JP5477715B2 - Highly transparent alumina ceramic and method for producing the same - Google Patents

Description

  The present invention relates to a material that is highly transparent to visible and infrared radiation, and a method of manufacturing the same.

  Among materials transparent to visible light and infrared radiation, sapphire, which is a single crystal material, is transparent to infrared radiation and has excellent mechanical properties. However, the price is so expensive that many applications do not pay. Patent Document 3 proposes polycrystalline alumina containing zirconium oxide. This material is described as being transparent in the visible range. Patent Document 1 describes an alumina-based polycrystalline ceramic, and the average size of the crystal particles is not larger than 1 μm. Patent Document 1 also describes the possibility of introducing an oxide of a metal selected from Group 3A and Group 4A in a proportion of less than 2 mol%. In order to obtain highly transparent pure alumina, hot isostatic pressing (HIP) at a temperature of 1150 to 1300 ° C. and a pressure of up to 200 MPa has been traditionally used (Non-Patent Documents 1 and 2). ). By using the HIP method, the porosity can be easily reduced to less than 0.05% and the particle size can be kept below 1 μm. By using the triaxial pressure application method with an optimized powder processing route, a pure alumina that is maximally dense and transparent with a particle size of 300 nm and an in-line transmittance of up to 71% is obtained. (Non-Patent Document 1). In general, the linear transmittance of HIP-treated pure alumina exceeds 50% (Non-patent Documents 1 and 2, Patent Documents 1 to 3).

  In the current state of the art (Non-Patent Documents 1 and 2, Patent Documents 1 to 3), several difficult steps are required to produce a translucent or transparent material. a) preparing a slurry from alumina powder, b) pouring the slurry into a porous mold, drying and removing from the mold to obtain a sintered object, i.e., a sintering precursor, c) an extracted sintered object Let things dry. d) Debinding at a temperature of 350 ° C. to 600 ° C. e) Sintering at a temperature between 1100 ° C. and 1350 ° C. until a sintered product having a density of at least 92% of the theoretical density is obtained, and f) Hot isostatic pressing known as “HIP” is performed at a temperature between 950 ° C. and 1300 ° C. and a pressure of 100 MPa to 380 MPa.

  However, recently, transparent / translucent alumina has been produced using SPS (Non-Patent Documents 3 to 7). Kim et al. Obtained a linear transmittance of 47% at a wavelength of 640 nm in the case of a sample sintered at 1150 ° C. and an applied pressure of 80 MPa (Non-patent Document 4). Since SPS employs graphite punches, the maximum pressure at SPS is generally limited by the compressive strength of the material used for the die. Typical high strength graphite has a compressive strength of 140 MPa (Non-Patent Documents 8 and 9). In 2006, Anselmi-Tamburini et al. Reported a method for producing high-density functional oxide compacts with crystal sizes in the range of 10 nm to 20 nm (Non-patent Document 10). Anselmi-Tamburini et al. Combined a relatively short thermal cycle (less than 10 minutes) with increasing pressure (up to 1 GPa) to advance high compression while greatly suppressing particle growth. However, the method filed by Anselmi-Tamburini et al. (Non-Patent Document 11) has not yet been adopted to produce a maximally dense and highly transparent material.

  Unlike the existing technology, the object of the present invention is to provide a simple method for obtaining a highly transparent alumina ceramic. Thus, this new method allows for the production of very transparent products in the visible and ultraviolet regions, where the dopant may or may not need to be added to the slurry. is there. This advantageously simplifies the manufacturing method in terms of the number of steps and the manufacturing process.

According to one aspect of the present invention , light having a wavelength of 645 nm when the porosity is less than 0.05% and the thickness is 0.8 mm, including the following steps (a) to (c): Is made transparent so that the actual linear transmittance is larger than 40% , and the average particle size is made of crystal particles of 200 nm or less. The crystal particles contain Al ₂ O ₃ , and the Al ₂ O ₃ has priority. having a crystal orientation, a method of manufacturing the alumina-based transparent polycrystalline ceramic is given.
(A) A precursor made of alumina powder is prepared.
(B) Applying a current directly to the precursor, the temperature of the precursor is a holding temperature that results in a high densification of the precursor but does not substantially result in particle growth in the precursor 950 The temperature is raised to a temperature range of not less than 1000 ° C and not more than 1000 ° C.
(C) While maintaining the precursor at the holding temperature, a pressure of 500 MPa or more and less than 1 GPa is applied to the precursor to densify the porosity to less than 0.05% .

  The present invention provides a much simpler method of producing a transparent alumina ceramic than the products of the majority of other existing more complex methods.

Schematic of high pressure die. Pure alumina powder was compressed between two conductive binderless WC punches pressed between two WC protective discs. The temperature was measured on the inner die surface (i.e., 0.75 cm away from the sample). A photograph of a transparent alumina sample. (A) A photograph in a state of being placed on a document. (B) Photo in a state of being 1.2 cm away from the document. The sample was sintered at 950 ° C. for 10 minutes under an applied pressure of 500 MPa. The sample thickness was 0.8 mm and both sides were polished. The document was not illuminated from behind. The graph which shows the linear transmittance | permeability of the alumina sintered at 950 degreeC and 1000 degreeC for 10 minutes under the applied pressure of 500 MPa. Also shown in the graph is comparative literature data for the real in-line transmission for the 645 nm wavelength of the sintered alumina sample. Furthermore, the sintering method and the applied pressure are shown in the legend. (A) FESEM image of raw TM-DAR alumina powder. (B) FESEM image of a fracture surface of a sample sintered at 1000 ° C. and 500 MPa for 10 minutes.

  A highly transparent alumina ceramic with a particle size smaller than 300 nm is made by high pressure discharge sintering. When the applied pressure is 500 MPa, highly transparent alumina having a real linear transmittance (wavelength of 645 nm) higher than 60% is obtained at a low temperature of 950 ° C. to 1000 ° C. By applying high pressure, highly transparent and maximally dense alumina can be obtained at low temperature and without large particle growth. The present invention provides a very simplified manufacturing method for obtaining an alumina-based transparent ceramic.

Commercially available α-A1 ₂ 0 ₃ powder (TM-DAR, Daimei Industrial Chemical Co., Ltd.) was used as the raw material alumina powder (Non-patent Document 12). FIG. 4A is a scanning electron microscope (SEM) image of the powder as delivered. From this, it was found that spherical particles were aggregated to form a cluster having a size of 5 to 50 μm. In the state as supplied from the vendor, the average BET specific surface area was 14.5 m ² / g.

The as-received powder is treated with a spark plasma sintering machine (spark plasma) without any pretreatment or additives.
sintering machine, SPS) (SPS-1050, Sumitomo Coal Mining Co., Ltd.) was sintered at 900 ° C., 950 ° C. and 1000 ° C. under a uniaxial pressure of 500 MPa. A schematic diagram of the high-pressure apparatus used here is shown in FIG. In this apparatus, a pressure exceeding 500 MPa can be applied to a sample having a diameter of 5 mm and a thickness of about 2.5 mm. According to the experimental results, it was found that the apparatus shown in FIG. 1 can apply a high pressure of 1 GPa, but 500 MPa is sufficient to make the sample as dense as possible. In a typical sintering experiment, 0.18 g to 0.2 g of alumina powder was poured onto the die. The apparatus has an internal graphite die and an external graphite die, and this powder is pressed between two WC punches, which are pressed between two intermediate WC discs. Apart from the protective disk and punch, the entire device is made of graphite. The temperature was accurately measured using a pyrometer that focused on the surface of the internal die (ie, 0.75 cm away from the sample). Graphite felt was used to reduce heat loss due to radiation. The powder was heated from room temperature to 700 ° C. in 10 minutes and then heated to the sintering temperature (ie 900 ° C., 950 ° C. and 1000 ° C.) in 10 minutes. The holding time was 10 minutes and the pressure was increased just before the holding time started. Heating was performed by a sequence of twelve power-on DC pulses followed by two power-off DC pulses. The duration of each pulse was 4.3 ms. The duty cycle of this pulse sequence was 12/14. That is, twelve pulses, or time intervals (each 4.3 ms long), were on and two time intervals were off. It is considered that the duty cycle of this pulse sequence does not greatly affect the transmittance of the completed sample. Over the entire duration of the experiment, the current was less than 1000 A and the voltage drop across the cooled ram was less than 4V.

  The sintered sample was processed into a disk having a diameter of 5 mm and a thickness of 1 mm, and both surfaces were carefully mirror-polished using a diamond slurry. The final thickness of the sample is 0.8 mm. The actual linear transmittance was measured in a wavelength range of 0.24 μm to 1.6 μm using a two-beam spectrophotometer (SolidSpec-3700DUV, Shimadzu Corporation). The distance between the sample and the detector was about 55 cm.

The microstructure was observed on the fracture surface of the sintered sample using a scanning electron microscope (SEM) (JSM-7100, JEOL Ltd.). The porosity was measured on a SEM image with a magnification of 10,000 times. Absolute density was not measured. This is because conventional methods such as the Archimedes method are insensitive to very low porosity. An electrically insulating SiC punch developed by Tamburini et al. (Goodfellow Cambridge, UK)
Unlike the limited die (SiC having a resistivity of 10 ² to 10 ³ Ωcm) (Non-Patent Documents 10 and 11), the configuration shown in FIG. 1 has a resistivity of 20 × 10 ⁻⁶ Ω.
Conductive punches and protective discs made of tungsten carbide (WC) with no binder in cm are employed. The configuration developed by Tamburini et al. Is more similar to a high pressure hot press than a high pressure SPS because of the high punch resistivity.

  At present, the sintering mechanism of other oxide powders under SPS conditions is not completely known. However, there is a possibility that the current plays an action during sintering that is not limited to Joule heating (Non-Patent Documents 13 to 15). Recently, Langer et al. Compared hot press (HP) technology and SPS technology for TM-DAR alumina powder (Non-Patent Document 13). Sample shape, heating schedule, applied pressure, and atmosphere were the same for both methods. The results show that at a given time, the SPS sample reached a higher density compared to HP. Campbell et al. (Non-Patent Document 15) and Conrad et al. (Non-Patent Document 15) studied the electro plasticity effect in particulate alumina. According to these reports, the electric field of 300 V / cm greatly affected the plastic deformation at 1450 ° C. and 1600 ° C. In fact, this electric field reduced the flow stress by up to 70% and generally increased the elongation to break. Since the current may promote densification, the inventor decided to use a conductive punch as shown in FIG.

  FIG. 2 shows a photograph of an alumina disk having a diameter of 5 mm and a thickness of 0.8 mm, which was sintered at 950 ° C. and 500 MPa for 10 minutes and polished on both sides. There is no illumination from the back of the document. In FIG. 2, (a) is a photograph placed on a document, and (b) is a photograph 1.2 cm away from the document. As can be seen from these photographs, the text, images and holograms of the document behind it can be clearly seen through the sample, even when placed directly on the paper or when it is lifted from the paper by 1.2 cm. FIG. 3 is a graph showing the linear transmittance of alumina sintered at 950 ° C. and 1000 ° C. for 10 minutes under an applied pressure of 500 MPa. As can be seen from this graph, 0. 0 ° C. sintered at 950 ° C. and 1000 ° C. The linear transmittance of the 8 mm sample at a wavelength of 645 nm was 63.3% and 64%, respectively. The transparency of the sample sintered at 950 ° C. and the sample at 1000 ° C. were almost the same. Since the sample sintered at 900 ° C. did not show transparency, the transparency was not measured. FIG. 3 also shows comparative data at a wavelength of 645 nm reported in the literature for transparent alumina when the sample thickness is 0.8 mm.

The actual linear transmittance obtained in this example is only inferior to the actual value reported by Apetz et al. (Non-Patent Document 1), and all other values reported in the literature (Non-Patent Document 2). ~ 7) is won. Apetz et al. Prepared fine particle samples by slip casting or pressure casting Al ₂ O ₃ powder with an average particle size of 150 nm. Apetz et al. Sintered alumina samples at 1150 ° C. and 1250 ° C. for 2 hours to obtain closed pores. Finally, the sample was subjected to hot isostatic pressing (HIP) treatment in argon at 1200-1400 ° C. and 200 MPa for 2 hours. Depending on the HIP treatment temperature, an average particle size of 0.3 μm to 8 μm was obtained.

  However, Apetz's work does not disclose the alumina powder manufacturer nor the initial powder shape (ie, the size and distribution of the initial powder particles and the particle shape). In contrast, Krell et al. (Non-Patent Document 2) uses the same powder (TM-DAR) as in this example. Krell et al. Obtained a low linear transmittance of 55% by sintering this powder at 1280 ° C. for 2 hours and then HIP treatment (200 MPa) at 1200 ° C. for 15 hours.

  Compared with the HIP method in Zpetz et al. (Non-Patent Document 1) and Krell et al. (Non-Patent Document 2), the method proposed in the present invention is very fast and simple. That is, i) no powder deagglomeration / pretreatment is required, the delivered powder is simply poured into the die as it is, and ii) 4-17 hours of the prior art (Non-Patent Documents 1 and 2) In comparison, the retention time is only 10 minutes.

  In the present invention, the transparency is superior to that obtained by SPS (Non-Patent Documents 3 to 7) of less than 80 MPa. Kim et al. (Non-patent Document 5) have the highest real linear transmittance of 47% in the case of a sample sintered with SPS of less than 80 MPa at a heating rate of 2 ° C./min (ie, sintering time of about 5 hours). Reported by. Reference data (Non-Patent Document 4) was measured for a wavelength of 640 nm on a 0.88 mm thick sample. Amen et al. (Non-Patent Document 7) obtained an actual linear transmission of 40% by optimizing the sintering object forming process. More recently, Steer et al. (6) studied the effect of Mg, Y, and La doping on the transparency of SPS treated alumina, but in the case of pure alumina, the transparency was less than 5%. It was.

  FIG. 4 compares a micrograph of TM-DAR powder with a micrograph of a fracture surface of a sample sintered at 1000 ° C. and 500 MPa for 10 minutes. As shown in FIG. 4 (a), the initial powder is strongly agglomerated, the particle size is distributed between 150 nm and 250 nm, and the initial particles are almost circular particles having an average particle size of 200 nm. FIG. 4 (b) is a transgranular fracture surface of the sintered sample, and the particle size is approximately the same as the original powder. Bernard-Granger et al. (Non-Patent Document 16) and Langer et al. (Non-Patent Document 13) studied the particle size / relative density curves for TM-DAR powders for pressureless sintering and SPS, respectively. According to their report, sintering time higher than that described in this application (that is, pressureless sintering at 1220 ° C. for 21 minutes (Non-Patent Document 16), or 1100 ° C., 50 MPa, treatment time of 1 hour) Even in the case of the powder sintered in (Non-patent Document 13)), there was no particle growth. However, the relative densities were 83% and 81.5%, respectively. In the present invention, by increasing the pressure to 500 MPa, a compact body having a maximum particle size of about 200 nm and a porosity of less than 0.05% was obtained.

  Since the alumina-based transparent polycrystalline ceramic of the present invention is very transparent and dense, it has a wide range of applications such as observation window shielding for very harsh environments.

European publication EP 1053 983 A2 US Publication US 2009/0137380 International Publication WO 2004/007398 A1 International Publication WO2006 / 113354 A2

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Claims (1)

Including the following steps (a) to (c), when the porosity is less than 0.05% and the thickness is 0.8 mm, the actual linear transmittance is 40 for light having a wavelength of 645 nm. % is transparent to be larger than an average particle size consists of the following crystal grains 200 nm, said crystalline particles comprise Al ₂ O _3, the Al ₂ O ₃ has a preferred crystal orientation, alumina-based transparent multi method for producing a crystalline ceramic.
(A) A precursor made of alumina powder having a particle size of 150 to 250 nm or less is prepared.
(B) Applying a current directly to the precursor, the temperature of the precursor is a holding temperature that results in a high densification of the precursor but does not substantially result in particle growth in the precursor 950 The temperature is raised to a temperature range of not less than 1000 ° C and not more than 1000 ° C.
(C) While maintaining the precursor at the holding temperature, the precursor is densified by applying a pressure of 500 MPa or more and less than 1 GPa to the precursor.