July 18, 2008
Fabrication of Porous Copper With Directional Pores Through Thermal Decomposition of Compounds
By Nakajima, Hideo Ide, Takuya
Lotus-type porous copper with aligned long cylindrical pores was fabricated by unidirectional solidification in an argon atmosphere. The hydrogen dissolved in molten copper through thermal decomposition of titanium hydride contained in the mold, which then formed hydrogen gas that evolved into the gas pores in the solidified copper. On the other hand, titanium may form oxides in the melt that serve as nucleation sites for insoluble hydrogen. The porosity and pore size decreased with increasing atmospheric argon pressure during the solidification, which can be explained by the Boyle-Charles law and the possible suppression of the decomposition due to external pressure. The addition of titanium hydride was more effective when it was added just before the melt solidified than when it was added to the melt. Moreover, the thermal decomposition method (TDM) is superior to the conventional fabrication method, which requires high pressure hydrogen gas. Thus, TDM is a promising fabrication technique for various lotus metals. DOI: 10.1007/s11661- 007-9402-4(c) The Minerals, Metals & Materials Society and ASM International 2008
POROUS and foamed metals exhibit various characteristics such as an inherent low-density and large surface area, which differ from bulk metals. Hence, these metals should be applicable as light- weight materials, catalysts, electrodes, vibration and acoustic energy damping materials, impact energy absorption materials, etc.[1- 3] Recently, a new type of porous metal, called lotus-type porous metal, has attracted much attention due to the long cylindrical pores aligned in one direction. Lotus-type porous metals are fabricated by a unidirectional solidification process in a pressurized gas atmosphere such as hydrogen.[4-7] The pores evolve from insoluble gas when the molten metal dissolving the gas is solidified. Lotus-type porous metals not only have the properties of conventional porous metals, but also have unique properties originating from their directional pores. In particular, these metals exhibit superior mechanical properties compared to conventional porous metals, which have nearly spherical, isotropical pore shapes. Thus, porous metals are attracting considerable attention for various industrial applications.[7,9-11] Shapovalov and the present authors' group[5,7] have fabricated lotus-type porous metals with homogeneously distributed pores using a mold casting technique (Gasar method) in high pressure gas. Although this technique is a simple process, it is difficult to control the solidification velocity, which consequently prevents a uniform pore size and porosity for metals with a lower thermal conductivity. In order to improve this shortcoming, we have developed a continuous zone melting technique[13-15] and a continuous casting technique.[16- 18] Because these techniques can control the solidification velocity, various types of lotus metals with homogeneously distributed long pores can be fabricated.
Although our fabrication techniques are quite advanced, one large technical barrier remains; high pressure hydrogen gas must be used. Employing high pressure hydrogen gas has inherent risks because it may lead to inflammable and explosive accidents if oxygen is mixed. Therefore, a technique that does not require high pressure hydrogen to fabricate lotus metals is highly desirable. In order to overcome this difficulty, we propose an alternative, but simple method to fabricate such lotus-type porous metals by using a thermal decomposition method (TDM) of compounds containing gas elements in a nonhydrogen atmosphere under nearly atmospheric pressure. The present article reports the principle of TDM and the first versatile method to control the pore morphology, including pore size and porosity of lotus-type porous metals.
II. EXPERIMENTAL PROCEDURES
Copper (99.99 pct pure) was melted by radio-frequency induction heating in a graphite crucible under an argon atmosphere from 0.1 to 0.5 MPa. The melting temperature was maintained at 1573 K by monitoring with an infrared pyrometer. Then the melt was poured into the mold, which had a copper bottom plate cooled by water and lateral side walls made of 0.1-mm-thick cylindrical stainless steel. A few pellets of titanium hydride, which ranged between 0.075 and 0.25 g in mass, were set on the bottom plate of the mold. Unidirectional solidification occurred in the mold so that a lotus- type porous copper ingot was obtained, as illustrated in Figure 1(a). The ingot size was 28 mm in diameter and had a height of maximum 90 mm. The ingot was cut by a spark-erosion wire cutting machine parallel and perpendicular to the solidification direction. The sectional views were observed by an optical microscope. The porosity epsilon was measured by epsilon = (1-rho/rho^sub 0^) x 100 (pct), where rho and rho^sub 0^ are, respectively, the apparent density of porous copper and nonporous copper measured by the Archimedes method.
Another method was also used to supply the hydrogen source in the melt. As shown in Figure 1(b), a few pellets of titanium hydride were set in the crucible along with copper. During the copper melt, hydrogen, which decomposed from titanium hydride, was dissolved into the melt. The molten copper was poured from the crucible into the mold without pellets of titanium hydride for unidirectional solidification. The ingots were investigated using the same method described previously.
III. RESULTS AND DISCUSSION
Figure 2 shows the optical micrographs of the cross-sectional views of lotus-type porous copper parallel and perpendicular to the solidification direction. The pore growth direction is coincident to the direction of the unidirectional solidification, which is consistent with that of the conventional high pressure gas method (PGM). This result suggests that the principal mechanism of this pore evolution is similar to that of PGM; when a molten metal that is dissolving a gas is solidified, the insoluble gas leaves the solid and evolves into gas pores in the solid metals.
Figure 3 shows the dependence of the porosity and the average pore diameter on the mass of titanium hydride for the lotus-type porous copper fabricated by TDM in a 0.1 MPa argon atmosphere. Adding 0.10 g of titanium hydride abruptly increased the porosity, but the porosity became constant near 55 pct upon adding more titanium hydride. When 0.10 g of titanium hydride was added to 200 g of molten copper, the concentration of hydrogen in the melt was evaluated as 0.128 at. pct. Directly comparing the hydrogen concentration from the titanium hydride to the available data on the solubility in molten copper just above the melting temperature (T^sub m^), 1083 K, is impossible, because the atmospheric hydrogen pressure of the TDM process differs from that of the solubility measurement (0.1 MPa). However, the concentration of dissolving hydrogen in the TDM process is comparable to the solubility of hydrogen in the liquid phase of copper near T^sub m^. If more than 0.1 g of titanium hydride is added to the melt, the supersaturated hydrogen may generate gas bubbles, which are then liberated from the melt to the atmosphere. The hydrogen dissolved in the melt evolves pores at the solid/liquid interface during solidification. If less than O.I g of titanium hydride is added, then all hydrogen can dissolve in the melt without bubbling so that some of the hydride forms pores in the solid/liquid interface, but the porosity may be smaller than that when more than 0.1 g of the hydride is added. Such redistributions of hydrogen in the liquid phase to the solid phase and the atmosphere are exhibited in Figures 4(a) and (b), depending on the mass of titanium hydride. For comparison, Figure 4(c) shows the PGM case. Thus, Figure 4 indicates that the TDM process is based on a nonequilibrium state, but the PGM process is based on an equilibrium state in the chamber where a constant high pressure gas is maintained. On the other hand, as shown in Figure 3, the average pore diameter is constant and nearly independent of the mass of titanium hydride. It has been previously confirmed that the pore size is affected by the supercooling rate from the liquid to the solid, which can be controlled by the solidification velocity in the PGM.[14,16,18,20] The same reasoning can be used to interpret the present data.
Next the atmospheric pressure effect was investigated. For this study, the mass of titanium hydride was constant at 0.25 g, and argon was selected as the atmospheric gas, which served as the external pressure. The argon pressure was varied from 0.1 to 0.5 MPa. Figure 5(a) shows sectional views of lotus copper parallel and perpendicular to the solidification direction as functions of argon pressure. The effect of external pressure is obvious, and the pore growth is suppressed at higher pressures. Figure 6 shows the dependence of the porosity and the average pore diameter as functions of argon pressure. Both the porosity and the average pore diameter decrease with increasing argon pressure. The pore volume v, which is the porosity, is inversely proportional to the external argon pressure p, which can be described by the Boyle-Charles law, v = nRT/p, where n, R, and T are the hydrogen molar number, the gas constant, and the temperature, respectively. Therefore, the pore diameter can be written as d [proportional to] p^sup -1/3^. The tendency of the pressure dependence of the porosity and the pore diameter are explained by the law. However, it seems that such a pressure effect may be more significant than that predicted by the Boyle-Charles law. This difference may be attributed to the possibility that the molar number of hydrogen is not constant under changing pressure. The decomposition rate of titanium hydride should be retarded as the external pressure of argon increases. Thus, the porosity and the pore diameter may decrease remarkably. Hence, we tried a different approach to adding titanium hydride into molten copper during the melting process for 600 seconds in an argon atmosphere at 0.1 MPa. Figure 5(b) shows the pore morphology of the cross sections of lotus copper perpendicular and parallel to the solidification direction for two cases. In case I, the pellets of titanium hydride were immediately added during solidification after the molten copper was poured from the crucible to the mold, as shown in Figure 1(a), while in case II, the pellets of titanium hydride were added during the melting of copper in the crucible, which was heated by the induction heating coil, as shown in Figure 1(b). The same amount of titanium hydride (0.125 g) was added in both processes, but the porosity of lotus copper in the addition to the mold (process I) is much higher than that in the crucible (process II). Although titanium hydride is completely decomposed during the melting of process II, most of the hydrogen gas escapes to the atmosphere in order to maintain the equilibrium of the hydrogen concentration in the melt with the atmospheric partial hydrogen pressure. On the other hand, in process I, the decomposition of titanium hydride in the molten copper and the solidification occurs almost simultaneously, and most of the hydrogen does not escape into the atmosphere; the hydrogen concentration in the molten copper is not in equilibrium with the atmospheric partial pressure of hydrogen. Therefore, process I is efficient and effective, which should lead to the scalable mass production of lotus metals and alloys.
Finally, we examined the role of the other metallic element, Ti, through the thermal decomposition of titanium hydride. Titanium hydride is decomposed into hydrogen, and titanium. The latter is a very reactive element, which easily reacts with residual oxygen in the molten copper. Consequently, titanium oxide particles are formed and dispersed, which may serve as the nucleation sites for the hydrogen pores in the solid/ liquid interface during unidirectional solidification. It is well known that the pores evolve by heterogeneous nucleation in metal melts in the presence of small amounts of foreign particles. Therefore, the same reasoning can be applied to the present case, and it is expected that the pore size and porosity become homogeneous by uniformly distributed nucleation sites. Thus, TDM exhibits another advantage to produce lotus metals with more homogeneous pore size and porosity than those by PGM, which does not provide intentional nucleation sites.
A new fabrication process using a thermal decomposition method of titanium hydride is a simple, yet versatile route to obtain porous metals with directional pores, which are also called lotus-type porous copper. This fabrication method can be operated in an atmosphere as low as atmospheric pressure without hydrogen gas. Hence, TDM is superior to the conventional method (PGM), because PGM uses high pressure hydrogen gas with inflammable and explosive risks and gas pressurization in a high-pressure chamber. Therefore, TDM may realize the production of lotus materials for applications such as heat sinks, high-damping, lightweight materials, biomaterials, etc.
The present work was supported by a grant-in-aid for scientific research (category S) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 17106009).
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HIDEO NAKAJIMA, Professor, and TAKUYA IDE, Graduate Student, are with The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: [email protected]
Manuscript submitted May 21, 2007.
Article published online January 9, 2008
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