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1.
Infrared absorption spectroscopy has been applied for the identification of the minerals associated with natural goethite
(α-FeO·OH) from Saudi Arabia. The thermal transformation of natural goethite as well as the effects of the presence of the
associated minerals on the reaction products were investigated. The results revealed that at 300°C protohematite is formed
which at 600°C is slightly crystallized. Further recrystallization and hematite formation takes place at 1000°C. It was also
found that the presence of other minerals found in goethite samples has no effect on the reaction products. 相似文献
2.
Minerals have been used as pigments for thousands of years. Red and yellow pigments are generally associated with iron oxides or, specifically, hematite (α-Fe 2O 3) and goethite (α-FeOOH). It is well known that, under heating, goethite dehydrates forming hematite. An interesting question yet to be answered is whether the pre-historical artists used this knowledge to obtain other shades of red and yellow or used the raw mineral directly.Raman spectroscopy was employed to address this question and XRD, TEM and TG were used as supporting techniques. Ex situ and in situ Raman spectra were obtained and it was observed that in the 250–300 °C temperature range, broad hematite features appears as a consequence of goethite dehydration. In the spectra of the heated sample a band at 657 cm −1 is of particular interest, as it is much more intense than in natural hematite; the possibility that it could be assigned as a magnetite band was discarded. At higher temperatures (900–1000 °C) the disordered structure is perfected and a Raman spectrum similar to a crystalline natural hematite sample is obtained.Temperatures in the 600–700 °C range can be easily reached, thus disordered hematite could be obtained from goethite heating even in ancient times, however, heat is not the only agent able to produce disordered hematite, since grinding, biodegradation and weathering can produce the same effect. Raman spectra obtained from weathered samples are also representative of disordered hematite.The data here reported indicate that it is not possible to differentiate heated goethite from other disordered hematites. 相似文献
3.
Phase changes of iron containing solid wastes from steel mill pickling
lines after thermal treatments were investigated aiming the determination
of the appropriate conditions for its transformation to be useful for industrial
raw materials. Above 275°C, the thermally treated wastes contain a mixture
of α-Fe 2O 3 (hematite)
and γ-Fe 2O 3 (maghemite)
in different proportions, depending on the maximum heating temperature of
the thermal treatment. Increasing the maximum temperature the maghemite participation
is decreased through its transformation to hematite. Above 850°C hematite
is the main constituent, suggesting that thermal treatment of the wastes in
this temperature will give a product that could be used as red iron pigment. 相似文献
4.
The thermal conversion of lepidocrocite (γ-FeOOH) into maghemite (γ-Fe 2O 3)and hematite (α-Fe 2O 3) has been studied by dynamic (DSC) and static heating experiments. Dynamic heating defines two main regions: conversion of
lepidocrocite to maghemite (endothermal signal peaking at 255°C) and conversion of maghemite to hematite (exothermal signal
peaking at 450°C). In addition, an exotherm following the lepidocrocite to maghemite endotherm is observed. The maghemite
phase appears as porous aggregates of nanocrystals characterized by an extensive spin-canting. We suggest that the additional
exotherm is associated with structural changes and a decreasing extent of spin-canting in the maghemite phase.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
High energy ball-milling methods were employed in the synthesis of anatase-doped hematite xTiO 2(a) · (1− x)α-Fe 2O 3 ( x = 0.1, 0.5, and 0.9) ceramic system. The thermal behavior of as obtained ceramic system was characterized by simultaneous
DSC–TG. The pure anatase phase was found to be stable below 800 °C, but there is a 10.36% mass loss due to the water content.
Two exothermic peaks on DSC curves of pure anatase indicate the different crystallization rates. The pure hematite partially
decomposed upon heating under argon atmosphere. Ball-milling has a strong effect on the thermal behaviors of both anatase
and hematite phases. For x = 0.1 and 0.5, there is gradual Ti substitution of Fe in hematite lattice, and the decomposition of hematite is enhanced
due to the smaller particle size after ball-milling. The crystallization of hematite was suppressed as the enthalpy values
decreased due to the anatase-hematite solid–solid interaction. For x = 0.9, most of the anatase phase converted to rutile phase after long milling time. The thermal behavior of xTiO 2(a) · (1− x)α-Fe 2O 3 showed smaller enthalpy value of the hematite transformation to magnetite and anatase crystallization due to the small fraction
of hematite phase in the system and hematite–anatase interaction, while the mass loss upon heating increased as a function
of milling time due to more water content absorbed by the smaller particle size. 相似文献
6.
The binary phase, porous, nanocomposite xα-Fe/(1 − x)Ce 0.9–K 0.1–O ( x = 0.05–0.2) catalysts and the catalyst-coated honeycomb ceramic device have been prepared by the citrate-gel thermal decomposition-reduction
process and the sol–gel assisted dip-coating method, respectively. The nanocomposite of fluorite-type structure CeO 2 nanoparticles about 18–51 nm and α-Fe nanoparticles about 32 nm is obtained at 600 °C for 2 h in a deoxidization atmosphere
and the α-Fe in nanocomposite has the suppression effect on grain growth of CeO 2. With Fe content increasing from 0.05 to 0.1, the specific surface area for the nanocomposites increases dramatically from
about 4.4 to 43.0 m 2/g, reaching a maximum value 57.7 m 2/g at x = 0.15, and the pores vary from macropores to micro- or mesopores. Due to the presence of nano α-Fe, all the catalysts exhibit
a very high soot catalytic activity, with the lowest T 20 (255 °C) and T 50 (291 °C) for the nanocomposite with x = 0.15, and it is confirmed by the bench test under practical diesel exhaust gases. 相似文献
7.
Preliminary tests of the chlorination of two iron oxides (wüstite and hematite) in various chlorinating gas mixtures were performed by thermogravimetric analysis (TGA) under non-isothermal conditions. Wüstite started to react with chlorine from about 200 °C generating ferric chloride and hematite as the final reaction products. The presence of a reducing and oxidizing agent in the chlorinating gas mixtures influenced the chlorination reactions of both iron oxides, during non-isothermal treatment, only at temperatures higher than 500 °C.The chlorination kinetics of hematite with Cl 2 have been studied in details between 600 and 1025 °C under isothermal chlorination. The values of the apparent activation energy ( Ea) were about 180 and 75 kJ/mol in the temperature ranges of 600–875 and 875–1025 °C, respectively. The apparent reaction order with respect to Cl 2 was found to be 0.67 at 750 °C. Mathematical model fitting of the kinetics data was carried out to determine the most probable reaction mechanisms. 相似文献
8.
The objective of present research was to sinter nanosized Mn–Zn ferrites (MZF) at low temperature (≤1,000 °C) by avoiding
the formation of nonmagnetic phase (hematite). For this purpose, MZF powder was synthesized by sol–gel auto combustion process
at 220 °C and further calcined at 450 °C. In calcined powder, single phase (spinel) was confirmed by X-ray diffraction analysis.
Pellets were pressed, having 43% of the theoretical density and showing 47 emu gm −1 saturation magnetization ( M
s). Various combinations of heating rate, dwelling time and gaseous environment were employed to meet optimum sintering conditions
at low temperature (≤1,000 °C). It was observed that sintering under air or N 2 alone had failed to prevent the formation of nonmagnetic (hematite) phase. However, hematite phase can be suppressed by retaining
the green compacts at 1,000 °C for 180 min in air then further kept for 120 min in nitrogen. Under these conditions, spinel
phase (comprising of nano crystallites), 90% of theoretical density and 102 emu gm −1 of saturation magnetization has been achieved. 相似文献
9.
Most of the wall paintings from Pompeii are decorated with red and yellow colors but the thermal impact of 79 AD Mount Vesuvius eruption promoted the partial transformation of some yellow-painted areas into red. The aim of this research is to develop a quantitative Raman imaging methodology to relate the transformation percentage of yellow ochre (goethite, α-FeOOH) into red color (hematite, α-Fe2O3) depending on the temperature, in order to apply it and estimate the temperature at which the pyroclastic flow impacted the walls of Pompeii. To model the thermal impact that took place in the year 79 AD, nine wall painting fragments recovered in the archeological site of Pompeii and which include yellow ochre pigment were subjected to thermal ageing experiments (exposition to temperatures from 200 to 400 °C every 25 °C). Before the experiments, elemental information of the fragments was obtained by micro-energy dispersive X-ray fluorescence (μ-ED-XRF). The fragments were characterized before and after the exposition using Raman microscopy to monitor the transformation degree from yellow to red. The quantitative Raman imaging methodology was developed and validated using synthetic pellets of goethite and hematite standards. The results showed almost no transformation (0.5% ± 0.4) at 200 °C. However, at 225 °C, some color transformation (26.9% ± 2.8) was observed. The most remarkable color change was detected at temperatures between 250 °C (transformation of 46.7% ± 1.7) and 275 °C (transformation of 101.1% ± 1.2). At this last temperature, the transformation is totally completed since from 275 to 400 °C the transformation percentage remained constant. 相似文献
10.
Samples of akaganeite (β-FeOOH) and goethite (α-FeOOH) have been studied after heating at various temperatures up to 800 °C. X-ray diffraction and Mössbauer spectroscopy measurements showed that slightly below the temperatures at which the samples transform to hematite (α-Fe 2O 3) the oxyhydroxide phases become nanocrystalline. 相似文献
11.
Diphasic cordierite gels were prepared from colloidal silica, aluminum and magnesium nitrates and citric acid. The mechanism
of xerogel decomposition was studied by infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The thermal
decomposition of the xerogel forms a solid mixture of MgO, Al 2O 3 and SiO 2 at around 250 °C. Cordierite crystallization was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA).
Xerogels were initially thermally treated, and this sample crystallized to μ-cordierite at 850 °C, at 900 °C α-cordierite
crystallizes and at 1150 °C α-cordierite is the major phase and μ-cordierite is totally consumed. The apparent activation
energy for cordierite crystallization process was determined based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, Ligero
methods and the Arrhenius law for dependence of activation energy with temperature. The apparent activation energy was (466.8 ± 34.3) kJ/mol,
the exponent of Avrami was (1.9 ± 0.2) and the frequency factor was (1.55 × 10 20) s −1. The Avrami value indicates a nucleation controlled process, which can be a consequence of the high xerogel homogeneity,
a consequence of the early and simultaneous formation of the MgO, Al 2O 3 and SiO 2 mixture. 相似文献
12.
Thermogravimetry combined with evolved gas mass spectrometry has been used to characterise the mineral crandallite CaAl 3(PO 4) 2(OH) 5·(H 2O) and to ascertain the thermal stability of this ‘cave’ mineral. X-ray diffraction proves the presence of the mineral and
identifies the products of the thermal decomposition. The mineral crandallite is formed through the reaction of calcite with
bat guano. Thermal analysis shows that the mineral starts to decompose through dehydration at low temperatures at around 139 °C
and the dehydroxylation occurs over the temperature range 200–700 °C with loss of the OH units. The critical temperature for
OH loss is around 416 °C and above this temperature the mineral structure is altered. Some minor loss of carbonate impurity
occurs at 788 °C. This study shows the mineral is unstable above 139 °C. This temperature is well above the temperature in
the caves of 15 °C maximum. A chemical reaction for the synthesis of crandallite is offered and the mechanism for the thermal
decomposition is given. 相似文献
13.
The thermal decomposition of mineral goethite is investigated at various temperatures between 25 and 700°C in vacuo. The mineral used in the present work consists of the agglomerates in which primary particles of goethite are tightly bound to each other. At temperatures below 200°C, the water molecules that blocked the interstice within the primary particles are removed and, as a result, the slit-shaped voids of 3.7 nm in width are opened between the primary particles. The thermal decomposition of mineral goethite starts at the temperatures above 200°C. At 230–300°C, the silt-shaped micropores of uniform size (0.8 nm in width) are progressively opened in the course of decomposition reaction. The microporous texture obtained in the present work completely coincides with that obtained by decomposition of the synthetic goethite fine particles reported previously. The process of formation of the microporous texture has been examined in the light of the structural relationship of topotaxy between goethite and hematite. On the basis of the results of micropore formation, the process of nuclei formation of hematite in the goethite matrix is discussed. 相似文献
14.
This article reports the possibility of detoxification of chrysotile asbestos through a low temperature heating and grinding
treatment. The effect of thermal treatment at different temperatures in the range from 500 to 725 °C for 3 h on raw natural
asbestos was characterized by thermal analysis, X-ray diffraction, and scanning electron microscopy. It was found that an
isothermal treatment at 650 °C caused the complete dehydroxylation of chrysotile Mg 3Si 2O 5(OH) 4. Transformation of the dehydroxylated phase to forsterite Mg 2SiO 4 was obtained by heat treatment in the range 650–725 °C. The study of microstructure changes of heated asbestos show the destruction
of characteristic fibers of chrysotile and formation of strips of forsterite. It is easily milled to pulverulent-shape material
by mechanical milling in vibratory mill. 相似文献
15.
A novel sol–gel synthetic route using water-soluble precursor salts is presented as a synthetic path for a high-purity negative
thermal expansion material, ZrW 2O 8. This synthetic route involves a sol–gel method with the use of EDTA as complexing agent. The aqueous solution is transformed
into a ceramic material after a two-step heat treatment: gelation at 60 °C and reactive sintering at 1,180 °C. The decomposition
of the gel is monitored with infrared spectroscopy and TGA. The high-temperature heat treatment results in ZrW 2O 8 with its characteristic negative thermal expansion behaviour α [75–130 °C]: −9.8 ± 1.6 μm/m °C and α [175–300 °C]: −1.2 ± 0.2 μm/m °C. 相似文献
16.
We have investigated tension wood cellulose obtained from Populus maximowiczii using X-ray diffraction at temperatures from room temperature to 250 °C. Three equatorial and one meridional d-spacings showed a gradual linear increase with increasing temperature. For temperatures above 180 °C, however, the equatorial
d-spacing increased dramatically. Thus, the linear and volume thermal expansion coefficients (TECs) below 180 °C were determined
from the d-spacings. The linear TECs of the a-, b-, and c-axes were: α
a
= 13.6 × 10 −5 °C −1, α
b
= −3.0× 10 −5 °C −1, and α
c
=0.6× 10 −5 °C −1, respectively, and the volume TEC was β = 11.1× 10 −5 °C −1. The anisotropic thermal expansion in the three coordinate directions was closely related to the crystal structure of the
wood cellulose, and it governed the macroscopic thermal behavior of solid wood. 相似文献
17.
Differential scanning calorimetry, differential thermogravimetry, X-ray analysis and electronic microscopic studies of thermal
transformations of PbO 2 were carried out. Formation of fine dispersed (less than 100 nm) particles of α-PbO was observed at PbO 2 thermal decomposition at heating to 580°C. Reverse reaction of Pb 3O 4 formation from PbO was found at cooling and annealing at 400°C in air. At heating of α-PbO to 650°C the particle growth to
1 μm with formation of β-PbO took place. Thermal decomposition with formation of β-PbO particles with size from 0.3 to 1 μm
at PbO 2 heating to 650°C was observed. Transition from PbO to Pb 3O 4 at cooling of sample heated to 650°C was not detected. Interpretation of observed phenomena from the point of view of particle
size influences on the shift of α-PbO↔β-PbO phase transition temperature and on the chemical activity of phases are presented. 相似文献
18.
Sodium zirconium phosphate (NZP) composition Na 1−x
Li
x
Zr 2(PO 4) 3, x = 0.00–0.75 has been synthesized by method of solid state reaction method from Na 2CO 3·H 2O, Li 2CO 3, ZrO 2, and NH 4H 2PO 4, sintering at 1050–1250 °C for 8 h only in other to determine the effect on thermal properties, such as the phase formation
of the compound. The materials have been characterized by TGA and DTA thermal analysis methods from room temperature to 1000 °C.
It was observed that the increase in lithium content of the samples increased thermal stability of the samples and the DTA
peaks shifted towards higher temperatures with increase in lithium content. The thermal stability regions for all the sample
was observed to be from 640 °C. The sample with the highest lithium content, x = 0.75, exhibited the greatest thermal stability over the temperature range. 相似文献
19.
In this
paper we present a study on the synthesis of Fe(III) oxide, by thermal decomposition
of some complex combinations of Fe(III) with carboxylate type ligands, obtained
in the redox reaction between some polyols (ethylene glycol (EG), 1,2-propane
diol (1,2PG), 1,3-propane diol (1,3PG) and glycerol (GL)) and NO 3
–
ions (from ferric nitrate). Fe 2O 3
was obtained by thermal decomposition of the synthesized metal-organic precursors
at low temperatures. γ-Fe 2O 3
was obtained as nanoparticles at 300°C, while at higher temperatures α-Fe 2O 3
starts to crystallize and becomes single phase at ~500°C.
The
formation of the metal-organic precursors and their thermal decomposition
were studied by thermal analysis and FTIR spectroscopy. 相似文献
20.
The thermal dehydroxylation of natural Al-bearing geothite was investigated by IR spectroscopy. Venezuelan lateritic bauxites (which in addition to goethite contain kaolinite, gibbsite, ilmenite and quartz), as well as chemically isolated samples of Al-goethites, were heated to 300, 600 and 1000°C. The spectral features of the iron oxides formed during the thermal treatment depend on the heating temperature, showing that the first dehydroxylation product is Al-bearing protohematite which at temperatures above 600°C is recrystallized to Al-bearing hematite. Part of the aluminum which is occuled in this hematite originates from the gibbsite and to a smaller extent from the kaolinite. 相似文献
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