JP6432032B2 - Group IV clathrate production method - Google Patents

Description

  The present invention relates to a method for efficiently producing a group IV clathrate using a group IV element such as germanium or silicon as a host atom.

  A clathrate (inclusion compound) is a compound in which guest atoms are encapsulated in a three-dimensional cage-like structure formed by host atoms, and has characteristics different from those of conventional crystal structures. The clathrate is classified from type I (structure 1, also referred to as Type I) to type VIII (also referred to as structure 8, Type VIII), depending on its three-dimensional structure.

For example, when a silicon clathrate whose host atom is silicon contains sodium as a guest atom, a silicon clathrate containing a large amount of sodium has metallic properties, but a silicon clathrate with a reduced sodium content Is known to clearly show band gap characteristics as a semiconductor. When a silicon clathrate containing sodium is produced, a clathrate mainly composed of a mixture of type I and type II (structure 2, also referred to as Type II) is generated. The I-type silicon clathrate has a cubic structure composed of silicon ₂₀ having a dodecahedron structure and silicon ₂₄ having a tetrahedral structure. The type II silicon clathrate has a cubic structure composed of silicon ₂₀ having a dodecahedron structure and silicon ₂₈ having a hexahedral structure. Type I silicon clathrate has a characteristic that once sodium atoms are included, sodium atoms are hardly removed in subsequent steps. On the other hand, it is known that the type II silicon clathrate can remove sodium and reduce the content by heat treatment or the like. Therefore, in order to improve convenience as a semiconductor, a technique for manufacturing a II-type silicon clathrate in a film shape with high efficiency is required.

  In addition, since the clathrate in which the host atom is composed of germanium exhibits characteristics close to that of gallium arsenide, application to a light absorbing material for solar cells is expected. In addition, some type II group IV clathrates function as direct transition semiconductors and can change light absorption characteristics from infrared to visible light by alloying. To do. Therefore, a highly versatile technique that can efficiently produce a film-like group IV clathrate is required.

  The inventor has disclosed a novel method for producing a film-like silicon clathrate in Patent Document 1 so far. Patent Document 2 discloses a novel method for producing a type II clathrate having silicon and germanium as host atoms. Furthermore, Patent Document 3 discloses a novel method for producing germanium clathrate.

Japanese Patent No. 5626896 Japanese Patent No. 5641481 JP 2014-43599 A

  The techniques disclosed in the above patents and patent applications by the inventors have made it possible to produce film-like silicon clathrate and germanium clathrate. However, sometimes the produced clathrate is in a powder form, and a film-like clathrate may not be produced. The present invention has been made in view of such problems of the conventional manufacturing method, and an object to be solved is to provide a manufacturing method capable of more stably manufacturing a homogeneous film-like clathrate.

The present invention relates to a method for producing a clathrate. The method for producing a clathrate according to the present invention comprises a container having an opening containing guest atoms and a substrate made of a host atom of a group IV element placed in a reaction container and heated to 500 ° C. in an inert gas atmosphere. A first heating step of heating at a heating time of 0.5 hours to 24 hours at the above temperature; a cooling step of cooling a reaction vessel in which a container containing guest atoms and a substrate made of host atoms are arranged; And a second heating step of heating the substrate at a temperature of 300 ° C. or higher and 450 ° C. or lower for 1 hour or longer under a negative pressure of 10 ⁻² Pa or lower. The clathrate production method of the present invention is characterized in that, in the first heating step, a substrate composed of host atoms is disposed at a position lower than the opening of the container.

The present invention relates to a method for producing a clathrate. The method for producing a clathrate according to the present invention comprises a container having an opening containing guest atoms and a substrate made of a host atom of a group IV element placed in a reaction container and heated to 500 ° C. in an inert gas atmosphere. A first heating step of heating at a heating time of 0.5 hours to 24 hours at the above temperature; a cooling step of cooling a reaction vessel in which a container containing guest atoms and a substrate made of host atoms are arranged; And a second heating step of heating the substrate at a temperature of 300 ° C. or higher and 450 ° C. or lower for 1 hour or longer under a negative pressure of 10 ⁻² Pa or lower. In the clathrate production method of the present invention, in the first heating step, the opening of the container containing guest atoms is provided on the upper surface of the container, and the substrate made of host atoms is removed from the opening of the container. It is also characterized by being arranged at a lower position.

  Furthermore, in the clathrate production method of the present invention, the cooling step is preferably a slow cooling step, and the cooling rate of this slow cooling step is preferably 5 ° C./hr or more and 20 ° C./hr or less. The cooling rate of 5 ° C./hr or more and 20 ° C./hr or less is a significantly lower cooling rate than normal natural cooling, during which a clathrate precursor having a preferred structure is formed.

  In the clathrate production method of the present invention, it is particularly preferred that the host atom is silicon, the guest atom is sodium, and the clathrate produced is a silicon clathrate.

  In the clathrate production method of the present invention, in the first heating step, the host atom vapor and the host atoms are disposed by arranging the host atom substrate at a position lower than the opening of the container containing the guest atoms. It can be made to react more reliably. By performing the second heating step subsequent to the first heating step of the present invention, the clathrate can be more stably produced in a film form than before.

  Furthermore, the manufacturing method of the clathrate of this invention makes the cooling process between a 1st heating process and a 2nd heating process the slow cooling process whose cooling rate is 5 degreeC / hr or more and 20 degrees C / hr or less. Further, the production of the film-like clathrate is more reliably performed.

  According to the method for producing clathrate of the present invention, it is possible to produce a large amount of clathrate having a film-like and II-type structure.

  In the clathrate production method of the present invention, the silicon atom can be stably produced by using silicon as the host atom and sodium as the guest atom.

FIG. 1 is a flowchart showing a preferred method for manufacturing a clathrate. FIG. 2 is a longitudinal sectional view of the clathrate manufacturing apparatus used in the embodiment. FIG. 3 is a diagram showing X-ray diffraction patterns of silicon clathrate manufactured by the manufacturing methods of Example 1 and Comparative Example. 4A is a drawing-substituting photograph in which the surface of the silicon clathrate manufactured by the manufacturing method of the comparative example is photographed, and FIG. 4B is an optical view of the surface of the silicon clathrate manufactured by the manufacturing method of Example 1. It is a drawing substitute photograph which shows the result image | photographed with the microscope. FIG. 5 is a diagram illustrating the temperature conditions of the first heating process and the cooling process of Example 2. FIG. 6 is a diagram showing the results of analyzing the silicon clathrate produced by the production method of Example 2 by Raman spectroscopy. FIG. 7A is a drawing-substituting photograph in which the surface of the silicon clathrate produced under the cooling condition of 20 ° C./hr in Example 2 is photographed, and FIG. 7B is a cooling condition of 10 ° C./hr. It is the drawing substitute photograph which image | photographed the surface of the manufactured silicon clathrate. FIG. 8 is a diagram showing the results of analyzing the silicon clathrate produced by the production method of Example 3 by Raman spectroscopy. FIG. 9 is a drawing-substituting photograph showing the result of photographing the surface of the silicon clathrate produced by the production method of Example 3 with an optical microscope.

  Hereinafter, embodiments of the present invention embodied as a manufacturing method for manufacturing a film-like silicon clathrate using silicon as a host atom and sodium as a guest atom will be described with reference to the drawings. FIG. 1 is a flowchart showing a preferred manufacturing method for manufacturing a film-like silicon clathrate. As shown in FIG. 1, the preferred clathrate manufacturing method of the present embodiment includes a pretreatment process, a first heating process, a cooling process, and a second heating process.

  In the pretreatment process shown in Step 1 (S1) of FIG. 1, first, the silicon substrate is cleaned and sodium is accommodated in a container having an opening. In a preferred embodiment, the opening is the entire top surface of the vessel so that sodium vapor is efficiently fed into the reaction vessel. In the following, a container having an opening accommodating sodium is also referred to as a sodium-containing container. The cleaned silicon substrate and the sodium-containing container are accommodated in the reaction container.

  FIG. 2 shows a cross-sectional view of the reaction vessel 1 containing the silicon substrate 2 and the sodium-containing vessel 3. The silicon substrate 2 is disposed in the vicinity of the bottom surface of the reaction vessel 1 so as to be positioned lower than the opening 5 on the upper surface of the sodium-containing vessel 3 in the reaction vessel 1.

  In the first heating step shown in Step 2 (S2) of FIG. 1, the silicon substrate 2 and the sodium-containing container 3 accommodated in the reaction vessel 1 are heated at a temperature of 500 ° C. or higher for 0.5 hours to 24 hours. Heat. Sodium vapor is supplied to the surface of the silicon substrate 2 by the first heating step of the present embodiment. It has been confirmed that the clathrate is not generated even if the subsequent steps are performed at a temperature of less than 500 ° C., and it is necessary to perform the treatment at a temperature of 500 ° C. or higher in the first heating step.

  In the cooling step shown in Step 3 (S3) of FIG. 1, preferably slow cooling is performed. When performing slow cooling, it is preferable to cool the whole reaction vessel 1 at a rate of 5 ° C./hr or more and 20 ° C./hr or less while containing the silicon substrate 2 and the sodium-containing vessel 3. This cooling process is a process in which it is estimated that the sodium which has become vapor in the first heating process is condensed and reacts with the silicon substrate. However, when an appropriate amount of sodium is supplied onto the silicon substrate 2, this cooling process can be a normal natural cooling process, not slow cooling.

In the second heating step shown in Step 4 (S4) of FIG. 1, the silicon substrate 2 on which the clathrate precursor has been generated by the steps up to here is 300 ° C. or higher and 450 ° C. under a negative pressure of 10 ⁻² Pa or lower. Heat to the following temperature. The heating time is set within 1 hour and within 48 hours. During the second heating step, the precursor on the silicon substrate 2 changes to a silicon clathrate containing sodium. In the second heating step, a part or all of the sodium atoms contained in the silicon clathrate as guest atoms are removed in order to always maintain the pressure by performing suction with a vacuum pump. As a result, a silicon clathrate is produced in which the sodium content is reduced or the sodium is removed.

  In the method for producing a silicon clathrate according to the present embodiment, the silicon substrate 2 is disposed at a position lower than the opening 5 of the sodium-containing container 3 in the reaction vessel 1 and between the first heating step and the second heating step. By providing the slow cooling step, it is possible to manufacture a silicon clathrate having a II-type structure on almost all surfaces of the substrate.

  In the following, Examples 1 and 2 in which the present embodiment is made more concrete, and the silicon substrate 2 is disposed above the opening 5 of the container 3 containing sodium, and the silicon clathrate is formed under the same heating conditions as in Example 1. The manufactured comparative example is shown.

Example 1
In this example, a silicon clathrate was manufactured using the silicon substrate 2 having a crystal plane of 111 and a metal sodium piece. As a pretreatment step of Step 1, the surface oxide film of the silicon substrate 2 was removed. The metal sodium piece was accommodated in a container 3 having an opening 5 on the entire upper surface. Then, the silicon substrate 2 and the sodium-containing container 3 were accommodated in the reaction container 1. At this time, the silicon substrate 2 was disposed so as to be in contact with the bottom surface of the reaction container 1 so as to be disposed at a position lower than the opening 5 of the sodium-containing container 3.

As the first heating step of Step 2, the reaction vessel 1 was sealed in an argon atmosphere, and heating was performed under conditions of a temperature of 575 ° C., a treatment time of 48 hours, and a pressure of 10 ⁵ Pa or more. In the cooling process of Step 3, the temperature of the reaction vessel 1 was naturally lowered in a state in which the silicon substrate 2 and the sodium-containing vessel 3 were accommodated without performing slow cooling. It took about 6 hours for the natural temperature to fall. As the second heating step of Step 4, the silicon substrate 2 was heated at a temperature of 400 ° C., a processing time of 3 hours, and a pressure of 10 ⁻² Pa or less, and then naturally cooled.

(Comparative example)
A silicon clathrate was produced using a silicon substrate 2 having the same crystal face as in Example 1 and metal sodium pieces as raw materials. In the comparative example, when the silicon substrate 2 and the sodium-containing container 3 are accommodated in the reaction container 1, the silicon substrate 2 is relatively higher than the opening 5 of the sodium-containing container 3 by using a support member (not shown). Placed in position. Then, the same conditions as Example 1 were applied from the 1st heating process of Step 2 to the 2nd heating process of Step 4, and the silicon clathrate was manufactured.

(Contrast of silicon clathrate manufactured in Example 1 and silicon clathrate of comparative example)
3 shows an X-ray diffraction pattern of the compound on the silicon substrate 2 manufactured by the manufacturing method of the comparative example. The X-ray diffraction pattern of the compound on the silicon substrate 2 manufactured by the manufacturing method of Example 1 is shown in the middle stage (second chart from the top) in FIG. The bar graph shown at the bottom of FIG. 3 is a theoretical X-ray diffraction pattern of type II silicon clathrate and diamond type silicon (c-silicon). From the compound of Example 1, a strong X-ray diffraction pattern derived from a type II silicon clathrate was confirmed, and it was confirmed that the compound of Example 1 was a type II silicon clathrate. Further, in the compound of Example 1, the diffraction pattern derived from the raw material silicon substrate is significantly weaker, whereas from the compound manufactured by the comparative manufacturing method, the diamond structure of the raw silicon substrate is used. It was quantified that the peak of the diffraction pattern derived from the sample was strongly detected, and that the surface of the silicon substrate 2 of Example 1 was covered with a type II silicon clathrate thicker than the comparative example. .

  FIG. 4A shows a result of photographing the surface of the silicon clathrate produced by the method of the comparative example with an optical microscope. FIG. 4B shows the result of photographing the surface of the silicon clathrate produced by the method of Example 1 with an optical microscope. It was also confirmed by a drawing substitute photograph that the silicon clathrate obtained by the production method of Example 1 was uniformly formed in a film shape. On the other hand, the silicon clathrate obtained by the production method of the comparative example has a portion that is non-uniform on the substrate, and it has been found that the yield is lower than that of the first embodiment.

  In the first heating step of Example 1, it is presumed that sodium vapor having a higher density was present at the bottom of the reaction vessel 1 than at the top. Further, in the cooling process of Example 1, it is considered that the sodium once turned into vapor condenses and descends to react more efficiently with the silicon substrate 1. As a result, Example 1 in which the silicon substrate 2 is disposed below the opening 5 of the sodium-containing container 3 is more than the comparative example in which the silicon substrate 2 is disposed above the opening 5 of the sodium-containing container 3. It is considered that the silicon clathrate could be manufactured more efficiently.

(Example 2)
In the clathrate manufacturing method according to the present embodiment, the silicon substrate 2 is disposed below the opening 5 of the sodium-containing container 3 to perform the first heating step, and the cooling step is further set as a slow cooling step. The optimum conditions for producing silicon clathrate stably were identified. Since the pretreatment process of Step 1 and the second heating process of Step 4 of the present embodiment have the same configuration as that of Embodiment 1, duplicate description is omitted.

In the first heating step of Step 2, the temperature of the reaction vessel 1 was increased to 600 ° C. over 1 hour, and the reaction was further performed at 600 ° C. for 1 hour. The pressure at this time was 10 ⁵ Pa or more. The cooling process of Step 3 was a slow cooling process, the cooling rate was set to two stages of 10 ° C./hr and 20 ° C./hr, and the reaction vessel 1 was cooled to 100 ° C. to specify more preferable clathrate production conditions. . Here, after cooling the reaction vessel 1 to 100 ° C., natural cooling is performed. Then, the 2nd heating process of step 4 was performed. After the second heating step, the silicon clathrate was naturally cooled.

  In FIG. 5, the relationship between the temperature and time which were performed at the 1st heating process and cooling process of a present Example is shown. In FIG. 6, the analysis result by the Raman spectroscopy of the compound manufactured with the manufacturing method of a present Example is shown. As shown in FIG. 6, a peak corresponding to the type II silicon clathrate was confirmed more strongly in the compound produced by applying a slower cooling condition of 10 ° C./hr to the cooling step, and the compound was of type II. The silicon clathrate was confirmed. A peak corresponding to the type II silicon clathrate was also confirmed in the compound produced by applying a faster cooling condition of 20 ° C./hr. As a result, the type II silicon clathrate can be produced by the production method of this example, and preferable conditions for the cooling step for producing the silicon clathrate are a cooling rate of 5 ° C./hr or more and 20 ° C. / It was confirmed that it was less than hr, more preferably 10 ° C./hr.

  FIG. 7 shows a photo, which substitutes for a drawing, in which the surface of each silicon clathrate produced at a two-step slow cooling rate in this example is photographed with an optical microscope. FIG. 7A is a photograph of the surface of a silicon clathrate produced under a faster cooling condition of 20 ° C./hr. FIG. 7B is a photograph of the surface of a silicon clathrate produced under slower cooling conditions of 10 ° C./hr. The silicon clathrate obtained by the slower cooling condition of 10 ° C./hr was more uniform in film form than the silicon clathrate obtained by the faster cooling condition of 20 ° C./hr. On the other hand, the silicon clathrate obtained under the cooling condition of 20 ° C./hr is confirmed to have a wider film-like region than the conventional one, although there are some non-uniform portions on the substrate. It was done. That is, conventionally, in addition to the silicon clathrate synthesized in a film shape, powdery silicon clathrate or an amorphous substance is sparsely formed. On the other hand, in the silicon clathrate obtained under the cooling condition of 20 ° C./hr, an amorphous material was synthesized in addition to the film-like portion, but it was confirmed that the film-like region was wider than before. It was. In addition, the silicon clathrate obtained under the slower cooling condition of 10 ° C./hr increased the region of the silicon clathrate synthesized in a film form as compared with the silicon clathrate obtained under the cooling condition of 20 ° C./hr. . The silicon clathrate obtained under slower cooling conditions of 5 ° C / hr was confirmed to have a wider film-like silicon clathrate region, but similar to the silicon clathrate synthesized by the conventional method. Thus, a powdery silicon clathrate and an amorphous substance were confirmed.

  In the cooling step of Example 2, it is considered that sodium that once became vapor by performing slow cooling was condensed and reacted with the silicon substrate 2 more efficiently for a long time while maintaining that state. By making the cooling process a slow cooling process, it is possible to produce a more uniform thin-film silicon clathrate.

Example 3
For the purpose of shortening the cooling process, in the clathrate manufacturing method according to the present example, the silicon clathrate was manufactured by increasing the cooling rate of the cooling process. Since the pretreatment process in step 1 and the second heating process in step 4 are the same as those in the first and second embodiments, redundant description is omitted. As the first heating step in Step 2, the reaction vessel 1 was sealed in an argon atmosphere, and heating was performed under conditions of a temperature of 580 ° C., a treatment time of 48 hours, and a pressure of 10 ⁵ Pa or more. In the cooling process of Step 3, the reaction vessel 1 was rapidly cooled to room temperature in 2 hours in a state where the silicon substrate 2 and the sodium-containing vessel 3 were accommodated.

In FIG. 8, the analysis result by the Raman spectroscopy of the compound manufactured on the rapid cooling conditions of a present Example is shown. A peak corresponding to the clathrate was detected at 500 cm ⁻¹ , and it was confirmed that the clathrate was produced even when quenching was performed in the cooling process of step 2. FIG. 9 shows the result of photographing the silicon clathrate produced by the method of this example with an optical microscope. Part of the powder was in the form of powder, but silicon clathrate crystals were confirmed.

  As mentioned above, although the specific example of this invention was described in detail in the Example, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. For example, in the example, the case where the cooling process is performed at two levels of 10 ° C./hr and 20 ° C./hr has been described. However, as described in Example 2, when the cooling process is performed at 5 ° C./hr, a film-like silicon class It has been confirmed that a rate can be obtained. In this embodiment, the clathrate is produced using a silicon substrate and sodium. However, the clathrate can be produced by the method of this embodiment even when other group IV elements are used as host atoms.

  The clathrate production method of the present invention can produce a suitable film-like silicon clathrate. The resulting clathrate can be used as a solar electric field, a thermoelectric element, or a superconducting material.

Claims (2)

A method of manufacturing a clathrate,
A container having an opening containing guest atoms and a substrate made of a group IV element host atom are placed in a reaction container, and the temperature is 500 ° C. or higher in an inert gas atmosphere. A first heating step of heating with a heating time of less than or equal to hours;
A cooling step of cooling the reaction vessel in which the vessel containing guest atoms and the substrate made of host atoms are disposed;
A second heating step of heating the substrate for 1 hour or more at a temperature of 300 ° C. or more and 450 ° C. or less under a negative pressure of 10 ⁻² Pa or less;
With
In the first heating step, an opening of the container containing guest atoms is provided on the upper surface of the container, and the substrate made of host atoms is disposed at a position lower than the opening of the container ,
A method for producing a clathrate, wherein the host atom is silicon, the guest atom is sodium, and the clathrate produced is a silicon clathrate . The cooling step is a slow cooling step,
The method for producing a clathrate according to claim 1, wherein a cooling rate in the slow cooling step is 5 ° C / hr or more and 20 ° C / hr or less.