Richard Karmhag

Oxidation Kinetics of Nickel Particles: Comparison Between Free Particles and Particles in an Oxide Matrix

Abstract The degradation of selective solar absorbers through oxidation has been studied. We compare the oxidation kinetics of nickel particles of various sizes. Both free particles and particles embedded in an oxide matrix were studied. The oxidation kinetics of polycrystalline nickel nanorods was determined by IR spectroscopy in the temperature range 300–500°C. The particles were oxidized when situated in the porous alumina matrix of an electrochemically deposited solar absorber coating. The oxidation kinetics was compared to that of free nanometer-particles at the same temperature and to micron-sized polycrystalline nickel particles, which were studied by thermogravimetry in a wider temperature range. It was found that the rate constant was markedly lower for the particles in the matrix. Implications for the durability of selectively solar absorbing coatings are discussed.

Oxidation kinetics of nickel nanoparticles

The oxidation kinetics of nickel particles with an approximate median size of 15 nm was determined by thermogravimetry. The particles had a thin initial oxide shell amounting to 36 wt % of the sample. The kinetics was studied in pure oxygen in the temperature range 135–235 °C, giving power law behavior of each isotherm with an exponent between 0.48 and 0.58. By use of a master plot technique an apparent activation energy of 1.34 eV was determined. The power law exponent is considerably lower than the one expected for diffusion limited oxidation of a spherical particle with an initial oxide shell. The deviations from parabolic kinetics are discussed in terms of nonlinear diffusion, grain growth, and particle agglomeration.

Oxidation kinetics of small nickel particles

The oxidation kinetics of two kinds of polycrystalline nickel particles with an approximate median diameter of 5 μm but with different morphology, has been studied by thermogravimetric measurements in the temperature range 573–973 K. The oxidation was found to be thermally activated with an apparent activation energy of about 1.5 eV. The kinetics was compared with a homogeneous field, coupled-currents theory for oxidation of spherical metal particles. Calculations using a median particle size or a distribution of particle sizes do not give satisfactory fits to the experimental data. Possible explanations of the deviations in terms of space charge, grain boundary diffusion, grain growth, and sintering are discussed.

Titanium–aluminum–nitride coatings for satellite temperature control

Abstract Intense solar irradiation, radiative cooling to outer space, and internal heat generation determine the equilibrium temperature of a spacecraft. The balance between the solar absorption and thermal emittance of the surface is therefore crucial, in particular for autonomous parts directly exposed to the solar radiation and thermally insulated from the main thermal mass of the spacecraft. The material composition but also the coating thickness are found to influence the equilibrium temperature of an object in space. In this paper we report on a systematic search for a suitable composition and thickness of TixAlyNz alloy coatings prepared by reactive, unbalanced magnetron sputtering from targets consisting of differently sized titanium and aluminum sectors. The films were deposited on glass, glassy carbon, aluminum sheet metal, and on sputtered aluminum and TixAl(1−x) films on glass. The stoichiometry and sheet resistance of the films was determined with Rutherford backscattering and four-point probe measurements respectively. Reflectance spectra for the visible and infrared spectral ranges were used to obtain average solar absorptance and thermal emittance values used in model calculations of the equilibrium temperature. Neglecting internal heat contributions, the lowest calculated equilibrium temperature in orbit around the Earth, 32.5°C, was obtained for a 505-nm-thick Ti0.14Al0.47N0.40-film.

Highly transparent Ni–Mg and Ni–V–Mg oxide films for electrochromic applications

Abstract Thin films of oxides based on Ni, Ni–V, Ni–Mg, and Ni–V–Mg were made by reactive d.c. magnetron sputtering. Electrochemical cycling showed pronounced anodic electrochromism. The Mg addition was capable of yielding a significantly enhanced optical transparency in the 400

Oxidation kinetics of metallic nanoparticles

The performance of nanomaterials in specific applications can be greatly affected by a thin oxide layer on the surface. In this paper we put forward a model for the oxidation kinetics of metallic nanoparticles. For very thin oxide coatings on nanoparticles, it is necessary to take into account non-linear diffusion of the rate-determining ions. Initial oxide coatings on the particles are also taken into account. We develop a phenomenological theory of oxidation by non-linear diffusion and compare it to previous linear diffusion theories. Our model calculations are in good agreement with the measured oxidation kinetics of 15 nm nickel particles, as obtained by thermogravimetry.

Optical constants of sputtered hafnium nitride films. Intra- and interband contributions

Abstract Transparent and opaque films of hafnium nitride have been prepared by reactive magnetron sputtering in an argon-nitrogen atmosphere. The films were deposited upon heated Coming glass substrates with a deposition rate of about 2.1 nm/s. The optical constants of the films were calculated using Kramers-Kronig integration and ellipsometry for the opaque samples and a combined R T method for transparent samples. Detailed observation of the thickness variation in the optical constants revealed a clear trend: larger n-values and smaller k-values in the VIS-and NIR-range for thinner films. This was analysed within the framework of the classical Drude model and found to be an effect of shorter relaxation time for thinner films. The effect is as large as a factor of three in the film thickness interval 15–380 nm. The optical effective mass of the d-electrons was found to be in the interval 0.82–0.95 of the free electron mass, which is significantly lower than in TiN but similar to ZrN. The interband contribution to ϵ2(ω) was obtained by subtraction of the Drude part from the experimental dielectric function. It exhibits a sharp increase for λ

TIN-ALLOY COATINGS FOR TEMPERATURE CONTROL OF SPACE VESSELS

The problem of combining the temperature control of a space vehicle with the mechanical and chemical stability of the surface is addressed. With the absorption of solar radiation and the emission of thermal radiation considered the static and dominant factors that determine the equilibrium temperature of a spherical object, a simple model is formulated. Realistic variations of the two material-dependent parameters, solar absorptance alpha and hemispherical emittance epsilon, permit a large change in the equilibrium temperature, from less than -50 to more than +150 degrees C. It is pointed out that for a generalized gray surface, i.e., one made from a material whose reflectance-emittance has the same value within the visible and the thermal wavelength regions, the equilibrium temperature is approximately 5 degrees C, independent of the numerical value of the reflectance. With the requirement for electrical conductivity also taken into account, TiN alloys are identified as candidate materials. Measurements and calculations of some of them indicate that this group may contain a material that fulfills all the requirements. The experimental reflectance spectrum is used in estimating the equilibrium temperature for different TiN alloys; one Ti-Al-N-alloy with a gradient content of Al is found to have the lowest equilibrium temperature, 66 degrees C.

Oxidation kinetics of nickel solar absorber nanoparticles

The alumina matrix of an electrolytically deposited solar absorber coating was etched away leaving nickel nanorods standing on an aluminium support. The oxidation kinetics of the nickel nanorods, with an approximate diameter of 30 nm and a height of approximately 300 nm, was determined by infrared (IR) spectroscopy. The IR absorption by longitudinal optical phonons in NiO was measured before and after exposure to pure oxygen at 260, 280, 300, 320 and 350 °C for different times. The absorptance peak height was determined and used as a measure of the degree of oxidation. The five different isotherms showed power-law behaviour with an exponent varying between 0.52 and 0.69. A fit to the homogeneous linear diffusion equation, derived for spherical geometry, gave parabolic rate constants, which are in agreement with data for larger nickel particles and bulk nickel. The deviations in the exponent from that of the parabolic law are discussed in terms of particle geometry and particle agglomeration. The apparent activation energy was determined, by the use of a master plot technique, to be 1.73 eV.

TixAlyN coatings for temperature control of spacecraft

Thermal control is an important issue in small-scale satellite design, and thin film coatings suit the limited mass and volume constraints. Group IVB transition metal nitride films meet the criteria that the satellite surface must be mechanically and chemically stable, and electrically conducting. Thin film TixAlyNz coatings have been investigated and tailored for temperature control. The films were deposited by reactive sputtering on aluminum substrates in N2/Ar-atmosphere. The solar absorptance, (alpha) , and thermal emittance, (epsilon) , were calculated from spectral reflectance measurements. It was found that an optimization of film composition leads to a reduced equilibrium temperature. The composition temperature. The composition Ti0.16Al0.41N0.43 has a flatter reflectance curve than TiN, and was found to be close to optimal. By varying the film thickness, interference effects could further reduce the equilibrium temperature. A 650 nm Ti0.16Al0.41N0.43 film showed a reflectance interference minimum positioned at the maximum of the blackbody spectrum, resulting in an increased emittance. Neglecting internal heat contributions, the lowest calculated equilibrium temperature was 34.6 degrees C for this film.