Beryllium metal matrix composites for aerospace and commercial applications

Abstract

There is a growing need in both aerospace and commercial markets for lighter weight, higher stiffness, higher thermal stability materials to solve the design engineers’ problems of reduced mass, higher access speeds, improved mechanical and thermal stability for today's advanced technology. To address those needs, Brush Wellman Inc. has developed, characterised, and put into high volume production a family of beryllium metal matrix composites. There are two classes of materials that have been developed to provide these engineering benefits to the designer in both the commercial and aerospace markets. The first family of materials is aluminium beryllium (AlBeMet, which is a registered tradename of Brush Wellman). This material is a metal matrix composite consisting of pure beryllium and aluminium, with 20–62 vol.-%Be and the remainder aluminium. The material is produced by both powder metallurgy and net shape technologies such as investment casting and semi-solid forming. The materials properties that make it attractive for the design engineer are a density that is 25% less than that of aluminium, a specific stiffness four times those of aluminium, titanium, steel, and magnesium, a higher dampening capacity than aluminium, and a coefficient of thermal expansion almost 50%lower than aluminium. The second family of materials is a beryllium–beryllium oxide metal matrix composite, which are called E materials. This material was developed to address the thermal management needs of the electronic packaging design engineer. The material properties that make this material attractive to the electronic packaging engineer are: a density 20–25% that of Kovar, Invar, and CuMoCu, and 30% less than Al–SiC; a thermal conductivity ranging from 210 to 240 W m^-1 K^-1and a tailorable coefficient of thermal expansion, ranging from 6×10^-6 to 8.7×10^-6 K^-1.     

Reactive deposition of Al-N coatings in Ar/N2 atmospheres using pulsed-DC or high power impulse magnetron sputtering discharges

In this paper, the metal to ceramic transition of the Al-N₂ system was investigated using classical reactive pulsed-DC magnetron sputtering and HIgh Power Impulse Magnetron Sputtering (HIPIMS) at a constant average current of 3 A. Optical emission spectroscopy measurements revealed more ionised aluminium species in the HIPIMS discharge compared to pulsed-DC sputtering. It also showed excited N⁰ and ionised N⁺ species in reactive Ar/N₂ HIPIMS discharges. The corresponding evolution of the consumed nitrogen flow as a function of the N₂ partial pressure revealed that a higher amount of reactive gas is needed to achieve stoichiometric AlN with HIPIMS. Electron probe micro-analysis and X-ray diffraction measurements confirmed that a partially poisoned aluminium target is enough to allow the deposition of stoichiometric hcp-AlN thin films via HIPIMS. To go further in the comparison of both processes, two stoichiometric hexagonal aluminium nitride thin films have been deposited. High power impulse magnetron sputtered hcp-AlN exhibits a higher nano-hardness (18 GPa) than that of the coating realised with conventional pulsed-DC sputtering (8 GPa).     

SrSn1−xFexO3−δ (0≤x≤1) perovskites: a novel mixed oxide ion and electronic conductor

The electrical conductivity of SrSn_1−xFe_xO_3−δ increases with the Fe content and reaches a value of ∼10⁻¹ S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10⁻²² atm.     

Electrical properties of high purity tin dioxide doped with antimony

The electrical conductivity of high purity tin dioxide doped with antimony was studied at temperatures of 900 to 1200° C and partial pressures of oxygen between 10^–8 and 1 atm. For specimens having a dopant concentration over 1 × 10¹⁹Sb cm^–3, the electrical conductivity decreased slightly with temperature and independent of oxygen partial pressure. The electrical conductivity of specimens having a dopant concentration under 1 × 10^–8Sb cm^–3 increased with temperature and with decreasing partial pressure of oxygen. The significance of the dopant and the thermally created defects is discussed.     

Silicon nitride films prepared by reactive plasma sputtering

Silicon nitride films were prepared by reactive plasma sputtering in nitrogen at a pressure of 2×10^-4 Torr. The residual gas in the reactor during film sputtering was analysed. The chemical composition of the films was determined from infrared absorption spectra in the wavelength region 2.0–15.0 μm and by the elastic scattering of ³He particles.The best quality silicon nitride films were obtained in pure nitrogen at the minimum residual gas pressures. An absorption minimum at 11.0 μm in the infrared spectra, corresponding to the Si-N chemical bond in the Si₃N₄ molecule, was observed in our films, indicating that their composition was close to stoichiometric.With a residual hydrogen pressure above 10% or a residual oxygen pressure above 2% the generation of new chemical bonds Si-H, N-H and Si-O respectively was observed in the silicon nitride films.     

High oxygen ion conduction in some Bi2O3-Y2O3 (Er2O3) solid solutions

Highly densified bodies (~ 98 % theoretical density) of the Bi₂O₃-Y₂O₃(Er₂O₃) systems containing 30 moles % Y₂O₃ and 20 moles % Er₂O₃ respectively were prepared from both mixed oxides and coprecipitated oxalates. DC ionic conductivity and impedance complex plane analysis on sintered samples were performed. Under oxygen partial pressure ranging from 1 to 10 Pa was found that, samples containing 30 moles % yttria showed a pure ionic conductivity up to an oxygen partial pressure of 10⁻²⁰ Pa. Samples containing 20 moles % erbia, extended its ionic conductivity up to an oxygen partial pressure of 10⁻²² Pa. The impedance analysis shows the presence of only one semicircle at low tempertures and it is attributed to bulk conductivity contribution. The conductivity was higher for the Bi₂O₃-Er₂O₃ sintered solid electrolytes, in such a case, a conductivity as high as 1.38 ohm cm at 700ºC was obtained. The activation enthalpy for the conduction process was calculated in the temperature range from 200º to 700ºC in all the cases. Microstructural development in the sintered sample was also studied.     

IR and x-ray studies of ion-beam-synthesized aluminium nitride films

IR transmission spectroscopy in conjunction with X-ray diffraction was used to characterize the phase composition of aluminium films after nitrogen ion implantation. Aluminium films deposited onto single-crystal silicon and implanted with 30 keV nitrogen ions (¹⁴N₂⁺) to a dose of 10¹⁷-10¹⁸ ions cm^-2 were subsequently characterized for aluminium nitride (AIN) formation by IR spectroscopy. The formation of a stoichiometric AIN layer was evident from the IR absorption band observed at 648 cm^-1. Furthermore, X-ray diffraction of an aluminium foil after nitrogen implantation at 110 keV to a dose of 5.0 × 10¹⁷ ions cm^-2 on each side revealed the presence of a polycrystalline AIN phase. A thermal treatment at 700°C did not yield any new crystalline phases.     

Reduction of sodium-accelerated oxidation of silicon nitride ceramics by aluminium implantation

Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0×10¹⁵cm^-2 at 72 keV (1 at% peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3×10¹⁵cm^-2 at 87 keV (1 at% peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen at 1100 and 1300°C for 2–6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can be attributed to their different roles in modifying the structure and properties of the oxide formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electrical conductivity and defect structure of polycrystalline tin dioxide doped with antimony oxide

Electrical conductivity () of tin dioxide doped with antimony has been measured as functions of temperature and oxygen partial pressure (p0_2> ). Variation of electrical conductivity is explained by assuming that the antimony oxide forms a substitutional solid solution and doubly ionized oxygen vacancies are predominant defects. Above –10^–5 atm oxygen partial pressure antimony ions are present predominantly in the pentavalent state in tin dioxide lattice. However, it is converted to the trivalent state below this oxygen partial pressure accompanied by a sudden rise in conductivity.     

Electrical conductivity in Ta2O5

The defect structure of undoped polycrystalline Ta₂O₅ was investigated by determining the temperature [850–1050°C] and oxygen partial pressure [10⁰–10^?19 atm.] dependence of the electrical conductivity. The data were found to be proportional to the $\text{～?}\text{1}\text{4}\text{th}$ power of the oxygen partial pressure for the oxygen pressure range <10^?8 atm. and independent of the oxygen partial pressure for P_O2 > 10^?6 atm. The enthalpy of formation of doubly ionized oxygen vacancies plus two electrons is estimated to be 118.31 Kcal/mole [5.13 eV]. The observed conductivity data are explained on the basis of the presence of unknown acceptor impurities in the undoped samples.     

Effect of oxygen partial pressure on the electrical and optical properties of highly (200) oriented p-type Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O films by DC sputtering

Thin films of NiO (bunsenite) with (200) preferential orientation were synthesized on glass substrates by direct current sputtering technique in Ar+O₂ atmosphere. Nanostructural properties of the NiO films were investigated by X-ray diffraction and also by atomic force microscopic (AFM) studies. Electrical and optical properties of the deposited films were investigated as a function of different partial pressure of oxygen in the sputtering gas mixture during deposition. The films showed p-type electrical conduction and the conductivity depends on the partial pressure of oxygen. The electrical conductivity (σ_RT) was found to be .0615 S cm⁻¹ for films deposited with 100% O₂ and its value sharply decreased with the decrease the partial pressure of O₂; for example σ_RT for 50% O₂ was 6.139 × 10⁻⁵ S cm^-1. The mechanism of the origin of p-type electrical conductivity in the NiO film is discussed from the viewpoint of nickel or oxygen vacancies, which generate holes and electrons respectively. X-ray photoelectron spectroscopic studies supported the above argument. Corresponding optical properties showed that the transparency decreases with increasing oxygen partial pressure and the bandgap also decreases.     

Thermal conductivity of calcium-doped aluminium nitride ceramics

Aluminium nitride ceramics were prepared with the addition of up to 12wt% of calcium oxide as a sintering aid. Both the oxygen and the calcium content of the samples decreased during sintering with increasing sintering temperature and soaking time. Higher amounts of calcium oxide resulted in higher thermal conductivities, with values up to 142 W m^–1 K^–1. Moderate sintering temperatures, short temperature soaking times and the use of inexpensive Ca-based sintering additives should enable the production of aluminium nitride ceramics with sufficiently high thermal conductivity at relatively low cost.     

Influence of the nonstoichiometry of titanium nitride TiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> on the degree of its modification with oxygen

We investigate the influence of the nonstoichiometry (the degree of completeness) of titanium nitride TiN on the degree of its modification with oxygen. The completeness of titanium nitride was controlled by changing the nitriding temperature, nitriding time, and partial nitrogen pressure. It is shown that as the completeness of titanium nitride decreases, which leads to an increase in the intensity of oxynitriding, and as the nitrogen content in the ternary compound TiN _x O_{1 – x} decreases, the level of surface hardening of titanium alloys after oxynitriding diminishes.     

Ion beam reactively sputtered silicon nitride coatings

An ion beam source was used to deposit silicon nitride films by reactive sputtering a silicon target with an Ar+N₂ beam. The nitrogen fraction in the sputtering gas was 0.05 to 0.80 at a total pressure of 6 to 20×10^?5 torr. The ion beam current was 50 mA at 500 V. A rate of deposition of about 2 nm min^?1 (0.12 μm h^?1) was found, and the spectra indicated that Si₃N₄ was obtained for a fraction of nitrogen higher than 0.50. However, the AES spectra also indicated that the sputtered silicon nitride films were contaminated with oxygen and carbon and contained significant amounts of iron, nickel, and chromium, most probably sputtered from the holder of the substrate and target.     

Development of as-fired aluminium nitride substrates with smooth surface and high thermal conductivity

As-fired aluminium nitride (AIN) substrates with smooth and uniform surface have been developed by green sheet and firing technology. The effect of setting for firing on surface roughness was investigated. AIN substrates were fabricated by pressureless sintering of green sheets piled up and sandwiched between AIN plates in an AIN crucible. The thermal conductivity, surface roughness and bending strength of the substrate sintered at 1770 °C for 2 h under a pressure of 1 MPa nitrogen were 194 Wm^–1 K^–1, 0.15 (m and 353 MPa, respectively.     

Defect structure of acceptor-doped calcium titanate at elevated temperatures

The defect structure of acceptor (Al or Cr)-doped polycrystalline calcium titanate was investigated by measuring the oxygen partial pressure dependence (at 10° to 10–18 atm) of the electrical conductivity at 1000 and 1050° C. The observed electrical conductivity data were proportional to for the oxygen pressure range < 10^–10 atm and proportional to for the oxygen pressure range ( 10^–7 atm. The conductivity values were observed to increase with the acceptor concentration in the p-type region with the shift in the conductivity minima towards lower oxygen partial pressure. The absolute value of the electrical conductivity in the acceptor-doped samples were lower in the n-type region compared to the values in the undoped CaTiO₃. Aluminium and chromium were found to be equally effective in acting as acceptor impurities in CaTiO₃. The defect chemistry of CaTiO₃ is dominated by the added acceptor impurities for the entire oxygen partial pressure range used in this investigation.     

Electrical conductivity of (ZrO2)0.85(CeO2)0.12 (Y2O3)0.03

A number of zirconia-based materials show promise as electrode materials in magnetohydrodynamic (MHD) generators. As a part of an exploratory programme to find suitable materials for graded electrode applications in MHD generators, partially stabilized and fully stabilized sintered ceramic materials are prepared and characterized. The oxygen ion transference number t _ion(O^2–) and electrical conductivity of this material are measured up to 1670 K in the oxygen partial pressure range 1 to 10^–6 atm. The activation energies for conduction are determined. The electrical properties of this material are characterized by mixed conduction, ionic and electronic. The observed conductivity data are explained in terms of the defect equilibrium reactions between tetravelent Ce⁴⁺ and trivalent Ce³⁺ ions.     

Conductive Carbon Nitride for Excellent Energy Storage

Conductive carbon nitride, as a hypothetical carbon material demonstrating high nitrogen doping, high electrical conductivity, and high surface area, has not been fabricated. A major challenge towards its fabrication is that high conductivity requires high temperature synthesis, but the high temperature eliminates nitrogen from carbon. Different from conventional methods, a facile preparation of conductive carbon nitride from novel thermal decomposition of nickel hydrogencyanamide in a confined space is reported. New developed nickel hydrogencyanamide is a unique precursor which provides self‐grown fragments of ?N?C?N? or N?C? C?N and conductive carbon (C‐sp²) catalyst of Ni metal during the decomposition. The final product is a tubular structure of rich mesoporous and microporous few‐layer carbon with extraordinarily high N doping level (≈15 at%) and high extent of sp² carbon (≈65%) favoring a high conductivity (>2 S cm^?1); the ultrahigh contents of nongraphitic nitrogen, redox active pyridinic N (9 at%), and pyrrolic N (5 at%), are stabilized by forming Ni? N bonds. The conductive carbon nitride harvests a large capacitance of 372 F g^?1 with >90% initial capacitance after 10 000 cycles as a supercapacitor electrode, far exceeding the activated carbon electrodes that have <250 F g^?1.     

Conductivity and creep in acceptor-dominated polycrystalline Al2O3

Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al₂O₃ were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 m. Hole conduction shows a slight preference for grainboundaries; ionic conduction is slightly hindered by grain boundaries, indicating that fast oxygen grain-boundary diffusion involving charged species does not occur. Conductivity and creep are accounted for on the basis of a model in which there is fast grain-boundary migration by a neutral oxygen species.     

Electrical conductivity and defect structure of CeO2-ZrO2 mixed oxide

The electrical conductivity of CeO₂-ZrO₂ system was measured as functions of the temperature and oxygen partial pressure and of the composition. The ionic conduction was prevailing in the ZrO₂ rich phase due to the increase of ionic defect concentration via homovalent doping effect. The enhancement of n-type electronic conductivity was observed in intermediate and CeO₂ rich phase compared with pure CeO₂, which originated either from homovalent doping effect or increase of electronic mobility due to the change of transport mechanism.