RU2414017C1 - Method of producing composite piezoelectric material - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: powdered piezoceramic material is mixed with powdered blowing agent. The powdered blowing agent used is pre-sintered starting piezoceramic material with particle size of 10-20 mcm in amount of 40-60 wt %. A polyvinyl plasticiser is added. The workpiece is pressed and burnt.

EFFECT: reduction of pore size to 1-5 mcm, high relative porosity of the composite piezoelectric material, mechanical quality factor and wider operating band of the transducer made from the disclosed piezoelectric material.

2 cl, 3 dwg, 1 tbl

Description

The invention relates to microstructural technology, and in particular to methods for producing and processing porous piezoceramics and piezo-composite elements with a ceramic matrix, and can be used in broadband ultrasound transducers for medical diagnostic and therapeutic equipment, non-destructive testing and diagnostics operating in the frequency range of 10-20 MHz .

The possibilities of improving the properties of piezoceramics by changing the chemical composition of the material have been practically exhausted over the past 50 years. In connection with the need to improve the properties of materials in recent decades, the technology of composite piezomaterials has been developed that allows you to change the characteristics of the initial piezoceramics over a wide range without changing the chemical composition.

One of the classes of piezoelectric composite materials is porous piezoceramics and ceramic materials composites with a connectivity of 3-3, 3-0 based on them.

The electrophysical properties of porous piezoceramics are determined by the properties of the piezoelectric material, porosity, type of connectivity, shape and size of pores. Compared with porous lead zirconate titanate (PZT) ceramics, porous piezoceramics are characterized by lower values of acoustic impedance Z _a , mechanical Q factor Q _M , transverse piezoelectric module d ₃₁ and coupling coefficients k ₃₁ and k _p with almost unchanged value d ₃₃ and increased values of the coefficient the relationship of k _t , the longitudinal and volumetric piezoelectric sensitivities (g ₃₃ and g _v ), the volumetric piezoelectric module d _V = d ₃₃ + 2d ₃₁ and the reception factor (d _V g _V ). Therefore, porous piezoceramics are successfully used in acoustic receivers, hydrophones, pressure sensors and ultrasonic transducers (Lopatin S.S., Lupeyko T.G. Properties of porous piezoelectric ceramics such as lead zirconate titanate. Izv. AN SSSR. Ser. Inorg. Mater. - 1991. - V.27, N 9. S.1948-1951.) / 1 /.

The following methods are known for producing porous piezoceramics described in the review (Wersing W., Lubitz K., Moliaupt J. Dielectric, elastic and piezoelectric properties of porous PZT ceramics // Ferroelectrics. - 1986. - V.68, N 1/4. - P.77-79.) / 2 /.

1. A method of burning polymer granules (BURPS - burning out of plastic spheres). ZTS powder and polymer granules are mixed with an organic binder and pressed in the form of elements of the desired shape. The polymer granules are burned together with a binder at a low temperature, after which the ceramics are sintered. The porosity of the sample easily varies in size and number of polymer granules and can reach 70%.

2. A method based on the use of water-soluble granules. ZTS powder is mixed with water-soluble granules and an organic binder and formed into the necessary elements. The granules are washed from the billet with water, after which the ceramic is sintered.

3. The method of polymer foam. The suspension, consisting of PZT powder mixed with water, is reacted with a polymeric blowing agent to form a porous preform, which is slowly dried. Then the polymer is burned out, and the ceramic is sintered. This method allows to obtain ceramic frames with porosity up to 95%.

4. The cryochemical method, which consists in quickly freezing a mixture of salt solutions, followed by moisture removal by sublimation in vacuum and thermal decomposition of the salt product. This method allows to obtain highly porous ceramic frames, consisting of particles with a size of 5-7 microns with a porosity of up to 95%.

5. Methods based on the thermal decomposition of hydroxides, carbonates, nitrates or oxalates, organic compounds, as well as chemical etching and activation of carbon.

There is also known a method of manufacturing a porous piezoelectric ceramic material in which, in order to increase the reproducibility of properties, the powder of the piezoceramic material is mixed with a powder of a porogen — lithium carbonate in an amount of 1-10 wt.%, Polyvinyl plasticizer is added, the blanks are pressed and heat treated to burn the porogen, followed by sintering ceramics (JP 1089486 (A), HO1L 41/22, HO1L 41/24, C04B 38/02, 1985-04-03) / 3 /. When burned, lithium carbonate decomposes with the release of carbon dioxide, however, the remaining lithium reacts with ceramic components with the formation of local unevenly distributed doped regions, which changes the properties of piezoceramics in an uncontrolled manner. A small amount of a pore-forming agent does not allow obtaining microporous ceramics with porosity above 10%, which limits the possibility of improving the electrophysical parameters of piezoceramics and application in devices with an operating frequency above 5 MHz.

A modification of the above method is a method of manufacturing a porous piezoelectric ceramic with a gradient porosity of 3-50% (CN 1953226 (A), HO1L 41/187, C04B 35/622, 2007-04-25) / 4 /, which consists in the formation of 3-5 layers of porous ceramics with a thickness of 0.2-0.5 mm with a different content of organic blowing agent 0-50%. An increase in porosity over the thickness of the sample allows one to reduce the acoustic impedance of the porous ultrasonic transducer for acoustic matching with biological tissues or water. However, the described method is not suitable for mass production and does not allow to obtain thin less than 1 mm piezoelectric elements for high-frequency ultrasonic transducers.

The closest in the method of forming a porous ceramic frame and the achieved result to the claimed invention is a method of manufacturing a porous piezoelectric ceramic based on the thermal decomposition of organic compounds (JP 4024971 (A), HO1L 41/24, 1992-01-28) 151, adopted as a prototype . In order to prevent cracking during sintering of ceramics, the synthesized PZT piezoceramics powder is mixed with a pore former in the form of a powder of spherical particles of paraffin with a diameter of 900 μm in an amount of 10-20 wt.%, Followed by burning and sintering of porous ceramics. The result is a large-porous structure of ceramics with closed porosity (3-0 connectivity) for low-frequency ultrasonic transducers. A small amount of a pore-forming agent does not allow obtaining microporous ceramics with a relative porosity above 10%, which will limit the possibility of improving the electrophysical parameters of piezoceramics and use in devices with an operating frequency above 1 MHz. An increase in the number of pore-forming agents and a decrease in the size of spherical particles of paraffin powder will lead to particle sticking, uneven distribution of pores, the formation of through holes and the inability to obtain piezomaterial.

The objective of the present invention is to provide a method for producing a microporous composite piezoelectric material for broadband ultrasonic transducers for operation in the frequency range 10-20 MHz.

The problem was solved with the achievement of a new technical result - reducing the pore size to 1-5 μm, increasing the relative porosity of the composite piezoelectric material by more than 10% due to the introduction of a non-shrinkable piezoelectric phase into the mixture of the piezoceramic material, which prevents the shrinkage of the piezoceramic matrix during sintering. The inventive method is universal and allows you to get microporous composite piezoelectric materials based on any piezoceramic compositions obtained by conventional ceramic technology, for example, PZT, lead titanate, lead meta- and magnoniobate, etc.

The specified technical result is achieved by the fact that in the known method for producing a porous composite piezoelectric material, which consists in mixing a powder of a piezoceramic material with a pore-forming powder in a weight ratio, providing a porous ceramic matrix with closed pores, adding a polyvinyl plasticizer, pressing and calcining the preform according to the invention as pore former powder use pre-sintered piezoceramic material powder with a particle size of 10-20 microns in an amount of 40-60 wt.%.

In the particular case of the method, the composition of lead zirconate titanate Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O _{5 is} used as the starting piezoceramic material.

Figure 1 shows an electron micrograph of SEM, Karl Zeiss, a porous composite piezoelectric material obtained by the claimed method, where 1 is a particle of a sintered piezoelectric material of the composition Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ , 2 — microporous ceramic matrix Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ , 3 — micropore.

Figure 2 shows the dependence of the shrinkage coefficient K _mustache. ^diam. the diameter of the sample of the porous composite piezoelectric material Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ obtained by the claimed method, the content of wt.% pre-sintered particles of the original piezoceramic material Pb _{0, 95} Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ .

Figure 3 shows the dependences of the relative porosity P% (curve 1), calculated (curve 2) and measured (curve 3) density ρ of the porous composite piezoelectric material Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ obtained by the claimed method, from the content of wt.% Pre-sintered particles of the starting piezoceramic material Pb _0,95 Sr _0,05 Ti _0,47 Sr _0,53 О ₃ + 1% Nb ₂ O ₅ .

The table shows the electrophysical parameters of the porous composite piezoelectric material Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ obtained by the claimed method, with different contents of wt.% Pre-sintered particles of the original piezoceramic material Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ .

The method is as follows. From the synthesized powder of piezoelectric ceramics Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ , ceramic billets with a diameter of 20 mm and a thickness of 20 mm are made by conventional ceramic technology. Billets are sintered at a temperature of 1240 ° C in a muffle furnace. Sintered blanks are ground in a ball mill and calibrated using a set of sieves to a size of 10-20 microns.

In the initial synthesized powder of piezoelectric ceramics of lead zirconate titanate Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ add a pore former in the form of a powder of pre-sintered piezoceramic material of the same composition Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ in an amount of 40-60 wt.% And mixed in a ball mill for 12 hours to obtain a homogeneous mass. To the resulting mixture add 5% five percent aqueous solution of polyvinyl plasticizer, mix thoroughly and mold in a metal mold under pressure. Then, the obtained preforms are sintered at a temperature of 1240 ° C in a muffle furnace. Particles of pre-sintered piezoceramic material that have undergone heat treatment at sintering temperature do not shrink during repeated sintering and represent the piezoelectric non-shrink phase of the composite. During sintering of composite material blanks, the non-shrinking phase prevents shrinkage of the initial synthesized piezoceramic material, which leads to the appearance of microporosity due to micro-fractures of the continuous ceramic matrix. The result is a microporous composite piezomaterial with a pore size of 1-5 μm and a porosity of 15-20%.

Then, disks with a diameter of 20 mm and a thickness of 1 mm were made from the obtained material for measuring electrophysical parameters. Electrodes were deposited onto the main surfaces of the disks by burning silver-containing paste. The disks were polarized in silicone oil at a temperature of 170 ° C for 30 minutes in a field of 3 kV / mm. The electrophysical parameters, shrinkage coefficient, density, and relative porosity of the microporous composite piezoelectric material were measured by standard methods in accordance with OST 11 0444-87 (Piezoceramic materials).

As can be seen from figure 1, the obtained porous composite piezomaterial is a composite structure with a ceramic matrix with a 3-0 connectivity, containing particles of sintered piezomaterial 1 with a size of 10-20 μm, and micropores 3 with a size of 1-5 μm, uniformly distributed in the ceramic matrix 2 .

As can be seen from figure 2, the shrinkage coefficient of the porous composite material K _mustache. ^diam. rapidly decreases from 1.18 to 1.02 with an increase in the concentration of particles of pre-sintered piezomaterial from 0 to 100 wt.%, due to an increase in the concentration of the non-shrink phase in the ceramic matrix.

As can be seen from figure 3, the calculated density of the composite material does not change when changing the content of pre-sintered granules of the same composition and is 8 g / cm ³ (curve 2). At the same time, the measured density ρ of the porous composite piezoelectric material (curve 3) decreases from 8 g / cm ³ to 5.5 g / cm ³ , and the relative porosity P% (curve 1) increases from 0 to 30% with an increase in the concentration of particles previously sintered piezomaterial from 0 to 100 wt.%, which is due to a decrease in the shrinkage coefficient (figure 2), leading to microfractures of the ceramic matrix and the formation of micropores.

As follows from the table, the obtained microporous composite piezoelectric material with a content of sintered granules of 40-60 wt.% Has a relative porosity of 15-20% of the closed type, high values of the longitudinal piezoelectric module d ₃₃ and the electromechanical coupling coefficient of the vibration mode thickness k _{t of the} initial piezoceramic material at low the values of the electromechanical coupling coefficient of the planar vibrational mode k _p , the transverse piezoelectric module d ₃₁ and the mechanical Q factor Q _m , which is necessary to create broadband ultrasonic transducers with high efficiency, sensitivity and resolution, operating in the frequency range 10-20 MHz.

When the content of sintered granules is less than 40 wt.%, The resulting porosity of the composite material is less than 10%, which does not provide a sufficient decrease in the mechanical quality factor and transverse piezoelectric and electromechanical coefficients. With an increase in the content of sintered granules above 60 wt.%, The piezoelectric and electromechanical characteristics of the porous composite material deteriorate. Closed microporosity of 15-20% with an average pore size of 1-5 microns and a microgranule size of 10-20 microns allows the manufacture of thin piezoelectric elements 100-200 microns thick (operating frequency 10-20 MHz). The content of sintered granules in the microporous composite material in an amount of 40-60% provides additional scattering of high-frequency ultrasound on dense granules (Fig. 1) with dimensions of 10-20 μm, close to the ultrasonic wavelength of 100-200 μm at a frequency of 10-20 MHz, resulting to reduce the mechanical quality factor Q _m to 20-40 and, consequently, the expansion of the working bandwidth of the ultrasonic transducer.

Information sources

1. Lopatin S.S., Lupeyko T.G. Properties of porous piezoelectric ceramics such as lead zirconate titanate. Izv. USSR Academy of Sciences. Ser. Inorg. Mater. - 1991. - V.27, N 9. S.1948-1951.

2. Wersing W., Lubitz K., Moliaupt J. Dielectric, elastic and piezoelectric properties of porous PZT ceramics // Ferroelectrics. - 1986.- V.68, N 1/4. - P.77-79.

3. CN 1953226 (A), HO1L 41/187, C04B 35/622, 2007-04-25.

4. JP 1089486 (A), HO1L 41/22, HO1L 41/24, C04B 38/02, 1985-04-03.

5. JP 4024971 (A), HO1L 41/24, 1992-01-28 - prototype.

Table The electrophysical parameters of the porous composite piezoelectric material Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ obtained by the claimed method, with different contents of wt.% Pre-sintered particles of the original piezoceramic material Pb _{0, 95} Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O ₅ . Parameter / wt.% 0 twenty 40 60 one hundred Porosity,% 2.5 5.75 fifteen twenty thirty d ₃₃ , pCl / N 290 280 290 290 260 d ₃₁ pCl / N 120 98 75 45 twenty ε ₃₃ ^T , 10 ⁹ F / m 11.20 9.35 7.45 5.86 4.05 k _p 0.561 0.473 0.3 0.201 0.109 k _t 0.498 0.498 0.51 0.516 0.440 Q _m 650 67 40 22 16

Claims (2)

1. The method of obtaining a composite piezomaterial, which consists in mixing the powder of the piezoceramic material with the powder of the blowing agent in a weight ratio, providing a porous ceramic matrix with closed pores, adding polyvinyl plasticizer, pressing and firing the workpiece, characterized in that the pre-sintered starting material is used as the blowing powder piezoceramic material with a particle size of 10-20 microns in an amount of 40-60 wt.%. 2. The method according to claim 1, characterized in that the composition of lead zirconate titanate Pb _0.95 Sr _0.05 Ti _0.47 Sr _0.53 O ₃ + 1% Nb ₂ O _{5 is} used as the starting piezoceramic material.

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