JP6342251B2 - Method for producing tungsten oxide and metal tungsten fine particles and fine particles obtained thereby - Google Patents

Description

  The present invention relates to a method for producing tungsten oxide and metal tungsten fine particles and fine particles obtained thereby.

  Currently, in the photocatalytic field, anatase and rutile-type titanium oxide are often used, but titanium oxide can only be used in the ultraviolet region of the irradiation energy of sunlight due to its own visible light transmission characteristics. It is a problem.

  For this reason, in recent years, research has been conducted to increase photocatalytic activity by supporting a noble metal catalyst on tungsten oxide having visible light absorption characteristics or doping a small amount of impurities.

  The thermal decomposition method is mainly used as a method for producing tungsten oxide fine particles, but the convenience of the submerged plasma method that can dissolve these ion sources in the liquid when supporting noble metals or doping impurities, etc. Is expensive.

  Among the in-liquid plasma methods, the in-liquid plasma methods (Patent Documents 1 to 5) using microwaves as energy sources are known as methods for synthesizing nanoparticles.

JP 2010-121193 A JP 2011-056428 A JP 2011-058064 A JP 2012-036468 A JP2013-237582A

  In the submerged plasma method, metallic tungsten is used as an electrode, and a plasma state is created by concentrating energy at the tip of the electrode. In the plasma, the tungsten electrode reaches a high temperature and evaporates, and is oxidized by water vapor generated by the plasma, oxygen generated by water decomposition by the plasma, dissolved oxygen in the liquid, and the like to generate tungsten oxide. The produced tungsten oxide is cooled by a solvent and deposited as fine particles in the liquid.

  However, a method for producing fine particles by a submerged plasma method is still in the research stage, and a method for producing tungsten oxide has not been established.

  At present, the production rate is as low as 0.01 g / h, and the inside of the electrode for generating plasma is a closed system except for the opening in contact with the liquid, so the yield is reduced due to the product sticking inside. This contributed to a further decrease in the production rate.

  In addition, since the oxygen source is limited to water and water decomposes due to plasma heat to generate hydrogen and oxygen, blue products such as hydrogen tungsten bronze and oxygen deficient tungsten oxide are mainly generated. There was a problem that it was difficult to produce tungsten oxide.

  In addition, since the source electrode is placed in a high temperature field in water, tungsten vapor generated in the plasma is immediately oxidized and it is difficult to form metallic tungsten fine particles. Another problem is that the power required to generate plasma is very large.

  The present invention has been made in view of the circumstances as described above. The production method of fine particles of tungsten oxide and metal tungsten, which can increase the production rate and yield, and have high energy efficiency, and the fine particles obtained thereby. It is an issue to provide.

  In order to solve the above-mentioned problems, a method for producing tungsten oxide or metal tungsten fine particles according to the present invention is a method for producing tungsten oxide or metal tungsten fine particles using a pulsed microwave plasma apparatus using tungsten as an electrode. However, the plasma ignition cycle exceeds 100 Hz.

  According to the present invention, by accelerating the ignition cycle of the pulsed plasma in liquid, the interval between plasma ignitions is shortened, the tungsten electrode is wrapped in continuous bubbles, and the electrode temperature is maintained at a high temperature. Evaporation and reaction are promoted, and the production rate is improved.

  Further, when a gas containing oxygen is supplied around the electrode, fine particles of tungsten trioxide can be generated, and when an inert or reducing gas is used, fine particles of metallic tungsten can be generated. In the latter, dissolved oxygen and oxygen generated by water decomposition produce tungsten oxide in addition to metallic tungsten, but selective recovery by decantation is possible due to the difference in specific gravity between the two.

  In addition, by forming a gas flow field around the electrode, the reaction product that has traditionally moved due to concentration diffusion can be forcibly discharged out of the system. The rate drop is improved.

  Further, by accelerating the ignition cycle, energy corresponding to the phase change from the liquid to the gas can be saved, so that it can be used as a high energy efficiency method.

It is the schematic of a plasma apparatus in a microwave liquid. It is the figure which showed the structure of the microwave power source. It is a graph which shows the tungsten oxide production rate of an Example / comparative example. It is a high-speed camera photography image in each condition. It is a graph which shows the relationship between the oxygen concentration in an Example, and a tungsten oxide production rate. It is a graph which shows the yield of an Example / comparative example. It is a graph which shows the spectroscopic measurement result of a product. It is an XRD analysis result (Examples 1-3). It is an XRD analysis result (Examples 4-6). It is an XRD analysis result (Example 7). It is an XRD analysis result (Example 8). It is an XRD analysis result (comparative example 1). It is a TEM observation image (Example 3, Example 6). It is a TEM-EELS O1s spectrum (Example 3, Example 6).

  The present invention is described in detail below.

  The method of the present invention is characterized in that the gas is supplied around the electrode after the plasma ignition cycle is accelerated.

  As the gas, oxygen-containing gas is supplied when producing tungsten trioxide, and inert or reducing gas is supplied when producing metallic tungsten.

  Specifically, pulse plasma is generated using a microwave power source in a reaction vessel filled with water. When noble metal is supported or impurities are doped, by dissolving these source ions in advance, the source ions can be reduced by plasma and supported or doped.

  The pulse plasma is intermittently ignited and extinguished, and this ignition cycle exceeds 100 Hz. The lower limit of the ignition cycle is more preferably 110 Hz or more, and further preferably 120 Hz or more. The upper limit is practically preferably 1 kHz or less, and more preferably 300 Hz or less. In the present invention, the ignition cycle is particularly preferably around 200 Hz.

  In the ignition / extinguish cycle of pulsed plasma in liquid, when the ignition cycle is slow at 100Hz, the electrode is enveloped by hot bubbles during ignition, but until the reignition, the bubbles shrink and the electrode is submerged in the liquid to cool. In contrast, as the ignition cycle becomes faster, the interval is shortened and reignition occurs before the bubbles are fully contracted, keeping the electrode at a high temperature and promoting the reaction.

  Thus, the reaction rate is improved by accelerating the plasma ignition cycle and keeping the raw material electrode at a high temperature. If the ignition cycle is too slow, the raw material tungsten may be mixed into the sample due to low-frequency cavitation. If the ignition cycle is too fast, the electrode evaporating rate increases with high temperature and metal tungsten is deposited in addition to the target tungsten oxide. Therefore, considering this point, the ignition cycle is preferably 300 Hz or less.

  Further, by supplying a gas containing oxygen around the electrode, the oxidation reaction is promoted, and fine particles mainly containing tungsten trioxide can be generated. On the other hand, by supplying an inert or reducing gas around the electrode, it is possible to generate fine particles of metallic tungsten that could not be generated conventionally.

  Further, by flowing a gas around the electrode, the vapor of the reaction product that has conventionally been moved by concentration diffusion can be forcibly discharged out of the system by the gas flow.

  When the gas is supplied to the periphery of the electrode in this way, for example, as shown in the enlarged portion of FIG. 1, a rod-like shape is formed in the inner conductor constituting the coaxial waveguide connected to the coaxial waveguide converter. By providing a through hole through which the raw material tungsten passes and making a gas flow path between the inner wall of the inner conductor and the rod-shaped raw material tungsten in the through hole, a flow field by gas supply can be provided around the electrode.

  In addition, when the ignition cycle is slow as in the past, the plasma electrode is submerged in water at each ignition, so in order to generate plasma, a phase change of liquid → gas → ionized gas is required. On the other hand, when the ignition cycle is fast, bubbles around the electrode are retained, so that it is possible to create a plasma state with only a phase change of gas → ionized gas.

  In the present invention, the prior art such as the device disclosed in the above-mentioned patent document is referred to for the configuration of the “pulsed microwave plasma device”.

  The pulse microwave submerged plasma apparatus includes a microwave oscillator, a waveguide, a container for storing a liquid, and a submerged plasma source. Hereinafter, a description will be given with reference to the example of FIGS.

  The microwave oscillator 10 generates and outputs a microwave, a power source 40, 41, and 42 that supplies the magnetron 44 with power for generating microwaves, and sends a signal to the microwave power source. And a microwave power supply controller (function generator) 43 for adjusting and controlling a wave output and the like.

  The waveguide 20 propagates the microwave output from the microwave oscillator 10 to the container 30. A solid circuit such as a power monitor 11 that measures the microwave power of each of emission and reflection and a tuner (such as a sleeve tuner 12) that matches load impedance can be attached to the waveguide 20. Further, the end plunger 13 or the like can be used for the waveguide 20. Furthermore, the waveguide 20 has a coaxial waveguide converter 14.

  The container 30 is a container for storing a liquid. Plasma is generated in the liquid stored in the container 30. The liquid is supplied with electric power for generating fine particles by generating plasma in the liquid. This power is not a direct current pulse but a microwave having a single frequency spectrum such as 2.45 GHz, 5.8 GHz, and 9.5 GHz band. For this reason, high power supply efficiency becomes possible by optimizing the resonance structure and transmission line impedance, and the load on the electrodes can be reduced by using microwaves as the driving power. The microwave power is preferably pulsed with a plurality of periods as one pulse. When microwave power capable of steady plasma discharge is input to the plasma source, intense heat is generated by the power and the electrode is destroyed. However, the electric power for generating plasma is high, and in order to simultaneously realize these conflicting demands, the electric power supply needs to be a microwave pulse. On the other hand, if the pulse width of the microwave pulse is shortened, the plasma becomes corona discharge, that is, non-thermal equilibrium plasma, the temperature rise is suppressed, and electrode wear is remarkably reduced. However, since the energy given to the liquid is small, there is a possibility that the reaction rate becomes slow or fine particles are not generated depending on conditions.

  The in-liquid plasma source is a device for supplying the microwave propagating through the waveguide 20 to the liquid. This submerged plasma source has a coaxial tube 15 and a raw material tungsten electrode 16.

  The coaxial tube 15 constitutes a part of the coaxial waveguide converter 14 and receives microwaves from the waveguide 20 and propagates them. In the coaxial waveguide converter 14, the waveguide 20 and the coaxial tube 15 are connected vertically. For this reason, when the microwave is transmitted from the waveguide 20 to the coaxial tube 15, the propagation direction is changed to the vertical direction. The coaxial tube 15 has a coaxial tube structure and includes a coaxial tube inner conductor 15a and a coaxial tube outer conductor 15b. The coaxial tube inner conductor 15a has a tungsten wire serving as an electrode 16 inserted therein, and is disposed coaxially with the coaxial tube outer conductor 15b in the hollow of the coaxial tube outer conductor 15b. The coaxial pipe inner conductor 15a has a barrel portion formed in a cylindrical shape, one end portion thereof connected to the coaxial waveguide converter 14, the other end portion formed in a conical shape, and A raw material tungsten electrode 16 is extended. By exposing the tip of the raw material tungsten electrode 16 in the liquid as described above, plasma can be generated in this portion.

  An annular sealing member 17 may be provided to support the raw material tungsten electrode 16 and prevent the liquid from flowing into the coaxial tube 15. As the sealing member 17, for example, PTFE having a low dielectric loss in the microwave band, no excessive dielectric constant, and high heat resistance can be used.

  The method of the present invention includes a container 30 for containing a solution, a microwave oscillator 10 that outputs a microwave, and a raw material tungsten electrode that excites a plasma in the solution by applying the microwave to the solution. 16 is performed using a pulsed microwave plasma apparatus.

  After the solution is put into the container 30 of the pulsed microwave plasma device, the microwave oscillator 10 of the pulsed microwave plasma device is turned on, and the microwave is output from the magnetron 44. The microwave propagates through the waveguide 20 and is applied to the solution via the coaxial waveguide converter 14 and the submerged plasma source. Thereby, plasma is generated in the solution.

  The electrode 16 is composed of a movable tungsten rod that can move in the axial direction in the coaxial tube inner conductor 15a. Even when the electrode 16 is consumed due to a reaction, the electrode 16 is externally connected to the coaxial waveguide converter 14. It can be sent into the system by operation. A gas containing oxygen or an inert or reducing gas is supplied through the space surrounding the rod-shaped electrode 16, that is, between the inner wall of the coaxial tube inner conductor 15 a and the raw material tungsten electrode 16 (see the enlarged portion in FIG. 1). ).

The fine particles obtained by the production method of the present invention are expected to be applied to the following fields.
・ Photocatalysts, gas sensors, conductive materials, electrochromic windows, UV blocking materials, infrared blocking materials, blue and yellow pigments, conductive paints, thin film heaters

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.
Example 1 (200Hz, no gas supply)
An outline of the microwave plasma device is shown in FIG. The microwave generated by the 2.45 GHz microwave oscillator is conducted to the reaction vessel through the waveguide, and the plasma energy is generated in the liquid by concentrating the microwave energy at the tip of the tungsten electrode by coaxial waveguide conversion. .

  The electrode is composed of a movable tungsten rod and can be fed into the system even when the electrode is consumed by reaction.

  Gas is supplied through the space around the electrode rod (enlarged portion in FIG. 1). When gas is not supplied, the structure other than the opening contacting the liquid is a closed system.

  The plasma electrode was water-cooled except for the electrode tip to prevent breakage due to high temperature.

  The reaction vessel was a 2 L beaker, and cooling water was supplied to a constant temperature bath to control the temperature of the reaction solution at 70 to 80 ° C.

  The pulse plasma was generated using a microwave pulse power source shown in FIG. The filament power supply is used to energize the filament that is the electron source of the magnetron. After energizing the filament, the pulse power supply is activated, and an intermittent pulse microwave is generated by selecting an arbitrary pulse frequency by the function generator. Although a constant current source is not normally used, it was used as an auxiliary role when the output of pulsed plasma decreased due to load fluctuations.

  The electrode was subjected to a plasma treatment for 60 minutes in 2 L of ultrapure water at a tip power of 3.5 kW, a pulse frequency of 200 Hz, and a duty ratio of about 20% under the condition that a tungsten rod and gas were not supplied. The resulting liquid was pale in color, suggesting that a blue tungsten product was formed. The product was filtered and dried and collected as a powder for evaluation.

Example 2 (300Hz, no gas supply)
Plasma treatment was performed under the same conditions as in Example 1 except that the pulse frequency was 300 Hz. The produced liquid showed pale blue color, suggesting that blue tungsten oxide fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Example 3 (400Hz, no gas supply)
Plasma treatment was performed under the same conditions as in Example 1 except that the pulse frequency was 400 Hz. The produced liquid showed pale blue color, suggesting that blue tungsten oxide fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Example 4 (200 Hz, compressed air)
Plasma treatment was performed under the same conditions as in Example 1 except that compressed air was supplied to the periphery of the electrode at about 1 L / min. The produced liquid was yellow, suggesting that tungsten trioxide fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Example 5 (300 Hz, compressed air)
Plasma treatment was performed under the same conditions as in Example 1 except that compressed air was supplied to the periphery of the electrode at about 1 L / min and the pulse frequency was 300 Hz. The produced liquid was yellow, suggesting that tungsten trioxide fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Example 6 (400 Hz, compressed air)
Plasma treatment was performed under the same conditions as in Example 1 except that compressed air was supplied to the periphery of the electrode at about 1 L / min and the pulse frequency was 400 Hz. The produced liquid was yellow, suggesting that tungsten trioxide fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Example 7 (300 Hz, pure oxygen)
Plasma treatment was performed under the same conditions as in Example 1 except that pure oxygen was supplied around the electrode at about 1 L / min and the pulse frequency was 300 Hz. The produced liquid was yellow, suggesting that tungsten trioxide fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Example 8 (300 Hz, argon)
Plasma treatment was performed under the same conditions as in Example 1 except that argon was supplied as an inert gas around the electrode at about 1 L / min and the pulse frequency was 300 Hz. The produced liquid was blue and had a gray precipitate, suggesting that blue tungsten oxide and metallic tungsten fine particles were produced. The product was filtered and dried and collected as a powder for evaluation.

Comparative Example 1 (100Hz, no gas supply)
Plasma treatment was performed under the same conditions as in Example 1 except that the pulse frequency was 100 Hz and the pulse duty ratio was 30%. The resulting liquid was almost clear, suggesting very little reaction product. The product was filtered and dried and collected as a powder for evaluation.

Comparative Example 2 (commercially available product)
As a comparative reference example, evaluation was performed using tungsten trioxide fine particles having an average particle diameter of 40 nm manufactured by Beijing dekedaoking technology.

Evaluation 1 (generation speed)
FIG. 3 shows the results of examining the production rates of the tungsten oxides of the example and the comparative example. The production rate was calculated by “weight of powder collected by filtration / plasma treatment time”.

  In the system without gas supply, the generation rate improved to about 0.14g / h when the pulse frequency was increased. As shown in FIG. 4, the pulse plasma is intermittently ignited / extinguished.

  At 100 Hz (Comparative Example 1), it is enveloped in high-temperature bubbles at the time of ignition, but until the re-ignition, the bubbles contract and the electrode is submerged in the liquid to be cooled, whereas 200 Hz (Example 1) Then, the period until re-ignition was very short, and when 300 Hz (Example 2) was exceeded, bubbles were continuously present during the interval until re-ignition, and at the same time, black body radiation of the electrode could be confirmed. The electrode temperature indicated by blackbody radiation was calculated to be about 5000 ° C. from Planck's law, and it became clear that the electrode maintained a high temperature. By accelerating the plasma ignition cycle in this manner, it is considered that the time during which the electrode as a raw material is exposed to a high temperature increases, and the reaction rate is accelerated.

  In addition, when a gas containing oxygen is supplied, the electrode is always enveloped by the gas supply, so that a state similar to the above 300 Hz or higher can be created. For this reason, although the production rate does not depend on the pulse frequency, the production rate is 0.34 g / h for air, and 1.5 g / h for pure oxygen, depending on the oxygen concentration. As shown in FIG. 5, the relationship between the oxygen concentration and the production rate is linear, and it has been clarified that the production rate is improved by promoting the oxidation reaction by supplying oxygen around the electrode.

Evaluation 2 (Yield)
The yields of Examples and Comparative Examples were calculated in terms of tungsten trioxide based on the ratio of the weight of powder collected by filtration and the amount of wear of the tungsten electrode. As shown in FIG. 6, it can be confirmed that the system (Examples 4 to 8) in which the gas flowed has a higher yield than the system (Comparative Example 1 and Examples 1 to 3) in which the gas did not flow. By flowing a gas around the electrode, it is considered that there is an effect of forcibly discharging the vapor of the reaction product that has been moved by concentration diffusion in the past into the liquid by the gas flow.

Evaluation 3 (Production substance identification)
The obtained sample showed yellow in the system (Examples 4 to 7) to which the gas containing oxygen was supplied, whereas the system to which the inert gas was supplied or was not supplied (Examples 1 to 3). 8) showed blue. This phenomenon suggests that the product varies depending on the gas species, and we attempted to identify the substance by the following analysis.

Analysis 1) The absorption spectra of the sample powders of Examples 1 to 8 and Comparative Example 2 were measured. For the measurement, an absorption spectrum was calculated from the reflectance obtained from the diffuse reflection spectrum using an ultraviolet-visible spectrophotometer V-670 manufactured by JASCO. The results are shown in FIG.

Analysis 2) Powder X-ray diffraction measurement was performed on the samples obtained in Examples 1 to 8 and Comparative Example 1 to identify the product. The results are shown in FIGS.

Analysis 3) The samples obtained in Example 3 and Example 6 were measured for TEM-EELS spectra. EELS is a technique for analyzing constituent elements and electronic structure by measuring the energy lost by interaction with atoms when electrons pass through the sample. Sensitivity of light elements such as oxygen is good and energy resolution is low. The feature is high. FIG. 13 shows a TEM image of the measurement site, and FIG. 14 shows an O1s EELS spectrum.

  As reference data for WO3, data published in G. Mountjoy et.al., Inst. Phys. Conf. Ser. No. 138 Section 2, 35-38 (1993) was used. c-WO3 is crystalline tungsten trioxide, a-WO3 is amorphous tungsten trioxide, and Damaged a-WO3 is amorphous tungsten trioxide in which oxygen deficiency occurs. The materials obtained in Examples and Comparative Examples are crystalline tungsten oxides based on the XRD results, but it can be confirmed that there is no difference in the electronic structure from the crystalline and amorphous peaks in the above data. Comparison was made with measurement data using both -WO3 and Damaged a-WO3 spectra as references.

  In Examples 1 to 3 (a system in which no gas is supplied), the XRD pattern in FIG. 8 substantially matches with orthorhombic WO3. The crystal structure of WO3 depends on the temperature. From the low temperature side, triclinic crystal (-50 to 17 ° C), monoclinic crystal (17 to 330 ° C), orthorhombic crystal (330 to 740 ° C), tetragonal crystal (740 ° C to It can be said that these samples obtained by quenching the high-temperature WO3 vapor generated in the plasma exhibit orthorhombic crystals are reasonable results. In addition, the shoulder around 24.4 ° indicated by hydrogen tungsten bronze is also seen, suggesting this existence.

  Oxygen deficiency was difficult to identify with XRD, so it was judged from TEM-EELS spectral analysis. As shown in FIG. 13, the produced sample of Example 3 is composed of particles exceeding 20 nm and particles of 10 nm or less, but their O1s EELS spectra are different as shown in FIG. It was suggested that occurred.

  Since the spectroscopic measurement result (FIG. 7) has an absorption spectrum with strong absorption in the visible light region and a transmission peak in the blue region, the products of Examples 1 to 3 are mainly oxygen-deficient tungsten oxide, hydrogen tungsten bronze and It is thought that it is composed of a small amount of tungsten trioxide.

  In the system supplied with air (Examples 4 to 6), the XRD pattern in FIG. 9 can confirm the orthorhombic WO3 and the shoulder around 24.4 ° indicated by the hydrogen tungsten bronze. The product obtained in Example 6 is composed of particles around 10 nm and particles observed thinly below 10 nm (FIG. 13). From the O1s EELS spectrum (FIG. 14), the particles below 10 nm are oxygen. It was confirmed that a defect occurred. The spectroscopic measurement result (FIG. 7) showed similar characteristics to Comparative Example 2 (commercial product), although absorption was observed in the visible light range. Since there is a tendency to have a slightly transmission peak in the blue region, the products of Examples 4 to 6 are mainly composed of tungsten trioxide, and contain a small amount of oxygen-deficient tungsten oxide and hydrogen tungsten bronze. Conceivable.

  In the system supplied with pure oxygen (Example 7), the XRD pattern of FIG. 10 has a very clear orthorhombic WO3 pattern, and a shoulder around 24.4 ° indicated by hydrogen tungsten bronze can be confirmed. The spectroscopic measurement result (FIG. 7) showed similar characteristics to Comparative Example 2 (commercial product), although absorption was observed in the visible light range. Since there is a tendency to have a slight transmission peak in the blue region, it is considered that the product of Example 7 is mainly composed of tungsten trioxide and contains a small amount of hydrogen tungsten bronze.

  In the system to which inert (argon) gas was supplied (Example 8), it was separated into a gray precipitate and a blue supernatant solution, which were separated and recovered by decantation, and each analysis was performed.

  In the supernatant collection, the XRD pattern (FIG. 11) has an orthorhombic WO3 pattern, and a shoulder around 24.4 ° indicated by hydrogen tungsten bronze can be confirmed. The spectroscopic measurement result (FIG. 7) of the supernatant collected has a strong absorption in the visible light region and an absorption spectrum having a transmission peak in the blue region. Since the oxygen deficient tungsten oxide is contained in the precipitate to be described later, the supernatant recovered is mainly composed of oxygen deficient tungsten oxide and hydrogen tungsten bronze, and is considered to contain a small amount of tungsten trioxide.

  In the precipitate, the XRD pattern (FIG. 11) similarly shows an orthorhombic WO3 peak and a shoulder near 24.4 ° indicated by hydrogen tungsten bronze, but the peak of oxygen-deficient tungsten oxide was also clearly confirmed. It can also be confirmed that not only α-phase tungsten having the same composition as the raw material rod but also β-phase tungsten is produced. Since the recovered material is gray, it is mainly composed of α-phase and β-phase tungsten, and is considered to contain small amounts of oxygen-deficient tungsten oxide, hydrogen tungsten bronze, and tungsten trioxide. In plasma, water is decomposed to generate hydrogen and oxygen, which are hydrogen and oxygen sources such as hydrogen tungsten bronze, but by supplying inert gas, oxidation is suppressed and metal tungsten fine particles are obtained. It is thought that.

  In Comparative Example 1, the XRD pattern (FIG. 12) has an orthorhombic WO3 pattern, and a shoulder around 24.4 ° indicated by hydrogen tungsten bronze can be confirmed. Further, the pattern of orthorhombic WO3 hydrate that was not produced in the examples can be clearly confirmed. As can be seen from the high-speed camera image (Fig. 4), large bubbles filled with high-temperature water vapor are generated at the same time as plasma ignition, and it takes 4 ms or more to disappear. It is thought that water vapor is combined. In addition, a peak of α-phase tungsten derived from the raw material due to the tungsten electrode rod peeling off due to low frequency cavitation occurring at 100 Hz is seen.

  Further, in the XRD patterns of FIGS. 8 and 9, the peaks of α phase and β phase tungsten are seen at 300 Hz (Examples 2 and 5) and 400 Hz (Examples 3 and 6), whereas these peaks are It was not observed in Examples 1 and 3 performed at a pulse frequency of 200 Hz. Further, the peak of α-phase tungsten is also observed in Comparative Example 1 (100 Hz, FIG. 12). At 100 Hz, the electrode surface was peeled off due to low frequency cavitation. At 300Hz or higher, as shown in the high-speed camera image (Fig. 4), the bubble and the black body radiation of the raw material electrode are constantly observed. It is thought that there are many. The oxygen stoichiometry calculated from the amount of supply air and the wear rate of the tungsten electrode suggests that 44 and oxygen are sufficiently supplied, but the air flow is about 5000 ° C as described above. Therefore, the flow field is about 19 times faster than that at room temperature, and the reaction field is limited to the high temperature electrode surface, so only a part of the oxygen in the supplied air is used and discharged outside the system. it is conceivable that. For this reason, it is considered that part of the oxygen-deficient tungsten oxide and part of it deposited in the liquid as metallic tungsten. In such a case, it is considered that the oxygen concentration is more dominant than the supply gas amount, but in fact, Example 7 using pure oxygen has an XRD peak derived from metallic tungsten despite the same 300 Hz condition. It was confirmed that there was no.

  Regarding the plasma ignition cycle and input power, in Comparative Example 1 (100 Hz), a peak power of 3500 W and a duty ratio of 30% were required to generate a stable plasma, whereas Examples 1 to 8 (200 (~ 400Hz), only 20% duty ratio was required at the same peak power value. The average input power can be calculated as 1050W and 700W, and the power consumption was reduced by about 33% by accelerating the ignition cycle.

  The above results are summarized as follows.

  The generation rate is increased by accelerating the plasma ignition cycle and keeping the raw material electrode at a high temperature, thereby increasing the reaction rate (FIG. 3, Examples 1 to 3, Comparative Example 1). If the ignition cycle is too slow, the raw material tungsten is mixed into the sample by low-frequency cavitation. If the ignition cycle is too fast, the electrode evaporates at a high temperature and the metal tungsten is deposited in addition to the target tungsten oxide. Therefore, the ignition cycle is preferably higher than 100 Hz and lower than or equal to 300 Hz, and preferably around 200 Hz. The yield was improved by providing a flow field by gas supply around the electrode, and quickly discharging the reaction product out of the electrode system.

  As for the production of tungsten oxide, by supplying a gas containing oxygen around the electrode, the oxidation reaction is promoted and the production rate is greatly improved. There is a correlation between oxygen concentration and production rate (FIG. 5). When a gas containing oxygen is supplied, the generation rate does not depend on the ignition cycle. However, when air is used (oxygen concentration 20.95%), if the ignition cycle is too fast, the target tungsten oxide will be used depending on the oxygen concentration. In addition, since metallic tungsten is deposited, the ignition cycle is preferably higher than 100 Hz and lower than or equal to 300 Hz, and preferably around 200 Hz. When pure oxygen is used, there is no influence of the ignition cycle because 100% concentration of oxygen is constantly supplied to the reaction field.

  The resulting product differs depending on the supply state of the oxygen-containing gas to the electrode. When the gas is not supplied, a blue powder mainly composed of oxygen-deficient tungsten oxide and hydrogen tungsten bronze is obtained. When the gas is supplied, tungsten trioxide is mainly used. A yellow powder is obtained.

  The tungsten oxide thus obtained is expected to be used for a photocatalyst, a gas sensor, a conductive material, and an electrochromic window by making use of its absorption characteristics in the visible light region. In addition, since absorption is also seen in the infrared region, when this material is applied to exterior building materials, the heat energy of sunlight is shielded as an infrared shielding material to prevent an increase in indoor temperature, such as an air conditioner. It is possible to provide a function of suppressing the use of. Further, since the product color can be controlled by the oxygen concentration, it is expected to be used as an exterior building material having high pigment and color selectivity.

  When an inert gas was used instead of the gas containing oxygen, it was confirmed that metal tungsten particles containing β-phase tungsten in addition to α-phase tungsten could be generated. In general, the α phase is more stable than the β phase, and most of the bulk tungsten is occupied by the α phase. Since β-phase tungsten has very high characteristics in both heat resistance and resistivity, it is suitable as a constituent material for thin film heaters and is expected to be used in these fields. At present, oxides are generated even when inert gas is supplied by dissolved oxygen in water or oxygen generated by decomposition of water with plasma, but metal tungsten fine particles precipitate due to the difference in specific gravity, so they can be separated by a simple method such as decantation. Is possible. After the separation, the tungsten oxide contained in a small amount can be removed by alkali treatment to recover the metal tungsten fine particles containing the β phase. In addition, it is considered possible to increase the yield of the metal tungsten fine particles by supplying a reducing gas containing hydrogen or the like and suppressing the formation of oxides.

  In the examples, oxygen gas and inert gas are used. However, when other gases are supplied, it is considered that a reaction product can be selectively generated to some extent according to the gas type. For example, it is expected that tungsten nitride is supplied by supplying nitrogen and tungsten carbide is supplied by supplying a mixed gas of carbon dioxide and hydrogen.

  In addition, in the conventional process, the phase change leading to plasma generation was required for two phases of liquid → gas → ionized gas, whereas in this example, the bubbles around the electrodes were held by accelerating the ignition cycle, From gas to ionized gas, the amount of energy required for the phase change from liquid to gas can be saved and energy efficiency can be increased.

  In addition, it should be noted that these methods can be used as methods for producing other metal oxides, metal compounds, and metal fine particles, and are very highly producible production methods.

DESCRIPTION OF SYMBOLS 10 Microwave oscillator 11 Power monitor 12 Three tab tuner 13 Plunger 14 Coaxial waveguide converter 15 Coaxial pipe 15a Inner conductor 15b Outer conductor 16 Tungsten wire (electrode)
17 PTFE (sealing material)
18 Tungsten GND ring 19 Refrigerant 20 Waveguide 30 Beaker (container)
31 Water bath 32 Refrigerant 33 Magnetic stirrer 34 Thermocouple 40 Filament power supply 41 High voltage power supply (CW)
42 High Voltage Power Supply (Pulse)
43 Function Generator 44 Magnetron

Claims (5)

  A method for producing tungsten oxide or metal tungsten fine particles using a pulsed microwave submerged plasma apparatus using tungsten as an electrode, wherein the plasma ignition cycle exceeds 100 Hz. Manufacturing method.   2. The method for producing tungsten oxide or metal tungsten fine particles according to claim 1, wherein the plasma ignition cycle is more than 100 Hz and not more than 300 Hz.   The method for producing tungsten oxide or metal tungsten fine particles according to claim 1 or 2, wherein a gas is allowed to flow around the raw material tungsten electrode.   4. The method for producing tungsten oxide or metal tungsten fine particles according to claim 3, wherein tungsten oxide fine particles are produced using a gas containing oxygen as the gas. 4. The method for producing tungsten oxide or metal tungsten fine particles according to claim 3, wherein metal tungsten fine particles are produced using an inert or reducing gas as the gas .