JP6181520B2 - Alumina sintered body, withstand voltage member and microwave transmission window - Google Patents

Description

  The present invention relates to an alumina sintered body, a withstand voltage member, and a microwave transmission window that are less likely to cause creeping breakdown due to an applied voltage.

  Conventionally, a housing of an X-ray tube in which a high voltage is applied between a cathode and an anode used in medical devices such as CT scans and analyzers such as a transmission electron microscope, a wall surface member of a large accelerator and a high voltage introduction terminal, a microwave Various ceramics are applied to parts that require high insulation, such as a transmission window.

  For example, Patent Document 1 proposes using alumina ceramics as an insulating member in a high voltage bushing (soot tube) of an X-ray tube.

JP-A-8-106828

  Ceramics used between the cathode and anode of medical devices such as CT scans and analyzers such as transmission electron microscopes emit electrons from the cathode side to the anode side when a voltage is applied. Secondary electrons are emitted by colliding with the surface. Then, creepage breakdown occurs when the amount of secondary electrons reaching the anode side exceeds the allowable range.

  And when there are many deposits such as dust and gas on the surface of the ceramic, many secondary electrons will be emitted, and creeping breakdown will easily occur. When such creepage breakdown occurs, in a medical device such as a CT scan or an analyzer such as a transmission electron microscope, the voltage applied between the cathode and the anode drops instantaneously (hereinafter referred to as an instantaneous voltage drop). ) And loss of important medical and analytical data occurs. For this reason, ceramics used in such applications need to reduce the emission of secondary electrons and reduce the occurrence of creeping breakdown.

  In addition, from the viewpoint of accuracy improvement and weight reduction, miniaturization is desired in medical devices and analyzers, but creeping dielectric breakdown is likely to occur only by shortening the creeping distance between the cathode and the anode. Therefore, in order to meet the demand for miniaturization, it is necessary to increase the allowable voltage until creeping breakdown occurs.

  The present invention has been devised to satisfy the above-described requirements, and an object thereof is to provide an alumina sintered body, a withstand voltage member, and a microwave transmission window that are less likely to cause creeping breakdown due to an applied voltage. .

The alumina sintered body of the present invention contains alumina crystals , transition metal oxides and alkaline earth metal oxides , the alumina content is 80% by mass or more, and the secondary electron emission coefficient is 8%. Ri der hereinafter, the alumina alpha mol%, oxide beta mole% of the transition metal, when the oxide of the alkaline earth metals and gamma mol%, α + β + γ = 100, the ratio of beta / gamma The value is 0.5 or more and 2.0 or less .

Moreover, the withstand voltage member of the present invention is characterized in that an electrode is provided on the alumina sintered body having the above structure.

  Furthermore, the microwave transmission window of the present invention is characterized by comprising the alumina sintered body having the above-described configuration.

  According to the alumina sintered body of the present invention, since there is little emission of secondary electrons, creeping dielectric breakdown can be made difficult to occur.

  Further, according to the voltage withstanding member of the present invention, since the alumina sintered body of the present invention is provided with electrodes, creeping dielectric breakdown is unlikely to occur, so data loss due to an instantaneous voltage drop can be reduced. . In addition, since the allowable voltage to creepage breakdown is large, the creepage distance can be shortened, and the mounted equipment and device can be downsized.

  Furthermore, according to the microwave transmission window of the present invention, since it is made of the alumina sintered body of the present invention, the multi-pact phenomenon in which secondary electrons multiply in an avalanche manner, and the surface from which electrons accumulated by the multi-pactor are emitted Since there is little possibility of destruction by electric discharge, it is highly reliable and can be used for a long time.

  Hereinafter, an example of the alumina sintered body, the withstand voltage member, and the microwave transmission window of the present embodiment will be described.

  The alumina sintered body of the present embodiment includes alumina crystals, the alumina content is 80% by mass or more, and the secondary electron emission coefficient is 8 or less. Here, the secondary electron emission coefficient is obtained by detecting secondary electrons emitted from the sample surface when the sample surface is irradiated with an electron beam (primary electrons).

More specifically, the secondary electron emission coefficient includes, for example, an electron gun used for a scanning electron microscope (SEM), a Faraday cup for capturing secondary electrons, a microammeter, a drive sample holder, and the like. It can be measured using a secondary electron emission coefficient measuring device. In the measurement, a pulse beam obtained by high-speed blanking of the electron beam is used in order to avoid surface charging, and the measurement conditions include, for example, an acceleration voltage of 0.6 to 30.0 kV and an internal pressure of 2.0 × 10 ⁻⁴ Pa. Pulse beam current value (I ₁ ) 90-110 pA, pulse width 1-3 msec
The irradiation area is 0.1 to 0.3 mm ² . Then, secondary electrons emitted by the irradiation of the pulse beam are captured in the Faraday cup, the secondary electron current (I ₂ ) is measured with a micro ammeter, and the value of I ₂ is divided by the value of I _1. Can be obtained.

  The alumina sintered body of the present embodiment has a secondary electron emission coefficient of 8 or less, and emits less secondary electrons when electrons collide. can do. In addition, since the allowable voltage until creeping breakdown is large, it can be said that the alumina of this embodiment is interposed between the cathode and anode of a medical instrument such as a CT scan or an analyzer such as a transmission electron microscope. By using a sintered material, data loss due to instantaneous voltage drop can be reduced. At the same time, the creepage distance can be shortened, and the equipment and devices to be mounted can be reduced in size, so that the accuracy and weight can be reduced.

The allowable voltage until creeping breakdown can be confirmed by the creeping breakdown voltage. The creeping breakdown voltage is obtained by dividing the voltage that has caused a voltage drop due to creeping breakdown between the cathode and the anode by the creeping distance (distance between the cathode and the anode). According to the bonded body, the creeping dielectric breakdown voltage is 6 kV / mm or more.

  And the presence of the alumina crystal | crystallization in the alumina sintered compact of this embodiment can be confirmed by measuring and identifying with an X-ray-diffraction apparatus (XRD). In the alumina sintered body, the alumina crystal is the most abundant crystal and shows the highest peak in the obtained X-ray diffraction chart.

The content of alumina in the alumina sintered body is determined by pulverizing a part of the alumina sintered body, dissolving the obtained powder in a solution such as hydrochloric acid, and then ICP (Inductively Coupled Plasma) emission spectroscopic analysis. apparatus (ICP) were measured using a can be obtained by converting from a quantitative value of the resulting Al to _Al 2 _{O 3.}

  The alumina sintered body of the present embodiment preferably contains an oxide of a transition metal and an oxide of an alkaline earth metal. This is based on the finding that the inclusion of transition metal oxides and alkaline earth metal oxides reduces the value of the secondary electron emission coefficient, and the reason is not clear. Examples of transition metal oxides include oxides such as vanadium, titanium, chromium, manganese and iron. Examples of alkaline earth metal oxides include oxides such as beryllium, magnesium, calcium, strontium and barium. Is mentioned.

  Moreover, the alumina sintered body of the present embodiment preferably includes spinel which is an oxide crystal composed of an alkaline earth metal and aluminum, and the alumina and spinel preferably include a transition metal. Here, the presence of spinel may be identified by measurement using XRD, similarly to alumina.

In addition, “contains a transition metal” means that in mapping using an electron beam microanalyzer (EPMA), for example, when the transition metal is titanium, with respect to alumina, the presence of Al and O Existence is confirmed, and spinel refers to the case where titanium is confirmed at the location where Al, Mg and O are present when the alkaline earth metal is Mg (MgAl ₂ O ₄ ).

  Whether or not titanium is detected when a spot (φ1 nm) is applied to alumina or spinel by observation with a transmission electron microscope (TEM) and using an attached energy dispersive X-ray spectrometer (EDS). But it can be confirmed. Note that “contains a transition metal” includes a solid solution of a transition metal.

  In the alumina sintered body of the present embodiment, when the transition metal is contained in alumina and spinel, the value of the secondary electron emission coefficient can be further reduced. The reason for this is not clear, but it is considered that the transition metal fills oxygen vacancies present in alumina and spinel, resulting in fewer oxygen vacancies.

  In the alumina sintered body of this embodiment, the transition metal oxide is preferably titanium oxide, and the alkaline earth metal oxide is preferably magnesium oxide. When this combination is used, an alumina sintered body having a smaller secondary electron emission coefficient can be obtained.

The alumina-based sintered body of the present embodiment has α + β + γ = 100, where alumina is α mol%, transition metal oxide is β mol%, and alkaline earth metal oxide is γ mol%. The ratio β / γ is preferably 0.5 or more and 2.0 or less. When the value of the ratio β / γ is 0.5 or more and 2.0 or less, the value of the secondary electron emission coefficient is further reduced, and the value of the creeping breakdown voltage is increased. In particular, the value of the ratio β / γ is more preferably 0.8 or more and 1.5 or less. Moreover, it is preferable that aluminum oxide is 90 mol% or more.

And about these mol%, it is the value calculated | required as follows. For example, when the transition metal is Ti and the alkaline earth metal is Mg, measurement is performed using ICP, and the obtained quantitative values of Al, Ti, and Mg are converted into Al ₂ O ₃ , TiO ₂ , and MgO, respectively. . And it can obtain | require by calculating molar ratio from each molecular weight, calculating the sum total of molar ratio as a denominator, and calculating each molar ratio as a numerator. Further, the value of the ratio β / γ may be calculated using β, which is the mol% value of TiO ₂ , and γ, which is the mol% value of MgO.

  The alumina sintered body of this embodiment preferably has an average crystal grain size of 10 μm or less. Thus, when the average crystal grain size is 10 μm or less, an alumina sintered body having a smaller secondary electron emission coefficient can be obtained.

  The average crystal grain size may be obtained by a well-known planimetric method. First, a photograph of the sample surface is taken at a magnification of 1000 times using an SEM. Next, draw a circle with a known area on the obtained photo, count the number of particles in the circle and the number of particles on the circumference, and calculate the average crystal grain size of the alumina sintered body as follows: .

The numerical values of the area A of the circle, the number of particles Nc in the circle, the number of particles Nj applied to the circumference, and the magnification M are substituted into the following equation ((Nc + (1/2) × Nj) / (A / M ² )). Then, the number of particles Ng per unit area is obtained. Furthermore, the average crystal grain size can be determined by substituting the value of the number of particles Ng per unit area into the following equation (1 / √ (Ng)).

Next, an example of the manufacturing method of the alumina sintered body of this embodiment is demonstrated. First, an aluminum oxide powder having an average particle size of about 1 μm is prepared, and this is weighed so as to be at least 80% by mass. In addition to the aluminum oxide powder, a sintering aid powder can be included, and the aluminum oxide powder and the sintering aid powder are used as primary materials. And 1 to 1.5% by weight of a binder such as PVA, 100% by weight of a solvent, and 0.1 to 0.5% of 100% by weight of the primary raw material powder after weighing.
A mass% dispersant is placed in a stirrer and mixed and stirred to form a slurry, which is then granulated with a spray granulator (spray dryer) to obtain granules.

  Next, the obtained granule is molded into a predetermined shape by various molding methods such as a die press molding method and an isostatic press molding (rubber press) method, and after cutting as necessary, Firing is performed in a firing furnace at a maximum temperature of 1400-1700 ° C in an air atmosphere and finished by grinding.

Thereafter, the obtained sintered body is heat-treated in a vacuum or a reducing atmosphere. This heat treatment can remove deposits such as dust and gas on the surface of the alumina sintered body, and the deposits can be reattached even when exposed to the atmosphere after the heat treatment by modifying the surface. Can be made difficult. The heat treatment temperature is preferably 800 to 1400 ° C. and the treatment time is preferably 5 minutes to 1 hour. By performing such heat treatment, the secondary electron emission coefficient can be made 8 or less, and an alumina sintered body having a large creepage breakdown voltage can be obtained. Note that when heat treatment is performed in an air atmosphere, deposits are likely to remain, and deposits are likely to be reattached after the heat treatment.

Next, an example in which the transition metal is titanium and the alkaline earth metal is magnesium will be described as a method for producing an alumina sintered body containing a transition metal oxide and an alkaline earth metal oxide. First, in addition to the aluminum oxide powder, a titanium oxide powder and a magnesium hydroxide powder are prepared. Then, a predetermined amount of each powder is weighed. The aluminum oxide powder is weighed so as to be at least 80% by mass.

Further, in the weighing of the titanium oxide powder and the magnesium hydroxide powder, the alumina was α mol%, the transition metal (titanium) oxide was β mol%, and the alkaline earth metal (magnesium) oxide was γ mol%. In this case, it is preferable to weigh so that α + β + γ = 100 and the ratio β / γ is 0.5 or more and 2.0 or less.

Next, another example of weighing will be described. In order to include spinel, which is a crystal of an alkaline earth metal and aluminum oxide, and alumina and spinel to contain a transition metal, the average particle size of the aluminum oxide powder is 0.6 times the average particle size of the magnesium hydroxide powder. A particle having a particle size distribution of 80% cumulative in the particle size distribution and not more than 1.75 times the average particle size may be used. Thus, by using aluminum oxide powder having an average particle size smaller than that of magnesium hydroxide powder, the activity of alumina is promoted, and it is easy to form spinel, which is a crystal of an alkaline earth metal (magnesium) and aluminum oxide. Become. Moreover, transition metal (titanium) is likely to be contained in alumina and spinel by using a titanium oxide powder having an average particle size not larger than 0.6 times the average particle size of magnesium hydroxide powder. And about the preparation method after weighing, it carries out by the method similar to the above.

  In order to make the average crystal grain size of the alumina sintered body 10 μm or less, aluminum oxide powder whose primary particle size is 1.5 μm or less is used, and the firing temperature is 1400 to 1600 ° C. The temperature range may be relatively low.

  Examples of the present invention will be specifically described below, but the present invention is not limited to these examples.

  Samples were prepared with various changes in the alumina content and heat treatment conditions after firing, and tests were conducted to measure the secondary electron emission coefficient and creepage breakdown voltage. A method for manufacturing a sample is described below.

First, an aluminum oxide powder having an average particle size of 1 μm and a sintering aid were prepared as primary materials. Then, by adjusting the amount of sintering aid, a predetermined amount was weighed so that the alumina content would be the value shown in Table 1 to obtain a primary raw material powder. And, 100% by mass of the primary raw material powder after weighing is 1
A mass% PVA (polyvinyl alcohol), 100 mass% solvent, and 0.2 mass% dispersant were placed in a stirrer and mixed and stirred to form a slurry. Then, this slurry was granulated with a spray granulator (spray dryer) to obtain granules.

  And the obtained granule was filled in a metal mold and pressed, and then a cutting process was performed to obtain a molded body having a predetermined shape. Next, the obtained molded body was put in a firing furnace and fired at a maximum temperature of 1600 ° C. in an air atmosphere to obtain a sintered body.

  Thereafter, grinding was performed, and heat treatment was performed in the atmosphere, temperature, and time shown in Table 1, so that a disk-shaped sample No. 1 having an outer diameter of 20 mm and a thickness of 5 mm was obtained. A plurality of 1 to 22 were obtained. Sample No. No heat treatment was performed for 1.

Next, the secondary electron emission coefficient was measured for each sample. As a measuring method, first, a secondary electron emission coefficient measuring device composed of an electron gun used for SEM, a Faraday cup for capturing secondary electrons, a microammeter, a driving sample holder, etc. is manufactured. did. Then, the sample is set in the drive type sample holder, and the pulse beam current value (I ₁ ) 100 pA, the pulse width 1 msec, under the environment of the acceleration voltage 0.6-30 kV and the pressure in the container 2.0 × 10 ⁻⁴ Pa
Under the condition of an irradiation area of 0.1 mm ^2, the secondary electrons emitted by the irradiation of the pulse beam are captured in the Faraday cup, the secondary electron current (I ₂ ) is measured with a microammeter, and I ₂ / I ₁ Values were calculated and shown in Table 1.

Next, in order to calculate the creeping dielectric breakdown voltage, metal electrodes were joined to the ends of both main surfaces of each sample by metallization. Then, a high voltage was gradually applied by a large-capacity power supply (maximum voltage 120 kV) through a wiring previously connected from the power supply terminal to each metal electrode. Then, the creeping dielectric breakdown voltage was calculated by dividing the voltage leading to the voltage drop by the creeping distance (sample thickness 5 mm) which is the distance between the metal electrodes.

In addition, after pulverizing a part of each sample and dissolving the obtained powder in a solution such as hydrochloric acid, the ICP (manufactured by Shimadzu Corporation: ICPS-8100) is used to convert the quantitative value of Al into Al ₂ O ₃ . It was confirmed that the alumina content was as shown in Table 1.

  From Table 1, sample Nos. That were not heat-treated. 1 and Sample No. having an alumina content of less than 80%. For No. 12, the secondary electron emission coefficient value exceeded 8, and the creeping breakdown voltage reached was as low as less than 6 kV / mm. In addition, Sample No. which was heat-treated in the air atmosphere. 2 and 3, the secondary electron emission coefficient value exceeded 8, and the creeping breakdown voltage reached was as low as less than 6 kV / mm.

In contrast, sample no. 4-11, 13-22, the content of alumina is 80 mass% or more,
Since the secondary electron emission coefficient is 8 or less, the creeping breakdown voltage reaches a good value of 6 kV / mm or more, and the alumina sintered body of this embodiment is less likely to cause creeping breakdown. I understood it.

Moreover, when comparing the sample Nos. 13 to 22 with the same alumina content, the value of secondary electron emission coefficient and creeping breakdown voltage was 5 to 1 hour (60 minutes) at a temperature of 800 to 1400 ° C. It was found that heat treatment was preferred.

Next, sample no. When the transition metal oxide shown in Table 2 is β mol% and the alkaline earth metal oxide is γ mol%, the ratio β / γ is A sample added as 1.0 was prepared, and a test was conducted to measure the secondary electron emission coefficient and the creeping breakdown voltage.

  In addition, about the preparation methods of samples other than adding the transition metal oxide and the alkaline earth metal oxide each having an average particle diameter of 1 μm, the sample No. 1 in Example 1 was used. 5 was prepared in the same manner as in No. 5. The secondary electron emission coefficient and the creeping breakdown voltage were measured by the same method as in Example 1. Table 2 shows the results of secondary electron emission coefficient and creeping breakdown voltage. Moreover, it measured using ICP by the method similar to Example 1, and confirmed that it had the composition as weighed.

  Table 2 shows that the inclusion of transition metal oxides and alkaline earth metal oxides can further reduce the secondary electron emission coefficient value and increase the creepage breakdown voltage value. Further, from the values of the secondary electron emission coefficient and the creeping breakdown voltage, it was found that the transition metal oxide was particularly preferably titanium oxide and the alkaline earth metal oxide was preferably magnesium oxide.

  Next, samples with different ratios of the average particle diameter of the primary raw materials were prepared, and a test for measuring the secondary electron emission coefficient and the creeping breakdown voltage was performed.

  First, magnesium hydroxide powder having an average particle diameter of 1 μm and aluminum oxide powder and titanium oxide powder whose particle diameter ratio to the magnesium hydroxide powder is shown in Table 2 were prepared as primary materials. As the aluminum oxide powder, a powder whose cumulative particle size of 80% in the particle size distribution is 1.75 times the average particle size is used.

Then, _Al 2 _{O 3} is 80 mol%, TiO ₂ is 10 mol%, such MgO is 10 mol%, aluminum oxide powder, and a primary raw material is weighed titanium oxide powder and magnesium hydroxide powder.

  And about the process after weighing, the sample was produced by the method similar to Example 2. FIG. Then, the secondary electron emission coefficient and the creeping breakdown voltage were measured by the same method as in Example 1. Moreover, it measured using ICP by the method similar to Example 1, and confirmed that it had the composition as weighed.

  From Table 3, Sample No. As shown in the results of 35, 36, 39, it has alumina and spinel, and by including titanium in alumina and spinel, the value of secondary electron emission coefficient can be reduced, and creeping breakdown It was found that the voltage value can be increased.

  Next, samples with alumina content of 98% by mass and different transition metal oxide and alkaline earth metal oxide ratios β / γ were prepared, secondary electron emission coefficient, creepage breakdown A test was conducted to measure the ultimate voltage.

  For the preparation method of the sample other than changing the ratio β / γ of the transition metal oxide and the alkaline earth metal oxide, the sample No. 2 in Example 2 was used. The same method as in Example 23, the secondary electron emission coefficient, and the creeping breakdown voltage were measured by the same method as in Example 1. Table 3 shows the results of secondary electron emission coefficient and creeping breakdown voltage. Moreover, it measured using ICP by the method similar to Example 1, and confirmed that it had the composition as weighed.

From Table 4, the value of the ratio β / γ of the transition metal oxide and alkaline earth metal oxide is 0.5.
It was found that the value of the secondary electron emission coefficient can be reduced and the creeping breakdown voltage value can be increased by being -2.0. More preferably, the value of the ratio β / γ of the transition metal oxide and the alkaline earth metal oxide is 0.8 to 1.5.

Next, samples with different average particle diameters of aluminum oxide powder were prepared, and a test for measuring secondary electron emission coefficient and creeping dielectric breakdown voltage was performed. The aluminum oxide content was 98% by mass, and the ratio β / γ of the transition metal oxide and alkaline earth metal oxide was 1.0. The average particle diameter of the aluminum oxide powder is as shown in Table 5. Regarding other production methods, the sample No. The same method as in Example 23, the secondary electron emission coefficient, and the creeping breakdown voltage were measured by the same method as in Example 1. The average crystal grain size of the sample was measured by a planimetric method and shown in Table 5 together with the results of secondary electron emission coefficient and creeping dielectric breakdown voltage. Sample No. 50 and Sample No. 43 indicates the same sample.

  From Table 5, it was found that when the average crystal grain size of the alumina sintered body is 10 μm or less, the value of the secondary electron emission coefficient can be reduced and the value of the creeping breakdown voltage can be increased.

  As apparent from the results of these examples, the alumina sintered body of the present embodiment has a small secondary electron emission coefficient and a large creepage breakdown voltage value. X-ray tube housing, large accelerator wall member, and high voltage to which a high voltage is applied between the cathode and anode used in medical devices such as CT scans and analyzers such as transmission electron microscopes It has been found that the present invention can be suitably used for parts requiring high insulation, such as introduction terminals and microwave introduction windows of plasma processing apparatuses.

  Further, it was found that the voltage-resistant member provided with electrodes on the alumina sintered body of the present embodiment can reduce data loss due to an instantaneous voltage drop. Moreover, since the allowable voltage until creeping dielectric breakdown is large, it was found that the creeping distance can be shortened, and the equipment and apparatus to be mounted can be reduced in size.

Furthermore, the microwave transmission window made of the alumina sintered body of the present embodiment is highly reliable and long because it is less likely to break due to multi-pact phenomenon or surface discharge from which electrons accumulated by the multi-pactor are emitted. It has been found that it can be used over a period of time.

Claims (6)

Crystalline alumina, comprises an oxide and an alkaline earth metal oxide of a transition metal, the content of the alumina 80 wt% or more, the secondary electron emission coefficient Ri der 8 below, the alumina α mol %, When the transition metal oxide is β mol% and the alkaline earth metal oxide is γ mol%, α + β + γ = 100, and the ratio β / γ is 0.5 or more and 2.0 or less. alumina sintered body, characterized in that it. 2. The alumina sintered body according to claim 1, wherein the alumina-based sintered body includes spinel that is a crystal of an oxide of an alkaline earth metal and aluminum, and the alumina and the spinel include a transition metal. The oxide of the transition metal is titanium oxide, alumina sintered body according to claim 1 or claim 2 oxides of the alkaline earth metal is characterized in that magnesium oxide. The alumina sintered body according to any one of claims 1 to 3, wherein an average crystal grain size is 10 µm or less. A withstand voltage member comprising an electrode on the alumina sintered body according to any one of claims 1 to 4 . A high-frequency or microwave transmission window comprising the alumina sintered body according to any one of claims 1 to 5 .