The trapping and transport phenomena of hydrogen in nickel

Hydrogen thermal analysis experiments have been employed to study the trapping and transport phenomena of hydrogen in nickel. Dislocations in nickel act as trapping sites of hydrogen, and the hydrogen trap activation energy at dislocations appears to be lower than the activation energy for the bulk diffusion of hydrogen. It is suggested that both hydrogen trapping at grain boundaries and short-circuit diffusion through grain boundaries in nickel are present. The trap binding energy at grain boundaries is estimated as 20.5 kJ ⋅ mol^-1. Using the hydrogen thermal analysis experiments, the solubility and diffusivity of hydrogen in nickel have been measured. The temperature dependences of those are described by C (H atoms/Ni atom) = 1.57 × 10^-3 exp(-11.76 kJ ⋅ mol^-1/RT) and D (m² s^-1) = 7.5 × 10^-7 exp(-39.1 kJ ⋅ mol^-1/RT), respectively.     

Hydrogen trapping in thoria-dispersed nickel

Trapping of hydrogen in thoria-dispersed nickel has been investigated by the hydrogen thermal desorption technique. It has been found that voids are formed at the particle-matrix interface during cold-working. Annealing at high temperature removes these extra trapping sites, allowing an intrinsic trapping effect due to the thoria particle-matrix interface to be measured. The trapbinding energy and trap-activation energy of hydrogen at the ThO₂-nickel matrix interface are estimated as 35 kJ mole^-1 and 48.7 kJ mole^-1, respectively, using the mathematical models derived from the existing trap theory. The energy level of hydrogen around the ThO₂-lattice interface is derived from the above values. The saddle-point energy of hydrogen at the ThO₂-lattice interface, 13.7 kJ mole^-1, is lower than the activation energy for hydrogen diffusion through a normal lattice, 40.2 kJ mole^-1. It is suggested that during thermal desorption from thoria-dispersed nickel, some of the hydrogen atoms in normal lattice interstitial sites are retrapped at ThO₂-matrix lattice interfaces which are not occupied with hydrogen at the charging temperature. Trap sites at ThO₂-matrix interfaces are dilutely occupied under 1-atm hydrogen pressure in the temperature range of 598 to 773 K. The fractional occupancy of traps ranged from 0.14 to 0.05. SUNG-MAN LEE, formerly with the Korea Advanced Institute of Science and Technology     

Hydrogen in iron

The applicability of advanced permeation techniques to the study of hydrogen and deuterium in iron and iron alloys is described. Time lag measurements lead to detailed information about hydrogen transport processes, including lattice diffusivities and trap binding energies and densities. The experimental technique couples gas phase charging of palladium coated specimens with the sensitive electrochemical detection method. In both annealed and deformed iron the trap binding energy for hydrogen and deuterium is 50 to 58 kJ/mol, while the trap density varies from about 10²⁰ m⁻³ for annealed iron to over 10²³ m⁻³ for heavily deformed iron. For the metallic glass Fe₄₀Ni₄₀P₁₄B₆ hydrogen transport occurs between energetically equivalent sites, with no evidence of trapping. The site density was estimated as about 6 × 10²⁹ m⁻³ . The hydrogen concentrations studied were several orders of magnitude less. Hydrogen and deuterium in iron differs only in their lattice diffusivities. The diffusivity ratio conforms nearly to the classical inverse square root of mass ratio, but shows a slight temperature dependence. The solubilities, trap binding energies, and partial atomic volumes of the two isotopes in iron are identical.     

The identification of hydrogen trapping states in an Al-Li-Cu-Zr alloy using thermal desorption spectroscopy

Thermal desorption spectroscopy (TDS) was utilized to identify several metallurgical states in an Al -2Li - 2Cu-0.1Zr (wt pct) alloy, which trap absorbed hydrogen. Six distinct metallurgical desorption states for hydrogen were observed for tempers varying from the T3 to peakaged condition. Lower energy thermal desorption states were correlated with interstitial sites, lithium in solid solution, and δ′ (Al₃Li) precipitates. These states have trap-binding energies ≤25.2 kJ/mol. Under the charging conditions utilized, approximately 4 pct of the total (e.g., trapped and lattice) hydrogen content was associated with interstitial sites, consistent with the view that the intrinsic lattice solubility of hydrogen in aluminum is very low. In contrast, dislocations, grain boundaries, and T₁ (Al₂CuLi) particles were found to be higher energy-trap states with trap-binding energies ≥31.7 kJ/mol. Approximately 78 pct of all absorbed hydrogen occupied these states. Moreover, greater than 13 pct of the available trap sites at grain boundaries were occupied. Such a high hydrogen coverage at grain-boundary sites supports the notion that hydrogen contributes to grain-boundary environmental cracking in Al-Li-Cu-Zr alloys. Also, it points out the error in assuming that hydrogen cannot play a major role in cracking of Al-based alloys due to the low lattice solubility.     

Viscosity of niobium-containing oxide-fluoride melts

Influence of Nb₂O₅ on viscosity of the CaF₂-based and CaF₂-30% Al₂O₃-based melts was investigated using a vibration viscosimetry method in the range 1523–1873 K. It is found that viscosity of the melts rise and the crystallization range shifts to lower temperatures as the Nb₂O₅ concentration increases from 0 to 25 wt %. The activation energy of viscous flow increases from 34 to 110 kJ mol^?1 and from 56 to 148 kJ mol^?1 for the CaF₂-based and CaF₂-30% Al₂O₃-based melts, respectively. The obtained data indicate the complexing behavior of niobium in the oxide-fluoride melts.     

Hydrogen in iron

The applicability of advanced permeation techniques to the study of hydrogen and deuterium in iron and iron alloys is described. Time lag measurements lead to detailed information about hydrogen transport processes, including lattice diffusivities and trap binding energies and densities. The experimental technique couples gas phase charging of palladium coated specimens with the sensitive electrochemical detection method. In both annealed and deformed iron the trap binding energy for hydrogen and deuterium is 50 to 58 kJ/mol, while the trap density varies from about 10²⁰ m^-3 for annealed iron to over 10²³ m^-3 for heavily deformed iron. For the metallic glass Fe₄₀Ni₄₀P₁₄B₆ hydrogen transport occurs between energetically equivalent sites, with no evidence of trapping. The site density was estimated as about 6 x 10²⁹ m^-3 . The hydrogen concentrations studied were several orders of magnitude less. Hydrogen and deuterium in iron differs only in their lattice diffusivities. The diffusivity ratio conforms nearly to the classical inverse square root of mass ratio, but shows a slight temperature dependence. The solubilities, trap binding energies, and partial atomic volumes of the two isotopes in iron are identical.     

Hydrogen in iron

The applicability of advanced permeation techniques to the study of hydrogen and deuterium in iron and iron alloys is described. Time lag measurements lead to detailed information about hydrogen transport processes, including lattice diffusivities and trap binding energies and densities. The experimental technique couples gas phase charging of palladium coated specimens with the sensitive electrochemical detection method. In both annealed and deformed iron the trap binding energy for hydrogen and deuterium is 50 to 58 kJ/mol, while the trap density varies from about 10²⁰ m^-3 for annealed iron to over 10²³ m^-3 for heavily deformed iron. For the metallic glass Fe₄₀Ni₄₀P₁₄B₆ hydrogen transport occurs between energetically equivalent sites, with no evidence of trapping. The site density was estimated as about 6 x 10²⁹ m^-3 . The hydrogen concentrations studied were several orders of magnitude less. Hydrogen and deuterium in iron differs only in their lattice diffusivities. The diffusivity ratio conforms nearly to the classical inverse square root of mass ratio, but shows a slight temperature dependence. The solubilities, trap binding energies, and partial atomic volumes of the two isotopes in iron are identical.     

Quantitative analysis on hydrogen trapping of TiC particles in steel

The change in the hydrogen-trapping behavior of a TiC particle accompanying its coherent to incoherent interfacial-character transition in a 0.05C-0.20Ti-2.0Ni steel that was quenched and tempered in a partially protective argon atmosphere and in ultrahigh vacuum (UHV) has been studied by thermal desorption spectrometry (TDS). The results indicated that (semi)coherent TiC precipitates demonstrate distinctly different hydrogen-trapping features from that of incoherent TiC particles with respect to hydrogen capacity, interaction energy with hydrogen, locations available for hydrogen occupation, and the capability of hydrogen absorption from the environment. The broad (semi)coherent interface of the disc-shaped (semi)coherent TiC precipitate does not trap hydrogen during tempering in a partially protected argon atmosphere, but traps hydrogen during cathodic charging at room temperature. The semicoherent interface traps 1.3 atoms/nm² of hydrogen at the core of the misfit dislocation with short-time charging (1 hour), which is characterized by a desorption activation energy of 55.8 kJ/mol. The side interface of the (semi)coherent TiC precipitate acts like the broad interface when the precipitate is small. As the precipitate grows, the side interface gradually loses its coherency and results in a simultaneous increase in the trapping activation energy and the binding energy. An increase in the trapping activation energy, i.e., the energy barrier for trapping, makes hydrogen trapping more difficult in cathodic charging at room temperature, while an increase in the binding energy enhances the capability of hydrogen absorption from the atmosphere during heat treatment. An incoherent TiC particle is not able to trap hydrogen during cathodic charging at room temperature due to its high energy barrier for trapping, but absorbs hydrogen during heat treatment at high temperatures. The amount of hydrogen that is trapped by incoherent TiC particles depends on their volume, which strongly indicates that incoherent TiC particles trap hydrogen within them rather than at the particle/matrix interface. Octahedral carbon vacancies are supposedly the hydrogen trap sites in incoherent TiC particles.     

Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite

The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an Al-10 vol pct SiC_p composite has been studied. The Al-4 wt pct Mg-10 vol pct SiC_p composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol^-1. These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol^-1, at a constant stress, σ = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the Al-Mg-A1₂O_3f composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the Al-Mg-SiC_p, composite are compared with those of the Al-Mg-Al₂O_3f and 6061 Al-SiC_p.w, composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the Al-SiC_p composite.     

Hydrogen trapping at spheroidized and elongated sulphidic inclusions-matrix interfaces in mild steel

The present work is concerned with those factors which determine the hydrogen trapping at the interfaces between spheroidized and elongated sulphidic inclusions, and matrix in mild steel by using gas-phase charging and electrochemical detection techniques. Three kinds of specimens A, B and C were prepared from the calcium-treated mild steel by water quench from 950°C, and from the ordinary mild steel by water quench from 950 and 1150°C, respectively. Specimen A was characterized by the interface between the spheroidized sulphidic inclusions and matrix, but the specimens B and C were characterized by the elongated sulphide-matrix interface. The values of time-lag decreased with increasing hydrogen input pressure for the specimens A, B and C. The results indicated that the defects produced at the interfaces act as saturable trap sites for hydrogen. The hydrogen trap density and binding energy were obtained from the plot of [(t_T/t_L)–1] vs. . The trap densities for the specimens A, B and C were found to be about 5.0 × 10^?8, 2.1 × 10^?7 and 5.0 × 10^?7 mol cm^?3, respectively. The trap-binding energy was determined to be ?(56.4 ± 1.1) kJ mol^?1 for the specimens A, B and C as well. The experimental results indicated that the nature of the interfaces is determined by the number of defects produced in the interfaces per unit volume, regardless of the inclusion shape. The defects distributed in the interfaces included namely microvoids and water-quench-created dislocations which act as deep trap sites for hydrogen.     

Kinetics of Na2O evaporation from CaO–Al2O3–SiO2–MgO–TiO2–Na2O slags

The present work is carried out to study the evaporation of Na₂O from CaO–Al₂O₃–SiO₂–TiO₂–MgO–Na₂O slags with high basicity and high alumina in the temperature range of 1500–1560°C. The ratio of evaporation was determined by monitoring the Na₂O content change of the slag melt under isothermal reduction conditions. The results show that the evaporation ratio increases with increasing the temperature. Higher basicity and increasing concentrations of Na₂O, Al₂O₃ are also found to increase the evaporation ratio of Na₂O, while MgO addition only slightly enhances the evaporation ratio. With TiO₂ content increasing, the evaporation ratio first increases and then decreases. The evaporation rate of Na₂O appears to be controlled by chemical reaction at the slag/gas interface in the beginning, followed by a mixed reaction-mass transfer regime, and finally a liquid-phase mass transport step. The apparent activation energy is 134.74?kJ?mol^?1 for the chemical reaction regime and 268.53?kJ?mol^?1 for the liquid-phase mass diffusion step.     

Enthalpies of formation of borides of iron,cobalt, and nickel by solution calorimetry in liquid copper

The enthalpies of formation at 1385 ±2 K of the following crystalline borides have been determined by high temperature solution calorimetry using liquid copper as the calorimetric solvent. Fe₂B-67.87 ±8.05 kJ mol⁻¹, Co₂B -58.1 ±7.0 kJ mol⁻¹, Ni₂B -67.66 ±4.12 kJ ml⁻¹, FeB-64.63 ±4.34 kJ mol⁻¹, CoB -69.52 ±6.0 kJ mol⁻¹, and NiB -40.2 ±3.77 kJ mol⁻¹. The enthalpy of fusion of NiB has been determined to be 28.25 ±1.54 kJ mol⁻¹ at its melting point of 1315 K. New data are reported also for the enthalpies of solution of iron, cobalt, and nickel in copper, and for the enthalpies of interaction between these metals and boron in dilute solutions in liquid copper.     

Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

Influence of an electric field on the plastic deformation of fine-grained Al2O3

The influence of an electric field E = 300 V/cm on the plastic deformation at 1450 °C to 1600 °C of fine-grained alumina with ∼300 ppm MgO and d _o=1.4 to 2.5 μm was investigated. After removal of the effect of Joule heating, the field reduced the flow stress by ∼4 MPa relatively independent of temperature, representing a 25 to 70 pct reduction in the flow stress. The field had no effect on the stress exponent n=2.2, nor on the grain size exponent p=1.9. It did however have an appreciable effect on the combined constant AD _o of the Weertman-Dorn equation and the activation energy Q. The latter increased from 492 kJ/mole without the field (typically that for Al³⁺ ion lattice diffusion) to 880 to 1070 kJ/mole with the field, which is more typical of sub-boundary or grain boundary diffusion of either Al³⁺ or O²⁻-ions in alumina. It therefore appears that the field changed the rate-controlling, grain boundary sliding accommodation mechanism from the lattice diffusion of Al³⁺ ions to either sub-boundary or grain boundary diffusion of Al³⁺ or O²⁻-ions. The electric field caused isotropic swelling of the specimen when applied for an extended period of time without application of stress. Pores responsible for the swelling occurred along the grain boundaries. They may have resulted from electrotransport of Al³⁺ ions, leaving behind an Al-deficient layer from which free oxygen bubbles formed along the grain boundaries. Electric field-enhanced gas reactions at the pores may have also contributed to the swelling. It is proposed that swelling due to electrotransport may possibly be avoided by the addition of solutes, which increase the electronic or oxygen ion conductivity transference numbers compared to the aluminum ions.     

Coarsening of Al<Subscript>3</Subscript>Sc precipitates in an Al-0.28 wt pct Sc alloy

The Ostwald ripening of Al₃Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with aging time, t, by electrical resistivity. The coarsening kinetics of Al₃Sc precipitates obey the t ^1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with t are consistent with the predicted t ^−1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al₃Sc interface energy, γ, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of γ using coarsening data obtained from TEM and electrical resistivity measurements. The value of γ estimated from the LSW theory is 218 mJ m⁻², which is nearly identical to 230 mJ m⁻² from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined to be (7.2±6.0)×10⁻⁴ m² s⁻¹ and 176±9 kJ mol⁻¹, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

Isothermal martensite formation in an AISI 52100 ball bearing steel

The formation of isothermal martensite from the retained austenite in an AISI 52100 ball bearing steel was investigated. Optical microscopy reveals that there are mainly two types of isothermal martensite formation: the growth of the athermal martensite and the nucleation and growth of new martensite in the retained austenite. X-ray diffraction shows that during the isothermal transformation, the ratio of lattice constantsc/a decreases, and TEM verifies the precipitation of Fe, Cr)₃C in martensite. The kinetics of the isothermal transformation in the quenched steel also shows “C” shape characteristic. At the first stage of the isothermal formation, recovery of the athermal martensite occurs with an activation energy of 91.8 kJ mol^-1, implying that the diffusion of carbon in athermal martensite results in the precipitation of carbide and the relaxation of the strain energy at the martensite/matrix boundary. In the second stage, the activation energy for the isothermal formation is 130 kJ mol^-1; that may be the energy required for the rearrangement of the configuration of dislocations, forming preferred sites for nucleation. Formerly Graduate Student. Formerly Graduate Student, Department of Materials Science and Engineering.     

Chemical potentials of oxygen for fayalite-quartz-lron and fayalite-quartz-magnetite equilibria

The oxygen potentials corresponding to fayalite-quartz-iron (FQI) and fayalite-quartz-magnetite (FQM) equilibria have been determined using solid-state galvanic cells: Pt,Fe + Fe₂SiO₄ + SiO₂/(Y₂O₃)ZrO₂/Fe + \r"FeO,\l"Pt and Pt, Fe₃O₄ + Fe₂SiO₄ + SiO₂/(Y₂O₃)ZrO2/Ni + NiO, Pt in the temperature ranges 900 to 1400 K and 1080 to 1340 K, respectively. The cells are written such that the right-hand electrodes are positive. Silica used in this study had the quartz structure. The emf of both cells was found to be reversible and to vary linearly with temperature. From the emf, Gibbs energy changes were deduced for the reactions: 0.106Fe (s) + 2Fe_0.947O (r.s.) + SiO₂ (qz) → Fe₂SiO₄ (ol) δG‡= -39,140+ 15.59T(± 150) J mol^-1 and 3Fe₂SiO₄ (ol) + O₂ (g) → 2Fe₃O₄ (sp) + 3SiO₂ (qz) δG‡ = -471,750 + 160.06 T±} 1100) J mol^-1 The “third-law≓ analysis of fayalite-quartz-wustite and fayalite-quartz-magnetite equilibria gives value for δH‡₂₉₈ as -35.22 (±0.1) and -528.10 (±0.1) kJ mol^-1, respectively, independent of temperature. The Gibbs energy of formation of the spinel form of Fe₂SiO₄ is derived by com-bining the present results on FQI equilibrium with the high-pressure data on olivine to spinel transformation of Fe₂SiO₄.     

Preparation and properties of fine hematite powders by hydrolysis of iron carboxylate solutions

Submicrometer, crystalline hematite (α-Fe₂O₃) particles were prepared by hydrolysis of organic iron carboxylate solutions using water at 175 °C for 30 minutes. The particle size of hematite was significantly dependent on the liquid-phase stirring speed and the organic compositions. The precipitation rate of hematite from the organic solution followed first-order kinetics. The precipitation rate increased markedly with increasing temperature, and the activation energy for the process was 94.6 kJ mol⁻¹. At 220 °C, the hydrolysis of iron carboxylate solution led to a mixture of hematite and magnetite (Fe₃O₄). The iron oxides prepared at 175 °C to 220 °C were found to be free from organic contamination by the starting material.