Low Sulfur Pressures Research Articles

Microstructure Evolution and Crystal Growth in Cu2ZnSnS4 Thin Films Formed By Annealing Colloidal Nanocrystal Coatings

A potentially low-cost approach for making copper zinc tin sulfide (CZTS) solar cells relies on annealing colloidal nanocrystal films cast on suitable substrates in a sulfur-containing environment to form thin films with micrometer-sized grains. The film microstructure and grain size affects the solar cell performance. We conducted a systematic study of the factors controlling crystal growth and microstructure development during annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 μm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The ev...

Cobalt Base Clad Layer Resistance on the Corrosion under Low Sulfur Pressure

The paper considers the sulphidation of the clad layers made of the cobalt base powder and prepared by the multi-overlapped tracks and sublayers fabricated using laser cladding process applied at 800°C under sulphur partial pressure ps2= 10e-8 atm and ps2= 10e-10 atm for 24 hours. The phase compositions of the scale were different. In both cases changes in the layer under the scale exhibited similar tendency – decomposition of the carbides from the interdendritic regions and severe internal corrosion along the dendrite boundaries.

Compact Sulfur Lamps Operated by Antenna-Excited Microwave Discharge

An antenna-excited microwave discharge (AEMD) lamp has shown attractive properties compared with conventional AC discharge or electrodeless microwave discharge lamps. This paper concerns a compact AEMD sulfur lamp with such a unique structure that a pair of antennas are covered with thin quartz glass. It is found that a stable plasma is generated between the antennas regardless of whether the antennas are coated by dielectrics. The microwave power supplied into the AEMD sulfur lamp is approximately 50 W, which is much lower than that of conventional sulfur lamps that require higher microwave power to start and maintain the discharge. Observed spectral features showed that the radiant power distribution was in the UV range at low sulfur pressure and shifted to the visible light range with increasing sulfur pressure. Finally, the possibility of obtaining a compact AEMD sulfur lamp is discussed.

Microstructural study of Fe-Cr-Al/Al composite coatings during oxidation and sulfidation at 900 °C

The microstructure of a composite coating system, which was composed of an inner layer of Fe-Cr-Al and an outer layer of aluminum, was studied after it was respectively oxidized and sulfurdized at elevated temperatures. Apart from the Al 2 O 3 scale formed on the surface, the microstructure of the composite coatings exposed at 900°C in air for 4h was a three-layer structure. The first layer consisted of a solid solution of Cr and Fe in α aluminum and an intermetallic compound FeAl 3 , while the second layer was a single phase of the aluminide and the third layer still remained the same appearance as the original Fe-Cr-Al coating. The microstructural observation of the specimen tested at 850 −900°C at low oxygen pressure and high sulfur pressure for 576h revealed that the surface coatings of the specimen had transformed into a duplex structure containing an outer layer and a thicker aluminide layer beneath. X-ray diffraction results showed that the out layer was composed of Al 2 S 3 and Al 2 O 3 and that AlCrFe 2 was the main phase composition of the aluminide layer, with a few of Al 2 S 3 and Al 2 O 3 accompanied.

Effects of the sulfur pressure on pyrite FeS 2 films prepared by sulfurizing thermally iron films

The effect of sulfur vapor pressure in preparing the FeS 2 films has been discussed and some incongruous views about sulfur pressure have been clarified in this paper based on experimental results and theoretical analysis. It is shown that lower sulfur pressures than the saturation value only result in poorer crystallization and worse performances, and in other words the FeS 2 films could be optimized through improving the sulfur pressure till the saturation point. However for a certain temperature the sulfur pressure is limited by its saturated vapor pressure, and further increase of the sulfur quantity reacted with Fe films has little influence on the structure and properties of the pyrite films.

Simultaneous Synthesis of Iron Carbide and Rutile from Ilmenite Ore by Thermochemical Reactions

Simultaneous synthesis of iron carbides (mainly Fe 3 C) and rutile (TiO 2 ) from natural ilmenite ore (mainly FeTiO 3 ) were examined by thermochemical reactions with H 2 -CO or H 2 -CH 4 gas mixtures at temperatures of 1 073-1 273 K. The low sulfur pressures incapable of forming FeS was added to these gas mixtures to make stable iron carbides without free carbon. Iron carbide is currently interesting as alternative pure iron sources in steelmaking. The rutile is also available as a feedstock for titanium industry. First, partial conversion from ilmenite to rutile and metallic iron mostly proceeded and subsequently iron carbidization occurred together with further reduction of rutile. Finally, these processes with both gas mixtures provided iron carbide (Fe 3 C) and lower titanium oxides such as Magneli phases [Ti n O 2n-1 , 4≤n≤9 namely TiO 2-x ) and/or Ti 3 O 5 rather than rutile. Reaction time, H 2 /CH 4 ratio, sulfur potential, and temperature dependence of reaction behavior was examined further. The iron carbidization rates were analysed using the first order rate model. Ilmenite samples prepared from reagents showed slightly slower conversion rates to titanium oxides at 1 073 K than the natural ore ones, though both samples showed similar iron carbidization rates.

Oxidation and sulfidation of Ni 3Al

Oxidation and sulfidation of Ni 3Al has been studied in air and in 1% SO 2/air gas mixture atmosphere at 605, 800 and 1000 °C. The experimental results show that Ni 3Al appears to have a severe corrosion phenomena resulting from the formation of NiO for both oxidation environments. For the oxidation–sulfidation reaction mechanism of Ni 3Al, it was found that the substitution of sulfur on oxygen sites in NiO tends to decrease the electron hole concentration in the valence band. With a constant of oxygen partial, a decrease in concentration of electron hole will result in an increase in Ni cation vacancies and hence there will be a greater flux Ni to the gas-oxide boundary; the rate of self-diffusion of the Ni 2+ ions, which determines the rate of oxidation, would increase. And this would increase the rate of oxidation with low sulfur partial pressure in the atmosphere in this research work. When the specimen surface is exposed to the oxygen environment, a thin spinel NiAl 2O 4 layer is built up immediately; the spinel formation rate is initially faster than that of spinel decomposition, which is a constant value. Gradually, when a thick oxide scale is reached, a balance between the rate of decomposition and formation will be met. As the temperature increased the spinel formation rate was higher than the spinel decomposition rate. Therefore, at 1000 °C, it is possible to detect the existence of spinel. On the other hand, at 800 °C the spinel formed will be decomposed into NiO and Al 2O 3 (needle-like), and they are the only oxides found on the specimen surface.

Sulfidation Mechanism of Ni–20Cr and Ni–13.5Co–20Cr Alloys at 873 K under Low Sulfur Pressures in H2S–H2 Atmospheres

The sulfidation mechanism of Ni–20Cr and Ni–13.5Co–20Cralloys was investigated at 873 K under low sulfur pressures (10−4.0,10−5.5, and 10−7.0 Pa) inH2–H2S gas mixtures by marker measurements ofdiffusion in the scales, the composition profiles in the scale and alloy,and a determination of the structure and composition of the equilibratedsulfide phases. A Pt marker experiment on the Ni–20Cr alloy indicatedchanges in the marker morphology, with marker fragments in the inner Cr3S4layer and a swelled marker section in the outer Ni3S2layer where the composition ranged from pure Pt to a Pt-(76–80) at.%Nialloy. The changes in morphology and composition of the Pt markers wereexplained by the phase constitution of the Pt–Ni–S andPt–Cr–S systems. The growth of the multilayer sulfide structureson both alloys at sulfur pressures above 10−5.5 Pa wascontrolled by the outward diffusion of cations through the inner Cr3S4layer. While, the inward sulfur and outward-chromium diffusion in the alloysubstrate played an important role in the internal sulfidation and in theexternal scale growth at 10−7.0 Pa. The concentration profiles of eachelement were measured by EPMA and the corresponding diffusion paths areplotted in a phase diagram of the Ni–Cr–S system at 873 K.

Sulfidation Properties of Ni–20Cr and Ni–13.5Co–20Cr Alloys at 873 K under Low Sulfur Pressures in H2S-H2 Atmospheres

The sulfidation properties of Ni–20Cr and Ni–13.5Co–20Cralloys as well as a nickel-base superalloy AISI685 were investigated at873 K in H2S–H2 gas mixtures with sulfur partial pressures of10−4, 10−5.5, and 10−7Pa by mass-gain measurements,electron-probe microanalysis (EPMA), and X-ray diffraction (XRD)analysis. Sulfidation obeyed the parabolic rate law, and the parabolic rateconstants decreased in the order Ni–20Cr, Ni–13.5Co–20Cr,and AISI685 at each sulfur pressure. With decreasing sulfur pressure, therate constants first decreased slowly and then rapidly at a 10−7 Pasulfur pressure. At both 10−4 and 10−5.5 Pa sulfur pressures,Ni–20Cr formed a surface scale with a duplex structure of inner(Cr3S4) and outer (Ni3S2) layers, while Ni–13.5Co–20Cr formeda triplex structure of inner (Cr3S4), intermediate(Ni,Co)Cr2S4, and outer[Ni3S2+(Co,Ni)9S8] layers. Thesurface scale formed on AISI685 was verycomplex, comprising at least four layers, a fibrous (Co,Ni)9S8 top surface,outer [Ni3S2+(Co,Ni)9S8], and intermediate [(Cr,Ti)3S4] layers, as well asan inner layer containing Cr, Ti, Mo, Al, and S. At the 10−7 Pa sulfurpressure, which is lower than the dissociation pressure of Ni3S2, bothNi–20Cr and Ni–13.5Co–20Cr formed a surface scale ofCr3S4 covered by a thin NiCr2S4 layer, accompanied by copious internalsulfidation of Cr3S4 and/or CrS. On AISI685 there was a surface scale of(Cr,Ti)3S4 accompanied by the usual internal sulfidation. It is discussedthat diffusion of cations in the inner Cr3S4 layer is the rate-determiningstep for the growth of the multilayer structures. At the 10−7 Pasulfur pressure, diffusion of Cr and S contribute to form a thin surfacescale and internal sulfidation, respectively.

Production of Iron Carbide from Iron Ores in a Fluidized Bed.

Utilizing an ordinary bubbling fluidized bed, a batch of hematite ores (0.15∼0.21 cm diameter) was reacted at 923∼1 123 K with H2–CO mixtures to synthesize iron carbide. Sulfur of low pressures unable to form FeS was added to the mixtures. For almost all the conditions, roughly one hour treatment yielded conversion of some types of ores fully to iron carbides as mostly Fe3C, partly accompanying FexC (x ≈2.5), dependently of the reaction conditions. It was proved that small amounts of gaseous sulfur led to carbides rather than free carbon or metallic iron as final products. The conditions of high temperature and low sulfur pressure provided high quality iron carbides having nearly 0.03 mass% S as low as conventional reduced irons. Their conversion yields seemed to be almost insensitive to the H2/CO mole ratio and ore type. From rate analysis for carburization process obtained in a same reactor, it was suggested that this process has several times higher reaction rates than those measured in the conventional Iron Carbide Process gas.

Self-diffusion of manganese in nonstoichiometric manganous sulphide

Self-diffusion of manganese in nonstoichiometric manganous sulphide, ${\mathrm{Mn}}_{1\ifmmode\pm\else\textpm\fi{}y}\mathrm{S},$ has been studied as a function of temperature (1073--1373) K and sulfur activity ${(10}^{\ensuremath{-}11}--{10}^{3})\mathrm{Pa}$ using radioactive ${}^{54}\mathrm{Mn}$ isotope as a tracer. In agreement with the defect model of ${\mathrm{Mn}}_{1\ifmmode\pm\else\textpm\fi{}\mathrm{y}}\mathrm{S}$ it has been shown that at very low sulfur pressures, close to the Mn/MnS phase boundary, self-diffusion of cations in metal excess ${\mathrm{Mn}}_{1+y}\mathrm{S}$ proceeds by an interstitial mechanism, consisting of pushing a cation from its normal lattice site into neighboring interstitial position. At higher sulfur activities, on the other hand, cation self-diffusion in metal-deficit ${\mathrm{Me}}_{1\ensuremath{-}y}\mathrm{S}$ proceeds by a simple vacancy mechanism, consisting of ${\mathrm{Mn}}^{2+}$ ions jumping from the lattice sites into neighboring vacancies. It has been demonstrated that not only the temperature and pressure dependence of cation self-diffusion in metal-excess and metal-deficit manganous sulphide, but also absolute values of self-diffusion coefficients of this process are in excellent agreement with those calculated from manganese sulfidation and evaporation kinetics. This agreement clearly indicates that both these methods can successfully be utilized in studying the transport properties of transition-metal sulfides and oxides.

Synthesis of Iron Carbide by Reaction of Iron Ores with H2-CO Gas Mixtures Bearing Traces of Sulfur.

Particles of hematite ores in a porcelain boat were reduced and carburized at 823∼1223K by H2-CO mixtures under low sulfur pressures where metallic iron remained stable.Within major of these various conditions nearly one hour reaction could convert their particles completely to iron carbides such as mostly Fe3C. It was found that traces of gaseous sulfur makes iron carbides enough stable rather than free carbon or metallic iron. Their conversion yields were insensitive to inlet H2/CO mole ratio and ore type. Total sulfur contents in products obtained for lower sulfur pressures were as low as conventional reduced irons.

The sulfidation of manganese at low sulfur pressures at 700?950�C

The kinetics of manganese sulfidation has been studied in H2-H2S gas mixtures as a function of temperature (973–1223 K) and sulfur pressure (7×10−9to 3×10−4Pa), using a thermogravimetric technique. The sulfidation of manganese at low pressures follows the parabolic rate law similar to the behavior at high sulfur pressures (10−4–105Pa), although an initial nonparabolic incubation period, longer at lower sulfur pressures, was observed. The sulfidation rate constant increased with sulfur pressure and temperature according to the following empirical equation: kP=const P(S2)1/nexp(−E/RT)However, in disagreement with the results at high sulfur pressures, the exponent 1/n and the activation energy changed considerably with temperature and sulfur pressure. The results are analyzed in terms of a point-defect model of the single corrosion product—MnS—and of the possibility of a doping effect of MnS by hydrogen.

The effect of preoxidation of some Ni, Fe, and Co-Base alloys on subsequent sulfidation at 982�C in sulfur vapor

The effect of preoxidation was studied on the subsequent sulfidation in sulfur vapor at a pressure of 0.1 atm at 982°C on numerous iron, nickel, and cobalt-base alloys which were either chromia or alumina formers. In general, alumina films were much more protective than chromia films, but the efficacy of preoxidation in reducing sulfidation rates depended more upon perfection of the films and whether cracking and/or spatting occurred. Increasing oxidefilm thickness had a beneficial effect until either penetration of the films by sulfur or cracking occurred, after which sulfidation rates were sometimes greater than for nonpreoxidized samples. The enhanced sulfidation rates are attributed to sulfidation of a solute-depleted substrate, the solute having been selectively removed by oxide formation. One alloy, MA 956, containing 0.5 Y2O3 as fine dispersions which normally provide spatting resistance, still exhibited extensive cracking and spalling of the oxide and was not much better than alloys without dispersoids or reactive-metal additions. The use of preoxidation to reduce sulfidation rates is not viable under the extreme conditions used. Preoxidation is conceptually a good method for inhibiting sulfidation at lower temperatures and much lower sulfur pressures.

A method for long-term sulfidation of metal at low sulfur pressures and its application to sulfidation of an Fe-20 at.% Al alloy at 1023 K

A method of long-term Sulfidation that involves generating sulfur vapor from dissociation of Ag2S or from transformation of NiS to Ni3S2+x is described. Sulfidation tests of iron samples showed the approach to be essentially valid. At 1023 K, Ag2S dissociation of Ag2S gives rise to\(p_{s_2 } = 7.9 \times 10^{ - 6} \) atm and, in the presence of an inert carrier gas, efficient transport of sulfur to the metal was effected. Liberated silver grew in the form of discrete whiskers, hence did not interfere with continued dissociation of Ag2S. Equilibration of NiS with Ni3S2+x occurs at\(p_{s_2 } = 1.4 \times 10^{ - 4} \) atm at 1023 K but, with continued loss of sulfur to the metal, the nonstoichiometric Ni3S2±x predominates and the ambient\(p_{s_2 } \) is no longer constant. Thus for long-term studies, Sulfidation with NiS/Ni3S2+x mixtures is suitable only for metals that sulfidize slowly or at rates independent of the ambient\(p_{s_2 } \). Using mainly the NiS-Ni3S2+x mixture as sulfur source, sulfidation tests up to 72 h at 1023 K were conducted on an Fe-20 at.% Al alloy. Sulfidation was apparently insensitive to the ambient\(p_{s_2 } \)and formation of a three-layer FeS/(Fe, Al)3S4/Al2S3 scale with internal precipitation zone proceeded at similar rates, and a relationship involving coupled diffusion is proposed. At longer exposure times, transformation of FeS to (Fe, Al)3S4 became extensive. Mechanical failure of the scale, followed by rapid healing beneath the original precipitation, also occured after extended reaction periods (>50 hr).

Effect of 1 wt.% yttrium addition on high-temperature sulfidation behavior of Fe-15Cr-4Al alloy

High-temperature sulfidation studies have been carried out on Fe-15Cr-4Al with and without 1%Y in the temperature range 700-1000/sup 0/C in an H/sub 2/-H/sub 2/S environment over the sulfur pressure range of 10/sup -9/-10/sup -3/ atm. Two-layered and three-layered sulfide scales were observed in both alloys at low and high sulfur pressures, respectively. The pegging phenomenon, similar to that occurring in high-temperature oxidation, across the innermost layer and substrate was observed in the case of the yttrium-containing alloy. Yttrium was found to be associated with aluminum and chromium sulfides. The role of yttrium was more evident at low than at high sulfur pressures and was found to reduce the parabolic rate constants by a factor of about one-half to one-seventh, respectively.

Sulfidation properties of Cr23C6 at 1073 K in H2S-H2 atmospheres of 103.5?10?6 Pa sulfur pressure

The sulfidation behavior of chromium carbide, Cr23C6, was investigated in H2S-H2 gas mixtures over a sulfur partial pressure range of 103.5−10−6 Pa at 1073 K using thermogravimetry, optical and scanning electron microscopy, X-ray diffraction analysis, and electron-probe microanalysis. The kinetics were rapid for short time periods and followed a linear rate law at low sulfur pressures, whereas sulfidation tends to obey a parabolic rate law at high pressures. Sulfidation rates decreased with increasing carbon content in the carbide. Surface morphologies could be divided into three groups: (I) at high sulfur pressures, petal-like. crystals (Cr2S3); (II) at intermediate pressures, a twinlike structure (Cr3S4); (III) and at low pressures, a flat surface with numerous hexagonal pits (Cr1−xS). The scale consisted of two distinct layers: an external scale with a single or multilayer structure and an inner scale with a mixture of Cr1−xS, Cr3C2, and Cr7C3. These higher carbides, Cr3C2 and Cr7C3, may be formed by the sulfidation-carburization of Cr23C6. Pt-marker experiments indicated that the external scale grew by chromium diffusion and that sulfur migration played an important role in the growth of the inner scale.

Sulfidation kinetics and scale structures of chromium at temperatures of 973?1173 K and 104?10?6 Pa sulfur pressures

The sulfidation behavior of chromium was investigated over a temperature range of 973–1173 K in H2S-H2 gas mixtures of 104–10−6 Pa sulfur partial pressures using thermogravimetry, X-ray diffractometry, optical and scanning electron microscopy, and electron-probe microanalysis. Sulfidation kinetics are rapid for short periods and obey a linear rate law at low sulfur pressures, whereas at high sulfur pressures sulfidation tends to be parabolic. The surface morphologies can be divided into four types: at high sulfur pressures a petal-like crystal of Cr2S3(rho. and tri.) (type 1), at intermediate sulfur pressures a twinlike structure of Cr3S4 (type 2), at low sulfur pressures a flat surface with numerous hexagonal pits of Cr1−xS (type 3), and a fine twinlike structure of ordered Cr1−xS (type 4). At 973 K, the sulfur pressure ranges are type 1 at\(p_{S_2 } \) > 10−4, type 2 at\(p_{S_2 } = 10^{ - 5} \), and type 3 at\(p_{S_2 }< 10^{ - 6} Pa\). The critical sulfur pressure where type 2 was formed, 10−5 Pa at 973 K, shifts toward higherressures at higher temperatures and becomes 10−3 Pa at 1073 K and 10−1 Pa at 1173K. Type 4 is observed at 1173K and 10−6 Pa sulfur pressure. Thesulfide scale is composed of two distinct layers: an external layer, which is dense with a fine columnar structure, and an inner layer, which is porous with a layered structure of sulfides and voids. The external scale is composed offour layers at high sulfur pressures: at the scale-gas interface Cr2S3(rho.), next Cr2S3(tri.), third Cr3S4, and innermost Cr1−xS. With decreasing sulfur pressures,the number of layers in the external scale was reduced. Pt markers were positioned between the external and inner scales.

Transition from oxidation to sulfidation of Fe-Cr alloys in gases with low oxygen and high sulfur pressures

Fe-Cr alloys with 17–30% Cr show in H2-H2O-H2S mixtures at 1273 and 1073 K a transition from protective oxide scale formation to rapid sulfidation. The critical oxygen pressure to stabilize the oxide formation increases with increasing sulfur pressure of the gas and decreasing Cr content of the alloy. Cr2O3 with traces of Fe2O3 is formed under these conditions. Below the critical oxygen pressure, a primarily formed Cr2O3 film becomes overgrown by (Fe, Cr)S. The kinetic boundary of oxidation-sulfidation, which lies in the stability field (Fe, Cr)S + spinel Fe1+xCr2−xO4 of the Fe-Cr-O-S phase diagram, is explained with the help of the Fe-Cr-O-S phase diagram and the assumption that Fe diffuses faster through the (Cr, Fe)2O3 solid solution than does Cr.

Sulfidation properties of an Fe-23.4Cr-18.6Al alloy at temperatures 1073 and 1173 K in H2S-H2 atmospheres of sulfur at pressures 104-10?5 Pa

An investigation is reported on the sulfidation properties of an Fe-23.4Cr-18.6Al(at.%) alloy at 1073 and 1173 K in H2S-H2 atmospheres, 104 > PS2 ≥ 105Pa. The sulfidation kinetics exhibited an early transient period before onset of parabolic kinetics. Values of the parabolic sulfidation rate constants increased by as much as 105 from their smallest values at low sulfur pressures, PS2 ≦ 10−4 Pa, to maximum values at sulfur pressures PS2 ∼ 102 Pa. Multilayered scales were formed, the number and types of layers dependent on sulfur pressure. A fully developed scale at sulfur pressures PS2 > 10−3 Pa.