材料专业学术英语——4.Materials fabrication and shape memory alloy

Sol-gel processThe sol–gel (solution–gelation) process is a versatile solution-based process for making ceramic and glassy materials. In general, the sol–gel process involves the formation of a sol (colloidal suspension of ca. ≥ 200 nm solid particles) and subsequent crosslinking to form a viscous gel. Though this technique has been in practice since the 1930s, until only recently have the complex mechanisms involved in sol–gel been investigated. The most common starting materials, or precursors, used in the preparation of the sol arewater-sensitive metal alkoxide complexes (M(OR)x, where R = alkyl group (e.g., CH3,C2H5, CF3, etc.). Although original formulations used sodium silicates, the use of alkoxide precursors avoids undesirable salt byproducts that may only be removed through long, repetitive washing procedures. In addition, the nature of the metal and associated R groups may be altered to affect the rate and properties of the ultimate oxide material.Sol–gel syntheses are typically carried out in the presence of polar solvents such as alcohol or water media, which facilitate the two primary reactions of hydrolysis and condensation (Eqs. 15 and 16, respectively). During the sol–gel process, the molecular weight of the oxide product continuously increases, eventually forming a highly viscous three-dimensional network.(15) M-OR + H2O → M-OH + ROH(16) M-OR + M-OH → [M-O-M]n + ROHDepending on the post-treatment used for the sol, a wide variety of materials may be synthesized: ultra-fine powders, thin film coatings, ceramic fibers, microporous inorganic membranes, ceramics and glasses, or extremely porous materials. Thin films are easily generated on a substrate through simple spincoating or dip-coating of the gel, followed by slow evaporation to prevent extensive cracking. Alternatively, the gel may be retained in a mold and heat-treated to convert the material into a dense ceramic or glass. If the solvent of an alcogel is removed through slow evaporation, a porous material known as a xerogel is formed.Powder MetallurgyAlthough the origin of making metallic materials through flame sintering dates back to ca. 3,000 B.C., this method was not widely applied until the late 18th century. Until only recently, metallurgy focused on doping and strengthening bulk metallic materials. Nowadays, powders are frequently used as precursors for metallic materials. For instance, tantalum powder is used in the fabrication of capacitors for electronics and telecommunications applications, including cell phones.Modern powder metallurgy consists of placing metal powder(s) into a closed metal cavity, or die, repeatedly compacting under high pressure (typically 200–300+MPa), and sintering in a furnace to yield a metal with desired porosity and hardness characteristics. The sintering process effectively results in the welding together of powder particles to form a mechanically strong finished material.For powder metallurgy, the density of the powder strongly influences the strength of the material obtained from compaction. As one would expect, the density of the powder depends on both the shape and porosity of individual micron-sized particulates. We know that close-packed metals will have higher densities than simple cubic materials. Among the close-packed metals, the theoretical percentages of total space occupied by atoms, relative to voidspace for bcc (coordination number 8), fcc (coordination number 12), and hcp (coordination number 12) unit cells are 68%, 74%, and 74%, respectively. Even if the metal particulates have the same diameter and are completely spherical, the actual packing density is typically on the order of 55–60%. This value may be improved by introducing nanosized particles that will fill the voids among the larger particles. During subsequent high-temperature sintering, the larger particles will grow at the expense of the nanoparticles, leaving behind relatively small voids that are closed during the thermal treatment.As their name implies, shape-memory alloys are able to revert back to their original shape, even if significantly deformed (Figure 3.24). This effect was discovered in 1932 for Au–Cd alloys. However, there were no applications for these materials until thediscovery of Ni–Ti alloys (e.g., NiTi, nitinol) in the late 1960s. As significant research has been devoted to the study of these materials, there are now over 15 different binary, ternary, and quaternary alloys that also exhibit this property. Other than the most common Ni–Ti system, other classes include Au–Cu–Zn, Cu–Al–Ni, Cu–Zn–Al, and Fe–Mn–Si alloys.The shape-memory effect is observed when the temperature of a piece of alloy is cooled to below that required to form the martensite phase: Ms (initial martensite formation) until Mf (martensite formation complete), as seen in Figure 3.25. Upon heating the martensitic material, a reformation of austenite begins to occur at A s until a final temperature, A f, is reached (Figure 3.25). Since the martensite phase is a highly distorted structure relative to fcc austenite, the alloy is highly soft and ductile and may be easily deformed while in its low-temperature phase.。