WO2018021282A1 - エレクトライド化マイエナイト型化合物の製造方法 - Google Patents
エレクトライド化マイエナイト型化合物の製造方法 Download PDFInfo
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- WO2018021282A1 WO2018021282A1 PCT/JP2017/026817 JP2017026817W WO2018021282A1 WO 2018021282 A1 WO2018021282 A1 WO 2018021282A1 JP 2017026817 W JP2017026817 W JP 2017026817W WO 2018021282 A1 WO2018021282 A1 WO 2018021282A1
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Definitions
- the present invention relates to a method for producing an electride mayenite type compound.
- the mayenite type compound has a representative composition of 12CaO ⁇ 7Al 2 O 3 (hereinafter sometimes abbreviated as “C12A7”).
- C12A7 12CaO ⁇ 7Al 2 O 3
- conductive mayenite type compound a mayenite type compound having conductivity
- Patent Document 1 a mayenite type compound having an insulator
- the conductive mayenite type compound (hereinafter sometimes abbreviated as C12A7: e ⁇ ) is obtained by substituting free oxygen in the aforementioned cage of the mayenite type compound with an electron.
- the concentration is 2.3 ⁇ 10 21 cm ⁇ 3 .
- the conductive mayenite type compound can be referred to as an inorganic electride compound (Non-patent Document 2).
- the C12A7: e ⁇ that has been reported by the present inventors is the first electride that is stable at room temperature and in the atmosphere (Non-patent Document 3).
- a mayenite type compound is melted and held in an atmosphere having a low oxygen partial pressure, and then cooled and solidified to produce the conductive mayenite type compound (Patent Document 2), a mayenite type compound precursor, A method of mixing with a reducing agent and performing a heat treatment (Patent Document 3) has been proposed.
- C12A7: e ⁇ can be synthesized by spark plasma sintering without using a reducing agent such as Ti, in which the mayenite type compound is directly synthesized by the Ca metal reduction method (Non-patent Document 4)
- a method of synthesis in stages Non-Patent Document 5
- a method of forming a thin film of C12A7 electride by forming a film on a substrate by vapor deposition Patent Document 5
- an aluminum foil on the surface of a mayenite type compound
- a method of producing a conductive mayenite type compound while maintaining at a high temperature in an atmosphere of low oxygen partial pressure Patent Document 6).
- the mayenite type compound can be converted into a conductive mayenite type compound, that is, converted into an electride mayenite type compound by various methods using a reducing agent.
- a reducing agent As shown in FIG. 2, when free oxygen ions (O 2 ⁇ ) and, for example, metal Ti are used as a reducing agent, TiO 2 which is an oxide of the reducing agent is obtained by the reaction shown in the following reaction formula. And free oxygen ions are extracted from the cage and replaced with electrons (e ⁇ ).
- Patent Documents 1 to 4 and Non-Patent Document 3 a complicated operation such as mixing a reducing agent such as metal Ti or metal Ca with a mayenite type compound or applying it to the surface thereof. Necessary. In addition, these production methods usually require a high temperature reaction of 1000 ° C. or more, and further heating for a long time.
- Examples of the manufacturing method that does not use a reducing agent include the manufacturing method described in Non-Patent Document 5. However, since this manufacturing method uses a discharge plasma sintering method, a plasma discharge device is required. Moreover, this manufacturing method is not practical yet in that heating at 1000 ° C. or higher and vacuum equipment are required, as in the method using a reducing agent.
- the present invention has been made for the purpose of solving the above-described problems, and does not require a reducing agent, and can be easily performed in a short time, with less energy, in a short time and with less energy than conventional methods. It is to provide a manufacturing method that can be manufactured.
- the present inventors have found a method of electrifying a mayenite type compound by applying a voltage to the mayenite type compound under heating and passing a current.
- the gist of the present invention is as follows. [1] An electride mayenite compound, wherein a voltage is applied to the mayenite compound in a heated state, and the current is directly passed through the mayenite compound, thereby electretizing the mayenite compound. Manufacturing method. [2] The production method according to [1], wherein a voltage is applied until an electrical resistivity of the mayenite type compound is 1.0 ⁇ 10 5 ⁇ ⁇ cm or less. [3] The production method according to [1] or [2], wherein a positive electrode and a negative electrode are applied to the mayenite type compound to directly pass a current.
- the present invention it is possible to provide a production method that does not require a reducing agent and can electretize a mayenite type compound at a lower temperature, in a shorter time, and with less energy than conventional methods. Moreover, since the mayenite type compound can be converted into an electride by passing a current directly through the mayenite type compound, the finely processed conductive mayenite type compound can be easily produced.
- FIG. 3 It is a schematic diagram which shows the concept of the heating / voltage control process (A) and the heating / current control process (B) in the production method of the present invention. It is a figure which shows the concept of the conventional method which draws out free oxygen ion by the reduction method. It is a conceptual diagram of the method of manufacturing an electride using a heating furnace.
- FIG. 3 it is a graph which shows the time (horizontal axis) change of the sample temperature (left vertical axis) and the oxygen detachment pressure (right vertical axis) by In sensor and Out sensor measurement in Comparative Example 1.
- FIG. 3 shows the experimental method shown in FIG.
- FIG. 13 is an enlarged view of a resistance transition temperature (925 K) portion in FIG. 12.
- 5 is a graph showing a relationship (PT curve) between a change in oxygen detachment pressure (vertical axis) and heating temperature (horizontal axis) under Area II current control and CD control. It is a schematic diagram (B) which shows the state in which the monoatomic oxygen desorbed on the positive electrode side oxidizes molybdenum (Mo) of the positive electrode. 5 is a graph showing the relationship (PT curve) between the change in oxygen detachment pressure ratio (vertical axis) and heating temperature (horizontal axis) under Area II current control and CD control, compared with the case of no voltage application.
- 5 is a graph showing heating temperature (left vertical axis), power consumption (right vertical axis), and oxygen dissociation pressure time (horizontal axis) change (TW-Time curve) under Area II current control and CD control.
- 6 is a graph showing the relationship between applied voltage and current in Example 2. It is a graph which shows the relationship of the elapsed time at the time of the current control in Example 2, and an electric current.
- 10 is a graph showing the relationship between applied voltage and current in Example 3. It is a graph which shows the relationship of the elapsed time at the time of the current control in Example 3, and an electric current.
- the mayenite type compound used in the present invention has a representative composition represented by 12CaO ⁇ 7Al 2 O 3 .
- the crystal of the mayenite type compound is formed by a cage structure (cage) sharing its wall surface and three-dimensionally connecting.
- an anion such as O 2 ⁇ is contained inside the cage of the mayenite type compound, but these can be replaced with another anion or conduction electron by chemical treatment. Examples of the anion is not particularly limited, for example, O 2-ion, F - ions, Cl - ions, and the like.
- the cage forming the skeleton of the mayenite type compound usually contains cations such as Ca and Al, and these cations may be partially or entirely substituted with other cations.
- the shape of the mayenite type compound used in the present invention is not particularly limited, and usually includes powder, fine particles, granules, bulk, thin film obtained by vapor phase method, and the like.
- a sintered body obtained by sintering a powdery mayenite type compound, or a single crystal obtained by a floating zone (FZ) method or the like may be used as an example of a bulk type.
- a conductive mayenite type compound can also be manufactured consistently from this raw material.
- the calcium component include calcium carbonate, calcium hydroxide, calcium oxide, or metallic calcium.
- the aluminum component examples include aluminum oxide, aluminum hydroxide, and metal aluminum. A mixture of these raw materials according to the stoichiometry of the target product can be used. Moreover, mixed oxides of calcium and aluminum such as CaAl 2 O 4 and Ca 3 Al 2 O 6 can also be used. Among these, in the present invention, the shape of the mayenite type compound is preferably a single crystal because the movement of anions in the cage such as oxygen ions is smooth.
- the production method of the present invention is characterized in that the mayenite type compound is electrified by applying a voltage to the mayenite type compound in a heated state and passing a current directly through the mayenite type compound. . This will be specifically described below.
- a voltage is applied to the mayenite type compound and a current is passed directly.
- the current flows directly to the raw material mayenite type compound.
- the positive electrode and the negative electrode are provided to the mayenite type compound, and these electrodes are provided.
- a voltage can be applied from directly to pass a current through the mayenite type compound.
- the material of the electrode is not particularly limited, but usually a material that is durable under high temperature conditions by heating or a material that is resistant to oxidation is selected, and C (carbon), Ti, Ni, Mo, W, Ta, Pt Ni—Cr alloy and the like, and preferably a metal such as Pt, Ti, Ni, etc. that can be applied by a vapor deposition method such as sputtering.
- the method of applying the electrode is not particularly limited, and specifically, there are a method of attaching the electrode by mechanical contact and a method of attaching by vapor deposition such as sputtering.
- the deposition method is preferably used because an electrode having an arbitrary size (for example, an extremely small area) can be attached to an arbitrary location of an arbitrary size sample.
- the mayenite type compound may be filled in a conductive container, and a voltage may be applied to the container to directly flow the current to the mayenite type compound.
- the material of the conductive container include, but are not limited to, metal and graphite.
- the shape of the container is not particularly limited as long as electricity flows directly into the mayenite type compound filled therein.
- a specific method for example, a method of using a conductive material as a lid on both ends of a cylindrical insulator container, filling a mayenite type compound between the lids, and passing a current through the lid, etc. It is done.
- the method of applying an electrode to the mayenite type compound and allowing an electric current to flow is preferable in that the operation is easy and the required amount of energy can be reduced.
- the mayenite type compound when a voltage is applied to the mayenite type compound, the mayenite type compound is placed under a heated state.
- free oxygen ions in the cage of the mayenite type compound are dissociated outside the cage, and the inclusion of electrons instead is referred to as electride.
- the mayenite type compound to which a voltage is applied in a heated state is referred to as an object to be heated.
- the heated body may be a mayenite type compound provided with the electrode.
- the heating method is not particularly limited, but the object to be heated can be externally heated, such as a heater, and the heat generated by flowing an electric current to the object to be heated (hereinafter sometimes referred to as self-heating). It can also be heated.
- the heating temperature of the object to be heated is not particularly limited and can be appropriately selected according to the degree of progress of the electride. Usually, it is a temperature lower than the melting point of the mayenite type compound to be used, for example, C12A7. For example, the temperature is lower than 1450 ° C.
- the heating temperature is advantageous in that the higher the motility of free oxygen ions in the mayenite type compound, the lower the heating temperature, but the lower the temperature, the better the energy efficiency and the higher the need for a high-temperature heat source. Moreover, 400 degrees C or less is more preferable at the point which can prevent the oxidation of the electride mayenite type compound.
- the temperature history of heating is not particularly limited within the range in which the effect of the present invention is obtained, and may be performed continuously or intermittently, and even when performed at a constant temperature, the temperature is increased stepwise. It may be lowered.
- the voltage to be applied to the object to be heated is not particularly limited and can be appropriately set as long as the effect of the present invention can be obtained. However, it is usually over 0 V, and a current flows through the mayenite type compound as will be described later. Thus, if a voltage can be applied, the voltage is not specified.
- the preferred voltage depends on the ionic conductivity of the mayenite type compound at the heating temperature. The upper limit is lower than the voltage causing dielectric breakdown. Considering the convenience of the power supply device, 200 V or less is preferable.
- the voltage application history is not particularly limited as long as the effect of the present invention is obtained, and may be performed continuously or intermittently, and even when performed at a constant voltage, the voltage is increased stepwise. It may be lowered.
- the value of the current that flows directly to the object to be heated is not particularly limited as long as the effect of the present invention is obtained. Specifically, it is only necessary to confirm that a current flows directly through the heated body, but the current density is usually 0.001 Acm ⁇ 2 or more and 10 Acm ⁇ 2 or less. Usually, since a mayenite type compound is an insulator, current hardly flows. A current usually flows through the mayenite type compound while heating and applying a voltage, specifically by a method described later. A specific method for supplying current will be described later.
- the production method of the present invention can be produced by appropriately combining the heating method, the voltage application method and the current application method, and the specific combination of the methods is not particularly limited, Preferably, for example, the following methods 1 to 3 are mentioned.
- Condition 1 Method of increasing the voltage stepwise by setting the heating temperature to a predetermined temperature Specifically, first, heating is performed to a predetermined temperature in a state where no voltage is applied to the object to be heated, and then the object to be heated In this method, the voltage applied to the heating body is applied stepwise from 0V.
- Condition 2 A method of increasing the heating temperature stepwise by setting the voltage to a predetermined voltage. Specifically, a voltage to be applied to the heated object is set at room temperature, and then the temperature is gradually increased from room temperature. Is the method.
- Condition 3 Method of raising both heating temperature and voltage Specifically, heating and voltage are applied to the object to be heated while simultaneously adjusting the heating temperature and applied voltage from a state where normal temperature and voltage are not applied to the object to be heated. This is a method of applying the voltage stepwise.
- the mayenite type compound is converted into an electride by heating and applying a voltage to the object to be heated, and the process of converting into an electride preferably goes through the following steps.
- the mayenite type compound is usually an insulator at normal temperature, almost no current flows at the beginning of voltage application, and only a very small amount of current flows.
- a decrease in the applied voltage is observed with a rapid increase in the current density flowing through the object to be heated next.
- the current density there is no particular limitation on the current density at this time, and since it usually changes rapidly, it is relatively difficult to specify the value, but about 1 Acm -2 is a standard, but usually it is 0.1 Acm -2 or more. is there.
- a process until the above phenomenon is observed is referred to as a voltage control process under heating (hereinafter referred to as a heating / voltage control process).
- the heating / voltage control step voltage application to the object to be heated is usually controlled according to the applied voltage (hereinafter, voltage control). This is because, in this process, almost no current flows through the heated body, and it is easier to manage the manufacturing by controlling the applied voltage.
- the current density increase point After the time point when the increase in current density and the decrease in applied voltage are observed (hereinafter referred to as the current density increase point), the object to be heated changes into a state in which the current flows more easily than when heating and voltage application are started. It is thought that.
- the method of controlling the voltage application is changed from matching the applied voltage to controlling the current value flowing to the heated body (hereinafter referred to as current control) while continuing to heat the heated body. Transition.
- current control controlling the current value flowing to the heated body
- transition By shifting to the current control and passing a current through the heated body, oxygen in the heated body is desorbed and electrons are supplied. Thereby, electride conversion of the mayenite type compound proceeds.
- a conductive mayenite type compound can be obtained by performing the current control up to a desired electron concentration.
- a process from when the current density increase point is reached until a desired conductive mayenite type compound is obtained and voltage application is finished is referred to as a current control process under heating (hereinafter referred to as a heating / current control process).
- the heating / current control step can be appropriately performed until the heated body reaches a desired electron concentration, but normally, the resistivity of the heated body decreases as the electrification progresses. Continue until the rate is sufficiently reduced.
- the specific value of the resistivity is not particularly limited, but it is a standard that the resistivity is usually 1.0 ⁇ 10 5 ⁇ ⁇ cm or less.
- the electrical conductivity of the object to be heated usually increases as the electride progresses. Therefore, the electrical conductivity can be used as a standard for electrification, and it is usually continued until the electrical conductivity reaches 1.0 ⁇ 10 ⁇ 4 S / cm or more.
- the object to be heated can also be determined by observing the color of the heated object. Specifically, it is only necessary to pass a current until the object to be heated has a desired color according to the grade described later. For example, if a grade C conductive mayenite type compound described later is desired, the object to be heated is heated. The heating / current control step may be continued until the body turns black.
- the method for shifting from the heating / voltage control step to the heating current control step is not particularly limited as long as the effects of the present invention are obtained, and can be performed by appropriately combining known methods. Specifically, for example, a method of switching appropriately using a power supply device corresponding to high power such as 100V / 10A, a method of preparing voltage application devices used in both steps, and switching after passing through the current density increase point, etc. Is mentioned.
- the production method of the present invention can be produced without any particular limitation on the pressure state, and can be produced under atmospheric pressure, reduced pressure conditions, and pressurized conditions.
- the production method of the present invention can be produced without any particular restrictions on the surrounding atmosphere.
- the lower the oxygen partial pressure the more the free oxygen ions are dissociated from the mayenite type compound, and therefore the heated body is a low oxygen partial pressure atmosphere of 100 Pa or less, particularly an atmosphere with extremely low oxygen partial pressure. It is preferable to maintain an extremely low oxygen partial pressure atmosphere.
- the extremely low oxygen partial pressure atmosphere is an atmosphere in which the oxygen partial pressure is reduced to about 10 ⁇ 15 Pa (10 ⁇ 20 atoms) or less in an inert gas atmosphere, or a vacuum degree of 10 ⁇ 5 Pa or less. State.
- the inert gas is not particularly limited, and nitrogen, argon and the like are used, and preferably argon.
- nitrogen, argon and the like are used, and preferably argon.
- the heating temperature exceeds 400 ° C., if the oxygen partial pressure is high, the electride mayenite type compound is likely to be oxidized. Therefore, an extremely low oxygen partial pressure atmosphere is preferable, and the heating temperature is If it is 400 degrees C or less, an atmosphere will not be restrict
- the conductive mayenite type compound can be classified into the following three grades A to C, based on the electron concentration per unit volume.
- the conductive mayenite type compound obtained by the production method of the present invention can be produced in any of the above three grades, but preferably has the highest electron concentration and the high electron emission capability per unit deposition. Suitable for manufacture of C class. Specifically, since the theoretical maximum value of the electron concentration is 2.3 ⁇ 10 21 cm ⁇ 3 , the electron concentration is 1.0 ⁇ 10 20 / cm 3 or more and 2.3 ⁇ 10 21 cm. -3 or less conductive mayenite type compound can be obtained.
- the obtained conductive mayenite type compound may have a uniform or non-uniform electron concentration, but is preferably uniform.
- the grade of the non-uniform conductive mayenite type compound is determined by visual color, and the electron concentration is a measured value of the highest electron concentration.
- the electrical conductivity at room temperature of the conductive mayenite type compound obtained by the production method of the present invention is about 1500 S / cm as its theoretical maximum value, it is preferably 1.0 S / cm or more, More preferably, it is 100 S / cm or more, preferably 1500 S / cm or less.
- FIG. 1 is a schematic diagram showing the concept of the heating / voltage control step (A) and the heating / current control step (B).
- free oxygen ions (O 2 ⁇ ) in the cage of the mayenite type compound are activated by heating to increase ion conductivity to increase oxygen ions. Turn into a conductor.
- Electrons are stripped from each of the above steps to change from an atomic polarization state to an electronic polarization state, the electrons after electron polarization remain in the cage as they are, and oxygen atoms try to deviate from the mayenite type compound.
- the dissociation pressure increases with heating.
- Oxygen dissociation refers to a phenomenon in which free oxygen ions leave the cage through a plurality of steps.
- the “oxygen dissociation pressure” in this specification is an O 2 partial pressure in an inert gas atmosphere such as Ar.
- FIG. 3 conceptually shows one aspect of producing electride using a heating furnace.
- a heating furnace is used, but the heating may be external heating such as a heater or internal resistance heating.
- C12A7: O 2 ⁇ is charged as an object to be heated 2 into the quartz tube 1 through which Ar having an extremely low oxygen partial pressure flows.
- An extremely low oxygen partial pressure control device 4 is attached to the heating furnace 3.
- the extremely low oxygen partial pressure control device 4 comprises an oxygen-containing gas inlet side oxygen sensor 41, an oxygen pump 42, a gas outlet side oxygen sensor 43, a circulation pump 44, and the like, and the extremely low oxygen partial pressure gas is heated. It is supplied into the furnace 3.
- the heating atmosphere is an extremely low oxygen partial pressure argon gas atmosphere flowing into the quartz tube 1. From the heated object 2 placed in this atmosphere, O 2 is desorbed by voltage control under heating and current control under heating, and an argon gas containing the desorbed oxygen flows out of the quartz tube 1, It circulates to the extremely low oxygen partial pressure control device 4.
- the heating / voltage control step when a voltage is applied in a range of 200 V or less, the higher the heating temperature, the higher the ionic conductivity (lowering the resistance). .
- the current in the mayenite type compound increases in proportion to the high voltage load.
- the temperature is increased to about 300 ° C. or higher, the resistance decreases as the temperature increases, so the load voltage decreases, the oxygen ion conductivity increases, and the current in the mayenite type compound increases. Therefore, the voltage range may be controlled in the range of 0 V to 200 V in relation to the temperature.
- the movement of the oxygen anion between the cages is performed while crossing the energy barrier in order to spread a spatially narrow bottleneck between the cages.
- the reaction mechanism of the production method of the present invention is estimated as follows.
- the oxygen atoms constituting the cage of the mayenite type compound are firmly bonded to calcium atoms and aluminum atoms and do not desorb unless the crystal lattice is broken.
- free oxygen ions in the cage are bound very loosely.
- the cage cannot move, but oxygen ions move while being replaced with oxygen ions on the cage wall by being drawn by the positive electrode (+) in the direction of current flow.
- the heating temperature and / or voltage is increased stepwise until the resistance of the object to be heated decreases rapidly and the current density increases rapidly.
- the ionic conductivity in that case is calculated
- the mayenite type compound near the positive electrode is electrolyzed into a conductive mayenite type compound and oxygen, and as a result, oxygen is released out of the cage.
- Example 1 [Voltage control under heating and current control under heating] A C12A7 single crystal obtained by the FZ method was cut into a size of 10 mm ⁇ 5 mm ⁇ 1 mm, and a platinum (Pt) thin film was deposited on both ends (end surface area: 5 mm ⁇ 1 mm) by sputtering as shown in FIG. Both the electrode and the negative electrode were attached with a molybdenum (Mo) wire to form an object to be heated. A voltage was applied to the object to be heated in a heated state according to the history shown in FIG. Specifically, high-temperature treatment up to 1000 ° C. and 1200 ° C. was performed under voltage control and current control.
- the treatment atmosphere was Ar gas (Ar flow rate: 3 L / min, average flow rate 5.2 cm / sec, atmospheric pressure, normal temperature) controlled at an extremely low oxygen partial pressure.
- Ar gas Ar flow rate: 3 L / min, average flow rate 5.2 cm / sec, atmospheric pressure, normal temperature
- a device manufactured by Canon Machinery Co., Ltd. was used as an extremely low oxygen partial pressure control device. As shown in FIG. 3, the oxygen partial pressure was measured by an Out sensor and an In sensor. The same applies to the extremely low oxygen partial pressure control.
- FIG. 11 shows changes in current / voltage in the body to be heated under the current control of Area II and the CD control. From about 18V at the start of current control (730K), a rapid change in the sample voltage is observed when the heating temperature reaches about 819K, and when the temperature drops to about 7V at 925K, the current in the body to be heated is gradually controlled by CD control. Increased to.
- FIGS. 12 and 13 partially enlarged view of FIG. 12
- FIG. 14 shows the relationship between the oxygen dissociation pressure (O 2 partial pressure in Ar) and the heating temperature. A rapid increase in oxygen detachment pressure is observed at 3T, and an increase in oxygen detachment pressure is observed as the heating temperature increases after 3T.
- FIG. 15 is a schematic view showing a state in which oxygen separated from the Ar atmosphere on the positive electrode side oxidizes molybdenum (Mo) of the molybdenum wire attached to the positive electrode.
- Mo molybdenum
- FIG. 16 shows the relationship between the equilibrium divergence pressure ratio (PI O2 / PII O2 ) and the heating temperature (T) in the case of CD control (voltage application 10 V) and in the case of a heated object in which no electrode is attached (no voltage applied). The comparison results are shown, and it can be seen that the 3T temperature is lowered (CD effective) when CD is inserted.
- PI O2 and PII O2 are respectively, In the sensor with the measured O 2 partial pressure (PI O2) and O 2 partial pressure measured by Out sensor (PII O2).
- FIG. 17 shows changes over time in temperature, power consumption, and oxygen detachment pressure after CD control.
- the power consumption peaked in the second half of the 3T temperature maintenance state in the current control state, and the oxygen detachment pressure was after the peak value.
- the power consumption once decreased, when the current in the body to be heated was increased by CD control, it turned to increase and saturated at a maximum of about 500 W / cm 2 .
- a C12A7 single crystal obtained by the FZ method was cut into a size of 10 mm ⁇ 5 mm ⁇ 1 mm, a platinum thin film was deposited on both ends thereof by sputtering, and each end was used as a positive electrode and a negative electrode, respectively.
- Molybdenum lead wires were attached to the positive electrode and the negative electrode to form an object to be heated.
- a current of about 1 pA current density 20 pA ⁇ cm ⁇ 2
- the power source was switched, and a power source capable of applying a maximum voltage of 100 V and a maximum current of 30 A was used.
- the temperature of the heated object was maintained, and a voltage was applied up to 100 V at each temperature. This was repeated until 180 ° C was reached.
- the minimum current detectable with this power supply is 0.1 mA, but no current was observed under the above conditions.
- the object to be heated was blackened and C12A7 was electrified. In the heated object, non-uniformity in electron concentration occurred.
- the maximum location was about 1.0 ⁇ 10 21 / cm 3 and the lowest location was about 1.0 ⁇ 10 19 / cm 3 .
- the electric conductivity at the site of maximum electron concentration of the electride-to-be-heated object was 5 ⁇ 10 2 S / cm.
- the heating temperature of C12A7 was raised from room temperature to 210 ° C. in the same procedure as in Example 2, and then held at 210 ° C. After that, when an experiment similar to that in Example 1 was performed, about 70 V was applied. A current of 0.2 mA (current density of 4 mA ⁇ cm ⁇ 2 ) was observed (FIG. 20). The resistivity of the heated object at this time is 5.0 ⁇ 10 4 ⁇ ⁇ cm, which indicates that the heated object is changed to an electride of grade B or higher. When the voltage was maintained at 75 V, a sudden increase in current of about 1100 mA was observed after about 7 seconds, and the current control was started (FIG. 21).
- the state of the sample was confirmed about 100 seconds after the current control, it turned black and C12A7 was electride. There was no difference in the electron concentration in the to-be-heated body, and it was an electride having a uniform electron concentration of 1.0 ⁇ 10 21 cm 3 .
- the electrical conductivity of the electride-to-be-heated body was 5 ⁇ 10 2 S / cm.
- Example 4 The object to be heated was prepared in the same manner as in Example 1, and when the voltage was applied to the sample, the sample was placed in an electric heating furnace capable of heating up to 1000 ° C., and the inside of the heating furnace was heated to 1000 ° C. Then, Ar (flow rate: 3 L / min, average flow velocity 5.2 cm / sec, atmospheric pressure, room temperature) controlled to an extremely low oxygen partial pressure atmosphere up to 10 ⁇ 29 Pa (10 ⁇ 34 atom) in the heating furnace. Washed away. With the extremely low oxygen partial pressure control device, the oxygen is extracted from the cage, and oxygen mixed in the Ar atmosphere controlled by the extremely low oxygen partial pressure is extracted from the Ar atmosphere by the principle of the oxygen pump. It was poured into a heating furnace.
- Ar flow rate: 3 L / min, average flow velocity 5.2 cm / sec, atmospheric pressure, room temperature
- the oxygen concentration in the argon at that time was measured, a remarkable oxygen detachment from the sample was observed after the electric resistance suddenly decreased. At the same time, a remarkable light emission phenomenon was observed on the sample positive electrode side. This means that the oxygen dissociated from the sample and the molybdenum wire connected to the positive electrode cause an oxidation reaction.
- the transparent object to be heated was blackened and converted to grade C electride, that is, an electride having an electron concentration of 10 20 / cm 3 or more was obtained. I confirmed.
- the electrical conductivity of the electride-to-be-heated body was 5 ⁇ 10 2 S / cm.
- Comparative Example 1 A C12A7 single crystal obtained by the FZ method was cut into a size of 10 mm ⁇ 5 mm ⁇ 1 mm to obtain an object to be heated. Using a device as shown in FIG. 3, the object to be heated was subjected to heat treatment in a heating furnace.
- the treatment atmosphere was an extremely low oxygen partial pressure Ar atmosphere (Ar flow rate: 3 L / min, average flow rate 5.2 cm / sec, atmospheric pressure, normal temperature).
- the object to be heated was heated from room temperature to 900 ° C. in 1 hour and then held at 900 ° C. for about 25 hours. Then, the said to-be-heated body was heated up to 1000 degreeC, and also after standing for 5 hours, it stood to cool naturally to normal temperature.
- the object to be heated after the heat treatment was light yellow. From this, the electron concentration of the mayenite type compound after the heat treatment was estimated to be less than 1.0 ⁇ 10 17 / cm 3 , and the grade was judged to be A class. From this, it was found that sufficient electride formation of the mayenite type compound cannot be achieved only by heat treatment.
- Comparative Example 2 The same process as in Comparative Example 1 was performed with a history as shown in FIG. 5 except that a polycrystalline C12A7: O 2 -powder sample was used in place of the C12A7 single crystal of Comparative Example 1. Similar to Comparative Example 1, the grade of the mayenite type compound after the heat treatment was class A. As with the C12A7 single crystal, the mayenite type compound could not be sufficiently electrified only by heat treatment.
- the process can be shortened compared to the conventional method, and an electride having a particularly high electron concentration can be produced at a low temperature, in a short time and with less energy.
Abstract
Description
本願は、2016年7月25日に、日本に出願された特願2016-145078号に基づき優先権を主張し、その内容をここに援用する。
マイエナイト型化合物は、12CaO・7Al2O3(以下、「C12A7」と略記することがある)なる代表組成を有する。前記C12A7の結晶は、2分子を含む単位胞にある66個の酸素イオンのうち、2個の酸素イオンが、結晶骨格で形成される正の電荷を有するケージ内のサブナノ空間に、対アニオンとして「フリー酸素」(O2-)を包接するという、特異な結晶構造を持つことが報告されている(非特許文献1)。
この方法では、図2に示すように、フリー酸素イオン(O2-)と、例えば金属Tiを還元剤として用いた場合、以下の反応式に示す反応により、還元剤の酸化物であるTiO2を形成し、フリー酸素イオンをケージ内から引き抜いて電子(e-)に置換している。
Ti+2O2-→TiO2+4e- ・・・(1)
しかし、例えば、特許文献1~4及び非特許文献3に示す製造方法では、金属Tiや金属Ca等の還元剤を、マイエナイト型化合物との混合や、その表面に塗布する等の煩雑な作業が必要となる。またこれらの製造方法は、通常は1000℃以上の高温反応が必要であり、さらには長時間の加熱が必要である。
エレクトライドの実用化のためには、還元剤を用いず、より低温かつ短時間で少ないエネルギーでマイエナイト型化合物をエレクトライド化することができる製造技術が希求されていた。
[1]マイエナイト型化合物に、加熱状態下、電圧を印加し、前記マイエナイト型化合物に直接電流を流すことにより、前記マイエナイト型化合物をエレクトライド化することを特徴とする、エレクトライド化マイエナイト型化合物の製造方法。
[2]前記マイエナイト型化合物の電気抵抗率が1.0×105Ω・cm以下になるまで電圧を印加する、前記[1]に記載の製造方法。
[3]前記マイエナイト型化合物に、正極及び負極を付与して直接電流を流す、前記[1]又は[2]に記載の製造方法。
[4]前記エレクトライド化を不活性ガス雰囲気下で行なう、前記[1]~[3]のいずれか1に記載の製造方法。
[5]前記エレクトライド化マイエナイト型化合物の電子濃度が、1.0×1020/cm3以上である、前記[1]~[4]のいずれか1に記載の製造方法。
また、マイエナイト型化合物に直接電流を流すことで、マイエナイト型化合物をエレクトライド化できることから、微細加工した導電性マイエナイト型化合物を簡便に製造することができる。
前記マイエナイト型化合物の結晶は、籠状の構造(ケージ)がその壁面を共有し、三次元的に繋がることで構成される。通常、マイエナイト型化合物のケージの内部にはO2-などのアニオンが含まれているが、化学処理によってそれらを別のアニオンや伝導電子に置換することができる。前記のアニオンとしては、特に限定はされないが、例えば、O2-イオン、F-イオン、Cl-イオン等が挙げられる。 マイエナイト型化合物の骨格を形成する前記のケージには、通常、Ca、Al等のカチオンが含まれ、それらのカチオンはその一部又は全部が別のカチオンで置換されていてもよい。
・Ca、Al及びO原子のみで形成されているもの;
・Caのすべてが、それ以外のアルカリ土類金属イオン等のカチオンで置換されているもの、例えば、ストロンチウムアルミネート(Sr12Al14O33);
・Caの一部が別のカチオンと置換されているもの、例えば、CaとSrの混合比が任意に変化された混晶であるカルシウムストロンチウムアルミネート(Ca12-xSrxAl14O33);
・Alがそれ以外のカチオンで置換されたもの、例えば、シリコン置換型マイエナイト型化合物であるCa12Al10Si4O35;又は
・カチオンとアニオンがともに置換されたもの、例えば、ワダライトCa12Al10Si4O32:6Cl-。
またマイエナイト型化合物を製造するための原料を用いることで、該原料から一貫して導電性マイエナイト型化合物を製造することもできる。この場合は具体的には、C12A7を例に取った場合、そのカルシウム成分及びアルミニウム成分となる原料を用いればよく、カルシウム成分としては炭酸カルシウム、水酸化カルシウム、酸化カルシウム又は金属カルシウム等が挙げられ、アルミニウム成分として酸化アルミニウム、水酸化アルミニウム、金属アルミニウム等が挙げられる。これらの原料をカチオン比が目的物の化学量論どおりに混合したものを使用することができる。また、CaAl2O4、Ca3Al2O6等のカルシウムとアルミニウムの混合酸化物を用いることもできる。
これらのうち、本発明では、酸素イオン等のケージ内アニオンの移動がスムーズであることからマイエナイト型化合物の形状は、単結晶が好ましい。
具体的な方法として、例えば、円筒型の絶縁体容器に、両端の蓋として導電性の材質を用い、その蓋の間にマイエナイト型化合物を充填し、前記の蓋に電流を流す方法等が挙げられる。
このうち、操作が容易で、必要とするエネルギー量を小さくすることができる点で、マイエナイト型化合物に電極を付与して、電流を流す方法が好ましい。
通常、マイエナイト型化合物は絶縁体であるため、電流は極めて流れにくい。マイエナイト型化合物に、具体的には後述する方法等で、加熱及び電圧印加するうちに、通常、電流が流れるようになる。具体的に電流を流す方法については後述する。
条件1:加熱温度を所定の温度として、電圧を段階的に上昇させる方法
具体的には、前記被加熱体に電圧を印加していない状態でまず所定の温度への加熱をし、引き続き前記被加熱体に印加する電圧を0Vから段階的に印加していく方法である。
条件2:電圧を所定の電圧として、加熱温度を段階的に上昇させる方法
具体的には、前記被加熱体に常温下で印加する電圧を設定し、引き続き温度を常温から段階的に上げていく方法である。
条件3:加熱温度と電圧を共に上昇させる方法
具体的には、前記被加熱体に、常温及び電圧を印加しない状態から、加熱温度と印加電圧を同時に調整しながら前記被加熱体に加熱及び電圧印加を段階的にしていく方法である。
前記加熱・電圧制御工程では、通常、前記被加熱体への電圧印加は、印加電圧見合いで制御(以下、電圧制御)する。通常、この工程では、前記被加熱体にほとんど電流が流れないためであり、印加電圧を制御する方が製造を管理しやすいためである。
前述の電流密度増加及び印加電圧減少が観察された時点(以下、電流密度増加点という)以降は、前記被加熱体が、加熱及び電圧印加を開始したときに比べて電流が流れやすい状態に変化したと考えられる。
前記加熱・電流制御工程は、前記被加熱体が所望の電子濃度に至るまで、適宜行なうことができるが、通常は、前記被加熱体の抵抗率はエレクトライド化が進むにつれ減少するので、抵抗率が十分に減少するまで継続する。具体的な抵抗率の値は、特に限定はされないが、通常、1.0×105Ω・cm以下となるまで行なうことが目安となる。
本発明の製造方法は、周囲の雰囲気について特に制限されることなく製造を行なうことができる。好ましくは、酸素分圧が低いほどマイエナイト型化合物からフリー酸素イオンの乖離を促進されるため、前記被加熱体を100Pa以下の低酸素分圧雰囲気、特に酸素分圧が極度に少ない雰囲気である、極低酸素分圧雰囲気に保つことが好ましい。前記極低酸素分圧雰囲気として具体的には不活性ガス雰囲気下で、酸素分圧を10-15Pa(10-20atom)以下程度まで低下させた雰囲気、又は真空度10-5Pa以下の状態をいう。不活性ガスとしては特に限定されず、窒素、アルゴン等が用いられ、好ましくはアルゴンである。
特に前記の通り、加熱温度が400℃を超える場合は、酸素分圧が高いと、エレクトライド化されたマイエナイト型化合物が酸化を受けやすくなるため、極低酸素分圧雰囲気が好ましく、加熱温度が400℃以下であれば、雰囲気は特に制限はされない。
具体的には、電子濃度の理論的な最大値は、2.3×1021cm-3であることから、電子濃度が1.0×1020/cm3以上、2.3×1021cm-3以下の導電性マイエナイト型化合物を得ることができる。
なお、得られた導電性マイエナイト型化合物は、その電子濃度が均一でも不均一でもよいが、均一のものが好ましい。また不均一な導電性マイエナイト型化合物の前記グレードの判定は、目視による色で判定し、電子濃度は、その最高の電子濃度の測定値とする。
マイエナイト型化合物のケージを構成する酸素原子は、カルシウム原子やアルミニウム原子と強固に結合しており、結晶格子を崩さない限り脱離しない。一方、ケージ内のフリー酸素イオンは極めて緩く結合している。
電圧をマイエナイト型化合物に印加すると、ケージは移動できないが、酸素イオンは電流の流れる方向、具体的には正極(+)に引かれてケージ壁の酸素イオンと置き換わりながら移動する。その際、Ca-Al-Oで構成されるマイエナイト型化合物のケージの壁の酸素イオンと置き換わりに必要な電位障壁(ΔE)を超える電圧印加が必要になる。そこで被加熱体の抵抗が急激に下がり、電流密度が急激に増加するまで、加熱温度及び/又は電圧を段階的に上昇させる。なおその際のイオン伝導性は電気抵抗の計測により求められる。
正極付近のマイエナイト型化合物が、導電性マイエナイト型化合物と酸素に電気分解され、その結果酸素がケージ外に放出される。加熱によりフリー酸素イオンのイオン伝導性が上昇することにより、正極付近に移動してくるフリー酸素は遂次電子と置換され、エレクトライド化が進行するものと推定される。
これにより従来は、還元剤を用いて乖離させていたフリー酸素イオンが、還元剤を使うことなく乖離させ、エレクトライド化できるものと考えられる。
以下に本発明の製造方法について実施例に基づいて詳細に説明する。
[加熱下の電圧制御並びに加熱下の電流制御]
FZ法で得られたC12A7単結晶を10mm×5mm×1mmの大きさにカットし、図1に示すように、この両端(端面面積:5mmx1mm)にスパッタリングにより白金(Pt)薄膜を蒸着し、正電極及び負電極の両電極をモリブデン(Mo)ワイヤーにより取り付け、被加熱体とした。該被加熱体を、図6に示すような履歴に従い、加熱状態下で電圧を印加した。具体的には1000℃まで、及び1200℃までの高温処理を電圧制御下及び電流制御下で実施した。処理雰囲気は、極低酸素分圧制御したArガス(Ar流量:3L/分、平均流速5.2cm/sec、大気圧、常温)とした。極低酸素分圧制御装置としてキャノンマシナリー(株)社製装置を用いた。酸素分圧は、図3に示すように、OutセンサーとInセンサーで計測した。極低酸素分圧制御については以下同様である。
そこで、一定加熱温度下における印加電圧と試料内電流の関係(I-V曲線)を印加電圧を増・減変化させることにより調べてみた。その結果、図9、図10に示すように、高温加熱状態において電圧増加変化と減少変化においてヒステリシスが生じていることがわかった。すなわちこの不可逆変化は被加熱体内にフリー酸素イオンが空間的に偏在していることを意味している。
図11に、Area IIの電流制御並びにCD制御下における被加熱体内の電流・電圧の温度変化を示す。電流制御開始(730K)の約18Vから、加熱温度が約819Kに到達した段階で試料電圧の急激な変化が見られ、925Kで約7Vまで下がった段階でCD制御により被加熱体内の電流を徐々に増加させた。
図17に、CD制御以降の温度、消費電力並びに酸素乖離圧の時間変化を示す。消費電力がピークとなるのは電流制御状態における3T温度維持状態の後半であり、酸素乖離圧がピーク値以降であった。その後、一旦消費電力は減少するものの、CD制御により被加熱体内電流を増加させた場合、増加に転じ最大500W/cm2程度で飽和した。
まず、当該被加熱体に、大気圧下で、100Vの印加電圧をかけたところ、試料温度30℃付近から1pA程度の電流(電流密度20pA・cm-2)を観測した。
次に電源を切り替え、最大100Vの電圧を印加及び最大30Aの電流が流せる電源とした。常温から10℃刻みで昇温した後、前記被加熱体の温度を保持し、各温度において100Vまで電圧を印加した。これを180℃に到達するまで繰り返した。本電源で検出可能な最小電流は0.1mAであるが、上記条件において電流は観測されなかった。
ここで、被加熱体の電圧印加方法を、前記電圧制御から、前記電流制御に移行した(図19)。電流制御になってから約100秒後に被加熱体の様子を確認したところ、被加熱体は黒色化し、C12A7はエレクトライド化していた。この被加熱体中には電子の濃度に不均一が生じた。最大の箇所は1.0×1021/cm3程度、最低の箇所は1.0×1019/cm3程度であった。前記エレクトライド化された被加熱体(本実施例のエレクトライド化マイエナイト型化合物)の電子濃度最大の部位の電気伝導度は5×102S/cmであった。
実施例1と同様に前記被加熱体を準備し、上記試料への電圧負荷時は、最高1000℃まで加熱できる電気加熱炉内に上記試料を置き、加熱炉内を1000℃まで加熱した。そして、最高10-29Pa(10-34atom)まで極低酸素分圧雰囲気に制御されたAr(流量:3L/分、平均流速5.2cm/秒、大気圧、室温)を加熱炉内に流した。極低酸素分圧制御装置によって、ケージから抜け出し、極低酸素分圧制御されたAr雰囲気に混入した酸素を酸素ポンプの原理でAr雰囲気中から抜き出し、極低酸素分圧状態でのArを再度加熱炉内に流した。
FZ法で得られたC12A7単結晶を10mm×5mm×1mmの大きさにカットして被加熱体とした。図3に示すような装置を用いて、この被加熱体に、加熱炉内で加熱処理を実施した。処理雰囲気は極低酸素分圧Ar雰囲気(Ar流量:3L/分、平均流速5.2cm/秒、大気圧、常温)とした。
このことから加熱処理のみでは、マイエナイト型化合物の十分なエレクトライド化はできないことがわかった。
比較例1のC12A7単結晶に替えて、多結晶であるC12A7:O2―粉末試料を用いた以外は、比較例1と同様の処理を、図5に示すような履歴でおこなった。比較例1と同様に、加熱処理後のマイエナイト型化合物のグレードはAクラスであった。C12A7単結晶同様、加熱処理のみではマイエナイト型化合物の十分なエレクトライド化はできなかった。
Claims (5)
- マイエナイト型化合物に、加熱状態下、電圧を印加し、前記マイエナイト型化合物に直接電流を流すことにより、前記マイエナイト型化合物をエレクトライド化することを特徴とする、エレクトライド化マイエナイト型化合物の製造方法。
- 前記マイエナイト型化合物の電気抵抗率が1.0×105Ω・cm以下になるまで電圧を印加する、請求項1に記載の製造方法。
- 前記マイエナイト型化合物に、正極及び負極を付与して直接電流を流す、請求項1又は2に記載の製造方法。
- 前記エレクトライド化を不活性ガス雰囲気下で行なう、請求項1~3のいずれか1項に記載の製造方法。
- 前記エレクトライド化マイエナイト型化合物の電子濃度が、1.0×1020/cm3以上である、請求項1~4のいずれか1項に記載の製造方法。
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