JP3208440B2 - Method and apparatus for producing zinc oxide whiskers and zinc oxide whiskers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc oxide whisker made of needle-like crystals longer than before, and a method and an apparatus for producing the same.

[0002]

2. Description of the Related Art Zinc oxide (ZnO) is a chemical product having various uses, and is widely used as a pigment in paints and paints, as well as vulcanization accelerators for rubber, materials for electronic components such as varistors and ferrites, and the like. It is used for additives such as synthetic resins, additives for ceramics products, and additives for cosmetics and pharmaceuticals. Further, zinc oxide can also be used as a reinforcing material such as metal, ceramics, and resin, a filter material, a catalyst carrier, a temperature sensor, an electromagnetic wave shielding material, and the like, by utilizing the characteristics of the crystal.

[0003] The crystals of zinc oxide have a hexagonal wurtzite structure. It is known that the growth rate of the crystal in the c-axis direction is significantly higher than that in the a-axis direction, so that the zinc oxide crystal tends to be acicular. Therefore, various methods for producing zinc oxide whiskers have been studied.

Japanese Patent Application Laid-Open No. 50-6597 discloses a method of growing an zinc oxide whisker on a base material by heating an alloy or a mixture of zinc and a metal having a higher boiling point in an atmosphere containing oxygen. Is disclosed. This publication discloses a method of heating a Zn—Cu alloy, which has a diameter of 3 mm.
It is described that whiskers having a length of 0 to 80 μm and a length of 10 to 30 mm are obtained.

On the other hand, Japanese Patent Publication No. 60-5529 discloses a method for growing needle-like crystals of zinc oxide in a gas phase. According to the production method described in the publication, after zinc metal is jetted into an oxygen-containing atmosphere to oxidize and burn zinc, the produced zinc oxide is immediately cooled at a cooling rate of 480 ° C./sec or more. It is characterized by. The length of the acicular crystal formed by this method is about 1 μm to several μm.

[0006] JP-A-2-192499, JP-A-Hei-4
-132700, JP-A-4-144995, JP-A-4-170399, JP-A-5-975
No. 97 and JP-A-6-92797 also disclose a production method and a production apparatus in which zinc vapor is oxidized in a gas phase supplied with zinc vapor to obtain zinc oxide whiskers. The zinc oxide whiskers produced in these methods and devices have a core and a different 4
It has a tetrapod-like structure consisting of an axially extending needle-like crystal part. In the obtained crystal, the diameter of the core is 0.
The length of the needle-shaped part is several tens to several hundreds of micrometers.

On the other hand, the production methods disclosed in Japanese Patent Application Laid-Open Nos. 1-252599 and 1-252600 heat a zinc powder having an oxide film in an atmosphere containing oxygen to form a tetrapod-shaped zinc powder. It is characterized by generating zinc oxide whiskers. The whisker produced by this method is composed of a core portion and a needle-like crystal portion extending from the core portion in different four-axis directions, wherein the base of the needle-like crystal portion has a diameter of 0.7 to 14 μm and a needle. The length of the crystal part is 3
２００200 μm.

[0008]

A whisker, which is an elongated needle-like single crystal, is an ideally perfect crystal with almost no structural defects, and has very excellent mechanical strength. For this reason, it is useful as a material for a composite material and in applications utilizing its physical properties. When applying zinc oxide whiskers to these applications, those with a larger size than before, especially longer whiskers, have a wide application range,
Reinforcement materials, filter materials, catalyst carrier materials, sensor materials,
It is more useful as a material such as an electromagnetic wave shielding material.

It is an object of the present invention to provide zinc oxide whiskers that are larger in size than conventional ones, especially longer zinc oxide whiskers.

It is another object of the present invention to provide a method by which longer zinc oxide whiskers can be produced easily and reproducibly.

[0011] It is a further object of the present invention to provide a method and apparatus for continuously producing longer zinc oxide whiskers.

[0012]

According to the present invention, there is provided a method for producing zinc oxide whiskers, comprising the steps of: providing a zinc source together with a reducing agent in the presence of lead oxide in an environment having a substrate for holding whiskers to be grown; By heating the substrate, zinc oxide whiskers are grown on the substrate.

In the present invention, zinc itself and zinc oxide, such as powdered zinc and zinc oxide, can be used as the zinc source. Further, as a zinc source, zinc-containing wastes such as galvanized sludge, incineration ash (bottom ash and fly ash) generated in a cleaning plant, and old tire incineration ash can be used. Further, a mixture of zinc metal and zinc oxide, a solid solution of zinc metal and zinc oxide, an oxidized zinc metal, and the like can also be used.
Further, a mixture of a zinc compound such as zinc chloride and metallic zinc or zinc oxide may be used as a zinc source. The zinc source can be used in various forms, for example in the form of a powder or granules.

In the present invention, the zinc source is heated together with the reducing agent. Reducing agents include carbon, hydrogen, carbon monoxide and the like. In particular, carbon is inexpensive and is a preferred reducing agent in the present invention. Carbon sources include carbon itself, waste wood,
Charcoal obtained by carbonizing waste such as thinned wood, driftwood, agricultural residue, food residue, and plant residue can be preferably used. Since it is desirable that these carbon sources come into more close contact with the zinc source, finely ground carbon sources are more preferable.

In the present invention, the zinc source is heated together with a reducing agent in the presence of lead oxide. Pb _{3 in} lead oxide
O ₄ , Pb ₂ O ₃ , PbO ₂ , PbO and the like are included. Under the heating condition, PbO ₂ → Pb ₂ O ₃ → Pb ₃ O ₄ → P
It is believed that bO conversion occurs, and PbO ₂ or PbO is particularly preferably used in the present invention. Also, a material containing lead oxide can be used as appropriate, and for example, incineration ash (for example, fly ash) can be used as such a material.

In the present invention, the lead oxide may be mixed with a zinc source or may be present on the side of the substrate supporting the whisker to be grown. When lead oxide is present on the substrate side, the lead oxide can be contained in the substrate or attached to the substrate. In order to deposit lead oxide on the surface of the base material, vapor deposition or application of a solution containing lead oxide or the like can be performed.

It is desirable that the base material for holding the whiskers is made of a material that does not undergo a chemical change at the heating temperature and retains its shape. Examples of such materials include heat-resistant stainless steel and ceramics. Ceramics include alumina, silicon nitride, silicon carbide and the like. The substrate can be formed into various shapes, and for example, a net shape, a needle shape, a punching metal, or the like is preferable. Further, a flat plate can be used as the base material.

In the present invention, the zinc source is heated together with the reducing agent. Also, a mixture containing a zinc source, a reducing agent and lead oxide may be heated. The heating temperature is, for example,
750 ° C to 1200 ° C, preferably 750 ° C to 1000 ° C
° C, more preferably 800 ° C to 900 ° C. Also,
The substrate is likewise 750 ° C to 1200 ° C, preferably 750 ° C.
It can be maintained at a temperature in the range of from 1000C to 1000C, more preferably from 800C to 900C. The heating time is, for example, 30 to 120 minutes. In such a short time, sufficient crystal growth is observed. By heating, components for whisker generation are vaporized and deposited on the substrate, and whisker growth occurs.

Air can be preferably used as an atmosphere for heating. Further, a reducing atmosphere such as carbon monoxide and hydrogen can be preferably used, and an inert atmosphere such as nitrogen and argon can be used.

In the present invention, lead oxide contributes to the crystal growth of zinc oxide, and as a component that contributes to further better crystal growth, it is selected from the group consisting of calcium hydroxide, silicon oxide and mixtures thereof. Materials. Heating a zinc source with calcium hydroxide, silicon oxide or a mixture thereof in the presence of a reducing agent and lead oxide can promote needle crystal growth.

In the present invention, the amount of each raw material used is, for example, 10 parts by weight or more, preferably 50 to 200 parts by weight of lead oxide, 100 parts by weight of zinc source, and 5 parts by weight of a reducing agent such as carbon.
It can be 0 parts by weight or more, preferably 30 to 800 parts by weight. Further, for example, 1 part by weight or more of calcium hydroxide, preferably 70 parts by weight,
0 to 1500 parts by weight, 1 part by weight or more of silicon oxide, preferably 1 to 500 parts by weight can be added.

The method for producing zinc oxide whiskers according to the present invention can include the following steps. First, a composition containing a zinc source, a reducing agent and lead oxide is prepared. The prepared composition is heated for a predetermined period of time in an environment having a substrate for holding a crystal to be grown, thereby evaporating the components of the composition, and a gas phase containing the components of the composition. To grow zinc oxide whiskers on the substrate. After moving the substrate with the whisker to another location, the whisker is separated and recovered from the substrate. As the various conditions in such a process, those described above can be used. This manufacturing method can be carried out in any of a batch system and a continuous system. In the case of the continuous method, the step of preparing the composition, the step of growing the whisker, and the step of separating and collecting the whisker are performed continuously.

According to the present invention, an apparatus for producing zinc oxide whiskers can be provided. The apparatus of the invention comprises a zinc source,
A material containing a reducing agent, lead oxide and necessary additives are stored in different places, respectively, and they are mixed appropriately, and a raw material supply means for supplying the obtained mixture and a mixture from the raw material supply means are received. A transport heating means for heating for a predetermined time while transporting, and a component provided from the mixture heated on the transport heating means, which is provided opposite to the transport heating means, to grow whiskers of zinc oxide. Substrate, means for moving the substrate, means for separating and recovering the whisker adhering on the moved substrate from the substrate, and residue of the mixture discharged from the transport heating means Means for collecting the This device is
In particular, it can be applied to a continuous manufacturing method.

The device according to the invention is more particularly shown in FIG. The zinc oxide whisker manufacturing apparatus 10 includes a raw material supply unit 20, a raw material transport / heating unit 30, a whisker growth / collection unit 40, a whisker separation / recovery unit 50, and a raw material recovery unit 60.
It can be divided into five. The raw material supply unit 20 contains a zinc source, a reducing agent, lead oxide, and an additive, respectively,
Containers 11a, 11b, 11 capable of discharging a predetermined amount
a mixing / supplying machine 1 for mixing c and 11d with the materials discharged from the containers and sending the mixed materials to the conveying / heating unit 30 for the raw materials
2 is provided. In this part, a predetermined amount of raw materials are mixed and continuously supplied. Material transfer / heating unit 30
Includes, for example, a conveyor 21 and a heater (not shown). The mixture 14 continuously supplied from the raw material supply unit 20 is placed on a conveyor 21 and transported by a predetermined distance. Meanwhile, the mixture 14 is heated to a predetermined temperature (for example, 750 ° C. to 1000 ° C.) by a heater (not shown).
° C). A whisker growing / collecting unit 40 is provided above the conveyor 21. The whisker growing / collecting section 40 can also be formed by the conveyor 31. The belt 32 constituting the conveyor 31 can function as a base material for supporting the whisker. The belt is desirably made of a heat-resistant material such as stainless steel and ceramics. The moving speed of the conveyor 21 and the conveyor 31 is adjusted to obtain a desired whisker size and yield. The raw material mixture is heated on the conveyor 21, and zinc oxide crystals grow on the belt 32 from the space 33 where the components are evaporated. The space 33
Conveyors 21 and 3 to maintain the atmosphere and temperature of
Between them, partitions 34a and 34b are provided. Note that a heater can be provided in the space 33. The whisker 35 grown on the belt 32 is moved in the direction of the arrow, and is separated from the space 33 for crystal growth. The whisker 35 is separated from the belt 32 by a scraping means 41 (for example, a scraping plate) at a predetermined position on the conveyor 31 (for example, at the end of the conveyor). Then, the whisker 35 is stored in the collection container 42. The collecting section 50 described above includes the scraping means 41 and the collecting container 42. On the other hand, the raw material residue remaining on the conveyor 21 is transported as it is, and is stored in a predetermined location, for example, a collection container 61 at the end of the conveyor. With the above apparatus, zinc oxide whiskers can be continuously produced.

The zinc oxide whisker produced by the production method of the present invention has a length of, for example, 30 mm or more, more preferably 35 mm or more. More specifically, the thickness is 15 to 35 μm and the length is 30 to 60 μm.
Zinc oxide whiskers consisting of mm needle crystals can be provided according to the present invention. Whisker of such a size is obtained for the first time by the above-described manufacturing method. The resulting needles are the result of c-axis growth in a hexagonal system.

[0026]

The mechanism by which long whiskers are produced by the method of the present invention is not clear at present. As the examples described below show, the presence of zinc or zinc oxide, a reducing agent (eg, carbon) and lead oxide contributes to the growth of long whiskers. It is considered that the conversion of zinc oxide to zinc by the action of the reducing agent and the supply of oxygen to zinc via lead oxide play an important role in the generation of long whiskers.

Although the present inventors are not certain, due to heating and reduction of the zinc source, the generated zinc receives oxygen via lead oxide at a good timing when being deposited on the substrate, and is oxidized. We speculate the mechanism by which zinc whiskers grow on the substrate. It is assumed that if the lead oxide is sufficient, zinc can be continuously supplied with oxygen, and longer zinc oxide whiskers are generated. In this process, it is speculated that lead oxide is also sublimated to some extent, but it is not certain. It is also considered that growing whiskers by vapor deposition on the substrate is important for obtaining longer whiskers. Further, the substrate facilitates whisker separation and recovery.

Further, the present inventors have found that silicon oxide (S
It has been found that iO ₂ ), calcium hydroxide (Ca (OH) ₂ ) or mixtures thereof promote whisker growth. As shown in the following examples, SiO ₂ seems to have an action mainly to help whisker grow straight, and Ca (OH) ₂ seems to have an action to promote whisker growth more. It is.

As shown in the following examples, according to the present invention, zinc oxide whiskers of several mm to several tens of mm can be generated in a short time. Depending on the conditions, it is expected that zinc oxide whiskers having a length of several tens of mm or more can be produced according to the present invention.

[0030]

EXAMPLES Example 1 Zinc oxide was used as a zinc source. Zinc oxide powder 0.3
g, lead oxide powder 0.1 g, Ca (OH) ₂ powder 2.5
g, by mixing SiO ₂ powder 1.0g and charcoal (powder) 1.5 g, and the mixture was housed in a crucible of 50ml capacity. A stainless steel wire or wire mesh was set up on the raw material in the crucible as a base material for whisker growth, and the crucible was covered with a lid. The crucible and the crucible lid were made of alumina.
This state is shown in FIGS. FIG. 2A is a cross-sectional view thereof, FIG. 2B is a front view seen from above, and FIG. 2C is a cross-sectional view schematically showing a state where whiskers have grown. A mixture 72 is contained and a wire 73 is upright. The crucible 71 is provided with a lid 74 for maintaining the atmosphere therein. FIG. 3 (a)
Is a cross-sectional view, (b) is a front view, and (c) is a cross-sectional view schematically showing a state when whiskers have grown. A wire mesh member 75 has a base material for whisker growth above the raw material mixture 72. It is provided as.

The crucible in which the raw material and the base material are put and the lid is put is placed in a heating furnace and kept at 800 ° C. for 60 minutes.
It returned to room temperature atmosphere and cooled. During the heating, it is estimated that the inside of the crucible became a reducing atmosphere due to the action of the charcoal. After cooling, the wire or wire mesh substrate was removed, and whiskers were recovered from the substrate. By gently touching the whisker with a finger or a glass rod, the whisker could be separated from the substrate. The obtained whiskers were needle-like crystals. It is considered that the whiskers grew in a tetrapod shape, but the collected whiskers were needle-shaped because the needle-like portions were very long. The thickness of the collected needle-shaped crystals having a large size is about 15 to 35 μm, and the length is 40 to 6 μm.
It was 0 mm.

The crystallinity of the obtained needle-like crystal was examined by X-ray diffraction. FIG. 4A is the obtained X-ray diffraction chart. FIG. 4B shows an X-ray diffraction peak of an ideal zinc oxide single crystal. FIG. 4A and FIG.
From the comparison of (b), it was confirmed that the obtained acicular crystal was a ZnO single crystal (whisker) having few dislocations and lattice defects. Fig. 5 shows the partial structure of the obtained whisker.
As shown in FIG. FIG. 5 is an electron microscope photograph showing a side surface of the whisker, FIG. 6 is an end portion of the whisker, and FIG. These photographs show that the obtained whiskers have good crystallinity. FIG. 8 shows the entire length of one of the obtained whiskers. FIG. 8 shows a whisker from which a long and thin white needle-like object seen below the difference in material is obtained. It can be seen that the whiskers shown here have a length of about 45 mm.

Example 2 Zinc oxide, lead oxide, Ca (OH) ₂ , SiO ₂ and charcoal were blended in the proportions shown in Table 1 (Nos. 1 to 24). A mixture of Zn, PbO ₂ , Ca (OH) ₂ and SiO ₂ (No. 25), Zn, charcoal, PbO ₂ , C
a (OH) ₂ and a mixture of SiO ₂ (No. 2)
6), galvanized sludge, PbO ₂ , charcoal, Ca (O
H) ₂ and a mixture of SiO ₂ (No. 27);
A mixture of fly ash (fly ash) and charcoal (No. 28) was also prepared. Whisker growth experiments were performed on the above 28 types of mixtures. Each mixture was placed in a 50 ml alumina crucible and covered with an alumina lid. In this experiment, the crucible and lid walls
Served as a substrate for whisker growth. The crucible with the lid was placed in a heating furnace, heated at 800 ° C. for 60 minutes, and then returned to room temperature and cooled. The amount, length,
Table 1 shows the colors, shapes, and locations of occurrence.

[0034]

[Table 1] In these experiments, no whiskers were generated from the sample (No. 1) to which PbO ₂ was not added. Further, ZnO, samples without addition of coal relative to PbO ₂ other (No.7), without the addition of charcoal, no whiskers were obtained from sample (No.25) using only Zn as zinc source Was. On the other hand, a sample to which no SiO ₂ was added (No.
19), and a sample with a small amount of addition (No. 20,
In 21), a flocculent product was obtained. And SiO
It was found that the addition of ₂ promoted the formation of needle crystals. It was also found that the addition of Ca (OH) ₂ promoted the formation of needle crystals. Furthermore, it was found that galvanized sludge and fly ash can also be applied as raw materials. During fly ash,
It is believed that it contains a zinc source and lead oxide.

In the above experiment, heating was performed on a mixture of all the raw materials. However, the same applies to the case where lead oxide was attached to the wall of a crucible or a lid and other raw materials were accommodated in the crucible and heated. Whiskers could be generated.

[0036]

As described above, according to the present invention, a long zinc oxide whisker having a length of several tens mm can be easily obtained with good reproducibility. Further, according to the present invention, zinc oxide whiskers can be generated in a shorter time of several tens of minutes. The method according to the present invention does not use a special reagent, is simpler, and can produce whiskers at a lower cost. Further, the present invention is suitable for producing a large amount of whiskers. The whiskers obtained by the present invention have good crystallinity and are longer. Therefore, the whisker of the present invention is composited with a metal, a resin, a ceramic, or the like, and a composite material having higher strength can be obtained. Further, the obtained composite material is expected to be less expensive than a composite material reinforced with silicon carbide or silicon nitride. The whisker obtained according to the present invention is more useful as a filter material, a catalyst carrier and the like.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a zinc oxide whisker manufacturing apparatus according to the present invention.

FIG. 2 is a diagram showing a state in which whiskers are generated in Embodiment 1 according to the present invention.

FIG. 3 is a diagram showing how whiskers are generated in Embodiment 1 according to the present invention.

FIG. 4 is a diagram showing an X-ray diffraction peak of zinc oxide whisker.

FIG. 5 is an electron micrograph showing the crystal structure of zinc oxide whiskers obtained according to the present invention.

FIG. 6 is an electron micrograph showing the crystal structure of zinc oxide whiskers obtained according to the present invention.

FIG. 7 is an electron micrograph showing the crystal structure of zinc oxide whiskers obtained according to the present invention.

FIG. 8 is a photograph showing a crystal structure of a zinc oxide whisker obtained according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Manufacturing apparatus of zinc oxide whisker 11a, 11b, 11c, 11d Vessel 12 Mixing / supply unit 14 Raw material mixture 20 Raw material supply unit 21 Conveyor 30 Raw material transfer / heating unit 31 Conveyor 32 Belt 35 Whisker 40 Whisker growth / collection unit 41 Scraping means 50 whisker separation / recovery section 60 raw material recovery section

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Akira Kojima 4-339-4, Honcho, Kiryu-shi, Gunma (56) References JP-A-50-6597 (JP, A) JP-A-3-150299 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) C30B 29/62 JICST file (JOIS)

Claims (10)

(57) [Claims] 1. In an environment having a substrate for holding a whisker to be grown, a zinc source is heated together with a reducing agent in the presence of lead oxide, whereby a zinc oxide whisker is formed on the substrate. A method for producing zinc oxide whiskers, characterized by growing whiskers. 2. The method for producing zinc oxide whiskers according to claim 1, wherein a mixture containing the zinc source, the reducing agent, and the lead oxide is heated. 3. The method for producing zinc oxide whiskers according to claim 1, wherein said lead oxide is contained in or adhered to said base material. 4. The method according to claim 1, wherein the zinc source is heated together with a material selected from the group consisting of silicon oxide, calcium hydroxide and a mixture thereof and the reducing agent. 3. The method for producing zinc oxide whiskers according to claim 1. 5. The method for producing zinc oxide whiskers according to claim 1, wherein the reducing agent is carbon. 6. The method according to claim 1, wherein said heating is performed at a temperature in a range of 750 ° C. to 1000 ° C.
The method for producing a zinc oxide whisker according to any one of the above. 7. A process for preparing a composition comprising a zinc source, a reducing agent and lead oxide, and heating for a predetermined time in an environment having a substrate for holding a crystal on which the prepared composition is to be grown. Evaporating the components of the composition by doing, a step of growing zinc oxide whiskers on the substrate from a gas phase containing the components of the composition, and placing the substrate with the whiskers in another place Separating the whisker from the base material after the transfer, and recovering the whisker from the base material. 8. The method for producing zinc oxide whiskers according to claim 7, wherein the step of preparing the composition, the step of growing the whiskers, and the step of separating and collecting the whiskers are performed continuously. 9. A raw material supply means for storing a zinc source, a reducing agent, a material containing lead oxide and necessary additives in different places, mixing them as appropriate, and supplying the resulting mixture; The mixture from the raw material supply unit, receiving and conveying heating means for heating for a predetermined time while conveying, provided opposed to the conveyance heating means, from the mixture heated on the conveyance heating means The components of
A substrate for growing whiskers of zinc oxide; a means for moving the substrate; and a means for separating and recovering the whisker adhering on the moved substrate from the substrate. Means for recovering the residue of the mixture discharged from the conveying and heating means. 10. A zinc oxide whisker comprising needle-like crystals having a length of 35 mm or more.