Preparation of nanocrystalline BiFeO3 and kinetics of thermal process of precursor

The Bi₂Fe₂(C₂O₄)₅·5H₂O was synthesized by solid-state reaction at low heat using Bi(NO₃)₃·5H₂O, FeSO₄·7H₂O, and Na₂C₂O₄ as raw materials. The nanocrystalline BiFeO₃ was obtained by calcining Bi₂Fe₂(C₂O₄)₅·5H₂O at 600 °C in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, FT-IR, X-ray powder diffraction, and vibrating sample magnetometer. The data showed that highly crystallized BiFeO₃ with hexagonal structure [space group R3c(161)] was obtained when the precursor was calcined at 600 °C in air for 1.5 h. The thermal process of the precursor in air experienced five steps which involved, at first, the dehydration of an adsorption water molecule, then dehydration of four crystal water molecules, decomposition of FeC₂O₄ into Fe₂O₃, decomposition of Bi₂(C₂O₄)₃ into Bi₂O₃, and at last, reaction of Bi₂O₃ and Fe₂O₃ into hexagonal BiFeO₃. Based on Starink equation, the values of the activation energies associated with the thermal process of Bi₂Fe₂(C₂O₄)₅·5H₂O were determined. Besides, the most probable mechanism functions and thermodynamic functions (ΔS ^≠, ΔH ^≠, and ΔG ^≠) of thermal processes of Bi₂Fe₂(C₂O₄)₅·5H₂O were also determined.     

Magnetic properties of nanocrystalline CuFe2O4 and kinetics of thermal decomposition of precursor

CuFe₂(C₂O₄)₃·4.5H₂O was synthesized by solid-state reaction at low heat using CuSO₄·5H₂O, FeSO₄·7H₂O, and Na₂C₂O₄ as raw materials. The spinel CuFe₂O₄ was obtained via calcining CuFe₂(C₂O₄)₃·4.5H₂O above 400 °C in air. The CuFe₂(C₂O₄)₃·4.5H₂O and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform FT-IR, X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray spectrometer, and vibrating sample magnetometer. The result showed that CuFe₂O₄ obtained at 400 °C had a saturation magnetization of 33.5 emu g^?1. The thermal process of CuFe₂(C₂O₄)₃·4.5H₂O experienced three steps, which involved the dehydration of four and a half crystal water molecules at first, then decomposition of CuFe₂(C₂O₄)₃ into CuFe₂O₄ in air, and at last crystallization of CuFe₂O₄. Based on KAS equation, OFW equation, and their iterative equations, the values of the activation energy for the thermal process of CuFe₂(C₂O₄)₃·4.5H₂O were determined to be 85 ± 23 and 107 ± 7 kJ mol^?1 for the first and second thermal process steps, respectively. Dehydration of CuFe₂(C₂O₄)₃·4.5H₂O is multistep reaction mechanisms. Decomposition of CuFe₂(C₂O₄)₃ into CuFe₂O₄ could be simple reaction mechanism, probable mechanism function integral form of thermal decomposition of CuFe₂(C₂O₄)₃ is determined to be 1 ? (1 ? α)^1/4.     

Thermal stability of the manganese-nickel mixed ferrite and iron phases in the Mn0.5Ni0.5Fe2O4/Fe composite/nanocomposite powder

The composite/nanocomposite powders of Mn_0.5Ni_0.5Fe₂O₄/Fe type were synthesized starting from nanocrystalline Mn_0.5Ni_0.5Fe₂O₄ (D = 7 nm) (obtained by ceramic method and mechanical milling) and commercial Fe powders. The composites, Mn_0.5Ni_0.5Fe₂O₄/Fe, were milled for up to 120 min and subjected to heat treatment at 600 °C and 800 °C for 2 h. The manganese-nickel ferrite/iron composite samples were subjected to differential scanning calorimetry (DSC) up to 900 °C for thermal stability investigations. The composite component phases evolution during mechanical milling and heat treatments were investigated by X-ray diffraction technique. The present phases in Mn_0.5Ni_0.5Fe₂O₄/Fe composite are stable up to 400–450 °C. In the temperature range of 450-600 °C, the interdiffusion phenomena occurs leading to the formation of Fe_1?xMn_xFe₂O₄/Ni–Fe composite type. The new formed ferrite of Fe_1?xMn_xFe₂O₄ type presents an increased lattice parameter as a result of the substitution of nickel cations into the spinel structure by iron ones. Further increases of the temperature lead to the ferrite phase partial reduction and the formation of wustite-FeO type phase. The spinel structure presents incipient recrystallization phenomena after both heat treatments (600 °C and 800 °C). The mean crystallites size of the ferrite after heat treatment at 800 °C is about 75 nm. After DSC treatment at 900 °C, the composite material consists in Fe_1?xMn_xFe₂O₄, Ni structure, FeO, and (NiO)_0.25(MnO)_0.75 phases.     

Solid state reactivity and mechanisms in oxide systems

Starting from oxalate mixed crystals Ni_xMn_3?x(C₂O₄)₃·6H₂O thermal decomposition at reduced oxygen partial pressure (po₂= 2%) leads to the formation of NiMn₂O₄ (x = 1) at metastable conditions. Ni_1.5Mn_1.5O₄ (x = 1.5) existing in the metastable state only has been also prepared. The spinel compounds both are of the highly inversed type. Following a sol-gel preparation route Mg₂TiO₄ has been also found to be formed in the metastable state. Annealing results in decomposition of the compounds providing NiMnO₃ and 1/2α-Mn₂O₃ or NiMnO₃ only or MgTiO₃ and MgO, respectively. The reaction rates observed are lower for NiMn₂O₄ and Ni_1.5Mn_1.5O₄ than for Mg₂TiO₄ decomposition. The reverse reaction of NiMn₂O₄ formation above 730°C shows an endothermic enthalpy of +61 kJ·mol^?1. For Mg₂TiO₄ formation above 1050°C an endothermic enthalpy of +19.3 kJ·mol^?1 is found. The results are discussed in terms of structural features of the oxides.     

Thermal analysis of some polynuclear coordination compounds as precursors of iron garnets (M3Fe5O12, M=Y3+ or Er3+)

The thermal behaviour of four coordination compounds (NH₄)₆[Y₃Fe₅(C₄O₅H₄)₆(C₄O₅H₃)₆]·12H₂O, (NH₄)₆[Y₃Fe₅(C₆O₇H₁₀)₆(C₆O₇H₉)₆]·8H₂O, (NH₄)₆[Er₃Fe₅(C₄O₅H₄)₆(C₄O₅H₃)₆]·10H₂O and (NH₄)₆[Er₃Fe₅(C₄O₆H₄)₆(C₄O₆H₃)₆]·22H₂O has been studied to evaluate their suitability for garnet synthesis. The thermal decomposition and the phase composition of the resulted decomposition compounds are influenced by the nature of metallic cations (yttrium-iron or erbium-iron) and ligand anions (malate or gluconate).     

A simple synthesis and room temperature magnetic properties of new binary Mn0.5Fe0.5(H2PO4)2·xH2O obtained from a rapid co-precipitation at ambient temperature

A new binary Mn_0.5Fe_0.5(H₂PO₄)₂·xH₂O powder was synthesized by simple and cost-effective method using phosphoric acid, manganese and iron metals as starting chemicals. The synthesized solid shows the complex thermal transformations and the final decomposition product is a new binary manganese iron cyclo-tetraphosphate, MnFeP₄O₁₂. The X-ray diffraction and FTIR results indicate that the synthesized new binary Mn_0.5Fe_0.5(H₂PO₄)₂·xH₂O and the decomposition MnFeP₄O₁₂ powders are a pure monoclinic phase with space group P2₁/n (Z = 2) and C2/c (Z = 4), respectively. The particle morphologies of Mn_0.5Fe_0.5(H₂PO₄)₂·xH₂O and MnFeP₄O₁₂ powders appear as the rod-like tetragonal shape and show a high agglomeration of small particles, which are similar to the case of Mn(H₂PO₄)₂·2H₂O and Fe₂P₄O₁₂, respectively. Room temperature magnetization results show a ferromagnetic behavior of the Mn_0.5Fe_0.5(H₂PO₄)₂·xH₂O and MnFeP₄O₁₂ powders, having the hysteresis loops in the range of ?10,000 Oe < H < +10,000 Oe with the specific magnetization values of 25.63 and 13.14 emu/g at 10 kOe, respectively. The lower magnetizations of Mn_0.5Fe_0.5(H₂PO₄)₂·xH₂O and MnFeP₄O₁₂ than those of Fe(H₂PO₄)₂·2H₂O and Fe₂P₄O₁₂ powders indicate the presence of Mn ions in substitution position of Fe ions.     

TG/DTA/MS Study of the thermal decomposition of FeSO4·6H2O

The thermal decomposition of FeSO₄·6H₂O was studied by mass spectroscopy coupled with DTA/TG thermal analysis under inert atmosphere. On the ground of TG measurements, the mechanism of decomposition of FeSO₄·6H₂O is: i) three dehydration steps FeSO₄·6H₂O FeSO₄·4H₂O+2H₂O FeSO₄·4H₂O FeSO₄·H₂O+3H₂O FeSO₄·H₂O FeSO4+H₂O ii) two decomposition steps 6FeSO₄ Fe₂(SO₄)₃+2Fe₂O₃+2SO₂ Fe₂(SO₄)3 Fe₂O₃+3SO₂+3/2O₂ The intermediate compound was identified as Fe₂(SO₄)₃ and the final product as the hematite Fe₂O₃.     

Theoretical studies of electronic structure and structural properties of anhydrous alkali metal oxalates

Nanocrystalline LiMn₂O₄ was synthesized by calcining LiMn₂(CO₃)_2.5·0.8H₂O above 600 °C in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, X-ray powder diffraction, and scanning electron microscopy. The result showed that highly crystallization LiMn₂O₄ with cubic structure [space group Fd-3m(227)] was obtained when the precursor was calcined at 600 °C in air for 1.5 h. The thermal process of the precursor in air experienced three steps which involved, at first, the dehydration of 0.8 water molecules, then decomposition of MnCO₃ into Mn₂O₃, at last, reaction of Mn₂O₃ and Li₂CO₃ into cubic LiMn₂O₄. Based on Starink equation, the values of the activation energies associated with the thermal process of LiMn₂(CO₃)_2.5·0.8H₂O were determined. Besides, most probable mechanism functions and thermodynamic functions (ΔS ^≠, ΔH ^≠, and ΔG ^≠) of thermal processes of LiMn₂(CO₃)_2.5·0.8H₂O were also determined.     

Thermal decomposition mechanism of iron(III) nitrate and characterization of intermediate products by the technique of computerized modeling

The nonahydrate of iron(III) nitrate shows no phase transitions in the range of ?40 to 0 °C. Both hexahydrate Fe(NO₃)₃·6H₂O and nonahydrate Fe(NO₃)₃·9H₂O have practically the same thermal behavior. Thermal decomposition of iron nitrate is a complex process which has a different mechanism than those described for other trivalent elements. Thermolysis begins with the successive condensation of 4 mol of the initial monomer accompanied by the loss of 4 mol of nitric acid. At higher temperature, hydrolytic processes continue with the gradual elimination of nitric acid from resulting tetramer and dimeric iron oxyhydroxide Fe₄O₄(OH)₄ is formed. After complete dehydration, oxyhydroxide is destroyed leaving behind 2 mol of Fe₂O₃. The molecular mechanics method provides a helpful insight into the structural arrangement of intermediate compounds.     

Thermal decomposition of natural iowaite

The thermal decomposition of natural iowaite of formula Mg₆Fe₂(Cl,(CO₃)_0.5)(OH)₁₆·4H₂O was studied by using a combination of thermogravimetry and evolved gas mass spectrometry. Thermal decomposition occurs over a number of mass loss steps at 60°C attributed to dehydration, 266 and 308°C assigned to dehydroxylation of ferric ions, at 551°C attributed to decarbonation and dehydroxylation, and 644, 703 and 761°C attributed to further dehydroxylation. The mass spectrum of carbon dioxide exhibits a maximum at 523°C. The use of TG coupled to MS shows the complexity of the thermal decomposition of iowaite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis,Characterization and Thermal Decompositions Studies of Some Malates Coordination CompoundsI. iron nickel compounds

The thermal behaviour of three coordination compounds, potential precursors of nickel ferrite [Fe₂Ni(C₄H₄O₅)_2.5(OH)₂]NO₃·5H₂O,[Fe₂Ni(C₄H₈O₃N₂)₄](NO₃)₈·24H₂O and (NH₄)[Fe₂Ni(C₄H₄O₅)₃(OH)₃]·3H₂O has been investigated to evaluate their suitability as precursors for nickel ferrite. For a complete and reliable assignment of the thermal transformations, the isolable solid intermediates and end products were characterized by IR, X-ray diffraction and Mössbauer investigations. A decomposition scheme is proposed.     

The solid-state thermal decomposition of Sr3[La(C2O4)3(H2O)2]2·11H2O

Strontium(II)diaquatris(oxalato)lanthanate(III)unidecahydrate, Sr₃[La(C₂O₄)₃(H₂O)₂]₂·11H₂O, has been synthesized and characterized by elemental, IR and electronic spectral studies. Thermal studies (TG, DTG and DTA) in air showed that all the crystal and coordinated water molecules are removed at ca. 225 °C. The final end product at 1,000 °C was shown to be a mixture of mainly SrCO₃, Sr₃La₄O₉ and La₂Sr₂O₅ along with oxides and carbides of both the metal, through the formation of an intermediate mixture of likely SrC₂O₄ and La₂(C₂O₄)_2.8 at 282 °C, and SrCO₃ and La₂O(CO₃)₂ at 540 °C. The multi-step dehydration and decomposition of the compound has been explored from the DSC study in nitrogen up to 670 °C, and the evaluated kinetic parameters are discussed.     

Preparation of CuCr<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowders using two different chromium sources

CuCr₂O₄ spinel powders were synthesized starting from different chromium sources, namely (i) chromium oxide (α-Cr₂O₃) and (ii) ammonium dichromate ((NH₄)₂Cr₂O₇). The copper source was a Cu(II) carboxylate-type complex. The Cu(II) carboxylate complex was obtained by the redox reaction between Cu(NO₃)₂·3H₂O and 1,3-propanediol (1,3PG) at 130 °C. In the first case (i), we have started from a mixture of α-Cr₂O₃, Cu(NO₃)₂·3H₂O and 1,3PG that upon heating formed the copper malonate complex, which decomposed around 220 °C forming an oxide mixture (CuO + α-Cr₂O₃). In the second case (ii), (NH₄)₂Cr₂O₇, Cu(NO₃)₂·3H₂O and 1,3PG were homogenously mixed. Heating this mixture at 130 °C resulted, in situ, in the Cu(II) complex. On controlled temperature increase, the violent decomposition of (NH₄)₂Cr₂O₇ took place at 180 °C along with the decomposition of the Cu(II) complex, leading to an amorphous oxide mixture of Cr₂O_3+x and CuO. By annealing the samples in the temperature range 400–1000 °C, the spinel phase (CuCr₂O₄) was obtained in both cases: (i) at 800 °C and (ii) at 600 °C as a result of the interactions between the precursors used, when the oxide system was amorphous and highly reactive. The presence of CuCr₂O₄ was highlighted by XRD and FTIR analyses.     

Thermal behaviour of two new salts of malonic acid: Cu(C3H2O4)·4H2O and Cu(NH4)2(C3H2O4)2

Two new salts of malonic acid have been prepared: the copper(II) malonate tetrahydrate and the copper(II)-ammonium double malonate. Their study by thermal analysis (TG and DTA) leads to the following results:Cu(C₃H₂O₄)·4H₂O: the dehydration is rather complex and it is only under careful conditions that an intermediate hydrate Cu(C₃H₂O₄)·3H₂O could be traced. At about 170°C the dehydration is not ended (the salt holds yet about 0.15H₂O) and the anhydrous salt occurs only at about 240°C. It decomposes immediately leading to residues the composition of which depends upon the surrounding atmosphere; the part played by the gas given off is discussed.Cu(NH₄)₂(C₃H₂O₄)₂: this salt melts and decomposes simultaneously at about 190°C. During the decomposition the copper nitride Cu₃N forms as intermediate compound (as well as copper metal). Concerning the final residues of the decomposition the results and the conclusions are the same as the ones of the previous case.     

New members of the [Mn6/oxime] family and analogues with converging [Mn3] planes

The synthesis, structural, and magnetic characterization of five new members of the hexanuclear oximate [Mn^III₆] family are reported. All five clusters can be described with the general formula [Mn^III₆O₂(R-sao)₆(R′-CO₂)₂(sol)_x(H₂O)_y] (where R-saoH₂ = salicylaldoxime substituted at the oxime carbon with R = H, Me and Et; R′ = 1-naphthalene, 2-naphthalene, and 1-pyrene; sol = MeOH, EtOH, or MeCN; x = 0–4 and y = 0–4). More specifically, the reaction of Mn(ClO₄)₂·6H₂O with salicylaldoxime-like ligands and the appropriate carboxylic acid in alcoholic or MeCN solutions in the presence of base afforded complexes 1–5: [Mn₆O₂(Me-sao)₆(1-naphth-CO₂)₂(H₂O)(MeCN)]·4MeCN (1·4MeCN); [Mn₆O₂(Me-sao)₆(2-naphth-CO₂)₂(H₂O)(MeCN)]·3MeCN·0.1H₂O (2·3MeCN·0.1H₂O); [Mn₆O₂(Et-sao)₆(2-naphth-CO₂)₂(EtOH)₄(H₂O)₂] (3); [Mn₆O₂(Et-sao)₆(2-naphth-CO₂)₂(MeOH)₆] (4) and [Mn₆O₂(sao)₆(1-pyrene-CO₂)₂(H₂O)₂(EtOH)₂]·6EtOH (5·6EtOH). Clusters 3, 4, and 5 display the usual [Mn₆/oximate] structural motif consisting of two [Mn₃O] subunits bridged by two O_oximate atoms from two R-sao^2? ligands to form the hexanuclear complex in which the two triangular [Mn₃] units are parallel to each other. On the contrary, clusters 1 and 2 display a highly distorted stacking arrangement of the two [Mn₃] subunits resulting in two converging planes, forming a novel motif in the [Mn₆] family. Investigation of the magnetic properties for all complexes reveal dominant antiferromagnetic interactions for 1, 2, and 5, while 3 and 4 display dominant ferromagnetic interactions with a ground state of S = 12 for both clusters. Finally, 3 and 4 display single-molecule magnet behavior with U_eff = 63 and 36 K, respectively.     

Thermal decomposition of the synthetic hydrotalcite iowaite

The thermal stability and thermal decomposition pathways for synthetic iowaite have been determined using thermogravimetry in conjunction with evolved gas mass spectrometry. Chemical analysis showed the formula of the synthesised iowaite to be Mg_6.27Fe_1.73(Cl)_1.07(OH)₁₆(CO₃)_0.336.1H₂O and X-ray diffraction confirms the layered structure. Dehydration of the iowaite occurred at 35 and 79°C. Dehydroxylation occurred at 254 and 291°C. Both steps were associated with the loss of CO₂. Hydrogen chloride gas was evolved in two steps at 368 and 434°C. The products of the thermal decomposition were MgO and a spinel MgFe₂O₄. Experimentally it was found to be difficult to eliminate CO₂ from inclusion in the interlayer during the synthesis of the iowaite compound and in this way the synthesised iowaite resembled the natural mineral.     

Thermal studies on yttrium,neodymium and holmium oxalates

The thermal decomposition patterns of Y₂(C₂O₄)₃ · 9 H₂O, Nd₂(C₂O₄)₃ · 10 H₂O and Ho₂(C₂O₄)₃ · 5.5 H₂O have been studied using TG and DTG. The hydrated neodymium oxalate loses all the water of hydration in one step to give the anhydrous oxalate while Y₂(C₂O₄)₃ · 9 H₂O and Ho₂(C₂O₄)₃ · 5.5 H₂O involve four or more dehydration steps to yield the anhydrous oxalates. Further heating of the anhydrous oxalates results in the loss of CO₂ and CO to give the stable metal oxides.     

Synthesis, characterization, infrared studies, and thermal analysis of Mn0.6Zn0.4Fe2(C4H2O4)3·6N2H4 and its decomposition product Mn0.6Zn0.4Fe2O4

Manganese zinc ferrous fumarato–hydrazinate precursor, Mn_0.6Zn_0.4Fe₂(C₄H₂O₄)₃·6N₂H₄ was synthesized for the first time and characterized by chemical analysis, infrared spectral studies, and thermal analysis. Infrared studies show band at 977 cm^?1 indicating bidentate bridging nature of the hydrazine in the complex. Thermogravimetric (TG) studies show two steps dehydrazination followed by two steps total decarboxylation. The precursor on touching with burning splinter undergoes self propagating autocatalytic decomposition yielding ultrafine Mn_0.6Zn_0.4Fe₂O₄. XRD studies confirms single phase formation as well as nanosize nature of “as prepared” Mn_0.6Zn_0.4Fe₂O₄. The saturation magnetization of the “as prepared” Mn_0.6Zn_0.4Fe₂O₄ was found to be 31.46 emu gm^?1, which is lower than the reported, confirms the ultrafine nature of the oxide.     

The kinetic parameters of thermal decomposition hydrated iron sulphate

The thermal decomposition of iron sulphate hexahydrate was studied by thermogravimetry at a heating rate of 5°C min^?1 in static air. The kinetic parameters were evaluated using the integral method by applying the Coats and Redfern approximation. The thermal stabilities of the hydrates were found to vary in the order. Fe₂(SO₄)₃·6H₂O → Fe₂(SO₄)₃·4.5H₂O → Fe₂(SO₄)₃·0.5H₂O The dehydration process of hydrated iron sulphate was found to conform to random nucleation mass loss kinetics, and the activation energies of the respective hydrates were 89.82, 105.04 and 172.62 kJ mol^?1, respectively. The decomposition process of anhydrous iron sulphate occurs in the temperature region between 810 and 960 K with activation energies 526.52 kJ mol^?1 for the D3 model or 256.05 kJ mol^?1 for the R3 model.     

Thermal decomposition of lutetium propionate

The thermal decomposition of lutetium(III) propionate monohydrate (Lu(C₂H₅CO₂)₃·H₂O) in argon was studied by means of thermogravimetry, differential thermal analysis, IR-spectroscopy and X-ray diffraction. Dehydration takes place around 90 °C. It is followed by the decomposition of the anhydrous propionate to Lu₂O₂CO₃ with evolution of CO₂ and 3-pentanone (C₂H₅COC₂H₅) between 300 °C and 400 °C. The further decomposition of Lu₂O₂CO₃ to Lu₂O₃ is characterized by an intermediate constant mass plateau corresponding to a Lu₂O_2.5(CO₃)_0.5 overall composition and extending from approximately 550 °C to 720 °C. Full conversion to Lu₂O₃ is achieved at about 1000 °C. Whereas the temperatures and solid reaction products of the first two decomposition steps are similar to those previously reported for the thermal decomposition of lanthanum(III) propionate monohydrate, the final decomposition of the oxycarbonate to the rare-earth oxide proceeds in a different way, which is here reminiscent of the thermal decomposition path of Lu(C₃H₅O₂)·2CO(NH₂)₂·2H₂O.