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1.
Summary Previous investigations have shown that the total evaporation of impurities from silicon carbide powder can be achieved by its thermochemical transformation to BaSiO3, BaF2 and CoSi (CoSi2) under the action of the modifier CoF2 + BaO, through in situ processing in an electrothermally heated graphite crucible [13]. The aim of the present work has been to describe a methodology to perform a rational selection of various potential candidates for modifiers, to make successful combinations between various chemical additives, to understand the reason(s) for an effective/ineffective action of a particular additive, i.e. to find the optimum conditions for each type of ceramic matrix chemical modifier. A combination of scanning electron microscopy (SEM), energy dispersive X-ray analyses (EDX) and X-ray diffraction analyses was used to confirm the identification of the alterations to the silicon carbide matrix. Indirectly, the present work supports the view that the aim justifies the means particularly when the aim is to reach a complete evaporation of the impurities from a carbide matrix, which as is well known, is not an ordinary or easy task. 相似文献
2.
Summary The aim of this work was to develop a method based on ICP-OES for the determination of trace elements in alumina-based ceramic powders containing SiO2 and MgO, respectively, and to study the evaporation and excitation of several elements by using external electrothermal vaporization. This method has the advantage of easy preparation, no dilution and short experiment durations. The evaporation mechanism was investigated separately from the excitation processes by means of radioactive59Fe and115Cd isotopes. The influence of different thermochemical additives, as well as third elements and matrix compositions was determined.
Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday 相似文献
Untersuchung von Aluminiumoxid-haltigem keramischem Material mit Hilfe der ICP-OES mit externer elektrothermischer Verdampfung
Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday 相似文献
3.
V. G. Pimenov D. V. Lukin A. V. Bondarenko V. S. Mikheev 《Journal of Analytical Chemistry》2002,57(2):138-141
A procedure for analyzing high-purity arsenic by atomic emission spectrometry (AES) was proposed that provides the preconcentration of silicon and other nonvolatile impurities in a crater of a graphite electrode using matrix distillation from 2-g sample portions as As2O3. The procedure is characterized by a low correction for the blank experiment. The detection limit for silicon was 4 × 10–7 wt %. 相似文献
4.
B. D. Joshi T. R. Bangia A. G. I. Dalvi 《Fresenius' Journal of Analytical Chemistry》1972,260(2):107-110
Summary An emission spectrographic method for the estimation of impurities of Al, As, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Sb, Sn, Te, Ti, V, Zn and Zr in high purity selenium at concentrations less than 1 ppm has been developed. The impurities were separated from 1 g of selenium by volatilizing Se from its solution in nitric acid and sulphuric acid. The residue containing impurities was dissolved in small amounts of 6 M HCl and loaded along with the washing on 10 mg mixture containing graphite (90%) and sodium chloride carrier (10%), in the electrode crater. The spectra were excited in a d.c. arc carrying 13 A. The detection limits lie in the range 0.005–0.5 ppm for the different elements. The precision of the method ranges from 7–18% for these elements.
Spektrographische Bestimmung von Verunreinigungen in Selen
Zusammenfassung Folgende Verunreinigungen werden in hochreinem Selen in Konzentrationen von <1 ppm bestimmt: Al, As, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Sb, Sn, Te, Ti, V, Zn und Zr. Das Selen wird von den Verunreinigungen durch Verdampfen abgetrennt (Lösen einer 1 g-Probe in HNO3/H2SO4). Der Rückstand wird in 6 M HCl gelöst und zusammen mit einem Graphit/NaCl-Gemisch (9010) in die Elektrodenbohrung gefüllt. Die Anregung erfolgt in einem Gleichstrombogen von 13 A. Die Nachweisgrenzen liegen im Bereich von 0,005–0,5 ppm, die Genauigkeit beträgt 7–18%.相似文献
5.
The formation of volatile fluorides of nonvolatile elements in the presence of the fluorine-bearing additive ZnF2 in the electrode crater of a direct-current arc was confirmed experimentally. It was shown that the use of fluorination reactions in the electrode crater lowers the detection limits of hardly volatile elements in aluminum and zirconium oxides by 1–1.5 orders of magnitude. It was also shown using the analysis of fullerene as an example that fluorination reactions can be used to standardize the procedures for determining impurities in different matrices similar in composition and temperature parameters using the same reference samples. 相似文献
6.
This work investigates the uptake of impurities during processing of Si3N4 and describes an analytical scheme for detecting sources of contamination. For this purpose a process as simple and short as possible was chosen, using commercial starting materials with a high standard of purity and reproducibility. The uptake of non-metallic and metallic contaminants was investigated by choosing elements which were specific for individual processing steps. This was difficult in the determination of metallic impurities in a powder consisting of Si3N4 with Y2O3/Al2O3 additives, because the powder mixture and the sources of contamination (milling balls, attritor disk, wall materials) were similar in composition and the available analytical methods were not precise enough to detect the small increase in concentration that occurred. Therefore pure Si3N4 powders were milled in order to get an indication of the kind and concentration of impurity introduced by the individual milling materials and steps. These elements can then be used as monitor elements to trace sources of contamination and to optimize processing parameters. Experience with the processing of Si3N4 with Y2O3/Al2O3 additives by cyclic milling, spray drying, burn-out and isopressing are reported. Contamination by carbon is unavoidable. Its concentration during the process is relatively high, as it is added in the form of processing aids (deflocculants, binders), but temporary, as it can be completely burned out. Oxygen is predominantly taken up during milling. Good deflocculation reduces the milling time and thus limits the uptake of oxygen. As a consequence of these findings the processing parameters could be optimized. Thus the uptake of metallic impurities, e.g. Fe could be limited to 10 g/g and the uptake of oxygen was found to be less than 0.2 wt%. 相似文献
7.
Measurements were carried out for the solution enthalpies of zirconium perchlorate, nitrile perchlorate, nitrile perchloratozirconate, and cesium perchloratozirconate in concentrated nitric acid. A thermochemical reaction cycle was established and the enthalpies of formation Hf
0 at 298.15 K were found for Zr(ClO4)4(s) (–745.5±2.7 kJ/mole), (NO2)2[Zr(ClO4)6](s) (–714.2±4.4 kJ/mole), and Cs2[Zr(ClO4)6](s) (–1656.3±4.5 kJ/mole).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 910–913, April, 1990. 相似文献
8.
Gannon P. Connor Daniel Delony Jeremy E. Weber Brandon Q. Mercado Julia B. Curley Sven Schneider James M. Mayer Patrick L. Holland 《Chemical science》2022,13(14):4010
Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N2 splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium(iii) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH2)2Cl]+, or results in dehydrohalogenation to the rhenium(iii) amido complex, (PNP)Re(NH2)Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol−1, while DFT computations indicate a substantially weaker N–H bond of the putative rhenium(iv)-imide intermediate (BDE = 38 kcal mol−1). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH3 generation.Rhenium–PNP complexes split N2 to nitrides, but the nitrides do not give ammonia. Here, the thermodynamics of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, showing that the first H addition is the bottleneck. 相似文献
9.
L. K. Kaganov N. M. Mukhamedshina M. M. Usmanova 《Journal of Radioanalytical and Nuclear Chemistry》1997,220(2):213-216
Instrumental neutron activation techniques for the determination of 26–36 impurities in MoSi2, WSi2, TiB2, NbB2, TiC, NbC and SiC with detection limit of 0.1–0.000001 ppm in the range of contents of 100–0.001 ppm have been developed. The determination of this large number of elements was achieved with using complex conditions of irradiation, including varying neutron energy and choosing optimal time parameters. The contents of short-lived radionuclides also have been determined. Self-shielding and disturbance of neutron flux, absorption of -rays were taken into account both with the help of literature data and preliminarily obtained experimental results. 相似文献
10.
Chemical routes to synthesize inorganics can start with solutions where multiple elements remain well mixed during liquid evaporation, precursor decomposition and crystallization. Because crystallization generally occurs at temperatures where diffusion is very limited, a single phase can crystallize with a greatly extended solid-solution that does not achieve its equilibrium phase assemblage until much higher temperatures where diffusion is no longer constrained. Partitioning at these higher temperatures can lead to unique microstructures such as the nano-composite illustrated here for a metastable Zr(1–x)Al
x
O2–x/2 (x0.57) phase that partitions into a composite containing t-ZrO2 grains and -Al2O3 plates. 相似文献
11.
Impurity profiling of 1–(3,4-methylenedioxyphenyl)-2-nitropropene, an intermediate in the synthesis of 3,4-methylenedioxymethamphetamine (MDMA), has been studied by SPE combined with TLC. Extraction of impurities was performed on C18 columns. TLC separation was performed on 0.2 mm silica gel 60 plates, with fluorescent indicator F254, in a horizontal developing chamber. To improve the quality of the profile (increase the TLC sensitivity) SPE extracts were concentrated by evaporation in a stream of nitrogen. The usefulness of methanol, ethanol, ethyl acetate, chloroform, acetonitrile, and their mixtures as mobile phases was tested by use of Gibbss triangle. Spots of the separated impurities were observed under UV light (exc=254 and 366 nm). The proposed characteristics of profile quality (optimization criteria) are based on matrix presentation of the TLC pattern. They take into account, simultaneously, the number of spots revealed, differences between RF values, and the intensity of fluorescence. 相似文献
12.
Max Knothe 《无机化学与普通化学杂志》1983,503(8):213-223
Precipitation of PdII as [Pd(NH3)2Cl2] and the Behaviour of Various Impurities The dependence of [Pd(NH3)2Cl2] precipitation upon reaction conditions (pH, Cl? content, reaction time, temperature) has been studied. The dependence of residual Pd content in the mother liquor upon these parameters was found to be significant only for the precipitation temperature (cPd at 20°C: 1.65 ± 0.11 mM; at 50°C: 6.70 ± 0.58 mM). The increase of Pd concentration was due to the formation of Pd(NH3)Cl3?. Among the impurities studied Cr, Ru, and Au were largely precipitated in the NH3 medium. In subsequent precipitation of [Pd(NH3)2Cl2] the following order of coprecipitation was found: The first four elements could be separated only incompletely by repeated reprecipitation. The coprecipitation of the platinum-group metals and of Au was highly dependence upon preceding formation of ammine complexes of these elements. The considerable coprecipitation of PtIV is presumably due to the formation of mixed Pd/Pt compounds, whereas the other impurities are adsorbed by [Pd(NH3)2Cl2]. 相似文献
13.
Summary The influence of organic modifier and ion interaction reagent (IIR) concentrations on analyte retention in ion interaction chromatography (IIC) was investigated via a bivariate approach. The system examined is as follows: ODS-Hypersils was used as stationary phase; the eluent was a variable mixture of a phosphate buffer pH 2.1 and methanol ranging 0%–40% (v/v). The organic modifier decreases both the analyte and the IIR free energy of adsorption hence their interaction is lower when the organic modifier concentration is higher. Retention equations describe how these two factors interact with each another. The estimated equilibrium constant for the IIR adsorption corresponds to G° = –17.3 KJ mol–1. This value is reliable because it makes sense physically. The chi-squared (k
calc – k
exp)2 is 1.11E + 00 and 2.85E-02, for octopamine and 2-naphthalenesulfonate, respectively. Retention equations were compared with those obtained from the outstanding retention models in IIC. For the modelled system, the extended thermodynamic approach is able to explain experimental evidence that cannot be rationalised by existing retention models: it proves superior for fitting experimental data and is also able to predict when the purely electrostatic approach may work properly. In the absence of IIR the present retention equation reduces, as expected, to the relationship that describes the influence of organic modifier on retention behaviour in reversed-phase liquid chromatography. 相似文献
14.
Wim Penninckx Desire L. Massart Johanna Smeyers-Verbeke 《Fresenius' Journal of Analytical Chemistry》1992,343(6):526-531
Summary A combination of 6 g palladium and 15 g magnesium nitrate is proposed as chemical modifier for lead determinations in biological materials and foodstuffs. The applicability of this modifier was investigated by the analysis of several types of samples, as compared to the classical NH4H2PO4 and Mg(NO3)2 modifier. Direct determinations of lead against aqueous standard solutions can be performed in 3-fold diluted urine, 2-fold diluted milk and 6-fold diluted blood, when the proposed modifier is added. A method for lead determinations in potatoes using the combined palladium and magnesium nitrate modifier, after a microwave acid digestion, is described. The optimum GFAAS pyrolysis temperature remains dependent on the matrix and should be determined for each type of sample. A wider linear range of the calibration curve is observed when the proposed Pd modifier is used. 相似文献
15.
A practical approach to ferrocenyl naphthaquinone derivatives involving thermal rearrangement of variously substituted 4-aryl-4-hydroxycyclobutenones was described. The reaction of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione with different aryl lithiums gave the corresponding 4-aryl-4-hydroxycyclobutenones, which were heated in p-xylene at reflux open to the air to yield ferrocenyl naphthaquinones. The redox chemistry of the ferrocenyl naphthaquinones was studied by electrochemical and in situ spectroelectrochemical techniques in CH2Cl2 solution and in CH3CN solution with water, weak and strong acidic additives. Ferrocenyl naphthaquinones displayed reversible two reduction processes involving semiquinone radical anion (Fc-snq−), dianion (Fc-nq2−) species and a one-electron oxidation process based on the ferrocenium/ferrocene (Fc+-nq/Fc-nq) couple in CH2Cl2. The redox reaction mechanism of the ferrocenyl naphthaquinones in the presence of the additives proceeded via hydrogen bonding or proton-coupled electron transfer. Effects of the substituents on the reduction potentials and intramolecular charge-transfer bands of ferrocenyl naphthaquinones were also discussed. 相似文献
16.
Ustynyuk L. Yu. Ustynyuk Yu. A. Laikov D. N. Lunin V. V. 《Russian Chemical Bulletin》2001,50(11):2050-2053
Model reactions of silica-supported zirconium hydrides (Si—O—)3ZrH and (Si—O—)2ZrH2 with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (Si—O—)3ZrCH3 and (Si—O—)2Zr(H)CH3 as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (Si—O—)2ZrH2 are more reactive toward the methane C—H bonds than zirconium monohydrides (Si—O—)3ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (Si—O—)2ZrH2 are in better agreement with the known experimental data for the Yermakov—Basset catalytic system. 相似文献
17.
Investigation of the separation of scandium and rare earth elements from red mud by use of reversed-phase HPLC 总被引:1,自引:0,他引:1
Tsakanika LV Ochsenkühn-Petropoulou MT Mendrinos LN 《Analytical and bioanalytical chemistry》2004,379(5-6):796-802
A chromatographic method has been developed for separation and determination of scandium (Sc) and rare earth elements (REEs) in samples from a red mud (RM)-utilization process. Reversed-phase high-performance liquid chromatography (RP-HPLC) with post-column derivatization using 4-(2-pyridylazo)-resorcinol (PAR) and UV–visible detection at 520 nm was tested using different gradient elution profiles and pH values to optimize separation and recovery, primarily for Sc but also for yttrium and the individual lanthanides, from iron present in the samples. The separation was performed in less than 20 min by use of a mobile phase gradient. The concentration of -hydroxyisobutyric acid (-HIBA), as eluent, was altered from 0.06 to 0.4 mol L–1 (pH 3.7) and 0.01 mol L–1 sodium salt n-octane sulfonic acid (OS) was used as modifier. Very low detection limits in the nanogram range and a good resolution for Sc and REEs except for Y/Dy were achieved. Before application of the method to the red mud samples and to the corresponding bauxites, Sc and REEs were leached from red mud with 0.6 mol L–1 HNO3
and mostly separated, as a group, from the main elements by ion exchange/selective elution (6 mol L–1
HNO3) in accordance with a pilot-plant process developed in this laboratory. After evaporation of the eluent to dryness the extracted elements were re-dissolved in the mobile phase. By use of this chromatographic method Sc, which is the most expensive of the elements investigated and occurs in economically interesting concentrations in red mud, could be separated not only from co-existing Fe but also from Y/Dy, Yb, Er, Ho, Gd, Eu, Sm, Nd, Pr, Ce and La. All the elements investigated were individually recovered. Their recoveries were found to be nearly quantitative. 相似文献
18.
V. V. Avdonin E. I. Kirpichev Va. I. Rubtsov L. B. Romanova M. E. Ivanova L. T. Eremenko 《Russian Chemical Bulletin》1996,45(10):2342-2344
The thermochemical study of cubane-1,4-dicarboxylic acid (1), diethyl cubane-1,4-dicarboxylate (2), diisopropyl cubane-1,4-dicarboxylate (3), and bis(2-fluoro-2,2-dinitro)ethyl cubane-1,4-dicarboxylate (4) was performed. The standard enthalpies of combustion (c
H°) and formation (f
H°) of these compounds were estimated using the method of combustion in a calorimetric bomb in an oxygen atmosphere. Using the additive group method, calculated values for f
H° of these substances which agreed satisfactorily with the experimental ones were obtained. The strain energies (E
s) of the cubic structure of derivatives1–4 were calculated. It was concluded thatE
s did not change on substitution of hydrogen atoms in cubane for various functional groups and was equal toE
s of the structure of cubane itself. The reliability of the single published value of f
H° in the cubane crystal state, 541.8 kJ mol–1 (129.5 kcal mol–1), was confirmed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2471–2473, October, 1996. 相似文献
19.
Summary An emission spectrographic method for the estimation of Ga, In and Tl in semiconductor grade selenium at less than a ppm concentration has been developed. The impurities were separated from 1 g of selenium by volatilizing it from its solution in nitric acid and sulphuric acid. The residue containing the impurities was dissolved in small amounts of 6 M HCl and was loaded along with the washings on 10 mg mixture containing graphite (90%) and sodium chloride carrier (10%) in the electrode crater. The spectra were excited in a d.c. arc carrying 13 A current. The detection limits lie in the range of 0.0025–0.005 ppm. The precision of the method ranges from 7–12% for these elements. The recoveries of impurities from selenium were confirmed using radioactive tracers.
Spektrographische Bestimmung von Gallium, Indium und Thallium in Halbleiterselen
Zusammenfassung Eine 1 g-Probe wird in Salpeter- und Schwefelsäure gelöst und die Verunreinigungen durch Verflüchtigung des Selens isoliert. Sie werden in 6 M HCl gelöst und zusammen mit einem Graphit/NaCl-Gemisch (91) in die Elektrode eingebracht. Zur Anregung dient ein Gleichstrombogen (13 A). Die Nachweisgrenzen liegen im Bereich von 0.0025–0.005 ppm, der Variationskoeffizient bei 7–12%. Die Wiedergewinnung der Verunreinigungen wurde mit Hilfe von Radioindicatoren geprüft.相似文献
20.
Changes in the chemical composition of surface were studied and the rates of formation of gaseous degradation products during polypropylene treatment in oxygen plasma and its flowing afterglow were determined by means of attenuated total internal reflectance IR spectroscopy and mass spectrometry. It was found that surface oxidation at minimal degradation rates is reached upon the joint action of O2(a 1Δ g ) molecules and ground-state oxygen molecules, whereas O(3 P) atoms participate in both oxidation processes and the processes of degradation of oxygen-containing groups leading to the formation of gaseous products. It was shown that only the action of plasma leads to the formation of vinyl and vinylidene double bonds, which disappear in reactions with O2(a 1Δ g ) molecules and O(3 P) atoms. 相似文献