Ferrite powder for backing material of ultrasonic vibrator

Abstract

To provide ferrite powder from which a backing material of an ultrasonic transducer having excellent performance and productivity can be obtained.SOLUTION: A ferrite powder for backing material of ultrasonic transducer includes 8.0% by mass or more and 9.0% by mass or less of strontium (Sr) and 58.0% by mass or more and 63.0% by mass or less of iron (Fe), and having a volume average particle diameter in the range of 30 μm or more and 50 μm or less, and the proportion of particles with a particle size of less than 16 μm is 15% by volume or more and 35% by volume or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to ferrite powder for a backing material of an ultrasonic vibrator.

Background Art

[0002] An ultrasonic vibrator is an electroacoustic transducer (electroacoustic transformer) that converts an electrical high-frequency signal into a mechanical ultrasonic vibration or, conversely, converts an ultrasonic vibration into an electrical signal. Ultrasonic vibrators are used in a wide range of fields, including ultrasonic probes used in medical ultrasonic diagnostic devices and imaging inspection devices, underwater acoustic transducers such as sonars, and industrial transducers for non-destructive inspection. In particular, with the recent progress of medical technology, the importance of medical ultrasonic vibrators (probes) has been attracting attention, and the development of their technology has been promoted.

[0003] An example of the specific configuration of a medical ultrasonic vibrator (probe) will be described with reference to FIG. 1. The ultrasonic probe (100) has a structure in which a laminate in which a piezoelectric body (8), an impedance matching layer (12), and an acoustic lens (14) are laminated in this order between a backing material (4), a back electrode (6), and a front electrode (10) is encapsulated in a sealing material (2). When a high-frequency pulse signal is input to the electrodes (6, 10) with the ultrasonic probe (100) in contact with a subject such as a living body, the dimensions of the piezoelectric body (8) fluctuate (vibrate) to generate ultrasonic waves. The generated ultrasonic signal is transmitted to the subject through the impedance matching layer (12) and the acoustic lens (14) and converges at a predetermined position. The converged ultrasonic signal changes its signal characteristics according to the internal structure of the subject and is reflected as a reflected signal (echo signal). The reflected echo signal is received by the piezoelectric body (8), and this is analyzed and / or imaged as a received signal to examine the internal structure of the subject.

[0004] The backing material, also called a damper, supports the piezoelectric element and serves to attenuate the ultrasonic waves radiated to the back side of the piezoelectric element. That is, the piezoelectric element radiates ultrasonic waves not only to its front surface but also to its back surface. If no backing material is provided, the ultrasonic waves radiated to the back side are reflected by the sealing material and re-enter the piezoelectric element, where they may be detected as an error signal. By providing a backing material, it becomes possible to suppress the error signal and increase the S / N ratio of the received signal, that is, the reception sensitivity. Therefore, the backing material is required to sufficiently attenuate ultrasonic waves, that is, to have excellent attenuation performance. If the attenuation performance of the backing material is poor, the ultrasonic waves radiated from the piezoelectric element to the back side are not sufficiently attenuated, which causes an error signal.

[0005] In addition, the backing material serves to suppress the vibration of the piezoelectric element. By appropriately suppressing the vibration of the piezoelectric element, short ultrasonic pulses can be obtained, and as a result, it becomes possible to increase the resolution of the echo signal. To suppress the vibration of the piezoelectric body, it is effective to increase the acoustic impedance of the backing material. However, if the vibration is excessively suppressed, the reception sensitivity of the echo signal may decrease. Therefore, the backing material is required to have its acoustic impedance controlled within an appropriate range. The acoustic impedance is represented by the product of the speed of sound in the backing material and the specific gravity of the backing material, as shown in the following equation (1).

[0006]

Equation

[0007] Thus, the backing material is required to have excellent ultrasonic attenuation performance and its acoustic impedance is controlled within an appropriate range. It is difficult to satisfy such requirements with a single material. Therefore, conventionally, a composite material in which a high specific gravity filler such as ferrite powder is dispersed in a matrix component such as rubber or resin is frequently used as the backing material. By using rubber or resin rich in flexibility as the matrix component, the absorption and attenuation of ultrasonic waves are promoted. In addition, by adding a high specific gravity filler, the acoustic impedance is controlled and the ultrasonic waves are scattered to improve the attenuation performance.

[0008] Patent Documents 1 to 3 disclose ultrasonic transducers provided with a backing material. Patent Document 1 discloses an ultrasonic transducer mainly composed of a piezoelectric body, a front electrode and a back electrode, an acoustic matching layer and an acoustic lens, and a backing material formed of, for example, rubber containing ferrite for attenuating ultrasonic waves to the rear side (

[0013] of Patent Document 1 and FIGS. 1 to 3). Further, Patent Document 1 describes that the ultrasonic transducer is mainly used for an ultrasonic endoscope that images a tomogram in a living body using an ultrasonic echo (

[0001] of Patent Document 1).

[0009] Patent Document 2 discloses an ultrasonic probe including a vibrator that transmits and receives ultrasonic waves and a damper that absorbs ultrasonic vibrations on one side of the vibrator, and describes that the damper is made of a flexible resin mixed with ferrite powder (Claim 1 of Patent Document 2). Further, Patent Document 2 describes that when a damper made of ferrite powder and a flexible resin is adopted, the acoustic impedance and the mechanical Q value of the damper are optimized (

[0010] of Patent Document 2). Patent Document 3 describes that, regarding an ultrasonic probe used for medical ultrasonic diagnosis, non-destructive inspection, etc., as the backing material, a material obtained by mixing ferrite powder into a rubber material has been put into practical use (the left column on the first page and the upper right column to the lower left column on the second page of Patent Document 3).

[0010] In addition, Patent Document 4 discloses a ferrite powder that, although not for use as an ultrasonic vibrator backing material, can be suitably used for manufacturing a molded body that can be stably detected by a metal detector. Specifically, it discloses a ferrite powder detectable by a metal detector, which contains hard ferrite particles containing 7.8 mass% or more and 9.0 mass% or less of Sr and 61.0 mass% or more and 65.0 mass% or less of Fe, and is characterized in that the amount of Na measured by ion chromatography is 1 ppm or more and 200 ppm or less (Claim 1 of Patent Document 4).

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0012] As described above, although ultrasonic vibrators equipped with backing materials (dampers) have been widely used conventionally, there has been room for improvement in these backing materials. That is, there is a desire to miniaturize ultrasonic vibrators, and accordingly, further miniaturization of the backing materials constituting ultrasonic vibrators has been required. In particular, some medical ultrasonic vibrators (probes) are inserted into body cavities for use, and strong demands for miniaturization of ultrasonic vibrators and backing materials have been made.

[0013] In order to miniaturize the backing material, it is effective to enhance the damping performance and vibration isolation performance of the material itself. Also, the workability of the material cannot be ignored when aiming for miniaturization. Particularly in recent years' backing materials for medical ultrasonic transducers, it is necessary to perform laser processing with a width of 100 μm within 3 mm, and fine workability is important. Furthermore, in order to miniaturize the backing material, the surface property of the backing material is also important. This is because if there is a gap at the interface between the backing material and the piezoelectric element that constitutes the ultrasonic transducer, the ultrasonic waves generated from the piezoelectric element may be reflected at that interface, resulting in an error signal. Therefore, it is desirable that the backing material has a smooth surface so that it can closely adhere following the surface of the piezoelectric element.

[0014] However, it has been difficult to improve the damping performance, vibration isolation performance, workability, and surface property in a well-balanced manner with the conventionally proposed backing materials. In order to enhance the vibration isolation performance, it is effective to increase the acoustic impedance of the backing material, and for this purpose, it is realistic to increase the proportion of high-specific gravity fillers such as ferrite powder. However, when increasing the proportion of the filler, it is likely to cause problems when mixing, molding, and processing this with matrix components such as rubber. Also, when increasing the proportion of the high-specific gravity filler, the surface of the backing material becomes uneven, and the surface property is likely to deteriorate. Therefore, it has been difficult to fabricate an ultrasonic transducer that can be miniaturized while maintaining performance using conventional backing materials.

[0015] The inventors of the present invention conducted intensive studies in view of such conventional circumstances. As a result, they obtained the knowledge that the composition, particle size, and shape of the ferrite powder contained in the backing material of the ultrasonic transducer have a great influence on the performance of the backing material (damping performance, vibration isolation performance, acoustic impedance, surface property, and workability), and that by improving this, a backing material excellent in performance and productivity can be obtained.

[0016] The present invention has been completed based on such knowledge, and an object thereof is to provide ferrite powder capable of obtaining a backing material for an ultrasonic transducer excellent in performance and productivity.

Means for Solving the Problems

[0017] The present invention includes the following aspects (1) to (4). In this specification, the expression "~" includes the numerical values at both ends. That is, "X~Y" is synonymous with "X or more and Y or less".

[0018] (1) Containing strontium (Sr) in an amount of 8.0% by mass or more and 9.0% by mass or less, and iron (Fe) in an amount of 58.0% by mass or more and 63.0% by mass or less, A ferrite powder for a backing material of an ultrasonic vibrator, having a volume average particle diameter in the range of 30 μm or more and 50 μm or less, and the proportion of particles having a particle diameter of less than 16 μm being 15% by volume or more and 35% by volume or less.

[0019] (2) The ferrite powder according to claim 1, wherein the ferrite powder contains hollow particles, and the number ratio of the hollow particles is 5 to 90% with respect to the total number of particles in the ferrite powder.

[0020] (3) The BET specific surface area is 0.50 m 2 / g or more and 3.00 m 2 / g or less, of the ferrite powder according to (1) or (2) above.

[0021] (4) The pore volume is 100 mm 3 / g or more and 300 mm 3 / g or less, of the ferrite powder according to any one of (1) to (3) above.

Advantages of the Invention

[0022] According to the present invention, there is provided a ferrite powder capable of obtaining a backing material for an ultrasonic vibrator excellent in performance and productivity.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0024] Specific embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described. Note that the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention.

[0025] <<1. Ferrite Powder for Backing Material of Ultrasonic Transducer>> The ferrite powder of the present embodiment is used to produce a backing material for an ultrasonic transducer. That is, a backing material can be obtained by mixing and kneading this ferrite powder as a filler with a matrix component such as rubber. This ferrite powder contains strontium (Sr) in an amount of 8.0% by mass or more and 9.0% by mass or less, and iron (Fe) in an amount of 58.0% by mass or more and 63.0% by mass or less. Further, this ferrite powder has a volume average particle diameter in the range of 30 μm or more and 50 μm or less, and the proportion of particles having a particle diameter of less than 16 μm is 15% by volume or more and 35% by volume or less.

[0026] The ferrite powder of the present embodiment has a strontium (Sr) amount of 8.0% by mass or more and 9.0% by mass or less, and an iron (Fe) amount of 58.0% by mass or more and 63.0% by mass or less. This ferrite powder has a composition of strontium (Sr) ferrite. Sr ferrite has a hexagonal magnetoplumbite-type crystal structure and has a composition of SrO·6Fe2O3 in its stoichiometric composition. Also, Sr ferrite is a hard ferrite. That is, it is a hard magnetic material (hard material) with high saturation magnetization and magnetic anisotropy, and is widely used as a material for permanent magnets.

[0027] By using ferrite powder with a strontium (Sr) ferrite composition, it becomes possible to enhance the vibration damping performance while maintaining the acoustic impedance characteristics and workability of the backing material. As a ferrite having a hexagonal magnetoplumbite-type structure, barium (Ba) ferrite (BaO·6Fe2O3) is known in addition to strontium (Sr) ferrite. However, since Ba ferrite has a large atomic weight of Ba, its true specific gravity is large. Therefore, when Ba ferrite is used for the backing material, the addition amount of ferrite has to be suppressed in order to control the acoustic impedance within an appropriate range. Therefore, it is not possible to sufficiently impart excellent vibration damping performance based on ferrite.

[0028] By limiting the amounts of strontium (Sr) and iron (Fe) in the ferrite powder, it becomes possible to impart excellent attenuation performance, vibration damping performance, acoustic impedance, and workability to the backing material in a well-balanced manner. If the Sr amount is less than 8.0% by mass and the Fe amount exceeds 63.0% by mass, Fe becomes excessive and the packing property of the ferrite powder may decrease. That is, in the ferrite powder with excessive Fe, the proportion of the hard ferrite component becomes small, and conversely, the proportion of hematite (α-Fe2O3) becomes large. Therefore, the coercive force of the ferrite powder tends to decrease. When the coercive force of the ferrite powder is low, when this ferrite powder is molded to produce a molded body or when this molded body is processed, the molded body is weakly magnetized by mechanical stress. Since the magnetized molded body becomes permanently magnetized, particles tend to aggregate inside it. Also, when the coercive force is low, demagnetization progresses with the passage of time after molding, so the aggregation of particles can be released. Therefore, the packing property of the particles in the molded body tends to deteriorate. Even if the Sr amount exceeds 9.0% by mass or the Fe amount is less than 58.0% by mass, the packing property of the ferrite powder may decrease. This is because the ferrite powder with too little Fe has an increased proportion of the surplus SrO component and its coercive force tends to decrease. Similar to the case of excessive Fe, the packing property of the ferrite powder in the molded body tends to deteriorate.

[0029] From the viewpoint of enhancing the packing property, the Sr content is preferably 8.0 mass% or more, more preferably 8.2 mass% or more. The Fe content is preferably 63.0 mass% or less, more preferably 62.0 mass% or less. Also, the Sr content is preferably 9.0 mass% or less, more preferably 8.6 mass% or less. The Fe content is preferably 58.0 mass% or more, more preferably 60.0 mass% or more.

[0030] The ferrite powder is composed of oxides and contains oxygen (O) in addition to strontium (Sr) and iron (Fe). The ferrite powder may also contain additive elements and unavoidable impurities other than Sr, Fe, and O. Examples of the additive elements include barium (Ba), manganese (Mn), and magnesium (Mg).

[0031] The ferrite powder of this embodiment has a volume average particle diameter of 30 μm or more and 50 μm or less. By limiting the volume average particle diameter within this range, it becomes possible to enhance the productivity and durability of the backing material. Ferrite powder with a volume average particle diameter of less than 30 μm is inferior in fluidity. There are problems in metering handling property, and there is a risk of reducing productivity when producing the backing material. On the other hand, if the volume average particle diameter of the ferrite powder exceeds 50 μm, the strength and surface property of the backing material produced using it deteriorate, and thus there is a risk of impairing the durability of the backing material. From the viewpoint of enhancing the metering handling property, the volume average particle diameter is preferably 31 μm or more, more preferably 33 μm or more. Also, from the viewpoint of enhancing the durability, the volume average particle diameter is preferably 49 μm or less, more preferably 46 μm or less. Note that the volume average particle diameter is the cumulative 50% diameter (D50) in the volume particle size distribution curve. The volume average diameter (D50) is obtained by examining the particle size distribution of the ferrite powder by laser diffraction dry particle size distribution measurement.

[0032] The ferrite powder of this embodiment has a ratio of particles with a particle size of less than 16 μm (fine particle ratio) of 15% to 35% by volume. By limiting the fine particle ratio within this range, it is possible to impart excellent vibration damping function and productivity to the backing material. If the fine particle ratio is less than 15% by volume, the ratio of fine particles of the ferrite powder contained in the backing material becomes excessively small. As a result, the packing property of the particles is impaired, and the vibration damping function of the backing material may decrease at a specific frequency. On the other hand, if the fine particle ratio exceeds 35% by volume, the ferrite powder tends to aggregate. Therefore, when producing the backing material, the kneading time with a matrix component such as rubber may be prolonged. Furthermore, in some cases, the ferrite powder may be contained in the backing material as aggregates, which may also lead to performance degradation of the backing material. From the viewpoint of enhancing the vibration damping function of the backing material, the fine particle ratio is preferably 16% by volume or more, more preferably 18% by volume or more. Also, from the viewpoint of enhancing the productivity of the backing material, the fine particle ratio is preferably 34% by volume or less, more preferably 33% by volume or less.

[0033] The average sphericity of the ferrite powder is desirably 0.90 or more. This makes it possible to further enhance the productivity of the backing material. Ferrite powder with an average sphericity of less than 0.90 has poor fluidity. Therefore, there are problems with metering handling properties, and there is a limit to enhancing the productivity of the backing material.

[0034] The ferrite powder contains perforated particles, and it is desirable that the number ratio of the perforated particles is 5 to 90% with respect to the total number of particles in the ferrite powder. Perforated particles are particles having a through-hole on their surface and a hole diameter ratio of 2% or more. The hole diameter ratio is the ratio of the equivalent circular diameter of the hole (equivalent circular hole diameter) to the equivalent circular diameter of the particle (equivalent circular particle diameter) as shown in the following formula (2).

[0035]

Number

[0036] By increasing the proportion of perforated particles to 5% or more, it becomes possible to further enhance the productivity of the backing material. The perforated particles act as a filler that firmly connects matrix components such as rubber. If the proportion of perforated particles in the ferrite powder is less than 5%, the function of the perforated particles as a filler may be insufficient. As a result, when producing the backing material, it becomes difficult for the matrix components such as rubber to cure, and the productivity may decrease. From the perspective of productivity, the proportion of the number of perforated particles is more preferably 7% or more, and even more preferably 10% or more. On the other hand, by suppressing the proportion of the number to 90% or less, the durability of the ultrasonic probe can be enhanced. If the proportion of the number exceeds 90%, when the backing material is incorporated into the ultrasonic oscillator (probe), the moisture contained in the perforated particles or the number of contact points with peripheral components increases, which may accelerate the corrosion of other peripheral components. From the perspective of corrosion suppression, the proportion of the number of perforated particles is more preferably 85% or less, and even more preferably 80% or less.

[0037] Whether a particle is a perforated particle or not can be examined using a scanning electron microscope (SEM) such as a field emission scanning electron microscope. Specifically, the particle is observed with an SEM. When a through-hole is observed, an SEM image of the particle is taken with the opening of the hole as the center. Then, the projected area (S1) of the particle is obtained from the taken particle image, and the equivalent circle diameter of the particle is calculated according to the following formula (3) using the obtained S1. Also, the projected area (S2) of the hole is obtained by the same method, and the equivalent circle diameter (hole diameter) of the hole is calculated according to the following formula (4) using the obtained S2. Then, the hole diameter ratio is obtained from the equivalent circle diameter of the particle and the equivalent circle diameter of the hole according to the following formula (2), and particles with a hole diameter ratio of 2% or more can be judged as perforated particles.

[0038]

Number

Number

[0039] The BET specific surface area of the ferrite powder is preferably 0.50 m 23.00 m / g or more 2 / g or less. By setting the BET specific surface area within this range, it becomes possible to further enhance the productivity and / or processability of the backing material. When the BET specific surface area is less than 0.50 m 2 / g, the affinity decreases when kneading the ferrite powder with the matrix component, and the processability during molding may be impaired. From the perspective of processability, the BET specific surface area is more preferably 0.70 m 2 / g or more, and even more preferably 0.85 m 2 / g or more. On the other hand, when the BET specific surface area exceeds 3.00 m 2 / g, cracks may occur in the particles constituting the ferrite powder during kneading. From the perspective of suppressing particle cracking, the BET specific surface area is more preferably 2.50 m 2 / g or less, and even more preferably 2.00 m 2 / g or less.

[0040] The pore volume of the ferrite powder is preferably 100 mm 3 / g or more and 300 mm 3 / g or less. By setting the pore volume within this range, it becomes possible to further enhance the productivity and / or processability of the backing material. When the pore volume is less than 100 mm 3 / g, the affinity may decrease when kneading the ferrite powder with the matrix component. From the perspective of processability, the pore volume is more preferably 120 mm 3 / g or more, and even more preferably 130 mm 3 / g or more. On the other hand, when the pore volume exceeds 300 mm 3 / g, cracks may occur in the particles constituting the ferrite powder during kneading. From the perspective of suppressing particle cracking, the pore volume is more preferably 280 mm 3 / g or less, and even more preferably 270 mm 3 / g or less.

[0041] The true specific gravity of the ferrite powder is 5.00 g / cm 3 or more and 5.70 g / cm 3The following are preferred. As shown in the above formula (1), the acoustic impedance of the backing material is represented by the product of the sound velocity in the backing material and the specific gravity of the backing material. Therefore, by adjusting the true specific gravity of the ferrite powder contained in the backing material, it is possible to control the acoustic impedance of the backing material. The true specific gravity is 5.10 g / cm 3 or more and 5.65 g / cm 3 or less, more preferably 5.20 g / cm 3 or more and 5.60 g / cm 3 or less, and even more preferably the following.

[0042] The coercive force (Hc) of the ferrite powder is preferably 150 kA / m or more and 500 kA / m or less, more preferably 220 kA / m or more and 500 kA / m or less, and even more preferably 280 kA / m or more and 500 kA / m or less. If the coercive force is excessively low, the ferrite powder may aggregate and be crushed in the molded body, which may reduce the packing property of the ferrite powder. By limiting the coercive force within the above range, it becomes possible to further enhance the packing property. Also, the saturation magnetization (σs) is typically 40 Am 2 ·kg or more and 75 Am 2 ·kg or less, more typically 50 Am 2 ·kg or more and 60 Am 2 ·kg or less.

[0043] Thus, according to the ferrite powder of this embodiment, excellent attenuation performance, vibration damping performance, workability, and surface properties can be imparted to the backing material. Therefore, it becomes possible to obtain a backing material for an ultrasonic vibrator that is excellent in performance and productivity.

[0044] <<2. Method for manufacturing ferrite powder>> The ferrite powder of this embodiment is not limited in its manufacturing method as long as it satisfies the above-described requirements. However, preferably, it includes the following steps: a step of mixing a strontium (Sr) source, an iron (Fe) source, water, a binder, a dispersant, and, if necessary, an antifoaming agent and / or a pH adjuster to prepare a slurry (raw material mixing step), a step of spray granulating the obtained slurry to prepare a granulated product (granulation step), and a step of firing the obtained granulated product to prepare a fired product (firing step). Further, if necessary, a step of performing post-treatment on the obtained fired product (post-treatment step) may be provided. Details of each step will be described below.

[0045] <Raw Material Mixing Step> In the mixing step, a strontium (Sr) source, an iron (Fe) source, water, a binder, a dispersant, and, if necessary, an antifoaming agent or a pH adjuster are prepared as raw materials, and these raw materials are mixed to prepare a slurry. As the Sr source and Fe source, known ferrite raw materials such as oxides, carbonates, and hydroxides may be used. Preferably, strontium carbonate (SrCO3) and iron oxide (Fe2O3) are used. These are inexpensive. Also, they are stable under normal temperature and humidity and have excellent handling properties.

[0046] The binder is added for the purpose of improving the strength of the granulated product obtained after spray granulation. As the binder, resin compounds such as polyvinyl alcohol (PVA) and / or polyvinyl pyrrolidone (PVP) may be used. The addition amount of the binder is preferably 0.1% by mass or more and 3.5% by mass or less, more preferably 0.5% by mass or more and 3.0% by mass or less, and still more preferably 1.0% by mass or more and 2.5% by mass or less in terms of solid content based on the Sr source and Fe source.

[0047] The dispersant functions to uniformly disperse the Sr source and Fe source in the slurry. By adding the dispersant, component segregation in the granulated product can be prevented. As the dispersant, an acrylic copolymer ammonium salt, a polycarboxylic acid ammonium salt, and / or a hexametaphosphate salt, etc. can be used. A preferred dispersant is an acrylic copolymer ammonium salt. When adding an acrylic copolymer ammonium salt, the generation of perforated particles can be promoted. The reason is speculated as follows. During slurry granulation, the droplet-shaped slurry becomes spherical and dries due to surface tension to obtain spherical granulated products. At this time, the ammonium salt easily binds with chlorine (Cl) inevitably contained in the raw materials to form a stable and hardly volatile complex. When such hardly volatile components are contained in the slurry, when the spherical droplets (slurry) dry, the cohesive force of the primary particles is weakened by the hardly volatile components. The primary particles with weakened cohesive force move inside the droplet and gather at the outer periphery, thus forming the outer shell of the granulated product. On the other hand, the solvent remains inside the droplet together with the hardly volatile components, and the granulated product is formed by the evaporation of this remaining solvent. Since the granulated product shrinks when the solvent inside the droplet evaporates, the outer shell sinks inward, and as a result, it is considered that perforated particles are formed. The addition amount of the dispersant is preferably 0.05% by mass or more and 0.8% by mass or less, more preferably 0.1% by mass or more and 0.6% by mass or less, and still more preferably 0.3% by mass or more and 0.5% by mass or less in terms of solid content based on the Sr source and Fe source.

[0048] The defoaming agent is not necessarily essential. However, by adding the defoaming agent, the generation of bubbles in the slurry can be suppressed. Therefore, the generation of abnormal particles in the granulated product is suppressed, and as a result, the productivity of the ferrite powder can be improved. As the defoaming agent, for example, a polyether-based defoaming agent, etc. can be used. The addition amount of the defoaming agent is preferably 0.1% by mass or more and 0.5% by mass or less in terms of solid content based on the Sr source and Fe source.

[0049] The pH adjuster is not necessarily essential. However, by adding a pH adjuster, the pH of the slurry can be adjusted, thereby changing the dispersion state of the Sr source and Fe source in the slurry. This is because the zeta potential of the Sr source and Fe source changes according to the slurry pH, and the dispersion state changes accordingly. As the pH adjuster, ammonia, alkali hydroxide, alkali carbonate, etc. can be used. However, if the alkali component remains in the final product, it may have an adverse effect. Therefore, a suitable pH adjuster is ammonia. The pH of the adjusted slurry is preferably 8.0 or more and 11.0 or less, more preferably 8.5 or more and 10.5 or less, and even more preferably 9.0 or more and 10.0 or less.

[0050] The mixing of raw materials (Sr source, Fe source, water, binder, dispersant, defoamer, pH adjuster) may be carried out by known methods. For example, the Sr source, Fe source, and water may be mixed using a mixing and grinding machine such as a wet bead mill, and a binder, dispersant, water, and, if necessary, a defoamer or pH adjuster may be added to the obtained mixture. The solid content concentration of the resulting slurry is preferably 30% by mass or more and 75% by mass or less, more preferably 45% by mass or more and 65% by mass or less, and even more preferably 50% by mass or more and 60% by mass or less.

[0051] <Granulation process> In the granulation process, the obtained slurry is spray granulated to produce granules. The spray granulation may be carried out using a spray dryer. The granulation may be carried out under known conditions. For example, conditions such as a slurry discharge rate of 2400 g / min or more and 5000 g / min or less, an atomizer disk rotation speed of 10000 rpm or more and 12000 rpm or less, and a drying temperature of 100°C or more and 500°C or less can be mentioned.

[0052] <Firing process> In the firing process, the obtained granulated material is fired (main firing) to obtain a fired product. The firing can be carried out, although not limited thereto, in an air atmosphere. The firing temperature is preferably 900°C or higher and 1230°C or lower, more preferably 1000°C or higher and 1220°C or lower, and even more preferably 1100°C or higher and 1200°C or lower. The firing holding time may be 3 hours or more and 6 hours or less.

[0053] <Post-treatment process> If necessary, the fired product may be post-treated. Examples of the post-treatment include grinding treatment, heat treatment, and classification treatment. The post-treatment may be carried out alone or in combination. In this way, the ferrite powder of the present embodiment can be obtained.

Examples

[0054] The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to the following examples.

[0055] (1) Preparation of ferrite powder [Example 1] <Raw material mixing> Strontium carbonate (SrCO3) and iron oxide (Fe2O3) were used as raw materials, and they were weighed so that the molar ratio of these was SrCO3:Fe2O3 = 1:5.7. Next, water was added to the weighed raw materials, and they were finely pulverized using a wet bead mill equipped with 0.65 mmφ zirconia beads to obtain a slurry.

[0056] <Granulation> To the obtained slurry, polyvinyl alcohol (PVA, 15% aqueous solution) as a binder, an acrylic copolymer ammonium salt (BASF AA-4040, 40% aqueous solution) as a dispersant, and an aqueous ammonia solution (25% aqueous solution) as a pH adjuster were added. At this time, the addition amount of the binder (PVA) was 1% by mass in terms of solid content, and the addition amount of the dispersant (acrylic copolymer ammonium salt) was 0.3% by mass in terms of solid content. The concentration (solid content) of the obtained slurry was 55% by mass. Then, using a spray dryer, the slurry to which the binder, dispersant, and pH adjuster were added was spray granulated to obtain a granulated product. The spray granulation was carried out under the conditions that the slurry discharge amount of the spray dryer was 3400 g / min and the atomizer disk rotation speed was 11000 rpm.

[0057] <Firing> Next, the obtained granulated product was fired (main firing) to obtain a fired product. The firing was carried out under the conditions of 1150 °C × 4 hours in an air atmosphere. In this way, ferrite powder was produced. The manufacturing conditions of the ferrite powder are summarized in Table 1.

[0058] [Example 2] Weighing was performed so that the molar ratio of the raw materials was SrCO3:Fe2O3 = 1:5.4. Otherwise, ferrite powder was produced in the same manner as in Example 1.

[0059] [Example 3] Weighing was performed so that the molar ratio of the raw materials was SrCO3:Fe2O3 = 1:6.2. Otherwise, ferrite powder was produced in the same manner as in Example 1.

[0060] [Example 4] The conditions during spray granulation were changed, and the slurry discharge amount of the spray dryer was 3400 g / min and the atomizer disk rotation speed was 12000 rpm. Otherwise, ferrite powder was produced in the same manner as in Example 1.

[0061] [Example 5] The conditions during spray granulation were changed, and the slurry discharge rate of the spray dryer was set to 3400 g / min, and the atomizer disk rotation speed was set to 10000 rpm. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0062] [Example 6] The fired product obtained through this firing was subjected to a classification treatment to change the particle size distribution. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0063] [Example 7] Spray granulation was performed so that the slurry discharge rate of the spray dryer became 3700 g / min to obtain a granulated product. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0064] [Example 8] When adjusting the slurry to be used for granulation, the addition amount of the dispersant (acrylic copolymer ammonium salt) was changed to 0.7 mass% in terms of solid content. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0065] [Example 9] When adjusting the slurry to be used for granulation, the addition amount of the dispersant (acrylic copolymer ammonium salt) was changed to 0.1 mass% in terms of solid content. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0066] [Example 10] The firing temperature was changed to 1180 °C. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0067] [Example 11] The firing temperature was changed to 900 °C. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0068] [Comparative Example 1] The firing temperature was changed to 1250 °C. Further, the fired product obtained through this firing was sequentially subjected to a pulverization treatment and a heat treatment. The pulverization was performed using a dry bead mill. The heat treatment was performed under the conditions of 850 °C for 1 hour in the air. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0069] [Comparative Example 2] When mixing the raw materials, dry mixing was performed using a Henschel mixer instead of a wet bead mill. Further, granulation was not performed, and the mixture obtained by dry mixing was fired (this firing) to obtain a fired product. The firing was performed under the conditions of 1090 °C for 4 hours in an air atmosphere. Further, the obtained fired product was subjected to a heat treatment. The heat treatment was performed under the conditions of 900 °C for 1 hour in the air. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0070] [Comparative Example 3] Weighing was performed so that the molar ratio of the raw materials was SrCO3:Fe2O3 = 1:4.7. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0071] [Comparative Example 4] Weighing was performed so that the molar ratio of the raw materials was SrCO3:Fe2O3 = 1:7.0. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0072] [Comparative Example 5] Weighing was performed so that the molar ratio of the raw materials was SrCO3:Fe2O3 = 1:5.6. Further, the conditions during spray granulation were changed, and the slurry discharge amount of the spray dryer was set to 3400 g / min and the atomizer disk rotation speed was set to 13000 rpm. Ferrite powder was produced in the same manner as in Example 1 except for this.

[0073] [Comparative Example 6] The conditions during spray granulation were changed, and the slurry discharge amount of the spray dryer was set to 3400 g / min and the atomizer disk rotation speed was set to 9000 rpm. Ferrite powder was produced in the same manner as in Comparative Example 5 except for this.

[0074] [Comparative Example 7] The conditions during spray granulation were changed, with the slurry discharge rate of the spray dryer set at 3200 g / min and the rotational speed of the atomizer disk at 11000 rpm. Further, the obtained fired product was subjected to classification treatment to change the particle size distribution. Ferrite powder was produced in the same manner as in Comparative Example 5 except for this.

[0075] [Comparative Example 8] The conditions during spray granulation were changed, with the slurry discharge rate of the spray dryer set at 5500 g / min and the rotational speed of the atomizer disk at 11000 rpm. Ferrite powder was produced in the same manner as in Comparative Example 5 except for this.

[0076] [Example 12] When adjusting the slurry to be granulated, the addition amount of the dispersant (acrylic copolymer ammonium salt) was changed to 1.0% by mass in terms of solid content. Also, the conditions during spray granulation were changed, with the slurry discharge rate of the spray dryer set at 3400 g / min and the rotational speed of the atomizer disk at 11000 rpm. Ferrite powder was produced in the same manner as in Comparative Example 5 except for this.

[0077] [Example 13] When adjusting the slurry to be granulated, the dispersant (acrylic copolymer ammonium salt) was not added. Ferrite powder was produced in the same manner as in Example 12 except for this.

[0078] [Comparative Example 9] The firing temperature was changed to 1250°C. Also, the conditions during spray granulation were changed, with the slurry discharge rate of the spray dryer set at 3400 g / min and the rotational speed of the atomizer disk at 11000 rpm. Ferrite powder was produced in the same manner as in Comparative Example 5 except for this.

[0079] [Example 14] The firing temperature was changed to 800°C. Ferrite powder was produced in the same manner as in Comparative Example 9 except for this.

[0080]

Table 1

[0081] (2) Evaluation of ferrite powder For the ferrite powders obtained in Examples 1 to 14 and Comparative Examples 1 to 9, evaluations of various properties were conducted as follows.

[0082] <Particle size distribution> The particle size distribution of the ferrite powder was measured, and the volume-based average particle diameter and the ratio of fine particles (particles with a particle diameter of less than 16 μm) were determined. The measurement of the particle size distribution was carried out by dry dispersion at a dispersion pressure of 3.0 bar using a laser diffraction particle size distribution measuring device (JEOL Ltd., HELOS & RODOS particle size distribution measuring device).

[0083] <Chemical composition> The content of metal elements (such as Fe, Sr, etc.) contained in the ferrite powder was analyzed by the following procedure. First, 0.2 g of the ferrite powder was weighed, and a mixture was prepared by adding 20 ml of 1N hydrochloric acid and 20 ml of 1N nitric acid to 60 ml of pure water. Next, the obtained mixture was heated to completely dissolve the particles constituting the ferrite powder, and an aqueous solution was prepared as a sample. ICP emission analysis was performed using a high-frequency emission plasma analyzer (Shimadzu Corporation, ICPS-1000IV) to measure the content of metal elements in the sample.

[0084] <Ratio of the number of hollow particles> The ratio of the number of hollow particles in the ferrite powder was measured by the following procedure. First, using a field emission scanning electron microscope (FE-SEM; Hitachi High-Technologies Corporation, SU-8020), an SEM image of the ferrite powder was taken at a magnification of 200 times. From the obtained SEM image, 200 particles constituting the ferrite powder were arbitrarily counted, and the number of hollow particles / 200 was calculated and taken as the ratio of the number of hollow particles.

[0085] <BET specific surface area> The BET specific surface area of the ferrite powder was measured using a specific surface area measuring device (Mounttech Co., Ltd., Macsorb HM model-1208). First, as a pretreatment, about 20 g of the ferrite powder as the measurement sample was separately placed in a medicine wrapping paper, degassed to -0.1 MPa with a vacuum dryer, and after confirming that the degree of vacuum had reached -0.1 MPa or less, it was heated under the conditions of 200 °C for 2 hours. Next, about 5 g of the heated sample (ferrite powder) was put into a standard sample cell dedicated to the specific surface area measuring device, and its mass was accurately weighed using a precision balance. The sample cell was set in the measurement port and the measurement was started. The measurement was performed by the one-point method. When the mass of the sample was input at the end of the measurement, the BET specific surface area was automatically calculated. The measurement environment was set to a temperature of 10 to 30 °C and a humidity of 20 to 80% in relative humidity, without dew condensation.

[0086] The pore volume of the ferrite powder was determined using mercury porosimeters Pascal140 and Pascal240 (manufactured by ThermoFisher Scientific). More specifically, a dilatometer CD3P (for powder) was used, the sample was put into a commercially available gelatin capsule with multiple holes, and the capsule was put into the dilatometer. After degassing with Pascal140, mercury was filled and the low-pressure region (0 to 400 Kpa) was measured as the 1st Run. Next, degassing and measurement of the low-pressure region (0 to 400 Kpa) were performed again as the 2nd Run. After the 2nd Run, the combined weight of the dilatometer, mercury, capsule, and sample was measured. Next, the high-pressure region (0.1 Mpa to 200 Mpa) was measured with Pascal240. The pore volume of the ferrite powder was determined using the mercury intrusion volume obtained from the measurement of this high-pressure part.

[0087] <True specific gravity> The true specific gravity of the ferrite powder was measured by the gas displacement method in accordance with JIS Z8807. At this time, helium was used as the displacement gas, and the measurement was performed in an environment of room temperature 20 to 25 °C and humidity 50 to 60%.

[0088] <Magnetic properties> The magnetic properties (σs, σr, Hc) of the ferrite powder were measured using a vibrating sample magnetometer (Toei Industry Co., Ltd., VSM-C7-10A). The measurement was carried out according to the following procedure. First, the ferrite powder was packed into a cell (inner diameter 5 mm, height 2 mm) and set on the magnetometer. Next, an applied magnetic field was applied and swept up to 10 K·1000 / 4π·A / m, and then the applied magnetic field was decreased to draw a hysteresis curve. The saturation magnetization σs, residual magnetization σr, and coercive force Hc were read from the obtained curve.

[0089] (4) Preparation and Evaluation of Resin Compositions Resin compositions containing the ferrite powders obtained in Examples 1 to 14 and Comparative Examples 1 to 9 as fillers were prepared and their performance was evaluated. The resin composition contains a matrix component (resin material) and a filler (ferrite powder) dispersed in the matrix component. Such a resin composition can be suitably used for manufacturing a resin molded body excellent in processability, durability, and true specific gravity during molding. By using this resin composition or resin molded body, it becomes possible to evaluate the substitution of the backing material.

[0090] <Productivity> 70 parts by mass of ferrite powder, 27 parts by mass of a main agent (epoxy resin), and 3 parts by mass of a curing agent were dispersed and mixed using a ribbon mixer to prepare a resin composition. Then, the time required from the start of stirring until uniform dispersion (dispersion time) was measured, and the dispersibility was evaluated based on the obtained dispersion time. The determination of whether or not it was uniformly dispersed was made visually. And the productivity of the resin composition was evaluated from the dispersion time (the time required from the start of stirring until uniform dispersion). Also, based on the evaluation results, the samples were classified according to the following criteria.

[0091] ○: Dispersion time is less than 1 minute △: Dispersion time is 1 minute or more and less than 5 minutes ×: Dispersion time is 5 minutes or more

[0092] <Processability> After drying the resin composition obtained when evaluating productivity under the conditions of 120°C for 5 minutes, heat curing was promoted under the conditions of 180°C for 1 hour. The heat-cured resin composition was processed with an ion milling device (Hitachi High-Tech Corporation, IM-4000). Next, the processed sample was observed for its cross-section using FE-SEM (Hitachi High-Tech Corporation, SU-8020) under the conditions of an acceleration voltage of 1 kV, LA mode, and a magnification of 450 times. Ion milling was performed under the following conditions.

[0093] - DISCHARGE VOLTAGE (discharge voltage): 1.5 kV - ACCELERATION VOLTAGE (acceleration voltage): 6 kV - STAGE CONTROL (processing mode): C3 - DISCHARGE CURRENT (discharge current inside the ion gun): 380 - 450 μA - ION BEAM CURRENT (ion beam current): 110 - 140 μA - GAS FLOW (argon gas flow rate): 0.07 - 0.10 cm 3 / min - Processing time: 60 minutes

[0094] And, the processability of the resin composition was evaluated from the FE-SEM cross-section observation results. Also, based on the evaluation results, the samples were classified according to the following criteria.

[0095] ○: Less than 10 cracks in the ferrite particles in the matrix component △: 10 or more and less than 30 cracks in the ferrite particles in the matrix component ×: 30 or more cracks in the ferrite particles in the matrix component

[0096] <Packing property> From the FE-SEM cross-section observation results obtained in the processability evaluation, the packing property of the ferrite particles in the matrix component was evaluated. Also, based on the evaluation results, the samples were classified according to the following criteria.

[0097] ○: There is a uniform presence in the gap where there are few parts of ferrite particles in the visual field. △: Ferrite particles are evenly present in the visual field, but are distinguishable from the parts with only resin. ×: The parts with ferrite particles in the visual field and the parts with only resin are localized and distinguishable from each other.

[0098] <Rust prevention property> Regarding the resin composition obtained during the processability evaluation, in an environment of room temperature 20 - 25°C and humidity 50 - 60%, the presence or absence of rust components in a tin-free steel (TFS) container filled with the resin composition heat-cured for FE-SEM cross-section observation was evaluated. Note that the presence or absence of rust components was determined visually. The contact surface between the resin composition and the tin-free steel (TFS) container was observed, and those with a color change to a rusty red or burnt umber without metallic luster and the occurrence of unevenness without surface smoothness were judged as rust components. Also, based on the evaluation results, the samples were classified according to the following criteria.

[0099] ○: Rust components do not occur for more than 1 year. △: Rust components do not occur for more than 1 month and less than 1 year. ×: Rust components occur within less than 1 month.

[0100] <True specific gravity of the resin composition> Using a kneader and a pelletizer, the ferrite powder obtained in Examples 1 - 14 and Comparative Examples 1 - 9 and polyethylene as a resin material were mixed, kneaded, and pelletized at a mass ratio of 85:15. As a result, a resin composition (resin molded body) in the shape of pellets with a volume average particle size of 3 mm was obtained. The true specific gravity of the obtained resin composition (resin molded body) was measured by the gas displacement method in accordance with JIS Z8807. At this time, helium was used as the displacement gas, and the measurement was carried out in an environment of room temperature 20 - 25°C and humidity 50 - 60%.

[0101] The true specific gravity of the resin composition is preferably 2.0 g / cm 3 or more, more preferably 2.3 g / cm 3 or more, and most preferably 2.5 g / cm 3The above is more preferable. Although the true specific gravity of the resin composition is desirably high, there is a limit to the amount of ferrite particles present in the matrix component from the viewpoint of processing into a molded body.

[0102] (3) Evaluation Results The FE-SEM image of the ferrite powder obtained in Example 1 is shown in Fig. 2. The particles constituting the powder were almost spherical. Also, perforated particles having through holes were observed.

[0103] The characteristics of the ferrite powders of Examples 1 to 14 and Comparative Examples 1 to 9 are summarized in Table 2. Also, the evaluation results of the resin compositions containing the ferrite powders are summarized in Table 3.

[0104] In the sample where the Sr amount of the ferrite powder was excessively small and the Fe amount was large (Comparative Example 4), the coercive force of the ferrite powder was too small, resulting in poor packing properties. In the sample where the Sr amount of the ferrite powder was excessively large (Comparative Example 3), the coercive force of the ferrite powder was also too small. Also, in the samples where the average particle size of the ferrite powder was excessively small (Comparative Examples 1, 2, and 5), the fluidity of the ferrite powder was poor, and the productivity of the resin composition was inferior. On the other hand, the sample with an excessively large average particle size (Comparative Example 6) was inferior in the packing properties of the resin composition. The samples with an excessively small proportion of fine particles (Comparative Examples 7 and 9) were inferior in packing properties and processability. On the other hand, the samples with an excessively large proportion of fine particles (Comparative Examples 1, 2, and 8) were inferior in any of productivity, processability, and packing properties.

[0105] In contrast, the samples of Examples 1 to 14 that satisfy the Sr amount, Fe amount, average particle size, and proportion of fine particles specified in this embodiment were excellent in the productivity, processability, and packing properties of the resin composition. In particular, Examples 1 to 11 in which the proportion of perforated particles, BET specific surface area, and pore volume were within a predetermined range were excellent not only in terms of productivity, processability, and packing properties but also in suppressing the generation of rust components.

[0106]

Table 2

[0107]

Table 3

Explanation of Symbols

[0108] 2 Sealing Material 4 Backing Material 6 Back Electrode 8 Piezoelectric Body 10 Front Electrode 12 Impedance Matching Layer 14 Acoustic Lens

Claims

1. containing strontium (Sr) in an amount of 8.0% by mass or more and 9.0% by mass or less, and iron (Fe) in an amount of 58.0% by mass or more and 63.0% by mass or less, a ferrite powder for a backing material of an ultrasonic vibrator, having a volume-average particle diameter in the range of 30 μm or more and 50 μm or less, and the proportion of particles having a particle diameter of less than 16 μm being 15% by volume or more and 35% by volume or less.

2. The ferrite powder according to claim 1, wherein the ferrite powder contains hollow particles, and the number ratio of the hollow particles is 5 to 90% with respect to the total number of particles in the ferrite powder.

3. The BET specific surface area is 0.50 m ２ / g or more and 3.00 m ２ / g or less, the ferrite powder according to claim 1 or 2.

4. The pore volume is 100 mm ３ / g or more and 300 mm ３ / g or less, and the ferrite powder according to any one of claims 1 to 3.

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