Distribution of aluminum and indium impurities in crystals of Ge-Si solid solutions grown from the melt

The problem regarding the distribution of aluminum and indium impurities in bulk crystals of solid solutions with a variable composition Ge_1−x Si _x (0 ≤ x ≤ 0.3) is solved in order to establish regularities of the changes in the segregation coefficients of impurities with variations in the composition of the host lattice in the germanium-silicon system. Aluminum-and indium-doped crystals of Ge_1−x Si _x (0 ≤ x ≤ 0.3) solid solutions with a silicon content decreasing along the crystallization axis are grown by a modified Bridgman method with the use of a silicon seed. The concentration distribution of impurities over the length of the crystals is determined from Hall measurements. It is demonstrated that the experimental data on the concentration distribution of impurities in the crystals are in good agreement with the results obtained from the theory according to which the equilibrium segregation coefficients of impurities vary linearly with a change in the composition of Ge-Si solid solution crystals.

     

The distribution of Ga and Sb impurities in Ge-Si crystals grown by the Bridgman method using a feeding rod

Gallium- and antimony-doped Ge_{1 ? x} Si _x crystals (0 ≤ x ≤ 0.25) have been grown by the improved Bridgman method using a silicon seed and a macrohomogeneous feeding Ge-Si rod of the corresponding composition. The impurity concentration profiles along the grown crystals were determined from Hall measurements. The mathematical problem of impurity distribution along two-component mixed crystals grown from a melt with uniform and graded composition is solved in the Pfann approximation and within the virtual-crystal model for the solid solution. It is shown that the experimental impurity distributions in Ge_{1 ? x} Si _x crystals are described well by the data calculated on the assumption of linear change in the impurity segregation coefficient with the crystal composition.     

Electroactive complex in thermally treated Ge-Si 〈Cu,Al〉 crystals

It is shown by Hall measurements that quenching complexly doped Ge_{1 − x} Si _x 〈Cu, Al〉 (0 ≤ x ≤ 0.20) crystals from 1050–1080 K leads to the formation of additional electroactive acceptor centers in them. The activation energy of these centers increases linearly with an increase in the silicon content in the crystal and is described by the relation E _k ^x = (52 + 320x) meV. Annealing these crystals at 550–570 K removes the additional acceptor levels. It is established that the most likely model for the additional electroactive centers is a pair composed of substituent copper and aluminum atoms (Cu _s Al _s ) or interstitial copper and substituent aluminum atoms (Cu _i Al _s ). It is shown that the generation of additional deep acceptor levels must be taken into account when using the method of precise doping of Ge_{1 − x} Si _x 〈Al〉 crystals with copper.     

Effect of micron and nano-grade titanium dioxides on the efficiency of hindered piperidine stabilizers in a model oxidative reaction

Polymer additives often show many side reactions during the ageing of polymers which change the useful life-time of the materials. In this regard nano-grade titanium dioxide additives with high surface areas and nano-particle sizes appeared in recent years to be used in polymers where the side reactions have been found to markedly influence mechanisms of ageing. The interaction between titanium dioxide pigments and stabilizers therefore is proposed as a field of great importance.In this paper the influence of nano and micron particle grade anatase and rutile titanium dioxide pigments on the efficiency of a hindered amine stabilizer, Chimassorb 119FL and Chimassorb 119 has been investigated in the system comprising the model radical reaction of cumene initiated (2,2′-azobisisobutyronitrile, AIBN) oxidation. This model reaction was designed to simulate the thermo-oxidative processes in polymers.Kinetic measurements of oxidation rates in the absence of the pigments showed strong retarding activity of the stabilizers under conditions of the model experiments. The rates depend linearly on the reciprocal square root of the concentration of the stabilizers over a sufficiently wide range thereby fitting the mechanism of addition of cumylalkyl R radicals to the Chimassorb molecules. The rate constants for the addition of cumyl R radicals to Chimassorb 119FL and Chimassorb 119 were determined to be k_(333 K) = (1.4 ± 0.2) × 10⁸ and (1.2 ± 0.2) × 10⁸ M⁻¹ s⁻¹, respectively. Hence the Chimassorb 119 family is relatively powerful retarders of thermal oxidation.Further measurements of oxidation rates in the presence of titanium dioxide particles showed that there is a significant reduction in the retarding action of the stabilizers in the presence of titanium dioxides. It occurs as a result of the initiating action of titanium additives producing an additional concentration of free radicals and inducing an additional oxidation rate which may weaken the initial inhibiting efficiency of the added hindered piperidine stabilizers. The titanium nano-samples added at a 1-wt.% into the oxidized condensed system are able to completely decrease the initial inhibiting efficiency of the stabilizers. In terms of the degree of the sensitising action the titanium dioxides can be ordered as: nano-rutile > nano-anatase treated hydroxyapatite > nano-anatase untreated > micro-anatase > micro-rutile.The behaviour of titanium dioxide particles incorporated with hindered piperidine HAS stabilizers in condensed systems may thus be used for the assessment of their performance during the thermo-oxidative degradation of polymers.     

Modelling light stabilizers as thermal antioxidants

Light stabilizers often display some degree of antioxidant activity against thermal degradation of polymers both in the solid state and the melt. Although this capacity to date has been documented in some instances such features have not been kinetically modelled for many light stabilizers. An understanding of the mechanisms of this activity is crucial in polymer materials due to the close link between prior thermal behaviour and post stabilisation.This paper considers the potential antioxidant activity of three representative UV stabilizers using a model system initiated (2,2′-azo-bisisobutyronitrile, AIBN) cumene oxidation. Kinetic measurements of oxidation rates in the presence of the stabilizers showed that the antioxidant activity as well as the mechanism and mode of inhibition was different for each of the stabilizers. Thus, whilst a triazine UV absorber (Cyasorb UV 1164) did not display any antioxidant activity, a hindered phenol (Cyasorb UV 2908) operated as a peroxy radical acceptor, and a hindered amine (Cyasorb 3529) retarded the model reaction without an induction period like many HAS stabilizers.The Cyasorb 2908 revealed weak antioxidant activity with a rate constant for the addition of cumylperoxy RO₂ radicals to the functional group of the stabilizer k₇ = 10^6.2±0.1 e^{−(3900±600)/RT}, however, the inhibition index f (80 °C) is significantly higher than that of the commercial phenolic antioxidant Irganox 1076. Oxidation rate profiles in the presence of Cyasorb 3529 displayed a strong retarding activity by the stabilizer under conditions of the model experiments. The rates were found to depend linearly on the reciprocal square root of the concentration of the stabilizer over a sufficiently wide range thereby fitting the mechanism for the addition of cumylalkyl R radicals to the Cyasorb molecules. The rate constants for the addition of cumyl R radicals to the Cyasorb were determined to be k_(333-353 K) = (2.0 ± 0.8) × 10⁸ M⁻¹ s⁻¹. This value surpasses even the rate constants for other related HAS Chimassorb [Zeynalov EB, Allen NS. Effect of micron and nano-grade titanium dioxides on the efficiency of hindered piperidine stabilizers in a model oxidative reaction. Polym Degrad Stab 2006;91(4):931-9.] stabilizers and it follows that Cyasorb 3529 is a powerful retarder of thermal oxidation.     

Twin position sensitive ionization chamber,utilizing backgammon and time projection method

The anomalous dependence of average prompt fission neutron (PFN) multiplicity on the fission fragment total kinetic energy of ²⁵²Cf(sf) was investigated using digital signal processing (DSP). A twin Frisch-grid ionization chamber (TGIC) was used for fission fragment (FF) spectroscopy and PFN was registered with help of fast neutron detector (FND) on liquid scintillator base. The DSP apparatus was build up around the 8-channel fast synchronously sampling waveform digitizers (WFD). It was found that the main limitation of the experimental method, developed in ref. [1] for investigation of the PFN properties in neutron induced fission reaction ²³⁵U(n,f), ²³⁹Pu(n,f) etc is related to the FF energy loses of in the target backing. A modification of the method was suggested to overcome such a limitation.     

Synthesis of a conjugate of 3´-sialyllactoside with recombinant flagellin as a carrier protein and assessment of its immunological activity in comparison with that of a similar hemocyanin-based conjugate

In the framework of a project directed to the design of synthetic cancer vaccines based on carbohydrate chains of tumor-associated gangliosides, a new prototype cancer vaccine, a polyvalent carbohydrate-protein conjugate 1 with ～11 3´-sialyllactoside ligands conjugated to one molecule of recombinant flagellin was synthesized. Dendritic cell vaccines were designed using the conjugate 1 and a neoglycoconjugate 2 consisting of ～400 3´-sialyllactose residues linked to hemocyanin from a snail Megathura crenulata (KLH). Comparative ELISA study was conducted to assess the induction of carbohydrate-specific immunoglobulins IgM and IgG after immunization of mice with conjugates 1 and 2 and dendritic cells loaded with 1 and 2. Synthetic polyacrylamide carrier conjugated to 3´-sialyllactose ligands and B16F0 melanoma cells were used as coating antigens.     

Growth of single crystals of semiconductor solid solutions by double feeding of the melt method

Crystallography Reports - The problem of component distribution in solid solution crystals grown from a melt fed by rods made of the components of the system, with allowance for the dependence of...     

Radical scavenging efficiency of different fullerenes C60-C70 and fullerene soot

This investigation was undertaken to determine the antioxidant activity of a range of fullerenes C₆₀ and C₇₀ in order to rank them according to their comparative efficiency. The model reaction of initiated (2,2′- azobisisobutyronitrile, AIBN) cumene oxidation was used to determine rate constants for addition of radicals to fullerenes. Measurements of oxidation rates in the presence of different fullerenes showed that the antioxidant activity as well as the mechanism and mode of inhibition were different for fullerenes C₆₀ and C₇₀ and fullerene soot. All fullerenes - C₆₀ of gold grade, C₆₀/C₇₀ (93/7, mix 1), C₆₀/C₇₀ (80 ± 5/20 ± 5, mix 2) and C₇₀ operated as alkyl radical acceptora, whereas fullerene soot surprisingly retarded the model reaction by a dual mode similar to that for the fullerenes and with an induction period like many of the sterically hindered phenolic and amine antioxidants. For the C₆₀ and C₇₀ the oxidation rates were found to depend linearly on the reciprocal square root of the concentration over a sufficiently wide range thereby fitting the mechanism for the addition of cumylalkyl radicals to the fullerene core. This is consistent with literature data on the more ready and rapid addition of alkyl and alkoxy radicals to the fullerenes compared with peroxy radicals. Rate constants for the addition of cumyl radicals to the fullerenes were determined to be k_(333K) = (1.9 ± 0.2) × 10⁸ (C₆₀); (2.3 ± 0.2) × 10⁸ (C₆₀/C₇₀, mix 1); (2.7 ± 0.2) × 10⁸ (C₆₀/C₇₀, mix 2); (3.0 ± 0.3) × 10⁸ (C₇₀), M⁻¹ s⁻¹. The increasing C₇₀ constituent in the fullerenes leads to a corresponding increase in the rate constant.The fullerene soot inhibits the model reaction according to the mechanism of trapping of peroxy radicals; the oxidation proceeds with a pronounced induction period and kinetic curves are linear in semi-logarithmic coordinates.For the first time the effective concentration of inhibiting centres and inhibition rate constants for the fullerene soot have been determined to be fn[C₆₀−soot] = (2.0 ± 0.1) × 10⁻⁴ mol g⁻¹ and k_inh = (6.5 ± 1.5) × 10³ M⁻¹ s⁻¹ respectively.The kinetic data obtained specify the level of antioxidant activity for the commercial fullerenes and scope for their rational use in different composites. The results may be helpful for designing an optimal profile of composites containing fullerenes.     

Growth dynamics of Ge1?xSix single crystals obtained by directional constitutional supercooling of the melt

The problem of growth dynamics for crystals of binary solid solutions, obtained by the constitutional supercooling of the melt with a silicon feeding rod, has been solved within the Pfann approximation. The dependences of the change in the axial growth rate of Ge_1−x Si _x crystals (0 ≤ x ≤ 0.3) are calculated. It is shown that the Ge_1−x Si _x crystallization rate significantly changes during growth. The results make it possible to determine the optimal conditions and technological parameters for growing Ge_1−x Si _x single crystals (0 ≤ x ≤ 0.3) with a specified concentration gradient along the crystallization axis.