[0048] Figure 3 Fig. 3 is an initial phase field structure diagram of the 1-micron-diameter and 3-micron-diameter microcrystalline particles prepared in Example 1 (left) and Example 2 (right).

[0049] Figure 4 Fig. 4 is a phase field method simulation result of Example 1 (left) and Example 2 (right).

[0050] Figure 5 Fig. 5 is an SEM photograph of the potassium strontium niobate ferroelectric texture ceramic of Example 1 (left) and Example 2 (right). DETAILED DESCRIPTION

[0051] The present application is further illustrated by the following examples, but is not limited to the following examples.

[0052] Example 1

[0053] This embodiment is a preparation method for regulating and controlling the grain size of ferroelectric texture ceramic by using phase field simulation technology, and the specific process is as follows:

[0054] Step one: preparing microcrystalline powder with a certain size distribution;

[0055] The specific process is as follows:

[0056] (1) After mixing the analytical pure strontium carbonate and niobium pentoxide powders, potassium chloride powder is added to obtain a raw material mixture of strontium potassium niobate microcrystals; the molar ratio of strontium carbonate to niobium pentoxide is 4:5; the potassium chloride is 2 times the weight of strontium carbonate and niobium pentoxide;

[0057] (2) The raw material mixture of strontium potassium niobate microcrystals is placed in a polytetrafluoroethylene ball mill jar, anhydrous ethanol is added, and the ball mill is ball milled for 12 h to obtain strontium potassium niobate microcrystal wet material; the weight ratio of the anhydrous ethanol to the raw material mixture of strontium potassium niobate microcrystals is 1:1. The ball milled strontium potassium niobate microcrystal wet material is placed in an oven and dried at 70°C. The dried powder is ground. The ground powder is placed in a corundum crucible, heated to 1100°C at a heating rate of 5°C / min, and kept for 2 h to calcine the ground powder. After calcination, the furnace is cooled to room temperature to obtain the calcined powder.

[0058] (3) The calcined powder is repeatedly washed and filtered in distilled water at 100°C until no Cl - is detected in the filtrate. The washed powder is dried at 70°C to obtain needle-like strontium potassium niobate microcrystal powder with a diameter of 1 micrometer, as shown in the left graph. Figure 1 From the left graph, it can be seen that the diameter of the microcrystalline particles is uniform, and the length is about 1-10 micrometers.

[0059] (4) The obtained needle-like strontium potassium niobate microcrystal powder is placed in an alumina ball mill jar, anhydrous ethanol is added, and the ball mill is ball milled for 20 h. After drying, the microcrystalline powder with an average length of 2.5 micrometers and a size satisfying normal distribution is obtained.

[0060] Step two: setting an initial phase field structure satisfying the particle size distribution;

[0061] The specific process is as follows: fitting the size distribution curve of the strontium potassium niobate microcrystal powder to obtain a normal distribution function f(x),

[0062]

[0063] μ = 10, σ = 20 / 3, such as Figure 2 As shown, the curve on the left indicates that the average length of potassium strontium niobate microcrystalline powder is 10 grid points, or 2.5 micrometers, with a length range of 2.5–17.5 grid points, or 0.625–4.375 micrometers. First, a two-dimensional 200×300 grid system is established. Needle-shaped particles with a size distribution satisfying the specified parameters are added to the system. These needle-shaped particles are randomly positioned, each with a diameter of 1 micrometer, and are parallel and non-intersecting. Four grid points are defined as equal to 1 micrometer. Then, values ​​are assigned to these regions in the system using a set of order parameters η. i (r,t) (i=1,2,3…400) represents the grain orientation. Within each grain, only one order parameter is 1, and the rest are 0. Figure 3 As shown in the left figure, microcrystals of varying lengths are randomly distributed within the system, indicating that the actual situation has been accurately represented by the digital image, meaning that the experimental data has been successfully transformed into the initial structure of the phase-field model.

[0064] Step 3: Establish a phase-field model for grain orientation growth;

[0065] The specific process is as follows: through the gradient energy density coefficient k i Introducing the grain boundary energy anisotropy coefficient C, k i Discretized into k x and k z The relationship between the two can be defined as:

[0066] k x =Ck z

[0067] To simplify the phase-field model, we assume C to be constant. Keeping the modulus constant, the discretized k... x With k z Satisfy the following equation:

[0068]

[0069] The total free energy function can be expressed as:

[0070]

[0071] In the formula, f is the local free energy density function. Determine the phase field parameters and boundary conditions, and then programmatically calculate the phase field equations:

[0072]

[0073] In the formula, L iand M is a kinetic coefficient related to the interfacial mobility and the thermodynamic diffusion coefficient, and ▽ is the Hamiltonian operator. Solving the phase field equation can constantly update the value of all order parameters of each lattice point of the system. In order to show the microstructure evolution process, the function Visualization is realized in the following form:

[0074]

[0075] When is equal to 1, the microstructure is white, indicating a grain; When it is between 0 and 1, the microstructure is gray, indicating a grain boundary.

[0076] Step four: simulation of the grain orientation growth process;

[0077] The specific process is: the number of order parameters p is 400; other parameters are set as follows: α, β, γ and L are all 1.0, k i is 2.0, the spatial step Δx is 2 lattice points, the time step (dimensionless) is Δt is 0.1, and the grain boundary energy anisotropy coefficient C is 20. The simulation result is shown in the left graph of Figure 4 , the grains gradually become coarse and form a topological network structure. Through statistical analysis of the change trend of the grain size with the time step, it is found that it satisfies the normal growth curve. The initial grain size distribution is recorded as a guide for the experiment.

[0078] Step five: preparation of the film;

[0079] The specific process is:

[0080] (1) The sintering aid, the binder and the organic solvent are sequentially placed in a polytetrafluoroethylene ball mill jar, and after ball milling for 12 hours, the microcrystalline powder meeting the simulation guide size distribution is added for further ball milling for 2 hours to obtain a slurry. The sintering aid is bismuth trioxide, which is 4% of the weight of the microcrystalline powder; the organic solvent is prepared from toluene and anhydrous ethanol in a weight ratio of 2:1, which is 2 times the weight of the microcrystalline powder; the binder is prepared from anhydrous ethanol, toluene, isopropyl alcohol, dibutyl phthalate and polyvinyl butyral, which is 45% of the weight of the microcrystalline powder; the weight ratio of anhydrous ethanol to toluene in the binder is 1:1; isopropyl alcohol is 30% of the weight of anhydrous ethanol and toluene; dibutyl phthalate is 20% of the weight of anhydrous ethanol and toluene; polyvinyl butyral is 25% of the weight of anhydrous ethanol and toluene

[0081] (2) The filtered slurry is cast on a stainless steel plate by a conventional brushing method using a nylon brush with a width of 1.5 cm to obtain a film. The casting process parameters are: brushing rate is 2 cm / s, thickness is 20 μm, and drying temperature is 70°C.

[0082] Step six: preparing green bodies with microcrystalline phases distributed in parallel;

[0083] The specific process is as follows:

[0084] (1) The obtained microcrystalline film is cut into an oblong strip with a length of 100 mm and a width of 30 mm. The width direction is parallel to the brushing direction, and the length direction is perpendicular to the brushing direction.

[0085] (2) The obtained oblong strip is rolled along the width direction at 50℃ until it is completely rolled up, and then it is unrolled and rolled along the width direction of the other side of the strip. This process is repeated for 20 times. The strip roll is pressed through the vertical side of the tablet press; the pressure is 50 MPa, and the pressure holding time is 10 min.

[0086] (3) The pressed strip roll is cut into green bodies with a desired size by a cutting machine;

[0087] Step seven: preparing ferroelectric texture ceramics with a target grain size distribution;

[0088] The specific process is as follows:

[0089] (1) The green body is heated from room temperature to 100℃ for 4 h and then heated to 200℃ for 2 h. Then, the temperature is increased to 600℃ for 18 h, and the sample is kept at this temperature for 4 h. After the heat preservation is completed, the sample is cooled to room temperature in the furnace, and a degassed sample is obtained.

[0090] (2) The degassed sample is placed in a high-temperature box furnace and pre-fired at 1100℃ for 4 h. After pre-firing, the temperature is decreased to 500℃ at a rate of 2℃ / min, and then the sample is cooled to room temperature in the furnace. When the pre-fired sample is cooled to room temperature, the high-temperature box furnace is heated to 1300℃ at a rate of 5℃ / min, and the degassed sample is sintered for 6 h. A sintered sample is obtained, and a ferroelectric texture ceramic with a specific grain size distribution is obtained after polishing and polishing, as shown in the left image of FIG. 2. Compared with the left image of FIG. 2, it can be found that the grain microstructure is completely consistent, which indicates that the phase field method simulation successfully predicts the evolution process of the microstructure of the potassium strontium niobate ferroelectric texture ceramic, and can guide the repeated and stable preparation of the target microstructure. Figure 5 Figure 4

[0091] Example 2:

[0092] The embodiment is a preparation method for regulating the grain size of ferroelectric texture ceramics by using phase field simulation technology, and the specific process is as follows:

[0093] Step one: preparing microcrystalline powder with a certain size distribution;

[0094] The specific process is as follows:​​

[0095] (1) Analytical pure strontium carbonate, niobium pentoxide powder were mixed, then potassium chloride powder was added to obtain a raw material mixture of strontium potassium niobate microcrystals; the molar ratio of strontium carbonate to niobium pentoxide was 4:5; the potassium chloride was 2 times the weight of strontium carbonate and niobium pentoxide;

[0096] (2) The raw material mixture of strontium potassium niobate microcrystals was placed in a polytetrafluoroethylene ball mill jar, anhydrous ethanol was added, and the ball mill was ball milled for 12 h to obtain a strontium potassium niobate microcrystal wet material; the weight ratio of the anhydrous ethanol to the raw material mixture of strontium potassium niobate microcrystals was 1:1. The ball milled strontium potassium niobate microcrystal wet material was placed in an oven and dried at 70°C. The dried powder was ground. The ground powder was placed in a corundum crucible, heated to 1300°C at a heating rate of 5°C / min, and held for 2 h to calcine the ground powder. After calcination, the furnace was cooled to room temperature to obtain a calcined powder.

[0097] (3) The calcined powder was repeatedly washed and filtered in distilled water at 100°C until no Cl - was detected in the filtrate. The washed powder was dried at 70°C to obtain needle-shaped strontium potassium niobate microcrystal powder with a diameter of 1 micron, as shown in Figure 1 From the right graph, it can be seen that the microcrystal particles are uniform in diameter and about 1-15 microns in length.

[0098] (4) The obtained needle-shaped strontium potassium niobate microcrystal powder was placed in an alumina ball mill jar, anhydrous ethanol was added, and the ball mill was ball milled for 2 h. After drying, a microcrystal powder with an average length of 2.5 microns was obtained, and the size satisfied a normal distribution.

[0099] Step two: set the initial phase field structure that meets the particle size distribution;

[0100] The specific process is: fitting the size distribution curve of strontium potassium niobate microcrystal powder to obtain a normal distribution function f(x),

[0101]

[0102] μ = 15, σ = 40 / 3, as shown in Figure 2 The right curve shows that the average length of the strontium potassium niobate microcrystal powder is 15 lattice points, i.e. 3.75 microns, and the length range is 5-25 lattice points, i.e. 1.25-6.25 microns; first, a two-dimensional 200x300 lattice space system was created, and needle-shaped particles meeting the size distribution were added to the system. The positions of these needle-shaped particles are random, the diameter is 1 micron, and they are parallel and do not intersect; 4 lattice points are set to be equal to 1 micron; then the regions in the system are valued, and a set of order parameters η i(r, t) (i = 1, 2, 3…400) represents the grain orientation, only one order parameter is 1 in all the particles, and the rest are 0, as shown in Figure 3 As can be seen from the right graph, the microcrystals of different lengths are randomly distributed in the system, which shows that the actual situation has been truly expressed by the digital image, that is, the experimental data has been successfully converted into the initial structure of the phase field model.

[0103] Step three: Establish a phase field model of grain orientation growth;

[0104] The specific process is: through the gradient energy density coefficient k i , the grain boundary energy anisotropy coefficient C is introduced, k i is discretized into k x and k z , and their relationship can be defined as:

[0105] k x = Ck z

[0106] In order to simplify the phase field model, we assume that C is a constant. Keeping the modulus unchanged, the discretized k x and k z satisfy the following equation:

[0107]

[0108] The total free energy function can be expressed as:

[0109]

[0110] In the formula, f is the local free energy density function. Determine the phase field parameters and boundary conditions, and program to calculate the phase field equation:

[0111]

[0112] In the formula, L i and M are the dynamic coefficients related to the interface mobility and the thermodynamic diffusion coefficient, and ▽ is the Hamilton operator. Solving the phase field equation can continuously update the value of all order parameters of each lattice point in the system. In order to display the microstructure evolution process, the function is used to realize visualization, which is as follows:

[0113]

[0114] When is equal to 1, the microstructure is white, indicating the grain; Between 0 and 1, the microstructure is gray, indicating the grain boundary.

[0115] Step four: simulation of the grain orientation growth process;

[0116] The specific process is: the number of order parameters p is selected as 400; other parameters are set as: α, β, γ and L are all taken as 1.0, k i 2.0, the spatial step Δx is taken as 2 grid points, the time step (dimensionless) is taken as Δt is taken as 0.1, the grain boundary energy anisotropy coefficient C is taken as 20. The simulation results are shown in the left graph of Figure 4 Fig. 1, the grains gradually become coarse and form a topological network structure, and through statistical analysis of the change trend of the grain size with the time step, it is found that it satisfies the normal growth curve. The initial particle size distribution is recorded as an experimental guide.

[0117] Step five: preparing the film;

[0118] The specific process is:

[0119] (1) The sintering aid, binder and organic solvent are sequentially placed in a polytetrafluoroethylene ball mill jar, and after ball milling for 12 hours, the microcrystalline powder meeting the simulation guide size distribution is added and continues to be ball milled for 2 hours to obtain a slurry. The sintering aid is bismuth trioxide, which is 4% of the weight of the microcrystalline powder; the organic solvent is prepared from toluene and anhydrous ethanol in a weight ratio of 2:1, which is 2 times the weight of the microcrystalline powder; the binder is prepared from anhydrous ethanol, toluene, isopropyl alcohol, dibutyl phthalate and polyvinyl butyral, which is 45% of the weight of the microcrystalline powder; the weight ratio of anhydrous ethanol to toluene in the binder is 1:1; isopropyl alcohol is 30% of the weight of anhydrous ethanol and toluene; dibutyl phthalate is 20% of the weight of anhydrous ethanol and toluene; polyvinyl butyral is 25% of the weight of anhydrous ethanol and toluene.

[0120] (2) The filtered slurry is cast on a stainless steel plate by a conventional brushing method using a nylon brush with a width of 1.5 cm to obtain a film. The casting process parameters are: brushing rate is 2 cm / s, thickness is 40 μm, and drying temperature is: 70°C.

[0121] Step six: preparing a green body with microcrystalline parallel distribution;

[0122] The specific process is:

[0123] (1) The obtained microcrystalline film is cut into a rectangular strip with a length of 100 mm and a width of 30 mm. The width direction is parallel to the brushing direction, and the length direction is perpendicular to the brushing direction.

[0124] (2) The obtained rectangular strip is rolled up along the width direction at 50°C, and after the strip is completely rolled up, it is unfolded and rolled up along the width direction of the other side of the strip. Repeat this process 20 times. The strip is rolled through the vertical side of the tablet press; the pressure is 50 MPa, and the pressure holding time is 10 min.

[0125] (3) The pressed strip is cut into the desired size of green body by a cutting machine;

[0126] Step seven: prepare the target grain size distribution of ferroelectric texture ceramics;

[0127] The specific process is:

[0128] (1) The green body is heated from room temperature to 100℃ for 4h and kept for 1h, then heated to 200℃ for 4h and kept for 2h; when the keeping is finished, continue to heat to 600℃ for 18h and keep for 4h. After keeping, cool to room temperature with the furnace temperature, and get the sample after degassing.

[0129] (2) The degassed sample is placed in a high temperature box furnace and pre-fired at 1100℃ for 4h; after pre-firing, cool to 500℃ at a rate of 2℃ / min, then cool to room temperature with the furnace temperature; when the pre-fired sample is cooled to room temperature, heat the high temperature box furnace to 1300℃ at a rate of 5℃ / min, and sinter the degassed sample for 1h, get the sintered sample, and get the ferroelectric texture ceramics with specific grain size distribution after polishing, as shown in the right figure of Figure 5 Compared with the right figure of Figure 4 It can be found that the grain micro-morphology is completely consistent, which shows that the phase field method successfully predicts the evolution process of the microstructure of potassium strontium niobate ferroelectric texture ceramics, and can guide the repeated and stable preparation of the target microstructure.

Claims

1. A method for simulating the preparation of ferroelectric texture ceramic grain size by phase field method, characterized in that The method comprises the following steps: Step 1: preparing a series of potassium strontium niobate microcrystal powders with different size distributions; Step 2: setting an initial phase field structure meeting the particle size distribution; The specific process is: according to the size distribution of the obtained potassium strontium niobate microcrystalline powder sample, the initial phase field structure meeting the size distribution is set; firstly, a two-dimensional N1*N2 grid space system is opened, and needle-shaped particles meeting the size distribution are added in the system, the needle-shaped particles are parallel to each other and do not intersect; the size distribution function of the needle-shaped particles is f(x), and 4 grid points are equal to 1 microns; then the values of these regions in the system are assigned, and a set of order parameters η i (r,t) represents the grain orientation, i=1,2,3…p, and in all the particles, only one order parameter is 1 and the others are 0; Step 3: establishing a phase field model of grain orientation growth: according to the characteristics of grain boundaries formed between each crystal face of potassium strontium niobate crystal, an anisotropy function of grain boundary energy is constructed and introduced into the phase field model to obtain the phase field model of grain orientation growth, so that the grain orientation growth process is realized; Step 4: simulation and simulation of the grain orientation growth process; The specific process is: firstly, selecting one of the generated initial structures, then assigning values to the phase field parameters and performing simulation and simulation; by statistically analyzing the change trend of the grain size with the time step, whether it meets the normal growth curve is studied; the initial particle size distribution corresponding to the normal growth curve is recorded as the experimental guidance basis; then another initial structure is selected, the simulation and simulation are carried out, the change trend of the grain size is counted, and whether it meets the normal growth curve is studied; the operation is repeated in this way until all the initial structures are studied, and the potassium strontium niobate microcrystal powder sample corresponding to the normal grain growth is obtained; Step 5: preparing a film; Step 6: preparing a green body with parallel distribution of microcrystals; Step 7: preparing a ferroelectric texture ceramic with a target grain size distribution.

2. The method for preparing the ferroelectric texture ceramic grain size controlled by phase field method according to claim 1, characterized in that The specific process of step 1 is: (1) mixing the analytical pure strontium carbonate and niobium pentoxide powders, then adding potassium chloride powder to obtain a potassium strontium niobate microcrystal raw material mixture; the molar ratio of strontium carbonate to niobium pentoxide is 4:5; the potassium chloride is 2 times the weight of strontium carbonate and niobium pentoxide; (2) placing the potassium strontium niobate microcrystal raw material mixture in a polytetrafluoroethylene ball mill jar, adding anhydrous ethanol, and ball milling on a ball mill for 12 h to obtain a potassium strontium niobate microcrystal wet material; the weight ratio of the anhydrous ethanol to the potassium strontium niobate microcrystal raw material mixture is 1:1; the ball-milled potassium strontium niobate microcrystal wet material is placed in an oven and dried at 70℃; the dried powder is ground; the ground powder is placed in a corundum crucible, heated to 1100-1300℃ at a heating rate of 5℃ / min, and kept for 2h to calcine the ground powder; after calcination, the furnace is cooled to room temperature to obtain the calcined powder; (3) The calcined powder is repeatedly washed in distilled water at 100°C and filtered until no Cl is detected in the filtrate - ; the washed powder is dried at 70°C to obtain needle-like potassium strontium niobate microcrystalline powder; (4) placing the obtained needle-shaped potassium strontium niobate microcrystal powder in an alumina ball mill jar, adding anhydrous ethanol, and ball milling on a ball mill for 2-20h, and drying to obtain a series of potassium strontium niobate microcrystal powder samples with different size distributions.

3. The method for preparing the ferroelectric texture ceramic grain size simulated by the phase field method according to claim 1, characterized in that The specific process of step 5 is: (1) the auxiliary flux, the binder and the organic solvent are sequentially placed in a polytetrafluoroethylene ball mill jar, after ball milling for 12 hours, the strontium potassium niobate microcrystalline powder sample obtained by simulation guidance is added and ball milling is continued for 2 hours to obtain a slurry; the auxiliary flux is bismuth trioxide, which is 4% of the weight of the strontium potassium niobate microcrystalline powder; the organic solvent is prepared from toluene and anhydrous ethanol in a weight ratio of 2:1, which is 2 times the weight of the strontium potassium niobate microcrystalline powder; the binder is prepared from anhydrous ethanol, toluene, isopropyl alcohol, dibutyl phthalate and polyvinyl butyral, which is 45% of the weight of the strontium potassium niobate microcrystalline powder; the weight ratio of anhydrous ethanol to toluene in the binder is 1:1; isopropyl alcohol is 30% of the weight of anhydrous ethanol and toluene; dibutyl phthalate is 20% of the weight of anhydrous ethanol and toluene; polyvinyl butyral is 25% of the weight of anhydrous ethanol and toluene; (2) the filtered slurry is cast on a stainless steel plate by a conventional brushing method using a nylon brush with a width of 1.5 cm to obtain a film; the casting process parameters are: a brushing rate of 2 cm / s, a thickness of 20-40 μm, and a drying temperature of 70 °C.

4. The method for preparing the ferroelectric texture ceramic grain size simulated by the phase field method according to claim 1, characterized in that The specific process of step six is: (1) the obtained microcrystalline film is cut into a rectangular strip with a length of 100 mm and a width of 30 mm; the width direction is parallel to the brushing direction, and the length direction is perpendicular to the brushing direction; (2) the obtained rectangular strip is rolled up along the width direction at 50 °C, and then unfolded after the complete rolling up; the strip is rolled up along the width direction of the other side, and this is repeated for 20-50 times; the strip roll is pressed through a tablet press perpendicular to the side surface; the pressure is 50-200 MPa, and the pressure maintaining time is 10-30 min; (3) the pressed strip roll is cut into a green body of a desired size by a cutting machine.

5. The method for preparing the ferroelectric texture ceramic grain size by phase field method according to claim 1, characterized in that The specific process of step seven is: (1) the green body is heated from room temperature to 100 °C for 4 h and then heated to 200 °C for 2 h; when the heating is completed, the temperature is further increased to 600 °C for 18 h and then maintained for 4 h; after the heating is completed, the temperature is cooled to room temperature in the furnace, and a sample after the glue removal is obtained; (2) the sample after the glue removal is placed in a high-temperature box furnace and pre-fired at 1100-1200 °C for 0.5-4 h; after the pre-firing, the temperature is decreased to 500 °C at a rate of 2 °C / min, and then cooled to room temperature in the furnace; when the pre-fired sample is cooled to room temperature, the high-temperature box furnace is heated to 1300 °C at a rate of 5 °C / min, and the sample after the glue removal is sintered for 1-4 h to obtain a sintered sample, and the target grain size distribution of the ferroelectric texture ceramic is obtained after polishing and polishing.

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