Jacob I. Kleiman

Structural aspects and conformation of linear carbon polytypes (carbynes)

Abstract The apparent discrepancy that an element with the richest chemistry of all should possess only few polymorphs was resolved recently by the increasing recognition of carbynes, chain-like compounds with sp -configuration and conjugated triple or cumulated double bonds between the carbon atoms in the chain. A simple classification approach is suggested leading to a linear relationship between the number of atoms in the chain and the c-axes lengths, the cell volumes, and the a 0 c 0 ratio. Moreover, a new carbyne form was observed which may have originated from regraphitization of shock-formed diamond. The new carbon polymorph fits well in the gap between already known carbyne forms.

Shock compression and flash heating of graphite/metal mixtures at temperatures up to 3200 K and pressures up to 25 GPa

Different aspects of phase‐transition processes in carbon in dynamic conditions were studied. Samples of graphite/metal mixtures have been recovered and analyzed after exposure to the combined action of high temperature and pressure pulses generated by a unique flash‐heating hemispherical implosion system. Transmission electron microscopy together with x‐ray and electron diffraction examinations proved the existence of diamond, different forms of graphite, and carbynes in the samples. A mechanism of formation of diamond is proposed which relies on a solid‐vapor‐liquid‐solid (SVLS) sequence of phase transformations. The experimental results were found to be in reasonable agreement with the proposed SVLS model. A tetragonally crystallized diamondlike carbon phase ( p‐diamond) was identified in the course of the work as well as a new linear carbon polymorph (Carbon‐XIV).

Influence of content and structure of hydrocarbon polymers on erosion by atomic oxygen

From a comprehensive analysis of erosion data for materials exposed to low-Earth-orbit space environment, to fast atomic-oxygen beams, and in plasma facilities it is shown that different correlations can be found between the thermal and hyperthermal atomic-oxygen erosion yield of hydrocarbon polymers and their chemical structure and content. Correlations have been found of the hyperthermal atomic-oxygen erosion yield of many polymeric materials in flight experiments with their inverse mass density of effective (not bonded with oxygen) carbon atoms, and with their degree of aromaticity. These correlations were used to identify the rate-limiting factors of erosion processes and to predict the erosion rate for polymer-based materials in low Earth orbit. The first correlation was not found in the interaction of thermal atomic oxygen with a number of polymers, and the second is strongly pronounced. The results are explained on the basis of physical and chemical processes affecting differently the erosion rates of polymers by thermal and by fast atomic oxygen. Subthreshold bombardment-induced and -enhanced degradation and surface chemical etching are proposed to be the major mechanisms of erosion by fast atomic oxygen.

Erosion resistance and durability improvement of polymers and composites in space environment by ion implantation

Abstract Spacecraft designers use polymers and polymer-based composite materials extensively in electrical, thermal, and structural applications to address both weight and performance demands. Without protection from the deleterious effects of the space environment, in particular hyperthermal atomic oxygen (HAO), these materials suffer accelerated erosion from chemical interaction and experience a loss of mass and deterioration of performance. High dose implantation at energies in the 10–100 keV range using ions of metal or semiconductor materials was used as a method of modifying the surface of these polymeric materials to produce changes that can yield dramatic improvements in space environmental durability. The results of this study show that computer modelling of the ion implantation process combined with reasonable fluence estimates give a good basis for the choice of implantation conditions. This study presents the results for high-performance materials including Kapton ® , Mylar ® , PEEK, Lexan ® , and PEEK/carbon fibre composites using X-ray electron spectroscopy, scanning electron microscopy, and other surface analysis techniques, before and after treatment. The results show that implantation of silicon and aluminum (singly, binary, or in combination with boron) or yttrium implantation produces a stable, protective oxide-based layer following exposure to HAO. The improvement in chemical resistance of these materials assures performance without deterioration in long duration space missions and shows promise for improvement in terrestrial performance in highly reactive oxidative environments.

Protective coatings for LEO environments in spacecraft applications

Abstract Spacecraft operating in the low Earth orbit (LEO) are exposed to an environment characterized by very low pressure, various atomic species, temperature extremes, ultraviolet (UV) radiation, electromagnetic radiation and atomic oxygen (AO), which is produced by the dissociation of molecular oxygen by UV radiation. The destructive influence of AO on polymer-based materials and composites and the synergistic effects between AO and other environmental factors have been dramatically demonstrated in LEO flights and ground-based simulators. This paper investigates the effects of contamination, structure and the synergism between temperature and AO fluence on polymer-based materials, and provides an overview of the recent developments in the design and use of protective coatings for polymer and composite materials in the LEO environment and their testing in ground-based space environment simulators. Three trends in protective coatings research are identified and discussed: (a) the improvement of technologies for high-performance oxide-based coatings; (b) self-healing coatings based on special semi-organic polymers; (c) protective multilayered structures. An evaluation is made of the properties and behaviour of different protective coatings on the polymers and composite materials used in spacecraft applications.

Synthesis of carbon nitride thin films by vacuum arcs

Abstract Carbon nitride (CN) thin films were synthesized by combining vacuum arcs and plasma ion implantation techniques. Three methods were investigated: plasma ion implantation into carbon films deposited by anodic vacuum arcs (AAPII), continuous cathodic vacuum arc with plasma ion implantation (CAPII) and pulsed cathodic vacuum arc (PCA). The films were found to be amorphous by X-ray diffraction (XRD). X-Ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis indicated the formation of C N, C N and C≡N bonds. Calculations of the surface tension components (dispersion and polar) of the films using the contact angle measurement technique suggested the formation of covalent carbon-nitrogen bonds. The CN films exhibited improved adhesion relative to the pure carbon films as indicated by adhesion calculations and the reduction in interfacial tension between the films and the substrate. A hardness of 18.9 GPa was obtained by nanoindentation measurements for CN films with an N/C ratio of 0.135.

METAL ION IMPLANTATION AND DYNAMIC ION MIXING FOR THE PROTECTION OF HIGH-PERFORMANCE POLYMERS FROM SEVERE OXIDATIVE ENVIRONMENT

Low energy high-dose Plasma Immersion Ion Implantation, combining both ion and recoil implantation (dynamic ion mixing), was used to enrich thin surface layers of high-performance polymers with an appropriate amount of specially selected reactive metal element such as Al. Both oxygen plasma and fast (E∼2–3 eV) atomic oxygen (FAO) beam have been used as aggressive environments for testing the implanted polymers. The modified materials successfully survived these test environments, including FAO, which is the main danger for carbon-based materials in space, in low Earth orbit. The retained doses of implanted and recoil implanted elements were controlled by RBS. The content, structure and morphology of the modified protective surface layers were examined by XPS and scanning electron microscopy (SEM). It was shown that protective oxide(s)-based surface structures were formed. Implantation and conversion conditions were found for which the appearance and important thermo-optical properties of treated polymer films, such as solar absorptance and thermal emittance, were practically unchanged.

Carbon nitride thin films prepared by nitrogen ion assisted pulsed laser deposition of graphite using KrF excimer laser

Abstract Carbon nitride films were prepared by nitrogen ion assisted pulsed KrF excimer laser deposition of graphite onto Si(100) substrates. The energy of nitrogen ions was changed between 25 and 1500 eV. The transport ratio of carbon atoms to nitrogen ions at the substrate was 1.0. The dependence of the stoichiometry and formed chemical bonds on the nitrogen ion energy was investigated. The nitrogen content in prepared films increased with decreasing the nitrogen ion energy, and showed a constant value of 30 at.% below 200 eV. The peak position of C1s spectra as found by X-ray photoelectron spectroscopy (XPS) analysis shifted to higher binding energy with decreasing nitrogen ion energy. The N1s XPS peak was deconvoluted into three peaks with binding energies BE=398.3, 400.0 and 402.0 eV, which were assigned to sp 3 C–N and sp 2 C–N and N–N bondings, respectively. The ratio of sp 3 to sp 2 bonded nitrogen atoms increased with decreasing ion energy, and showed a maximum value in the energy interval between 50 and 75 eV. The carbon content with the sp 3 C–N bond type was estimated at 12.6 at.% from electron energy loss spectroscopy (EELS) analysis. The nitrogen content with the sp 3 C–N bond type was estimated at 18.0 at.% by XPS. The ratio of carbon to nitrogen atoms with sp 3 bonds was found to be 1.43 in the films grown at nitrogen ion energies of 50 eV, which is close to that of C 3 N 4 compound predicted as a superhard material.

Comparison of surface modification of polymeric materials for protection from severe oxidative environments using different ion sources

Abstract Mid-energy monoenergetic conventional single-, dual-, or triple ion implantation, implantation with a MEVVA ion source, as well as Metal Plasma Immersion Ion Implantation have been used for the treatment of high-performance polymers at specially selected regimes, derived from the results of computer simulation. Testing of the implanted advanced polymers and carbon fiber reinforced composites has been performed in a Space Simulator for accelerated testing by exposure to fast ( E ∼2–3 eV) atomic oxygen beam, as well as by exposure to oxygen plasma, and ozone+UV. After an intermediate stage of surface conversion, the treated materials did not show any mass loss or surface morphology change, thus indicating high-quality protection from these severe oxidative environments. A variety of complementary surface analysis techniques, such as RBS, SIMS, XPS and SEM have been used to examine the surface content and structure of the modified subsurface layers.

Modeling and Database Development of Conductive and Apparent Thermal Conductivity of Moist Insulation Materials

Computer simulation and modeling of coupled heat and moisture transfer are becoming increasingly significant for accurate calculations of heat and moisture transfer. However, to solve their equations, the true component, λ, of apparent thermal conductivity, λapp, as a function of temperature, moisture content, and material structure is required. An approach to the development of a comprehensive database for λ is discussed. Its key feature is demonstrated by the development of a theoretical model for λ and λ app of two common building insulation materials — expanded polystyrene insulation and highly porous calcium silicate — for a wide range of temperature and moisture conditions.