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1.
Aluminous hematites prepared in three different ways have been examined at 300K and 4.2K using the Mössbauer technique. The results indicate significant differences between the behaviour of aluminous hematites that have been subjected to high temperatures (>600° C) and those which have not. The magnitude of the room temperature quadrupole splitting of the former increases with aluminium content, approaching at ～16 mole percent substitution the value (?0.22 mm/s) exhibited by all of the low temperature specimens. This variation may be explained qualitatively in terms of a preferential c-axis contraction of the lattice upon incorporation of aluminium, which does not occur unless a c-axis defect structure is removed by subjection of the hematite to high temperatures. The solid solubility limits of high and low temperature hematites (～15 mol % and ≥19 mol % respectively) also differ, as do the room temperature decreases in hyperfine splitting (?0.82 kOe/mol % Al and ?0.86 kOe/mol % Al). At 4.2 K only low temperature hematite exhibits a decrease in hyperfine splitting with increasing Al content (?0.40 kOe/mol % Al). The absolute values of the recoil free fractions of hydrothermally prepared aluminous hematites have been determined at 4.2 K (0.70±0.02 — pure hematite, 0.82±0.02, 14±2 mol % Al substitution) and exhibit a similar increase with Al content to that previously observed for aluminous goethites. The room temperature recoil free fraction of pure hematite has been measured to be 0.64±0.02. The effects of particle size on both hyperfine splitting and recoil free fraction have been investigated.  相似文献   

2.
Three natural hematites, α-Fe2O3, from the region of Elba have been investigated by means of 57Fe Mössbauer spectroscopy at variable temperatures between 80 and 400K. The samples were selected on the basis of their different morphology as observed from powder X-ray diffraction and transmission electron microscopy. The mean crystallite diameters (MCD) along [104] are 1000, 280 and 40 nm respectively. Energy-dispersive analyses of X-rays revealed the presence of minor amounts of Si impurities in those two hematites having the largest MCD. All three hematites show the coexistence at low temperatures of antiferromagnetic-like (AF) and weakly-ferromagnetic-like (WF) spin states. The saturation values of the AF and WF magnetic hyperfine fields and quadrupole shifts have been determined, from which conclusions are drawn concerning the spin structure in relation to the crystallinity of the samples. The variations of the hyperfine parameters in the Morin-transition region indicate a gradual reorientation of both AF and WF spins towards the basal plane. As expected, the Morin transition itself is affected by the particle size. The two hematites exhibiting the largest particle dimensions still show an AF contribution for T>270K. It is suggested, and argued that this unusual behaviour is due to the presence of Si4+, and hence Fe2+, in the lattice. The characteristic Mössbauer temperatures and the intrinsic isomer shifts were evaluated from the temperature variation of the observed isomer shifts. Both parameters are not significantly affected by the morphology and are in excellent agreement with data obtained for synthetic hematites.  相似文献   

3.
Several goethites were obtained through the hydrolysis at 60 °C of Fe(III) solutions containing variable amounts of Mn(II) ions. The obtained samples were thermally treated at temperatures ranging from 180 to 310 °C until the complete phase transformation to hematite was achieved. The effect of Mn in the dehydroxylation process was investigated using X-ray diffraction (XRD) and the Rietveld refinement of XRD data together with scanning electron microscopy (SEM), differential thermogravimetric analysis (DTA) and Fourier transform infrared spectroscopy (FTIR). In all cases, the formed hematites retained the acicular shape of the precursor goethite. The dehydroxylation temperature increased with the increase of the Mn content in the parent goethite. The cell parameters of both phases decreased with the thermal treatment, however the decrease in the goethite b-parameter was more pronounced. This fact could be attributed to the distortion in the goethite structure by the presence of manganese. The band shifts in the FT-IR spectra of the goethites with different Mn substitution were analysed. The intensities of the hydroxyl vibrations were indicative of the degree of dehydroxylation.The chemical reactivity of all the samples, before and after the thermal treatment, was also studied. The kinetic experiments were carried out at 40 °C in 4 mol dm− 3 HCl. The acid dissolution of all Mn-goethites showed a congruent behavior indicative of a homogeneous distribution of Mn in the goethite crystals, this trend was not observed in the formed hematites presenting a high Mn content. The dissolution rate in goethites increased with the increase of Mn content, the opposite effect was observed in the corresponding hematites. The activation energy in both phases was also obtained and indicated that the Mn substitution produces an opposite effect on goethite- and hematite-phases. Different kinetic laws were applied in order to explain the dissolution behavior, but the modified first-order Kabai equation described the dissolution data best.  相似文献   

4.
Pure titanate nanotubes and titanate nanotubes doped with iron (III) and chromium (III) were fabricated by the hydrothermal treatment in methanol and sodium hydroxide mixture. The fabricated nano tubes have high surface area, high aspect ratio, consisted of very good surface morphology and high metals dispersion. The morphology, crysralline phase, composition were characterized by powdered X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Barrett-Joyner-Halenda methods and X-ray photoelectron spectroscopy. The results showed that nanotubes possess anatase phase and are composed up of 8–12 nm in diameter and 360–400 nm in length. The band gap of the titanium dioxide nanotubes was determined using transformed diffuse reflectance spectroscopy according to the Kubelka-Munk theory, showed pronounced band gap decrease on doped titanium dioxide nanotubes. The photocatalytic activity of doped nanotubes were evaluated in terms of degradation of phenol and photoreduction of carbon dioxide into methanol and ethanol under Ultra violet and Infra red irradiation. It was found that with iron (III) and chromium (III) doped titanium dioxide nanotubes exhibited much higher photocatalytic activity than undoped titanate nanotubes.  相似文献   

5.
A study of the characteristics of the Morin transition in aluminous hematites, α-(Fe1?xAlx)2O3, produced from thermally transformed lepidocrocites, is reported. Six compositions with Al contents between 0.2 and 10 at% have been considered. It is argued that these samples present the advantage that they contain smaller amounts of hydroxyl and water as compared to hematites obtained by other preparation methods. The samples were characterised by a variety of conventional techniques, including thermal analyses, X-ray diffraction, FTIR, TEM/EDX, BET surface-area measurements and diffuse reflectance spectroscopy. All results indicate that the Al is structurally incorporated in the hematite lattices. Transmission Mössbauer spectra were recorded at various temperatures between 80?K and room temperature in order to precisely determine the Morin-transition region and the spin structure in both the low-temperature antiferromagnetic and weakly ferromagnetic states. It was found that the Morin-transition temperatures are markedly higher as compared to similar hematites made from aluminous goethites and that a transition phenomenon persists to an Al substitution of up to at least 10 at%. This different behaviour is ascribed to lower concentrations of structural hydroxyl groups in these lepidocrocite-based hematites.  相似文献   

6.
A novel method for synthesis of aluminium hematites, based upon the homogeneous precipitation of Fe and Al oxinates in various proportions, is presented. The precursor precipitates are heated in air at 700?°C. X-ray diffraction, thermal analyses, BET, FTIR, optical reflection analysis, TEM and Mössbauer spectroscopy at room temperature and 80?K of the resulting products indicate that single-phase hematites are formed with structural Al substitution of up to 10 at%. Interestingly, the particle size (>100?nm) is not substantially reduced by the Al content. Although it remains difficult to obtain a homogeneously distributed Al substitution in the final hematite, this processing line offers a unique opportunity to separate the effects of grain size and Al substitution on the Morin transition temperature (T M) of Al hematite. From the comparison between the present hematites and a series of Al-substituted hematites with lepidocrocite as precursor, it could be shown that the effect on T M, associated with a change of a factor 10 in grain size, is about 1/3 of the effect caused by a change of 10 in the degree of substitution. Finally, it is suggested that proper thermal treatments under different conditions of the same precursors are likely to produce spinel phases.  相似文献   

7.
Two botryoidal hematites from the qtz-fl-hem veins at the Nonnenmattweiher cirque (zone of Badenweiler-Lenzkirch, Südschwarzwald) were dated using the (U + Th)-He method. One hematite gives an Early Triassic age (234 ± 5 Ma) and the second straddles the Cretaceous/Jurassic time boundary (126 ± 4 Ma). The ages bracket the ∼entire age spectrum reported to date for Mesozoic vein-type mineralizations in the Schwarzwald. The maximum sedimentary overburden and associated palaeo-temperatures (>130 °C) in the basement of the southern Schwarzwald during the Jurassic did not cause severe age-resetting of the hematites. This implies that both ages probably are formation ages rather than ages of cooling, provided that the 4He retentivities of both hematites are similar. The fact that two hematites from the veins at Nonnenmattweiher yield highly discordant Mesozoic ages suggests multiple hydrothermal activity during the Mesozoic. A previously published age of two adularia (∼155 Ma), intermediate between the hematite ages, supports this view. Received: 1 February 1996 / Accepted: 6 September 1996  相似文献   

8.
Laser-melted magnesium silicate droplets, supercooled 400–750°C below their equilibrium liquidus temperatures before crystallization, were examined to provide a comparison with meteoritic and lunar chondrules and to examine physicochemical parameters that may indicate the conditions of their formation. Internal textures of the spherules strikingly resemble textures observed in some chondrules. Definite trends in crystal morphology, crystal width and texture were established with respect to nucleation temperature and bulk composition. Such trends provide a framework for determining the nucleation temperature of chondrules. The only phase to nucleate from the supercooled forsterite-enstatite normative melts was forsterite, which was present in more-than-normative amounts. Highly siliceous glass (~65wt. % SiO2) was identified interstitially to the forsterite crystals in seven of the spherules and is thought to be present in all. The presence of enstatite and the large proportion of crystals in some meteoritic chondrules implies that they were maintained at temperatures considerably in excess of 600°C at some point in their history.  相似文献   

9.
The photoreduction efficiency of toxic hexavalent chromium into non-toxic trivalent chromium was studied using local low-cost material and modern technology. The materials involved different iron–titanium oxide nanopowders synthesized via simple hydrothermal–hydrolysis process. X-ray diffraction and high-resolution transmission electron microscope were employed to study the structural properties of the as-prepared samples. The effects of molar ratio (Fe/Ti) and hydrothermal temperature on spectroscopic properties have been investigated using Fourier transform infrared FT-IR spectroscopy. The photocatalytic performance of hexavalent chromium was systematically studied at various conditions including initial concentration of Cr(VI), hydrothermal temperature and Fe/Ti ratios of mixed iron–titanium oxide powders. It has been found that the highest photoreduction efficiencies of Cr(VI) were 95.7 and 86.2% at initial concentrations 10 and 60 ppm of Cr(VI), respectively. The synthesized mixed Fe2O3–TiO2 photocatalyst exhibited higher efficiency of about 88% under visible light irradiation. The as-prepared mixed oxide catalyst exhibited high photocatalytic conversion efficiency and recycling stability in comparison with different commercial catalysts.  相似文献   

10.
著名的“山西式”铁矿几乎分布于整个华北地区,其成因曾是一个古老的地质问题。本文在前人研究的基础上,通过野外露头观察,并结合镜下鉴定与能谱分析对太原西山七里沟剖面本溪组铁质层中铁质鲕粒结构及其特征进行了详细研究。研究表明,大部分铁质鲕粒具有放射结构,并能隐约看到同心圈层,同时,在鲕粒内部及其周围发现了大量的似蓝细菌丝状体,以及铁质凝块和球粒,具有明显的微生物成因特征。因此,认为铁质鲕粒的形成与微生物密切相关,微生物参与了铁矿的形成。  相似文献   

11.
The dissolution kinetics of three stoichiometric dolomite specimens (hydrothermal single crystal, microcrystalline sedimentary rock, coarse-grained marble) were studied in aqueous carbonate solutions. Hydrodynamic conditions were controlled through use of a rotating dolomite disk in which one face was exposed to solution and fluid flow regime was defined by spinning rate. The resulting mass transfer properties were uniform across the disk surface. The dissolution experiments were begun at an initially undersaturated condition set by CO2 at ~ 1 atm dissolved in deionized water. The reaction was followed by measuring concentrations of Ca2+, Mg2+, HCO3?, and pH over time in a free-drift type of experiment at 0, 15, and 25°C.Dissolution rates for all three samples were similar in form and value; grain size effects were insignificant. Ca/Mg was constant throughout each run at 0.81–0.96. From initial conditions, the dissolution rate decreased as the solution became more saturated. At solution conditions still far from equilibrium (ion activity product = 10?19), rate dropped off sharply to a very low value. Surface morphology, determined by SEM, showed deep narrow holes in the single crystal, while the rocks dissolved along grain boundaries. These features suggested preferential dissolution of energetically favored sites and surface reaction rate control. Initial rates were used to calculate an apparent activation energy of 32 kJ mol?1 (sedimentary dolomite) and 27 kJ mol?1 (single crystal).Initial dissolution rates at 25°C and pH ~ 4 for all samples varied with spinning speed and ranged from 1–3 μmol m?2 s?1 for laminar flow conditions to almost 3–6 μmol m?2 s?1 as the transition to turbulence began. At lower temperatures, the rate was lower, and increasing spinning velocity had less effect. The strongest spinning rate dependence occurred far from equilibrium, and it became a less important factor as the saturation state increased.  相似文献   

12.
In this study, the photocatalytic degradation of Congo red has been investigated in N-doped TiO2 (N-TiO2) aqueous suspensions under visible light irradiation. Visible light-active N-TiO2 was successfully prepared at three different weight contents (2.5, 5, and 7%) employing sol–gel method. It was able to harvest the visible irradiation with wavelength suitable for activation. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometer, diffused reflectance UV–Vis spectroscopy, nitrogen adsorption Brunauer–Emmert–Teller, Raman spectroscopy, photoluminescence and X-ray photoelectron spectrometer were used to characterize the doped catalyst. The samples had a relatively large specific Brunauer–Emmert–Teller surface areas of about 42 m2 g?1 with average X-ray diffraction crystalline size of 52 nm and showed visible light photocatalytic activity at about 408 nm. The impacts of several operating parameters on the Congo red photodegradation process were examined. Langmuir–Hinshelwood model exhibited pseudo-first-order degradation kinetics. N-TiO2-assisted plausible photodegradation mechanism has been suggested based on the qualitatively detected intermediate compounds.  相似文献   

13.
Transition metal-doped TiO2 nanoparticles are synthesized by sol–gel method. The as-prepared samples are characterized by various techniques to correlate structural and optical properties with chemical nature of dopants and their effect on photocatalytic degradation of diethyl phthalate esters. X-ray diffraction (XRD) reveals that all the samples are crystalline and exhibit anatase as a major phase. Chemical nature of dopants could not affect the formation of anatase and its volume fraction. The crystallite size of undoped and doped TiO2 nanoparticles varies between 10 and 12 nm as confirmed by XRD and transmission electron microscope. The lowest optical band gap observed is 2.47 eV in Mn-doped TiO2. Among all the samples, Ni-doped TiO2 sample shows better photocatalytic activity and degradation of diethyl phthalate due to its lower crystallite size and higher surface area than those of Mn- and Co-doped TiO2 samples.  相似文献   

14.
基于控制降温速率的动力结晶实验,对五大连池老黑山钾质玄武质熔岩的冷却结晶过程中橄榄石晶体形态随降温速率变化动力学演变进行了研究。实验研究表明,老黑山钾质玄武岩的熔融温度发生在1 022 ℃～1 260 ℃之间,峰值为1 166.3 ℃～1 186.5 ℃;实验产物主要由橄榄石和玻璃质组成,新生的橄榄石晶体形态差异较大,有针状骸晶、铁轨状骸晶、羽毛状雏晶、细长条状和粒状微晶。橄榄石晶体的生长、尺寸和形态、分布和实验产物的结晶程度与降温速率之间存在着密切关联。随着降温速率的变慢,橄榄石从针状（0.075 ℃ /min～0.5 ℃ /min）、铁轨状（0.075 ℃ /min～0.5 ℃ /min）、放射状与羽毛状（0.046 8 ℃ /min～0.075 ℃ /min）、细长条状（0.062 5 ℃ /min）等晶体演变为粒状微晶（0.03 ℃ /min）;同时,岩石的结晶程度逐渐增强,橄榄石晶体尺寸逐渐变大、自形程度逐渐变高,玻璃相和气孔逐渐减少。这种新生橄榄石形态的变化与降温速率的内在关联性,对认识老黑山熔岩流固结的动力结晶机制、熔岩流的运动机制有一定科学意义。  相似文献   

15.
The surface microtopography of garnet from metamorphic schists of the northern Ladoga region has been investigated. The morphology of garnet is distinguished by a rough stepped surface on dodecahedral faces {110} and by the absence of sharp crystal edges. As follows from experimental results on crystal growth under hydrothermal conditions, the microtopography pattern gives evidence for a highly oversaturated inter-granular medium and the important role of diffusion as a factor controlling garnet growth.  相似文献   

16.
黄龙嗜冷细菌两种胞外单糖对碳酸钙矿化影响   总被引：2，自引：0，他引：2  
为了探究黄龙水体中嗜冷细菌的胞外代谢产物对钙华沉积速率及沉积产物的影响,本文从黄龙钙华水体中分离到优势土著嗜冷细菌,并以其标志性胞外代谢产物D-葡萄糖和D-核糖作为研究对象,研究了两种单糖组分在低温环境下对钙华沉积的影响,并通过X射线衍射(XRD)、扫描电子显微镜(SEM)、红外吸收光谱(FT-IR)等分析手段对碳酸钙晶型、形貌以及结构组成进行了表征。实验表明:D-葡萄糖在低温条件下能促进方解石型碳酸钙的沉积,并加快沉积速率。低浓度下(20 mg/L和40 mg/L)的沉积产物均为方解石型碳酸钙并对其形貌有一定的影响;高浓度下(80 mg/L和160 mg/L)能诱导少量的文石型碳酸钙的合成。D-核糖在低温条件下能促进碳酸钙的沉积,加快沉积速率。与D-葡萄糖不同,D-核糖仅能合成方解石型碳酸钙,但对方解石晶面的生长表现为抑制作用,且随浓度增大抑制作用益明显。此结果可为黄龙钙华生物成因的探究提供一定的理论研究基础。  相似文献   

17.
Co3O4 nanoparticles were successfully deposited on the surface of natural halloysite nanotubes (HNTs) to produce Co3O4/HNTs composites. The structure and morphology of the samples were characterized using X-ray diffraction, field-emission scanning electron microscope, transmission electron microscope and Fourier transform infrared. The results indicated that Co3O4 nanoparticles were uniformly attached on the surface of HNTs with narrow size distribution. Co3O4/HNTs exhibited an excellent photocatalytic efficiency for degradation of methyl blue under UV light, better than Co3O4 and HNTs mixture, HNTs and pure Co3O4. The mechanism of enhanced photocatalytic activity of Co3O4/HNTs was also proposed.  相似文献   

18.
天然沸石对提高TiO2光催化活性的作用   总被引：1，自引：1，他引：0  
以天然沸石为载体,研究了其负载TiO2后对甲基橙溶液的光催化降解效果,探讨了TiO2与沸石的结合类型。通过对负载前后反应动力学模式变化的研究,阐释TiO2/沸石体系的光催化反应进程。从复合前后样品的AFM、IR、发射光谱变化上看,部分TiO2与沸石载体形成了新的Ti－O－Si或Ti－O－Al的化学键。光催化实验结果和动力学分析显示,在反应的初始阶段,当TiO2中引入多孔的沸石载体后,催化剂对甲基橙的降解过程由原来纯粹的吸附控制模式向反应控制模式转变,但还没有达到纯粹的反应控制的程度。沸石载体是通过提高吸附过程的速率,来提升初始阶段的光催化反应速率（R0）的。  相似文献   

19.
《Geochimica et cosmochimica acta》1999,63(23-24):3983-3995
Exact solutions to equations governing isothermal diffusive dissolution of a crystalline slab in a ternary liquid were obtained to include the effect of coupled chemical diffusion in the liquid. These analytical results, supplemented by approximate solutions valid for slow dissolving, provide new insights into the characteristics of diffusive dissolution in ternary systems. Dissolution rate is proportional to square root of time in diffusive dissolution. The coefficient of proportionality is a function of diffusion coefficients, liquidus relation, melt composition at the crystal–melt interface, and compositions of the dissolving crystal and starting melt. In the limit of slow dissolving, the dissolution rate can be written in terms of three dimensionless parameters that are functions of the aforementioned parameters. Dissolution rate is proportional to the diffusion rate of the slow eigen component in the melt when the diffusion rate of the minor eigen component is much slower than the diffusion rate of the major eigen component.Laboratory experiments of diffusive dissolution of single crystals and polycrystalline aggregates of quartz in a haplodacitic melt (25 wt.% CaO, 15 wt.% Al2O3, and 60 wt.% SiO2) were conducted at 1500°C and 1 GPa. Measured dissolution distances (Xb, in microns) are proportional to the square root of experimental run time (t, in seconds), Xb = −0.620 (±0.019) √t. Chemical concentration profiles measured from quenched melts are invariant with time when displayed against the distance (measured from the crystal–melt interface) normalized by the square root of time. The melt compositions at the crystal–melt interface, extrapolated from the measured diffusion profiles in the quenched melts, are within 0.2 wt.% of the independently measured quartz liquidus in the ternary CaO–Al2O3–SiO2 at 1500°C and 1 GPa. These results suggest that crystal and melt are in chemical equilibrium at their interface shortly after the onset of dissolution. Diffusive dissolution of quartz and quartzite is characterized by slow dissolving. Using quartz liquidus as one of the boundary conditions, it has been shown that the calculated dissolution distances and concentration profiles are in good agreement with the experimentally measured ones. Coupled diffusion played an essential role in quartz and quartzite dissolution in haplodacitic to haplobasaltic melts, and is likely to play an important role in diffusion-limited kinetic processes such as crystal growth and dissolution in natural melts of basaltic–rhyolitic compositions.  相似文献   

20.
The development of fibrous morphology and capability of fibres for tracking the opening trajectory were investigated using numerical simulations of a natural antitaxial fibrous vein. Starting from a non-unique best case, variation of fracture opening velocity, grain size, wall roughness, growth anisotropy and crystal growth velocity shows that these parameters differ in importance for crystal morphology and tracking capability. Fibrous veins can be simulated using crack–seal opening of the fracture. Grain boundaries track the opening trajectory if the wall roughness is high, opening increments are small and crystals touch the wall before the next crack increment starts.  相似文献   

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