MgO smoke

At high oxygen exposures at 295 K, the surface consists predominantly of hexagonal structures, but also present as a minor component are square lattice structures (Figure 4.10) reminiscent of the cubic structure associated with MgO smoke formed by the oxidation of magnesium at high temperature.20 Therefore, two pseudomorphic oxide overlayers form at Mg(0001) at room temperature, but what factors control their separate growth are not known. 

In transmission studies of small aluminum spheres, Batson (22) has shown details of the generation of surface plasmons in the aluminum and in the thin coating oxide layer. Marks (23) and Cowley (24) have examined the surface plasmons and surface state excitations of small MgO smoke crystals. It is clearly evident that these excitations may be produced by electron beams passing the crystal in the vacuum, 3 nm or more away from the surface. 

Genzel and Martin (1972, 1973) measured extinction by MgO smokes loosely packed on transparent substrates both in air and covered with the transparent oil Nujol. Their results showed absorption bands appreciably shifted from the bulk absorption band the peak frequencies agreed with calculations, but the widths were consistently greater than predicted by sphere theory. In addition, a narrower absorption feature always appeared at the... 

In Fig. 12.166 sphere and CDE calculations are compared with measurements on MgO cubes well dispersed in KBr neither is very satisfactory. The calculated position of peak absorption by spheres is fairly close to that measured but not coincident with it the CDE calculations show appreciable absorption over approximately the same frequency range as the measurements but no structure. If the optical constants we have used accurately place the Frohlich frequency, Fig. 12.166 suggests that neither spheres nor a broad distribution of shapes are good approximations for MgO particles. This is hardly surprising because electron micrographs reveal that MgO smoke is composed of cubes. These cubes are so nearly perfect that they have been used to quickly determine the resolution of electron microscopes degraded resolution results in apparently rounded corners. 

We used the dielectric function e of bulk MgO calculated from oscillator parameters determined by Jasperse et al. (1966), together with the dielectric function em of the KBr matrix given by Stephens et al. (1953) (corrected by June, 1972), to calculate the absorption spectrum (12.37) of a dilute suspension of randomly oriented MgO cubes. These theoretical calculations are compared with measurements on well-dispersed MgO smoke in Fig. 12.16c. Superimposed on a more or less uniform background between about 400 and 700 cm-1, similar to the CDE spectrum, are two peaks near 500 and 530 cm- , the frequencies of the two strongest cube modes. It appears that for the first time these two modes have been resolved experimentally. If this is indeed so we conclude that the widths of individual cube modes are not much greater than the width of the dominant bulk absorption band. Genzel and Martin (1972)... 

When the Mg is suitably complexed, or, should we say, coordinated, such as found in poly-crystalhne MgO smoke , the binding of CO is much reduced, cf. the 11 kJmol suggested for this binding by G. Spoto, E. Gribov, A. Damin, G. Ricchiardi and A. Zecchina, Surf. Scl, 540, L605 (2003). 

Fig. 6. Effect of sintering temperature on the IR spectra of 12CO adsorbed at 298 K (5.33 kPa) on (a) high-surface-area MgO, (b and c) progressively sintered MgO samples, and (d) MgO smoke. KD (trimeric) and P (polymeric) species evolve with time through fragmentation in O (oxidized, carbonate-like groups) and (Q, Q ) reduced counterparts [reprinted from Zecchina et al. (22) with permission of Elsevier Science Publishers].

Fig. 6 shows clearly that the anionic species formed at threefold coordinated sites are preferentially affected by the morphological modifications induced by the sintering and resulting from a dramatic decrease in the abundance of surface O T anions as the dimension and perfection of the microcrystals gradually increase (Fig. 5). Of course, on the nearly perfect microcrystals of MgO smoke, anionic species were not observed.

Fig. 16. Excitation spectra in vacuo of MgO smoke taken with 400-nm luminescence (a) sample outgassed at 1200 K and (b) sample in contact with water vapor and then outgassed at 1200 K.. (Figure according to Coluccia et al. (/JO).]...

Remarkably uniform MgO smoke was prepared by Coulomb and Vilches (1984) by burning magnesium ribbons in dry OjAi mixtures. The MgO particles were collected in die form of a coating on a clean aluminium surface and were subjected to heat treatment (at c. 950°C and pressures < 10 6 mbar). The specific surface area of the final MgO(l 0 0) powder was c. 8 m2 g-1 so that it was not difficult to undertake accurate physisorption measurements and also neutron scattering experiments. 

Fig. 19. Coverage dependence of the IR spectra of CO dosed at 60 K onto high-surface-area MgO (230m g ), sintered MgO (40m g ), and MgO smoke (10m g ) parts (a), (b), and (c), respectively. The reported spectroscopic regions (2120—2070 and 1700—1125 cm ) refer to the chemistry of CO interacting with low-coordinated basic centers. All spectra are vertically shifted for the sake of clarity. The decrease of the band intensity by moving from high (a) to low (c) surface area samples is remarkable note the nearly total absence of the 0 chemistry on the MgO smoke (c). (Adapted with permission from Spoto et al. (26).)...

The three sequences shown in Fig. 19 demonstrate that the decrease of the specific surface area is accompanied by a dramatic decrease in intensity of the spectra and by a drastic spectral simplification. In particular, all bands in the 2120-1100 cm interval, attributed to species formed at very reactive three-fold and four-fold coordinated oxygen sites located on edges and steps (O4J) and on corners (Of ) are strongly affected by sintering and are nearly totally absent on MgO smoke (Fig. 19c). A similar effect was discussed in Section III.C for the H2 adsorption and splitting on the same MgO samples (Fig. 11) and on the chemistry of CO adsorbed on Mg sites (spectral region not reported in Fig. 19) (12,26,177). 

Figure 21.13 TEM of commercial MgO smoke following calcination after prolonged storage in air. (Reproduced from [44] with permission).

Measurements of the spin concentration indicate that relatively high concentrations of A (equivalent to about 0.5% of the surface oxide ions) can be formed. Other sources of electrons such as TMI impurities may be ruled out because of their low concentrations. The formation of NB radical ions was also found to be much greater on MgO smoke that had been etched by water vapor than on normal MgO smoke (65, 72). Etching of the regular cubic particles composing MgO smoke led to a considerable increase in the lowest coordination surface oxide ions, i.c., O c-... 

The above considerations are borne out experimentally on most rocksalt ionic compounds. For example, when magnesium metal is burned, the tiny MgO smoke particles that are formed are almost perfect cubes (see Fig. 2.4 in Ref. 1). The need to form a non-polar surface and to maximize the ligand coordination of surface ions makes the (100) surface energy much lower than that of other possible surfaces in the rocksalt structure. This is also manifest in the cubic shape of grains of table salt, NaCl. The (110) surface of MgO, whose ions are only four-fold coordinated, is also much less stable than the (100) surface [24]. 

Photoluminescence spectra of high surface area samples of alkaline earth oxides have been reported by Tench et al. [39] and interpreted in terms of the radiative decay of surface excitons. Representative data obtained by Tench et al. [39c] with microcrystalline MgO smoke (obtained by combustion of the metal in oxygen) are reproduced in Fig. 5, together with their schematic model for surfaces of the cubic MgO microcrystallites and their modification by exposure to water vapour. 

One method used to examine the possibility of low-energy GBs is the classic MgO smoke experiment. Mg metal is burned in air and the resulting MgO smoke particles are caught on a grid. The relative orientations between cube... 

Matthews, John was best known for his work on misfit, epilayer growth, and MgO smoke. 

Chaudhari, P. and Matthews, J.W. (1971) Coincidence twist boundaries between crystalline smoke particles, J. Appl. Phys. 42, 3063. Original description of the MgO smoke experiment for GBs Zhu, Y. and Granick, S. (2001) Viscosity of interfacial water, Phys. Rev. Lett. 87(9), 096104. The idea is that the viscosity of water can be very different if it is constrained to be a film in a silicate grain boundary. 

For the production of nodular iron, nodularisation is performed. BAT for nodularisation is to select a nodularisation technique with no ofT-gas production or to capture the produced MgO smoke, using a lid or cover equipped with extraction equipment or by using a fixed or movable hood, and to... 

---