High-temperature aerospace

The beryUides continue to be of interest for high temperature aerospace appHcations because of their oxidation resistance, low density, and high strength at elevated temperature (7). The limited strain capacity of the materials, particularly at low temperatures, has thus far prevented actual use. 

Occasionally you might come across a compound called pseudo-cumene, which is a benzene ring connected to three methyl groups. This compound is an isomer of cumene known as 1,2,4-trimethyl benzene. Pseudo-cumene is a starting material for the manufacture of trimilletic anhydride, an important ingredient in alkyd resin paints and high temperature aerospace poiyimide resins. 

High-temperature, aerospace and impact resistant applications... 

This work was supported by contract FA9550-06-1-0125 with the High Temperature Aerospace Materials Program (Dr. Joan Fuller, program manager) in the Air Force Office of Scientific Research. 

For a general discussion of these high-temperature adhesives, compared with PBI, see Section 5.5. These adhesives are synthetic thermosetting resins formed by the reaction of a diamine and a dianhydride. As with PBIs, they were developed specifically for high-temperature aerospace applications. PI adhesives are superior to PBIs for long-term strength retention, as shown in Figure 5.4. ... 

Materials that come close to the ideal ladder structure are thermosetting polyimides and polybenzimidazoles. These are used primarily in high-temperature aerospace applications as composites and adhesives. 

Polyimides are an important class of polymers for high temperature aerospace applications. Thin polyimide films are ideal candidates for protective coatings on antenna reflectors and other electronic applications. Their properties, both physical and electrical, are expected to be strongly influenced by their morphology. We have developed a novel technique for monitoring microstructural characteristics of thin polymer films. It is based on the sensitivity of the positron lifetimes to the molecular architecture of the polymers. Specifically, positron lifetimes can be used to calculate free volume hole radii and free volume fractions in the test polymers. A free volume model has been developed to calculate dielectric constants of thin polyimide films. It has been tested on a series of special purpose polyimide films developed for aerospace communication networks. The results are described in the following sections. 

Fiber-reiaforced panels covered with PVF have been used for greenhouses. Transparent PVF film is used as the cover for flat-plate solar collectors (114) and photovoltaic cells (qv) (115). White PVF pigmented film is used as the bottom surface of photovoltaic cells. Nonadhering film is used as a release sheet ia plastics processiag, particularly ia high temperature pressing of epoxy resias for circuit boards (116—118) and aerospace parts. Dispersions of PVF are coated on the exterior of steel hydrauHc brake tubes and fuel lines for corrosion protection. 

High process temperatures generally not achievable by other means are possible when induction heating of a graphite susceptor is combined with the use of low conductivity high temperature insulation such as flake carbon interposed between the coil and the susceptor. Temperatures of 3000°C are routine for both batch or continuous production. Processes include purification, graphitization, chemical vapor deposition, or carbon vapor deposition to produce components for the aircraft and defense industry. Figure 7 illustrates a furnace suitable for the production of aerospace brake components in a batch operation. 

These provide thin films of a soHd, or a combination of soHds, interposed between two moving surfaces to reduce friction and wear. They are coming into more general use for high temperatures, vacuum, nuclear radiation, aerospace, and other environments that prohibit use of oils and greases. 

Other alloys have been developed for use in particular corrosive environments at high temperatures. Several of these are age-hardenable alloys which contain additions of aluminum and titanium. Eor example, INCONEL alloys 718 and X-750 [11145-80-5] (UNS N07750) have higher strength and better creep and stress mpture properties than alloy 600 and maintain the same good corrosion and oxidation resistance. AHoy 718 exhibits excellent stress mpture properties up to 705°C as well as good oxidation resistance up to 980°C and is widely used in gas turbines and other aerospace appHcations, and for pumps, nuclear reactor parts, and tooling. 

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

Carbon—carbon composites are used in high temperature service for aerospace and aircraft appHcations as weU as for corrosion-resistant industrial pipes and housings. AppHcations include rocket nozzles and cases, aircraft brakes, and sateUite stmctures. Carbonized phenoHc resin with graphite fiber functioned effectively as the ablative shield in orbital re-entry vehicles for many years (92). 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

Giaphite with its exceptional strength and thermal stabiUty at high temperatures is a prime candidate material for many aerospace and nuclear appHcations. Its properties, through process modifications, are tailorable to meet an array of design criteria for survival under extremely harsh environmental operations. 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

The PEEK resia is marketed as aeat or filled pellets for iajectioa mol ding, as powder for coatiags, or as preimpregaated fiber sheet and tapes. Apphcations iaclude parts that are exposed to high temperature, radiation, or aggressive chemical environments. Aerospace and military uses are prominent. At present, polyamideimide (PAl) resia and poly(arylene sulfides) are the main competitors for apphcations requiring service temperatures of 280°C. At lower temperatures, polyethersulfones, amorphous nylons, and polyetherimides (PEI) can be considered. 

Maturation as a technology does not mean that advancement and innovation has ceased. Adhesive bonding is so essential to the aerospace field that as long as there is a desire to go higher, faster and farther more efficiently, there will be an incentive to develop new materials and processes for adhesive bonding. Areas of particular interest for future applications are high-temperature adhesives, fiber-reinforced metal laminates and more efficient bond assembly techniques. 

If one amino group in o-phenylenediamine is converted to an amide group by formic acid, the intermediate benzimidazole is formed. This reaction, conducted with a wide range of reactants, produces resins (polybenzimidazoles) used as high-temperature adhesives for laminates in the aerospace industry. Heat insulation is made by including tiny bubbles of silica and all... 

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