Thermochemical Processes Episode 2

20 Thermochemical Processes: Principles and Models An alternative procedure which is effective in providing a controlled flux of metal atoms and condensing some of these on a clean substrate, is the method known as ion plating. An apparatus for this method of thin film formation consists of a plasma which is principally used to provide ions which clean the substrate surface by ion bombardment. A separately heated Knudsen cell or a heated filament provide the source of metal atoms to form the deposit, and these must diffuse through the plasma, either in the charged state, or as excited or neutral atoms. This procedure appears to be more efficient than the simple sputtering of a metal target due to the separation of the control of the metal atom source from the plasma, the first merely serving as a source, and the latter as a method for preparing a clean substrate to receive the deposit (Figure 1.7). Extractor plates Vacuum pumps Plasma generator Kundsen atomic vapour source Substrate Positive ion stream Figure 1.7 Ion plating device in which the substrate is cleaned by ion bombardment and the material to be deposited is supplied by a Knudsen cell These sputtering procedures have an advantage over Knudsen evaporation, since metallic alloys and some metal compounds can be directly sputtered without significant difference between the composition of the film and the source material. As there is most probably a difference between the sputtering efficiencies of the elements in the alloy, the one with the higher efficiency will be sputtered first, enriching the surface of the alloy with the other element, which will subsequently be sputtered, until the surface composition is restored. The relative rates of Knudsen evaporation and sputtering are, however, not significantly different in most practical cases. The independence of the sput-tering mechanism of the relative vapour pressures of the alloying elements clearly presents an experimental advantage over the use of a number of Knudsen cells, one for each element. The production of nanoparticles Films consisting of nanoparticles of between 1 and 102 nm in diameter can be produced by the evaporation of metals, using either a freely evaporating, Vapour deposition processes 21 heated, metal sample or a metal block which is heated with a focused laser beam, which evaporates into a low-pressure (10 2–10 3 atmos) inert atmo-sphere followed by condensation on a cold plate or liquid-nitrogen-cooled cold finger. In laser heating, energy densities of 106–107Wcm 2 are directed on the surface of the metal during a laser pulse of 10ns duration and 10 2 cm2 in area and at a repetition rate of 5000s 1 to cause the formation of a vaporized plume. This typically contains about 1015 atoms at an atom density equivalent to about 1018 atoms per cm3. As an example of this process, the evaporation of 1015 atoms of zinc would require about 10 9 of the molar heat of evaporation of zinc (130.4kJmol 1), or about 10 4 joule, which is negligible in comparison with the total energy imparted by the laser beam, and therefore most of this energy is used in raising the temperature of the gaseous products in the plume to temperatures as high as 10000K, to form a plasma containing free ions and electrons. The laser beam is scanned across the surface of the target in order to make the maximum use of the beam energy, and reduce the conduction of heat away from the surface and into the interior of the metal sample. If the surrounding inert gaseous atmosphere is heated from below the metal sample to cause convection currents, the plume of the evaporated metal is conveyed upwards to the cold plate, which is placed above the irradiated metal sample, for rapid condensation. Condensation of atoms occurs within the plume during ascent to the condensing plate to yield the fine particle deposit because the plume is super-saturated in metal vapour as the temperature of the plume approaches that of the surrounding atmosphere by radiation, and subsequently by collision, cooling. Metal oxide, nitride, and carbide nanopar-ticle films can be produced by adding oxygen, ammonia or methane to the inert atmosphere surrounding the metal target during irradiation, and scanning the laser beam to find unreacted metal. The final average size of the nano-particles can be controlled through the temperature of the condensing plate. For example, the average diameter of ZnO particles was found to increase from 10–20nm at 173K, to 50–60nm at 230K condensation temperature. Nanofilms of oxides can also be produced by heating gaseous metal compounds, such as halides, e.g. TiCl4, in an oxidizing flame. In this technique the gaseous compound is introduced into the central axis of an oxygen–methane flame, and the resultant product is condensed on a cold plate. Composite materials may also be prepared by mixing suitable gaseous compounds of the elements contained in the composite. A published example of this procedure is the formation of an Fe3O4/SiO2 composite by simultaneous oxidation of iron pentacarbonyl, Fe(CO)5 and hexamethylsiloxane, (CH3)3SiOSi(CH3)3, in a methane/oxygen/nitrogen flame (Goldstein 1997) (Figure 1.8). The potential technological importance of nanoparticles is due to the increase in sinterability with a substantial decrease in the sintering temperature 22 Thermochemical Processes: Principles and Models Water-cooled plate Laser beam Gas S inlet m e Vapour plume Gas outlet Convection current Heater Rotated sample is evaporated to form vapour plume Figure 1.8 Evaporation of a metal by laser beam irradiation to provide a source for the deposition of nanoparticles on a water-cooled substrate when compared to conventional materials since grain boundaries with high-diffusivity paths form a substantial fraction of a nanoparticle assembly. Also cold-compacted assemblies of nanoparticles have very much higher rates of atomic transport due to the fact that about 10% of the atoms in the compact are in the grain boundaries. Coating with thin diamond films The reaction between hydrogen and methane at high-temperatures has recently found an important application in the coating of cutting tools, for example, with thin films of diamond. A pre-requisite appears to be to be the inoculation of the tool surface with diamond paste to provide nuclei for the film formation. The film is grown in a gas mixture of >95% hydrogen and 1–4% methane, with or without the addition of a small partial pressure of water vapour. This gas mixture has been passed through a high-frequency discharge or over a tungsten filament held at about 2000–2500K before arriving at the tool surface. The substrate on which the film is to be formed must be held at a temperature between 1000 and 1500K. For the analysis of the process it is suggested that the high-temperature treat-ment of a hydrogen–methane mixture produces atomic hydrogen and acetylene as the important products. The thermodynamic analysis of the mixture shows the composition as detailed in Table 1.1, which indicates that atomic hydrogen is at a much lower concentration than molecular hydrogen. A proposed mecha-nism is that the surface of the diamond paste particles is covered with hydrogen atoms which are bonded to surface carbon atoms and these react with hydrogen atoms from the gas phase to produce hydrogen molecules which are desorbed Vapour deposition processes 23 Table 1.1 Equilibrium composition of gas mixtures at 2300K and 0.1atmos pressure (mole fractions of the major gases only) 95%H2 C 5%CH4 95%H2 C 4%CH4 C 1%H20 H2 0.947 H2 C2H2 0.0234 C2H2 H 0.0296 H CH4 4.69 ð 10 5 CO CH3 2.74 ð 10 5 CH4 CH3 0.947 0.0140 0.0296 0.00938 3.62 ð 10 5 2.12 ð 10 5 to the gas phase, leaving spare bonding electrons on the diamond surface. These bonds are then the sites of acetylene adsorption on the surface which couple to spread the diamond structure across the substrate. During the adsorp-tion of acetylene on the diamond nucleus surface, the bond nature of the adsorbed molecule changes from that found in the acetylene molecule to the graphite structure in the adsorbed state, and finally to bonds between the adsor-bate diamond substrate and the neighbouring adsorbed acetylene molecule. The alternative to diamond formation is, of course, the formation of graphite, which is the stable phase under these conditions. The structure of graphite involves triangular carbon–carbon bonding to form edge-joined benzene rings in flat planes, separated from one another by weak bonding which allows the planes to glide over one another easily. It will be seen that the carbon–carbon bond is stronger in benzene than in diamond, and in fact, the Gibbs energy of the transformation C(diamond) ! C(graphite) has the Gibbs energy change, at one atmosphere G° D 1372 4.53T Jmol 1 D 5905 Jmol 1 at 1000K The presence of water vapour in the ingoing gas mixture has been found to suppress the formation of graphite and thus to favour diamond formation. The significant change in composition when water vapour is added, is the presence of carbon monoxide in about half the proportion of hydrogen atoms. Plasma evaporation and pyrolysis of carbon to form Fullerenes The vapour phase in the evaporation of carbon at high temperatures contains a number of gaseous species C, C2, C3 and higher polymers. Of these the first 24 Thermochemical Processes: Principles and Models three molecules constitute the major species at 4000K, the relative partial pressures favouring the trimer at 1atmos pressure, and the monomer when the pressure is decreased to 102 Pa. This is a temperature which is typically achieved at the surface of the electrodes when carbon electrodes are used in a plasma heater. New materials have been synthesized from the condensates of carbon evaporation resulting from a DC plasma formed between carbon electrodes in a low pressure (about 103 Pa) inert gas such as helium or argon. In the discharge the anode is evaporated by electron bombardment, and the condensates are found, in part, on the anode as a thin web-like structure. These include the first complex of this kind to be discovered, the spherical C60 giant molecule and more recently single and double wall tubes of nanodimensions. The tube walls have the graphite six-membered ring structure, and the tubes are frequently sealed at each end by caps made of five-membered carbon rings, which can be removed by selective oxidation. The nanotubes have interesting electrical properties which suggest future applications in electronic microcir-cuits. Also of considerable interest are the nanoparticles which are formed when a 1:1 mixture of graphite powder and an oxide powder such as NiO, Fe2O3 or a lanthanide oxide such as Y2O3 is placed in a central cavity drilled into the carbon anode electrode. Nanoparticles of approximately spherical shape in which the metal carbide is encapsulated in a graphite layer of varying thickness are produced. Because of the low sinterability of graphite the parti-cles retain their shape at high temperatures, and the metal carbide particles function as catalysts for the preparation of organic compounds without the disadvantages of similar metal catalyst particles mounted on the surface of an inert carrier, such as a ceramic (see Chapter 4). The method of laser evaporation of carbon has also produced nanotubes, but a promising development of potentially great flexibility is in the thermal decomposition (pyrolysis) at temperatures around 1000–1300K of organic molecules containing C–C double bonds, such as benzene. A number of organic species have been tested in this way, and aligned nanotubes have been produced by the decomposition of iron ferrocene Fe(C5H5)2, in which an iron atom is sandwiched between two cyclopentadiene molecules. Given the enormous variation of organic molecules which can be tested in this proce-dure, many new possibilities of the production of nanotubes in a wide range of configurations appear available (Terrones et al., 1999). Materials science and the formation of thin films The formation of nuclei from the vapour phase The growth of deposits on a substrate requires the initial formation of nuclei and their subsequent growth and agglomeration into a film, most probably a ... - tailieumienphi.vn