WO2007120005A1

WO2007120005A1 - Method for manufacturing sintered body of alumina-spinel composite powder

Info

Publication number: WO2007120005A1
Application number: PCT/KR2007/001866
Authority: WO
Inventors: Shin Hu Kang; Ji Woong Kim; Tae Hyung Kim
Original assignee: Hyundai Rotem Company; Seoul National University Industry Foundation
Priority date: 2006-04-19
Filing date: 2007-04-17
Publication date: 2007-10-25
Also published as: KR100743952B1

Abstract

Provided is a method for manufacturing a sintered body of an alumina- spinel composite powder for bulletproof material, in which alumina (AI2O3) and magnesium oxide (MgO) raw materials are milled into an alumina powder and a spinel composition powder through high-energy milling, and thereafter heat- treating, mixing and sintering are sequentially performed. Since the method employs high-energy milling, it is possible to synthesize spinel phase through heat-treatment at a low temperature for a short duration. Furthermore, the small amount of additive is added during the sintering process so that it is possible to obtain a sintered body of a composite powder with a low density and a high densification degree at a low temperature.

Description

[DESCRIPTION]

[Invention Title]

METHOD FOR MANUFACTURING SINTERED BODY OF ALUMINA-SPINEL COMPOSITE POWDER

[Technical Field]

The present invention relates to a method for manufacturing a sintered body of an alumina-spinel composite powder for high-hardness structural material using high-energy milling. More particularly, the present invention relates to a method for manufacturing a sintered body of an alumina-spinel composite powder for high-hardness structural material by preparing an alumina (AI2O3) powder and a spinel (MgA^O₄) composition powder with high- energy milling using alumina (AI2O₃) and magnesium oxide (MgO) as raw materials, and then heat-treating, mixing, and sintering the prepared powder in sequence. [Background Art]

Korean Patent Publication No. 1998-72405 discloses a technology for at least one material selected from the group consisting of kaolinite, sillimanite and kyanite, a thermal shock resistant alumina composed of alumina and alkaline earth metal oxide, and a composition of mullite composite.

In addition, to manufacture an alumina-magnesia castable block which must undergo a firing process, Korean Patent Publication No. 1998-48883 discloses a technology in which 5-10% by weight of water is added to a composition consisting of alumina raw material, magnesia powder raw material and alumina cement. The composition with water added is then kneaded, and the kneaded mixture is molded into a predetermined shape, cured and dried, and thereafter fired in a temperature range of 1,300 °C to 1,750⁰C.

However, technology for mixing an alumina and a spinel-structured ceramic and using the mixture for high-hardness structural material has never been introduced. [Disclosure]

[Technical Problem]

The present invention provides a method for manufacturing a ceramic sintered body that has a high densification and a high degree of hardness and is also lightweight, so that it can be used for high-hardness structural material .

[Technical Solution]

Embodiments of the present invention provide a sintered body of an alumina-spinel composite powder for high-hardness structural material, the method including: milling alumina and magnesium oxide raw materials into an alumina powder and a spinel composition powder, respectively, by using high- energy milling; heat-treating the spinel composition powder of the milled powder at a temperature ranging from 1,100 °C to 1,300 °C for 0.5 to 1 hour; mixing the alumina powder into the heat-treated spinel composition powder; and sintering the mixed powder at a temperature ranging from 1,450 ⁰C to 1,550⁰C for 2 to 6 hours.

When the heat-treatment of the spinel composition powder is performed at a temperature lower than 1,100 °C or for less than half an hour, the spinel is not synthesized. On the contrary, when the heat-treatment is performed at a temperature higher than 1,300 °C or for 1 hour or more, powder particles agglomerate, which makes it difficult to sinter the powder. Therefore, it is preferable that the heat-treatment be performed within the given range.

Meanwhile, when milling a mixed powder of the present invention, high- energy milling is used. In general, high-energy milling is a milling process that uses a high-milling force that has a great impact on the powder. Examples of high-energy milling include vibration milling, attrition milling, and planetary milling.

It is preferable that planetary milling be used in the milling process of the present invention. The reason is that it is possible to effectively obtain a fine nanopowder using planetary milling even the case where a material such as alumina or the like, which is difficult to mill, is used.

Meanwhile, when a great impact is exerted on the powder, the crystal Unity of the powder is deteriorated. Due to theses factors, the powder adopts a very unstable state. The milled powder can be synthesized into a desired phase at a relatively low temperature during heat-treating compared to powder which is not greatly impacted by high-energy milling. Furthermore, it is possible to densify this milled powder at a low temperature during the sintering process.

In addition, it is preferable that the high-energy milling be performed using wet milling rather than dry milling. Ethanol or acetone is generally used as a solvent in wet milling. However, since acetone is highly volatile during milling, it is preferable to use alcohol in wet milling.

Also, embodiments of the present invention provide a sintered body of an alumina-spinel composite powder for high-hardness structural material characterized in that at least one of SiO₂ and CaO, of which content is 1-4 wt% by weight, is added into the mixed powder. According to an aspect of the present invention, 1-4% by weight of SiO₂ or CaO is added during the sintering process, which causes the density of the ceramic sintered body to be low and the densification degree to be high because SiO₂ or CaO is molten at 1,400 °C into a liquid phase so that liquid-phase sintering occurs in the case where SiO₂ or CaO coexists with alumina. However, if the content of SiO₂ or CaO is less than 1% by weight, liquid-phase sintering hardly occurs. Since mechanical properties are deteriorated if the content of the additive is greater than 4% by weight, it is preferable that the content of the additive is in the range of 1-4% by weight.

Also, embodiments of the present invention provide a sintered body of an alumina-spinel composite powder for high-hardness structural material characterized in that a mole fraction between the milled alumina powder and the spinel powder ranges from 86:14 to 28:72 (volume ratio is in the range of 8:2 to 2:8) during the mixing process. Also, embodiments of the present invention provide a method for manufacturing a sintered body of an alumina-spinel composite powder for high- hardness structural material characterized in that the mixed powder of the milled alumina and spinel powders has a composition of Al₂0₃-20~30mol% MgO during the mixing process, and 1-3% by weight of SiO₂ is added into the mixed powder and the mixed powder with the SiO₂ added is sintered at preferably at a temperature ranging from l,450°C to l,500°C for 2 to 3 hours during the sintering process.

Meanwhile, the sintering is not complete when performed at a temperature lower than 1,450 °C or for less than 2 hours. On the contrary, when the sintering temperature exceeds 1,550 ⁰C or the heat-treatment time is 6 hours or more, the particles are coarsened, causing the mechanical properties to be deteriorated. Therefore, it is preferable to perform the sintering process under the given conditions. [Advantageous Effects]

According to the method for manufacturing an alumina-spinel composite powder of the present invention, since a high-energy milling is used in milling a powder, it is possible to synthesize a spinel phase through heat- treatment at a low temperature for a short duration. Furthermore, the small amount of additive is added during the sintering process so that it is possible to obtain a sintered body of a composite powder with a low density and a high densification degree at a low temperature.

A sintered body of an alumina-spinel composite powder manufactured by the present invention has several advantages such as low density and high degree of hardness in comparison with a sintered body obtained by using only alumina. Thus, it is suitable as a ceramic for a high-hardness structural material, such as bulletproof material for an armored car. [Description of Drawings]

Fig. 1 illustrates an electron microscopy image and an X-ray diffraction graph of a commercial alumina powder in the image on the left, and those of an alumina powder milled using wet high-energy milling in the image on the right ;

Fig. 2 illustrates an electron microscopy image and an X-ray diffraction graph of a spinel composition powder milled using wet high-energy mi 11 ing;

Figs. 3A and 3B illustrate electron microscopy images of an alumina powder and a spinel composition powder milled using dry milling method, respectively;

Fig. 4 illustrates an X-ray diffraction graph of a spinel powder synthesized by heat-treating the milled spinel composition powder at 1,300 °C for 1 hour;

Fig. 5 illustrates a graph comparing the hardness and density of each sintered body according to embodiments (No.1-6) of the present invention with the hardness and density of the commercial alumina; and

Figs. 6A and 6B illustrate section electron microscopy images showing a sintered body of a commercial pure alumina, and Fig. 6C illustrates a section electron microscopy image of a composite sintered body with 2 wt% SiO₂ powder added as an inorganic additive in which a mole fraction (%) of alumina to spinel is 51:49 (volume ratio is 4:6). [Best Mode]

Hereinafter, embodiments of the present invention will now be described for more fully setting forth the present invention. The present invention may, however, should not be construed as being limited to the embodiments set forth herein.

A method for manufacturing a sintered body of an alumina-spinel composite powder for high-hardness structural material includes: milling alumina (Al₂O₃) and magnesium oxide (MgO) raw materials into an alumina (Al₂O₃) powder and a spinel (MgAl₂O₄) composition powder, respectively, by using high- energy milling; heat-treating the milled spinel composition powder; mixing the alumina powder and the spinel composition powder synthesized by the heat- treatment; and adding an additive into the mixed powder and sintering the mixed powder with the additive added.

Manufacture of nanopowder of alumina and spinel composition

A commercial alumina (AI2O3) powder and a magnesium oxide (MgO) powder are used as raw materials for manufacturing a nanopowder of an alumina and spinel composition.

An alumina ball is put into a plastic vessel such that a ratio of the alumina powder to the magnesium oxide is 10:1 by weight, and a solvent is then added. Thereafter, the resultant is milled at 250 RPM for 10 hours using a planetary mill so that the alumina powder and the magnesium oxide powder are mixed together at a molar ratio of 1:1, to thereby form a spinel (MgAl₂O₄) composition nanopowder. Likewise, an alumina nanopowder is obtained by using a commercial alumina in the same manner as the spinel powder. Results of an analysis of the obtained nanopowder using an electron microscope and an X-ray diffractometer are illustrated in Figs. 1 and 2. As illustrated in Fig. 1, results from the planetary milling show that the commercial alumina powder is very finely milled, which can be observed from the broadening of the width of the X-ray diffraction peak. Furthermore, as illustrated in Fig. 2, it can be observed that the spinel composition powder is also milled as finely as the alumina powder.

From results of analyzing the width of the diffraction peak shown in Figs. 1 and 2 using the Sherrer method, it can be observed that crystalline sizes of alumina and spinel are 50 run or smaller.

Meanwhile, comparing the powder prepared by wet milling illustrated in Figs. 1 and 2 with the powder prepared by dry milling illustrated in Fig. 3, it can be understood that the powder of the wet milling is more uniform and finer than the powder of the dry milling.

Heat-treatment of spinel composition nanopowder

The spinel composition nanopowder is heat-treated at 1,300 "C for 1 hour, and the milled spinel composition powder is analyzed by an X-ray diffractometer. The results are illustrated in Fig. 4.

Comparing Fig. 4 with Fig. 2, it can be observed that spaces between diffraction peaks of Fig. 4 are different from those of the spinel composition powder of Fig. 2. This result shows that the spinel powder is synthesized by the heat-treatment under the above conditions and its crystal Unity is improved.

Mixing and sintering of spinel composition powder and alumina nanopowder

The milled alumina powder and the heat-treated spinel composition powder are mixed at a predetermined ratio shown in table 1 below using a horizontal ball mill to manufacture a composite powder. After selectively adding an inorganic additive (SiO₂) into the composite powder, the resultant is sintered at a temperature ranging from 1,450 °C to 1,550 "C for 2 to 6 hours in the atmosphere.

[Table 1] Mixing and sintering conditions of composite powder

Density, porosity, and hardness are measured for the sintered body manufactured as above. First, table 2 below illustrates measurement results of the density, porosity and hardness of the sintered body manufactured by varying the mixing ratio of the alumina powder to the spinel composition powder .

[Table 2]

Density, porosity and hardness of sintered body according to the mixing ratio of powder

Mole fraction(%) of 86 14 78 22 70 30 51 49 40 60 28 72 alumina to spinel (82) (73) (6 4) (46) (37) (28) (volume ratio)

Densitv(g/cπf) 3. 63 3. 71 3. 67 3. 56 3. 52 3. 47

AϋDarent porositv 0. 00 0. 00 0. 00 0. 00 0. 14 0. 14

Vickers 12 .8 13 .2 12 .3 17 .3 14 .7 14 .3 hardness(GPa)

As illustrated in Fig. 2, the density of the sintered body of the composite powder manufactured by the inventive method is in the range of 3.47 to 3.71, which is lower than the density of pure alumina, i.e., 3.97. Thus, it can be understood that the sintered body of the composite powder is suitable for a lightweight high-hardness structural material.

In addition, the apparent porosity of the sintered body is in the range of 0.00 to 0.14. Thus, it can be understood that the sintered body has a dense texture and its Vickers hardness is 12.3 GPa or greater.

In comparison to the above measurement results and the density and hardness of the commercial alumina, the alumina-spinel composite (No. 1-6) has lower density and higher hardness than pure alumina, as illustrated in Fig. 5. In particular, it is observable that the sintered body has the highest hardness when the mole fraction (%) of the milled alumina powder to the heat-treated spinel composition powder is 51:49 (volume ratio is 4:6). That is, when the mole fraction of the alumina powder to the spinel powder is 51:49 (volume ratio is 4:6), the sintered body can be preferably used for a lightweight high-hardness structural material such as a bulletproof material, because of low density, high densification, and high degree of hardness. Furthermore, table 3 below illustrates the density, apparent porosity and hardness of the sintered body obtained by adding 1-4% by weight of SiO₂ into a mixed powder in which a mole fraction between alumina and spinel is 78:22 (volume ratio is 7:3), and then sintering the mixed powder with SiO₂ added at 1,450 °C for 2 hours.

[Table 3]

Density, porosity and hardness of sintered body according to content of additive

As illustrated in table 3, the density of the sintered body decreases as the content of SiO₂ increases. The hardness reaches a maximum level when the content of SiO₂ is 2% by weight, and the apparent porosity becomes 0 when the content of SiO₂ is 2% or more by weight. Thus, it can be understood that adding 2% by weight of SiO2 is ideal. Meanwhile, liquid-phase sintering does not easily occur when the content of SiO₂ or CaO is less than 1% by weight.

When the content of additive exceeds 4% by weight, mechanical properties are degraded.

In addition, table 4 below illustrates measurement results of the density and apparent porosity for the sintered specimen obtained by adding 2% by weight of SiO₂ into a mixed powder in which a mole fraction between alumina and spinel powder is 70:30 (volume ratio is 6:4), and then sintering the mixed powder with SiO₂ added for 2 hours, while the sintering temperature is varied from 1,450 °C to 1,550 °C .

[Table 4]

Density, porosity and hardness of sintered body according to sintering temperature

As illustrated in table 4, it can be understood that the density of the sintered body increases as the sintering temperature increases.

Furthermore, table 5 below illustrates measurement results of the density and apparent porosity for the sintered specimen obtained by adding 2% by weight of SiO₂ into a mixed powder in which a mole fraction between alumina and spinel powder is 70:30 (volume ratio is 6:4), and then sintering the mixed powder with SiO₂ added at 1,450 °C , while the sintering time is varied from 2 hours to 6 hours.

[Table 5]

Density, porosity and hardness of sintered body according to sintering time

Generally, the density of the sintered body increases with the increase of the sintering time. However, as illustrated in table 5, even in cases where the sintering time is varied from 2 hours to 6 hours in the embodiment of the present invention, the density does not distinctly increase. Accordingly, it can be understood that two-hour sintering is sufficient for densification. Meanwhile, when the heat-treatment is performed at a temperature lower than 1,450 °C or for less than 2 hours, sintering is not completely achieved. When the sintering temperature exceeds 1,550 °C or the heat-treatment is performed for more than 6 hours, particles in the sintered body are coarsened, which degrades mechanical properties.

Meanwhile, Figs. 6A and 6B illustrate section electron microscopy images of a commercial pure alumina sintered body, and Fig. 6C illustrates a section electron microscopy image of a sintered body obtained by adding 2% by weight of SiO2 into a mixed powder in which a mole fraction of alumina to spinel is 70:30 (volume ratio is 6:4), and then sintering the mixed powder with SiO2 added at 1,450 °C for 2 hours. As shown in the images, it can be understood that the sintered body of alumina-spinel composite powder according to the present invention has a denser structure with small porosity than the pure alumina sintered body. [Industrial Applicability]

A sintered body of a composite powder manufactured according to the present invention is suitable for use as a high-hardness structural material.

Claims

[CLAIMS] [Claim 1]

A method for manufacturing a sintered body of an alumina-spinel composite powder for high-hardness structural material, the method comprising: milling alumina and magnesium oxide raw materials into an alumina powder and a spinel composition powder, respectively, by using a high-energy milling; heat-treating the spinel composition powder of the milled powders at a temperature ranging from 1,100 °C to 1,300 °C for 0.5 to 1 hour; mixing the alumina powder into the heat-treated spinel composition powder ; and sintering the mixed powder at a temperature ranging from 1,450 °C to 1,550 °C for 2 to 6 hours. [Claim 2]

The method of claim 1, wherein the high-energy milling comprises a planetary milling. [Claim 3]

The method of claim 1 or 2, wherein at least one of S1O2 and CaO, of which content is 1-4% by weight, is added into the mixed powder. [Claim 4]

The method of claim 1 or 2, wherein a mole fraction between the milled alumina powder and the spinel powder ranges from 86:14 to 28:72 (volume ratio is in the range of 8:2 to 2:8) during the mixing of the alumina powder into the heat-treated spinel composition powder. [Claim 5]

The method of claim 1 or 2, wherein the mixed powder of the milled alumina powder and the spinel powder has a composition of Al₂0₃-20~30mol% MgO during the mixing of the alumina powder into the heat-treated spinel composition powder, and 1-3% by weight of SiO₂ is added into the mixed powder and the mixed powder with the SiU2 added is sintered at a temperature ranging from 1,450 to 1,500 "C for 2 to 3 hours during the sintering of the mixed powder. [Claim 6]

A sintered body manufactured by the method of claim 1 or 2, wherein the sintered body is used for a bulletproof material of an armored car.