RU2623693C2 - Method of obtaining composite piezomaterial - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: mixed powder synthesized piezoceramic material with a blowing agent powder in a weight ratio, which ensures obtaining of a porous ceramic matrix with closed pores, by adding polyvinyl plasticiser, moulding and calcination of the workpiece, characterised in that the powder is used as a blowing agent powder crystalline corundum (α-Al₂O₃) with a mean particle size of 40-400 microns in an amount of 10-20 vol. %.

EFFECT: reduction in mechanical quality factor of the piezoceramic element.

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Description

The invention relates to ceramic technology, and in particular to methods for producing and processing porous piezoceramics and ceramic-matrix piezocomposite elements, and can be used in broadband ultrasonic transducers for non-destructive testing and diagnostics, geophysics, medical diagnostic and therapeutic equipment operating in the frequency range from 500 kHz to 5 MHz.

State of the art

The possibilities of improving the properties of piezoceramics by changing the chemical composition of the material over the past 50 years are almost exhausted. In connection with the need to improve the properties of existing materials in recent decades, the development of technology of composite piezoelectric materials, which allow you to change the characteristics of the original piezoceramics over a wide range without changing the chemical composition.

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

In recent years, significant progress has been made in improving the mechanical properties of ceramic materials using the technology of ceramic composite. Structural ceramic-matrix composites with improved mechanical and thermal properties are widely used in the aerospace, automotive and energy industries [1, 2].

Functional ceramic composites, unlike structural composites, are much less studied and their use in the electronics industry is very limited. More recently, effective hardening mechanisms of structural ceramics have been transferred to the field of functional ceramics to improve their mechanical and electrical properties. However, the problem of the compromise of properties, namely, the deterioration of the piezoelectric properties of the composite due to changes in the microstructure of ceramics (suppression of grain growth and, as a result, clamping and decreased mobility of the domains), in particular, the electromechanical coupling coefficient, remains unresolved.

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 to 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) [3].

The following methods are known for producing porous piezoceramics described in a 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) [4]:

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 a 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 No. 1089486 (A), HO1L 41/22, HO1L 41/24, C04B 38/02, published 04/03/1985) [5].

When burned, lithium carbonate decomposes with the release of carbon dioxide, however, the remaining lithium reacts with ceramic components to form 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 a porosity higher than 10%, which will limit the possibility of improving the electrophysical parameters of piezoceramics and application in wideband low-frequency devices with an operating frequency in the range from 1 kHz to 2 MHz.

A modification of the above method is a method of manufacturing a porous piezoelectric ceramic with a gradient porosity of 3-50% (CN No. 1953226 (A), HO1L 41/187, C04B 35/622, published April 25, 2007) [6], 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 piezoelectric elements of sufficient thickness for broadband ultrasonic transducers.

There is also a known method of manufacturing a porous piezoelectric ceramic based on the thermal decomposition of organic compounds (JP No. 4024971 (A), HO1L 41/24, published January 28, 1992) [7]. 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 of more than 10%, which will limit the possibility of improving the electrophysical parameters of piezoceramics and use in broadband devices with an operating frequency from 1 kHz to 2 MHz. An increase in the number of pore former and a decrease in the size of spherical particles of paraffin powder leads to particle sticking, uneven pore distribution, the formation of through holes and the inability to obtain a piezomaterial. The closest in the method of forming a porous ceramic frame and the achieved result to the claimed invention is a method for producing a composite piezoelectric material, which consists in using a pre-sintered initial piezoelectric ceramic material as a pore former (RU No. 2414017 C1, HO1L 41/24, published March 10, 2011) [8 ] taken as a prototype. The method is as follows. In the initial synthesized powder of piezoelectric ceramics of lead zirconate titanate Pb _0.95 Sr _0.05 Ti _0.47 Zr _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 with a particle size of 10 -20 microns in an amount of 40-60 wt.%. After preparing the mixture, molding with the addition of a plasticizer and firing the blanks in a muffle furnace, a porous piezoceramic material is obtained. During sintering of composite material blanks, the non-shrinking phase (particles of pre-sintered piezoelectric material) 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%.

The content of sintered granules in the microporous composite material in an amount of 40-60 wt.% Provides additional scattering of high-frequency ultrasound on dense granules at a frequency of 10-20 MHz, leading to a decrease in the mechanical quality factor Q _M to 20-40 and, consequently, the expansion of the working bandwidth of the ultrasonic transducer. However, such a decrease in the mechanical quality factor Q _M is insufficient even for high-frequency vibrations, and for low-frequency converters it is not observed in principle because of the large ratio of wavelengths to the size of sintered granules and low acoustic contrast (sintered granules and ceramic matrix have the same chemical composition, density and elastic properties).

Disclosure of invention

The objective of the present invention is to provide a method for producing a composite piezoelectric material with a microporous ceramic matrix for broadband ultrasonic transducers operating in the frequency range from 500 kHz to 5 MHz.

The problem was solved with the achievement of a new technical result - a decrease in the mechanical quality factor of the piezoceramic element Q _M <5 due to the introduction of a non-shrink non-piezoactive filler with high acoustic contrast (the difference in the density and speed of sound in the matrix and filler materials), preventing the shrinkage of the piezoceramic matrix during sintering and leading to the formation of a microporous matrix with inclusions in the form of corundum particles scattering the ultrasonic wave.

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 piezoceramic material powder 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, crystalline corundum powder (α-Al ₂ O ₃ ) with an average particle size D, bl a short ultrasonic wavelength λ (0.1λ <D <λ) in a piezoelectric element in an amount of 10-20 vol.%. In the claimed frequency range from 500 kHz to 5 MHz (λ = 8 and 0.8 mm, respectively), the average particle size of α-Al ₂ O ₃ varies from 400 to 40 microns.

In the particular case of the method, the composition of lead zirconate titanate PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O _{3 is} used as the starting piezoceramic material.

The invention is illustrated by drawings.

In FIG. 1 shows an optical micrograph of a porous composite piezoelectric material obtained by the claimed method, where 1 is a particle of α-Al ₂ O ₃ , 2 is a microporous ceramic matrix PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O ₃ , 3 is micropore .

In FIG. 2 shows the dependence of the shrinkage coefficient K _U ^diam. by the diameter of the sample of the porous composite piezoelectric material PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O ₃ obtained by the claimed method, from the content of particles α-Al ₂ O ₃ vol.%.

In FIG. ^Figure 3 shows the dependences of the relative porosity P%, calculated ρ ^theor. and measured ρ ^exper. the density of the porous composite piezoelectric material PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O ₃ obtained by the claimed method, the content of α-Al ₂ O ₃ vol.%.

The table shows the electrophysical parameters of the porous composite piezoelectric material PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O ₃ obtained by the claimed method, with different particle contents of α-Al ₂ O ₃ vol.%.

The method is as follows. In the initial synthesized powder of piezoelectric ceramics of lead zirconate titanate PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O ₃ add a pore _former in the form of a powder of crystalline corundum (α-Al ₂ O ₃ ) with an average particle size of 150 μm in quantity 10-20 vol.% And mixed in a ball mill for 12 hours to obtain a homogeneous mass. A 5% aqueous solution of polyvinyl plasticizer is added to the resulting mixture, mixed thoroughly and molded in a metal mold under pressure. Then, the obtained preforms are sintered at a temperature of 1240 ° C in a muffle furnace. During sintering of composite material blanks, the non-shrinking phase (α-Al ₂ O ₃ particles) prevents the initial synthesized piezoceramic material from shrinking, 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%.

The content of particles of α-Al ₂ O ₃ with an average size of 150 μm in the microporous composite material in an amount of 10-20 vol.% Provides additional scattering of high-frequency ultrasound on particles of α-Al ₂ O ₃ having high acoustic contrast (density α-Al ₂ O ₃ and piezoelectric ceramics of lead zirconate titanate 4 and 8 g / cm ³ , and the speed of sound 4000 and 11200 m / s, respectively), leading to a decrease in the mechanical quality factor Q _M and, consequently, to an increase in the working passband of ultrasonic transducers.

To measure the electrophysical parameters, disks with a diameter of 20 mm and a thickness of 1 mm were made from the obtained material. 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 measurement of electrophysical parameters, shrinkage coefficient, density and relative porosity of the microporous composite piezoelectric material was carried out by standard methods in accordance with OST 11 0444-87: Piezoceramic materials. [9].

As can be seen from FIG. 1, a porous composite piezoelectric material, which is a composite structure with a porous ceramic matrix containing particles of α-Al ₂ O ₃ with an average size of 150 μm in an amount of 20 vol.% Is an inhomogeneous acoustic structure that can effectively scatter ultrasonic waves of certain frequencies.

As can be seen from FIG. 2, the shrinkage coefficient of the porous composite material K _U ^diam. rapidly decreases from 1.18 to 1.02 with an increase in the concentration of particles of pre-sintered piezomaterial from 0 to 60 vol.%, which is due to an increase in the concentration of non-shrink phase in the ceramic matrix.

As can be seen from FIG. 3, the estimated density ρ ^theor. composite material when changing the particle content of α-Al ₂ O ₃ from 0 to 60 vol.% varies linearly from 8 g / cm ³ to 5.5 g / cm ³ . At the same time, the measured density is ρ ^exper. the porous composite piezomaterial decreases from 8 g / cm ³ to 4 g / cm ³ and the relative porosity of the ceramic composite P% rapidly increases with increasing concentration of α-Al ₂ O ₃ from 0 to 10 vol.%, and with a further increase in the content of α- Al ₂ O ₃ up to 60 vol.% Grows almost linearly.

As follows from the table, obtained in accordance with the claimed method, the microporous composite piezomaterial with a particle content of α-Al ₂ O ₃ 10-20 vol.% Has a relative microporosity of 10-20% of the closed type, high values of the longitudinal piezoelectric module d ₃₃ and the electromechanical coupling coefficient the thickness of the vibration mode k _t exceeding the corresponding values for the initial piezoceramic material, with reduced values of the coefficient of electromechanical coupling of the planar vibration mode k _p transverse to the piezoelectric mode For d ₃₁ and mechanical quality factor Q _M , which is necessary for creating broadband ultrasonic transducers with high efficiency, sensitivity and resolution, operating in the frequency range 500 kHz - 5 MHz.

The content of particles of α-Al ₂ O ₃ less than 10 vol.% Does not provide sufficient porosity and lower mechanical quality and transverse piezoelectric and electromechanical coefficients. With an increase in the particle content of α-Al ₂ O ₃ above 20 vol.%, The piezoelectric and electromechanical characteristics of the porous composite material deteriorate.

The optimum content of particles of α-Al ₂ O ₃ in the microporous composite material in an amount of 10-20 vol.% With an average particle size of 150 μm provides additional ultrasound scattering on particles of α-Al ₂ O ₃ , which at a frequency of 2 MHz leads to a decrease in mechanical quality factor to record values of Q _M = 2-3 and allows you to get broadband ultrasonic transducers without additional damping of the piezoelectric element.

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, lead titanate zirconate, lead titanate, lead meta- and magnoniobate, etc.). The obtained composite piezomaterials can be used for the manufacture of broadband ultrasonic transducers operating in the frequency range 500 kHz - 5 MHz.

Information sources

1. Yang V., Chen H.M. Alumina ceramics toughened by piezoelectric secondary phase. J. Eur. Cer. Soc. 2000. V. 20. P. 1687-1690.

2. Xiang P.-H., Dong X.-L., Chen H., Zhang Z., Guo J.-K. Mechanical and electrical properties of small amount of oxides reinforced PZT ceramics. Ceramics International. 2003. V. 29. P. 499-503.

3. 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. N9. S. 1948-1951.

4. 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.

5. JP No. 1089486 (A), HO1L 41/22, HO1L 41/24, C04B 38/02, published 04/03/1985.

6. CN No. 1953226 (A), HO1L 41/187, C04B 35/622, published 04/25/2007.

7. JP No. 4024971 (A), HO1L 41/24, published January 28, 1992.

8. RU No. 2414017 C1, HO1L 41/24, published on 03/10/2011 - prototype.

9. OCT 11 0444-87: Piezoceramic materials.

Claims (2)

1. A method of obtaining a composite piezomaterial, which consists in mixing the synthesized powder of a piezoelectric material with a pore-forming powder in a weight ratio, providing a porous ceramic matrix with closed pores, adding a polyvinyl plasticizer, pressing and firing the workpiece, characterized in that crystalline powder is used as a pore-forming powder corundum (α-Al ₂ O ₃ ) with an average particle size of 40-400 microns in an amount of 10-20 vol.%. 2. The method according to p. 1, characterized in that the composition of lead zirconate titanate PbTi _0.6 Zr _0.336 W _0.006 Mn _0.0233 Nb _0.0347 O _{3 is} used as the starting piezoceramic material.

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