Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Sulphur transformation during pyrolysis of an Australian lignite

Transformation of various sulphur forms, including inherent and added pyrite, sulphates (CaSO₄ and FeSO₄) and organic sulphur, during pyrolysis of an Australian lignite was studied using TGA, TGA-MS and a fixed bed reactor, supplemented by sulphur form analysis. It was shown that hydrogen sulphide (H₂S) and a small quantity of sulphur dioxide (SO₂) were released during the pyrolysis of the pyrite-lignite blend. However, only SO₂ was detected during the pyrolysis of the lignite with high pyrite content. Inorganic matter was found to help retaining some of the inorganic sulphur, including pyrite, in the char. Inherent sulphates decomposed at much lower temperatures than the added sulphates, releasing SO₂ rather than H₂S. The inherent sulphates in the lignite were dominated by iron sulphates which started to decompose and release SO₂ at around 500 K and all sulphate had been decomposed at 1073 K. The retention of organic sulphur in the high organic sulphur lignite was higher than in its acid washed lignite sample, due to the interaction between inherent inorganic matter with the organic sulphur retaining the organic sulphur in the solid phase. SO₂ was the only sulphur gas produced during pyrolysis of acid washed lignite. A comprehensive mechanism of sulphur transformation during pyrolysis of lignite was proposed.     

Spatiotemporal distribution of nucleation events during crystal growth

We consider irreversible second-layer nucleation that occurs when two adatoms on a terrace meet. We solve the problem analytically in one dimension for zero and infinite step-edge barriers, and numerically for any value of the barriers in one and two dimensions. For large barriers, the spatial distribution of nucleation events strongly differs from rho(2), where rho is the stationary adatom density in the presence of a constant flux. Theories of the nucleation rate omega based on the assumption that it is proportional to rho(2) are shown to overestimate omega by a factor proportional to the number of times an adatom diffusing on the terrace visits an already visited lattice site.     

Effect of solid solution strengthening by iridium on the nucleation of dislocations in a molybdenum single crystal during nanoindentation

The nucleation of dislocations in single crystals of molybdenum and a Mo-1.5 at % Ir solid solution has been investigated by nanoindentation. In the curve of indentation of a Berkovich indenter into the single crystals, an abrupt transition from elastic to plastic deformation has been observed at a depth of 20–40 nm due to the nucleation of dislocations in the initially dislocation-free region under the contact. Alloying of molybdenum by iridium results in a twofold increase in shear stresses under which dislocations nucleated in the contact. Therefore, the solid solution impurity of iridium in molybdenum leads to an increase not only in the plastic strain resistance (to an increase in the hardness) but also in the elastic shear stresses under which dislocations are generated (homogeneously or heterogeneously) in the contact. The latter effect cannot be explained only by an increase in the elastic moduli because of its smallness; however, it is determined to a large extent by a higher degree of perfection of the solid solution crystal as compared to unalloyed molybdenum.     

Ultrasonic reactivation of phosphonate poisoned calcite during crystal growth

The effect of ultrasonic irradiation (42,150 Hz, 17 W dm(-3)/7.1 W cm(-2)) on the growth of calcite in the presence of the inhibitor nitrilotris(methylene phosphonic acid) (NTMP) was investigated at constant composition conditions. In seeded growth experiments, it was found that the inhibiting effect of NTMP on crystal growth could be seriously mitigated under influence of ultrasonic irradiation. An approximately twofold increase in volumetric growth rate was achieved during ultrasonic irradiation, and recovery of the growth rate following inhibition was strongly enhanced compared to growth experiments without ultrasonic irradiation. The results could be explained in part by the physical effect of ultrasound that causes breakage and attrition of poisoned crystals, which resulted in an increase in fresh surface area. Mass spectroscopy analysis of sonicated NTMP solutions revealed that there is also a chemical effect of ultrasound that plays an important role. Several breakdown products were identified, which showed that ultrasound caused the progressive loss of phosphonate groups from NTMP, probably by means of physicochemically generated free radicals and/or pyrolysis in the hot bubble-bulk interface.     

Self-organized superlattice formation during crystal growth from continuous beam fluxes

Alloy superlattice structures consisting of alternating Si-rich and C-rich layers form spontaneously during gas-source molecular beam epitaxy of Si(1-y)C(y) on Si(001) from constant Si2H6 and CH3SiH3 precursor fluxes at T(s)=725-750 degrees C. The self-organized patterning is due to a complex interaction among competing surface reactions. During growth of the initial Si-rich layer, strain-driven C segregation to the subsurface results in charge transfer from surface Si atom dangling bonds to C backbonds. This decreases the Si2H6 sticking probability, and, hence, the instantaneous deposition rate, thereby enhancing C segregation. The Si-rich layer continues until a critical C coverage is reached allowing nucleation of a C-rich layer which grows until the excess subsurface C is depleted. The process then repeats with periods tunable through the choice of T(s) and y(avg).     

A low-dimensional crystal growth model on an isotropic and quasi-free sustained substrate

A new crystal growth theoretical model is established for the low-dimensional nanocrystals on an isotropic and quasifree sustained substrate. The driven mechanism of the model is based on the competitive growth among the preferential growth directions of the crystals possessing anisotropic crystal structures, such as the hexagonal close-packed and wurtzite structures. The calculation results are in good agreement with the experimental findings in the growth process of the lowdimensional Zn nanocrystals on silicone oil surfaces. Our model shows a growth mechanism of various low-dimensional crystals on/in the isotropic substrates.     

Diffusionless crystal growth in a eutectic system during rapid solidification

Experiments on nonequilibrium rapid eutectic growth are surveyed. The applicability limits of the modern theoretical models describing rapid solidification of binary systems are assessed. A problem of rapid eutectic growth when the local equilibrium is violated in the solute diffusion field (in the bulk liquid and at the solid-liquid interface) is formulated. An analytical solution to the problem of rapid lamellar eutectic growth under local nonequilibrium conditions in the solute diffusion field is found. It is shown that the diffusion-limited growth of a eutectic pattern ceases as soon as a chemically homogeneous crystalline phase begins to grow when the critical point V=V _D is achieved (V is the solid-liquid interface velocity and V _D is the solute diffusion speed in the bulk liquid). At V ≥ V _D, eutectic decomposition is suppressed and the nascent homogeneous crystalline phase has the initial (nominal) chemical composition of the binary system.     

In situ observation of antisite defect formation during crystal growth

In?situ x-ray diffraction (XRD) coupled with molecular dynamics (MD) simulations have been used to quantify antisite defect trapping during crystallization. Rietveld refinement of the XRD data revealed a marked lattice distortion which involves an a axis expansion and a c axis contraction of the stable C11b phase. The observed lattice response is proportional in magnitude to the growth rate, suggesting that the behavior is associated with the kinetic trapping of lattice defects. MD simulations demonstrate that this lattice response is due to incorporation of 1% to 2% antisite defects during growth.     

Influence of an adsorbing gas on the layering transitions in crystal growth

The influence of an adsorbing gas on the layering transition of a film is studied. In agreement with an earlier publication, we find that the adsorbate lowers the critical temperature T_c(n) for the nth layer. We also find that a system which is dry near T = 0 may undergo an apparent wetting transition due to the adsorbate. Indirect evidence is given that the sequence T_c(n) ends at T_R, the roughening temperature, as conjectured by de Oliviera and Griffiths. Re-entrant behavior in the layering transitions is observed for sufficiently strong adsorption energy; similar behavior also appears in the roughening transition, for which there exists some experimental evidence.     

Graphene growth on h-BN by molecular beam epitaxy

The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.     

The interaction of two excitons in a benzene crystal

The paper deals with the investigation of pair interactions in a system of weakly interacting excitons in a benzene crystal as a function of the distance of molecules, while interactions due to phonons are neglected. It is demonstrated that this interaction has a repulsive character of the type [A/(R +a)]e ^–k (R+a), which determines the possibility of realizing the superfluid state of the exciton system provided that the density of excitons is sufficiently great.The author wishes to express her thanks to Prof. M. Trlifaj for suggesting the present theme and for useful discussions. Her thanks are also due to Miss A. Piútová for her help with the computations.     

Adsorption of nitrogen on single crystal faces of iridium,studied by field emission microscopy

The adsorption of nitrogen on iridium was studied with a field emission microscope equipped with a probe-hole assembly to enable emission experiments on individual emitter regions. The adsorption of nitrogen is markedly face-specific. Temperature programmed desorption reveals three binding states: γ₁ on the (100) face with a maximum heat of adsorption of 7–8 kcal/mole, γ₂ on the regions around (110) with a maximum heat of adsorption of 10–11 kcal/mole and γ₃ on the roughest tip regions (210), (320), (531) and (731) with a maximum heat of adsorption of 13–14 kcal/mole. Nitrogen adsorbed in the γ₁ and γ₃ states causes a decrease, but in the γ₂ state a small increase, in the work function. These results are discussed in relation with data on nickel, palladium, platinum and rhodium. While nitrogen is only weakly adsorbed on all these metals there is a marked difference in the nature of the pertinent adsorption complex.     

On the impact of stress on intrinsic defect formation during single crystal silicon growth

The incorporation of intrinsic point defects during silicon crystal growth from the melt is discussed using most recent data for intrinsic point defect thermal equilibrium concentration and diffusivity. It is shown that by taking into account the impact of stress on the thermal equilibrium concentration of vacancies and self-interstitials, the intrinsic point defect properties derived from self- and metal-diffusion experiments can be used to model point defect incorporation and clustering during silicon crystal growth from the melt. The solution is however not unique and a large uncertainty still remains on the real thermodynamic parameters of both intrinsic point defects. Only comparison with a wide range of experimental data from crystal pulling experiments—taking into account also the stress effects—will allow to further narrow down the uncertainty ranges on the intrinsic point defect properties in silicon.     

籽晶尺寸对宝石级金刚石单晶生长的影响

本文在国产六面顶压机上,在5.6 GPa, 1250—1450℃的高压高温条件下,分别选用边长0.8, 1.5和2.2 mm三种尺寸的籽晶,系统开展了Ib型宝石级金刚石单晶的生长研究.文中系统考察了籽晶尺寸对宝石级金刚石单晶生长的影响.首先,考察了籽晶尺寸变化对宝石级金刚石单晶裂晶问题带来的影响.研究得到了籽晶尺寸变大,裂晶出现概率增加的晶体生长规律.其次,在25 h的生长时间内,考察了上述三种尺寸籽晶生长金刚石单晶时,生长时间与单晶极限生长速度的关系.得到了选用大尺寸籽晶,可以提高优质单晶合成效率、降低合成成本的研究结论.借助扫描电子显微镜和光学显微镜,对三种尺寸籽晶生长金刚石单晶的表面形貌进行了标定.最后,傅里叶微区红外测试,对三种尺寸籽晶生长宝石级金刚石单晶的N杂质含量进行了表征.研究得到了选用大尺寸籽晶实现快速生长金刚石的同时,晶体的N杂质含量会随之升高的晶体生长规律.