The present invention relates to an oxide-dispersion strengthened platinum material in which zirconium oxide is finely dispersed in platinum. In particular, it relates to an oxide-dispersion strengthened platinum material that consists of coarse platinum grains and to its production process.
A platinum material exhibiting good high-temperature strength has been used mainly as a structural material for glass melting for a long time. High-temperature strength required to the platinum material is so-called creep strength. In particular, the most important objective in developing a platinum material is considered to be how long a durable time until creep rupture will be extended.
For improving creep strength, there has been conventionally used a technique that a particular oxide is finely dispersed in platinum. As such an oxide-dispersion strengthened platinum material, a material in which zirconium oxide is dispersed in platinum is known.
However, zirconium-oxide dispersion strengthened platinum material that has been conventionally known can ensure a certain level of creep strength, but is in the present state that the creep strength is required to be improved further.
Accordingly, the present invention is aimed at providing a platinum material in which creep strength is more improved than that in conventional materials by improving a metal grain shape in a conventionally known oxide-dispersion strengthened platinum material in which zirconium oxide is dispersed, and providing a process for producing the platinum material.
In order to solve the above problems, the inventors of the present invention paid their attentions to the fact that creep strength depends on the metal grain of a platinum material, that is, the size of a platinum grain and have completed a technique for further improving the creep strength by making crystal grains in a platinum material to be finished as a final product more coarse than those in the conventional product.
That is, the present invention is characterized in that in an oxide-dispersion strengthened platinum material in which zirconium oxide is dispersed in platinum and which is obtained through rolling and thermal recrystallization, platinum grains that constitute the platinum material have an average grain size in a rolling direction ranging from 200 to 1500 xcexcm and have an average grain aspect ratio of 20 or more.
Because a platinum material in the present invention is an oxide-dispersion strengthened material in which zirconium oxide is dispersed in platinum and which is obtained through thermal recrystallization after rolling, if being considered liken to a plate material, platinum grains in the platinum material texture are in a state of being stretched in a direction of a plate surface, that is, in a state of being extended in a longitudinal direction. That is, the platinum grains constituting a platinum material of the present invention are those in which the average grain size in the rolling direction, that is, in the plate thickness direction is in the range of 200 to 1500 xcexcm and the crystal grain aspect ratio, that is, the ratio of the crystal size in the plate surface direction to the crystal size in the plate thickness direction is 20 or more. As long as the inventors of the present invention. know, there is no platinum material that is composed of-such coarse platinum grains in conventional zirconium-oxide dispersion strengthened platinum materials.
According to the platinum material of the present invention, the creep strength is further improved compared to that in conventional platinum materials, and even when the material is used as a structural material for glass melting, the material can decrease the amount of platinum eluted into molten glass. Generally, because spots where creep rupture and an elution phenomenon of a platinum material occur are considered to be caused mainly by grain boundaries, the reason why the platinum material of the present invention can achieve the improvement of creep strength and the decrease in the elution phenomenon of platinum is considered that the number of grain boundaries themselves is extremely few in the material because platinum grains constituting the platinum material are coarse.
An oxide-dispersion strengthened platinum material according to the present invention can be obtained by the following production process. That is a process for producing an oxide-dispersion strengthened platinum material where zirconium oxide is finely dispersed in platinum, comprising the steps of pouring powdered platinum into water to prepare a platinum suspension; adding a zirconium nitrate solution and a pH adjusting liquid in the platinum suspension for adjusting the suspension to a given pH to precipitate zirconium hydroxide and thus to form zirconium hydroxide carrying platinum; collecting the zirconium hydroxide carrying platinum, which is then cold isostatic pressed into a molding; sintering and forging the molding under the conditions in which secondary recrystallization growth in a platinum grain is restrained, to form a platinum ingot; and cold-rolling the platinum ingot in a reduction ratio of 70% or more and then thermally recrystallizing the product.
A production process according to the present invention is characterized in that first, a given powdered platinum is prepared and then a zirconium-hydroxide carrying platinum in which zirconium hydroxide is supported is formed with the use of a chemical precipitation reaction. And in the process, using the powder of this zirconium hydroxide carrying platinum, forming, sintering, forging, cold rolling and thermally recrystallizing are sequentially conducted, but the process is characterized in that sintering and forging among these processes are conducted under the conditions whereby the secondary recrystallization growth of platinum is restrained to the utmost. In the following, a production process according to the present invention will be detailed one by one.
First, in contrast to so-called coprecipitation (a coprecipitation process), in a production process of the present invention, platinum is first processed into given powders; the powdered platinum is used to prepare a platinum suspension; a zirconium nitrate solution and a pH adjusting liquid are added for adjusting the suspension to a given pH to precipitate zirconium hydroxide and thus to form zirconium-hydroxide carrying platinum; and the zirconium-hydroxide carrying platinum is collected and is then cold isostatic pressed into a molding.
When a zirconium-hydroxide carrying platinum is formed by such a process, platinum alone is powdered in advance. Thus, platinum powders may be appropriately prepared as those having a particle size suitable for subsequent molding and sintering steps. In general, powdered platinum exhibits quite higher gas adsorption ability. However, according to the production process of the present invention, gas adsorption on a platinum surface may be reduced due to the presence of zirconium hydroxide supported on the platinum surface, so that the formation of unwanted pores due to adsorbed gas, which becomes an issue during molding and sintering, i.e., the formation of internal defects in the platinum material to be finally obtained, can be effectively prevented.
Further, it is preferable in the production process of the present invention to use heated powdery platinum when forming a zirconium-hydroxide carrying platinum. The heating process is conducted at temperatures of 400xc2x0 C. or higher. Such heating may considerably inhibit pore formation due to adsorbed gas during the subsequent molding and sintering processes. And, after the heating process, the surface of the powdered platinum becomes smooth, so that zirconium hydroxide can be homogeneously and finely supported by each platinum surface and thus zirconium oxide can be quite homogeneously and finely dispersed in a platinum material. This heating process may be conducted during or after the powdering process.
And when a zirconium-hydroxide carrying platinum is formed according to the production process of the present invention, a pH adjusting liquid added together with zirconium nitrate is sufficient if it has such character as to raise pH value. Because when pH of the mixed solution of a platinum suspension and a zirconium nitrate solution is raised, the zirconium nitrate changes to zirconium hydroxide to precipitate, and the precipitated zirconium hydroxide is supported on the surface of platinum. As a pH adjusting liquid, it is preferable to use any solution of ammonia, sodium hydroxide, calcium hydroxide and potassium hydroxide, or to use a urea solution.
When any solution of ammonia, sodium hydroxide, calcium hydroxide and potassium hydroxide is used as a pH adjusting liquid, it is preferable that a zirconium nitrate solution is added to a platinum suspension and the mixture is stirred, and then the pH of the mixture is adjusted while adding a pH adjusting liquid, including ammonia. When any of these pH adjusting liquids is used, a chemical precipitation reaction is caused without any need of especially raising the temperature of the solution.
Moreover, if a urea solution is used as a pH adjusting liquid, it is preferable that after a zirconium nitrate solution is added to a platinum suspension and the mixture is heated at the boiling temperature with stirring, a urea solution is added to adjust the pH of the mixture and then heating is stopped. In case of a urea solution, if the temperature of the mixture is too low, hydrolysis of urea is very slow, so that the nucleus growth of zirconium hydroxide may preferentially proceed while the nucleation little occurs. The mixture is, therefore, heated at the boiling temperature for promoting the nucleation of zirconium hydroxide and maximizing the nucleus growth rate.
And, pH adjusting with the above described pH adjusting liquid is preferably conducted so that the mixture is in the range of pH 4.5 to 11.0, more preferably pH 6.0 to 8.0 at the end of the chemical precipitation reaction. If it is less than pH 4.5, zirconium hydroxide is not formed, while if more than pH 11.0, carrying of zirconium hydroxide on a platinum surface may be interfered.
Moreover, in a process for producing an oxide-dispersion strengthened platinum according to the present invention, the particle size of platinum used for preparing a platinum suspension, i.e., platinum powder prepared by being powdered in advance, is preferably in the range of 0.05 to 10 xcexcm. Because platinum powders with the particle size of less than 0.05 xcexcm cannot be readily prepared and may tend to form a bridge by agglomeration. And if the particle size is more than 10 xcexcm, moldability in cold isostatic pressing may be deteriorated while zirconium hydroxide supported on each platinum powder surface may be poorly dispersed, leading to uneven secondary recrystallization growth during the final thermal recrystallization step. Thus, the use of platinum powders with a particle size in the range of 0.05 to 10 xcexcm may allow zirconium hydroxide to be evenly dispersed and supported on individual platinum particle surfaces. And when molding, sintering and forging are conducted using this zirconium-hydroxide carrying platinum in which zirconium hydroxide is evenly dispersed and supported, zirconium oxide will be evenly and finely dispersed in a platinum ingot. The zirconium oxide finely dispersed in a platinum ingot may suitably function as an inhibitor for controlling secondary recrystallization growth during the final thermal recrystallization step while it becomes a factor to improve creep strength of the platinum material.
The zirconium-hydroxide carrying platinum obtained by the above chemical precipitation method is collected by, for example, filtration and is then subjected to appropriate drying. And, in the production process of the present invention, cold isostatic pressing, sintering and forging are sequentially conducted using the collected zirconium-hydroxide carrying platinum. The steps of sintering and forging are conducted under the conditions whereby the secondary recrystallization growth of platinum is restrained to the utmost, as described above.
Secondary recrystallization refers to recrystallization of a small number of coarse crystal grains, which is driven by crystal grain boundary energy. In a process for producing an oxide-dispersion strengthened platinum material according to the present invention, first, cold isostatic pressing is conducted so as not to cause this secondary recrystallization when zirconium-hydroxide carrying platinum is formed.
In the studies by the inventors of the present invention, it has been ascertained that in the case where uniaxial compression molding, so-called mold pressing that is general as a fabrication method in powder metallurgy is used, secondary recrystallization growth occurs even if conditions of sintering temperature and forging temperature are modified when sintering and forging are conducted later. It is presumed that in case of mold pressing, because unevenness is easily induced in the platinum density distribution and internal stress in a molding, the unevenness may lead to secondary recrystallization growth. On the other hand, in the case where cold isostatic pressing is used, as long as subsequent sintering and forging are conducted under given temperature conditions, secondary recrystallization growth does not proceed before thermal recrystallization to be conducted finally.
There are no specific restrictions to the conditions of the cold isostatic pressing in this case, but it is preferable that collected zirconium-hydroxide carrying platinum is filled in a rubber mold, which is then molded at a molding pressure of 40 to 200 MPa (about 408 to 2040 kg/cm 2). Because if a molding pressure is less than 40 MPa, the material cannot be compressed into a molding with a given shape, so that grain growth by sintering cannot adequately proceed, and if a molding pressure is more than 200 MPa, secondary recrystallization growth tends to be caused by the change in particle. shapes.
And, after forming the molding by cold isostatic pressing, it is sintered and forged under the conditions whereby secondary recrystallization growth is restrained. By the sintering step, zirconium hydroxide in the molding is converted into zirconium oxide. In a production process of the present invention, collected zirconium-hydroxide carrying platinum is formed into a molding and is then sintered while converting zirconium hydroxide into zirconium oxide. Alternately, the collected zirconium-hydroxide carrying platinum may be sintered in advance to be converted into a zirconium-oxide carrying platinum, which is then used as a molding.
A sintering temperature in the sintering step is preferably 1000 to 1250xc2x0 C. The reason why a sintering temperature is made to be in the range of 1000 to 1250xc2x0 C. is that sintering at more than 1250xc2x0 C. tends to cause the secondary recrystallization growth of platinum grains, so that a platinum material comprising of coarse platinum grains according to the present invention cannot be obtained in thermal recrystallization to be conducted at the end, while at less than 1000xc2x0 C., binding between platinum particles by sintering or grain growth may be inadequate. There are no restrictions to an atmosphere during sintering.
And, it is preferable to conduct forging after heating to 1100 to 1250xc2x0 C. The heating temperature range of 1100 to 1250xc2x0 C. during forging is selected because at more than 1250xc2x0 C., similarly to the case of sintering temperature, the secondary recrystallization growth of platinum grains tends to be caused, so that a platinum material comprising of coarse platinum grains according to the present invention cannot be obtained in thermal recrystallization to be conducted at the end, while at less than 1100xc2x0 C., cracks tend to be generated during forging. For forging, a processing procedure is not specially limited, but forging by striking with an air hammer is preferable because the material is heated to an elevated temperature.
After forming a platinum ingot by molding, sintering and forging as described above, the ingot undergoes thermal recrystallization by cold rolling under the condition of a reduction ratio of 70% or more, preferably 90% or more. Because of properties of the platinum material, if thermal recrystallization is conducted at temperatures of less than 1200xc2x0 C., recrystallization tends to inadequately proceed. Thus, it is preferably conducted at temperatures of 1200xc2x0 C. or higher, and an optimal temperature for thermal recrystallization may be appropriately determined, depending on a reduction ratio or the like during cold rolling. After internal strain is loaded to platinum ingot by conducting such cold rolling in the reduction ratio of 70% or more, thermal recrystallization treatment can make secondary recrystallization growth proceed remarkably, resulting in producing an oxide-dispersion strengthened platinum material according to the present invention, that is, a platinum material comprising of coarse platinum grains that have the average grain size in the rolling direction ranging from 200 to 1500 xcexcm and the average grain aspect ratio of 20 or more.