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MATERIALS AND INTERFACES Carbon Combustion Synthesis of Ferrites: Synthesis and Characterization Karen S. Martirosyan and Dan Luss* Department of Chemical Engineering, UniVersity of Houston, Houston, Texas 77204
Submicrometer powders of Mn-Zn and Ni-Zn ferrites have been produced by carbon combustion synthesis of oxides (CCSO). The self-propagating temperature front had a maximum temperature of a up to 1300 °C and moved at a velocity 3.5 mm/s. Solid-state interactions between the precursors and crystal growth of the crystalline spinel ferrites started in the early period of the combustion and continued in the postcombustion zone. The crystalline Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 ferrites formed using reactant mixtures containing up to 40% carbon and did not require further calcination to get complete conversion. The product particle size increased with increasing carbon content in the reactants mixture and oxygen concentrations. At least twice the amount of required stoichiometric oxygen was needed to generate a stable combustion front and form pure spinel ferrite structure. The emission of carbon dioxide increased the porosity and friability of the products. The synthesized ferrites had soft magnetic properties which compared well with those obtained by other synthesis methods. Introduction
Table 1. Characteristics of the Reactants Used During Carbon Combustion Synthesis of Mn-Zn and Ni-Zn Ferrites
Ferromagnetic nickel-zinc and manganese-zinc ferrites MexZn1-xFe2O4, where Me ) Ni or Mn, at 0.1 < x < 0.9 are widely used in the manufacture of microwave and telecommunication equipment.1,2 Important properties of these ferrites are the initial permeability (100-2000), residual and saturation magnetization (up to 0.2 and 0.4 T, respectively), and the coercive field (less than 1 kA/m). These ferrites can be synthesized by several processes. The conventional and oldest one is by calcination of a mixture of oxides or carbonates in a furnace at a high temperature (up to 2000 °C) for 2-24 h.3,4 This process produces large particles and consumes extensive energy. In some cases the conversion is incomplete and the sintered material requires grinding and a second calcination. Uniform and homogeneous oxide powders can be produced by wet chemical methods such as sol-gel,5 spray-drying,6 hydrothermal,7 coprecipitation,8 citrate precursor,9 aqua combustion synthesis,10-12 forced hydrolysis method,13 cryochemical,14 microemulsion,15 and others. These processes require multiple steps, and their large-scale application is limited by the high cost of the reactants and byproduct disposal. Moreover, they produce, in general, amorphous powders that require hightemperature calcination to obtain crystalline material. Ferrites can be synthesized also by self-propagating hightemperature synthesis (SHS), also referred to as combustion synthesis.16-20 In SHS the reactant mixture contains in addition to oxides and/or carbonates a pure metal (Fe, Ni, Mn) powder which serves as a fuel and is incorporated in the product. The highly exothermic reaction (400-1000 kJ/mol) between the metal and oxygen generates a high temperature (up to 2500 °C) front that propagates through the reactant mixture at a velocity of 0.5-10 mm/s, converting it to a complex oxide. SHS has * To whom correspondence should be addressed. E-mail: dluss@ uh.edu.
reagent
purity (%)
ZnO MnCO3 NiO Fe2O3 carbon oxygen (extra dry)
99.9 99.9 99.9 99.0 96.0 99.99
av particle size (µm)
std dev (µm)
melting point (°C)
1 25 10 5 0.04
0.5 1.9 1.2 0.8 0.01
1975 350 (decomp.) 1955 1562 3800
source Aldrich Aldrich Aldrich Aldrich Alfa Aesar Aeriform Corp.
many advantages such as simplicity of process, energy efficiency, and high production rate.21 One disadvantage of complex oxides formation by SHS is the high price of the pure elements relative to that of oxides or carbonates. Thus, the production costs by SHS may exceed those by the calcination process, which uses less expensive metal-containing precursors, such as carbonates or oxides. Moreover, it is difficult to produce submicrometer ferrite particles by SHS due to partial product melting at the high reaction temperature. We developed a novel synthesis method of complex oxide ceramics, called carbon combustion synthesis of oxides (CCSO), which is a modified form of SHS.22,23 In CCSO the exothermic 298K oxidation of carbon (C + O2 ) CO2, ∆HCO ) -393.5 kJ/ 2 mol) generates a thermal reaction wave with relatively low temperatures 600-1200 °C that propagates at a velocity of 0.1-4 mm/s through the solid reactant mixture, converting it to the desired oxide product by the reaction n
∑ i)1
m
µiXi(s) + RC(s) + βO2(g) )
Pj(s) + δCO2(g) + ∑ j)1 (-∆H) (1)
where Xi(s) is a solid compound (such as an oxide, superoxide, nitride, or carbonate, chloride, or oxalate) containing the metal needed to form the oxide, Pj(s) is the solid complex oxide
10.1021/ie060571l CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007
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product, µI, β, and δ are stoichiometric coefficients, and -∆H is the heat of the reaction.
R)
x/12 (100 - x)/
∑
(2)
µiMi(s)
where x is the carbon weight percent in the mixture and Mi(s) the molecular weight of the ith reactant. Unlike SHS of metal carbides (TiC, ZrC, and others) or glass-carbide composites,24-27 the carbon used in the CCSO is not incorporated into the product and is emitted from the sample as carbon dioxide. The extensive emission of CO2 in CCSO affects the product structure, changes the local thermal properties (such as heat capacity and conductivity), and causes a loss of heat to the surrounding. CCSO maintains the advantages of SHS and circumvents some of its disadvantages. Specifically, it enables a more economical synthesis of complex oxides when the price of the pure metal is high relative to that of its precursor. In addition, the high rate of CO2 release helps form a highly porous (up to 0.7) and friable product, having a particle size in the range of 50-800 nm with a surface area up to10 m2/g.28 We produced, by CCSO homogeneous, submicrometer complex oxide powders such as BaTiO3, LaGaO3, and LiMn2O4. The goal of this study was to determine if CCSO can produce high-quality submicrometer powders of soft magnetic Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 ferrites. The price of carbon powder is at least 10 times less expensive than iron or manganese or nickel powders. Thus, the cost of producing ferrites by CCSO is lower than that by SHS. Experimental System and Procedure The CCSO synthesis of the Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 ferrites was via the reactions
0.35NiO + 0.65ZnO + Fe2O3 + R(C + O2) w Ni0.35Zn0.65Fe2O4 + RCO2v (3) 0.25MnCO3 + 0.75ZnO + Fe2O3 + R(C + O2) w Mn0.25Zn0.75Fe2O4 + (R+0.25)CO2v (4) where 2 < R < 14. The green mixture contained up to 40 wt % carbon and noncombustible oxide/carbonate precursors. Some properties of the reactants are reported in Table 1. All the reagents were dried at 115 °C for 5 h and thoroughly mixed with carbon powder by ball milling for about 60 min. The combustion synthesis was conducted by loading a loose mixture (relative densities of about 0.3) into a ceramic boat that was placed inside a cylindrical stainless steel vessel (70 mm i.d. and 60 mm length) fed by oxygen at a flow rate up to 12 L/min. A schematic of the experimental system is shown in Figure 1. Local ignition of the reactant mixture by an electrically heating coil generated a propagating temperature front. An infraredtransparent sapphire window on top of the vessel enabled viewing and recording of the sample surface radiation at a frequency of 60 frames/s by an infrared camera (Merlin, Indigo Systems). The spatial resolution of the camera was about 10 µm with a viewable area between 0.5 and 3600 mm2. The digital IR images were used to determine the temperature-time history, shape, and average velocity of the propagating combustion front. Image analysis software (ThermaGRAM) was used to determine the instantaneous and average velocity of the moving thermal front and temporal changes in the shape of the temperature front.
Figure 1. Schematic of the experimental setup.
The local combustion temperature (Tc) inside the reactants mixture was measured by an S-type (Pt-Rh) thermocouple of about 0.1 mm diameter inserted in the center of sample. The thermocouple readings were recorded and processed by an Omega data acquisition board connected to a PC. The changes in the chemical composition within the reaction zone were determined by rapid quenching of the combustion front.29,30 These experiments were conducted in a 62 mm high copper cylinder with a 20 mm conical hole at the top and 1 mm tip at its bottom. The cylinder was made of two separable halves to enable layer by layer characterization of the dependence of the chemical composition on the position in the cone. The copper block mass (2.5 kg) was much larger than that of the sample (about 15 g), enabling rapid heat removal from the sample (the thermal conductivity of Cu is about 400 W/mK). The quenching rate was estimated to be 1000 °C/s. The cylinder was located under a bell-shaped vessel into which oxygen was fed at a flow rate of 12 L/m. The sample was ignited at the base of the sample cone by an electrically heated coil. The combustion was extinguished at a critical position, at which the heat losses prevented the front from propagating any further. The carbon content of the as-synthesized product was determined by a carbon analyzer (Leco, WR-112). The composition and crystal structure of the products were determined by X-ray diffraction (Siemens D5000 diffractometer) with Cu KR radiation (λ ) 1.540 56 Å). Scans were taken at room temperature over a wide range of 5° < 2θ < 80° at 0.05° intervals. Particle morphological features and microprobe analysis were determined by scanning electron microscopy (SEM; JEOL JAX8600, Japan) of loose powder fixed to a graphite disk. Particle size distribution and the particle surface area were determined by a Coulter SA 3100 BET analyzer. The powders produced by CCSO were ground by ball milling for 2 h. The powders were mixed with 10 wt % poly(vinyl alcohol) solutions (8 wt % PVA concentration in distilled water as a binder) and granulated with a 60-mesh sieve. The granulated powders were pressed in a stainless steel die under 100 MPa into specimens (12 mm in diameter, 5 mm in length) and toroids (12 mm o.d., 6 mm i.d., and 5 mm thickness). The samples were sintered for 3 h in air under three temperatures of 1050, 1150, and 1250 °C. Magnetic measurements were conducted at room temperature with a fluxgate magnetometer, with applied fields of up to 400 kA/m. The ac conductivity was measured using an impedance
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Figure 2. Dependence of the maximum combustion temperature and average front velocity on the carbon concentration in the reactants mixture during the synthesis of Ni-Zn and Mn-Zn ferrites.
consumed by the decomposition of the manganese carbonate by the reaction 3MnCO3 f Mn3O4 + CO + 2CO2. Figure 4 shows macro and micro IR thermal images of the temperature front during synthesis of Mn-Zn ferrite from a reactant mixture containing 14 wt % carbon. The macro images (Figure 4a) show that the combustion front propagated at an average velocity of 1.5 mm/s with a maximum temperature of ∼950 °C. The magnified IR images (Figure 4b) show that chaotic local hot spots formed ahead of the moving temperature front. They grew from an initial size of 100 µm in the preheated zone to 0.5 mm in the reaction zone. Their motion was slower but similar to the “scintillating reaction wave” observed in gasless SHS.31-33 The moving temperature front was not exactly planar for both reactions 3 and 4, and a sharp boundary did not exist between the combustion and the postcombustion zone. The IR images for both systems suggest that increasing the carbon concentration in the reactants mixture up to 40 wt% increases the length of the reaction zone up to 50 mm. Both the maximum reaction temperature and front velocity increased as the oxygen flow rate to the reactor was increased (Figure 5). For reactions 3 and 4 about 8/3 mol of oxygen are needs for complete combustion of the carbon. Consequently, for complete conversion of the carbon to CO2 during CCSO the minimum oxygen flow rate (FO2, L/min) is
F O2 )
Figure 3. Local temporal temperature at the center of the sample generated by carbon combustion synthesis of Ni-Zn and Mn-Zn ferrites at 25 wt % the carbon concentration in reactants mixture. The oxygen flow rate was 12 L/min.
signal analyzer (Model HP4192A) at a frequency range of 100 Hz to 20 MHz with an alternating electrical field of ∼1 V/cm. The initial permeability (µ) was measured at 100 kHz by a circuit loaded with a toroid-shaped sample. Results Synthesis and Process Characterization. The maximum reaction temperature and average front velocity during the carbon combustion synthesis of Ni-Zn and Mn-Zn ferrites increases with increasing carbon concentration in the reactant mixture (Figure 2). Neither reaction mixture could be ignited with a carbon concentration less than 7 wt %. The combustion proceeded in an unstable mode, and the front extinguished after moving for about 3-5 mm for mixtures containing 7-9 wt % carbon. A self-sustaining reaction with a stable combustion front propagating at a constant velocity of 0.24 mm/s was attained for reactant mixtures containing more than 9 wt % carbon. At a carbon concentration of 40 wt % the combustion front velocity and maximum temperature approached asymptotic values of 3.4 mm/s and 1300 °C, respectively. Typical temporal temperatures at the center of a sample for the two combustion reactions are shown in Figure 3. The temperature in the reaction zone increased at a rate of 70 and 40 °C/s during the synthesis of Ni-Zn and Mn-Zn ferrites, respectively. The maximum combustion temperatures and rate of temperature rise during the synthesis of Ni-Zn ferrite by reaction 3 were higher than those of Mn-Zn ferrite by reaction 4 at the same carbon concentration. Probably, the lower value was caused by the heat
8 mc 3 tr F O 2
(5)
where mc is the carbon weight in the mixture in grams, tr the total reaction time in minutes, and FO2 ) 1.429 kg/m3 the oxygen density under normal conditions. Thus, the minimum oxygen flow needed for complete conversion of 1.8 g carbon (12 wt % of the reactant mixture) to CO2 by reaction 3 at tr of 1 min is 3.36 L/min that corresponds to a linear velocity of 87.3 cm/ min. The reaction did not ignite at oxygen flow rates lower than 2.2 L/min (linear velocity of 57.2 cm/min). In the range of 2.23.4 L/min, i.e., in the region in which the oxygen flow rate was not sufficient for complete combustion of the carbon, the combustion is unstable. At FO2 of 8 L/min the front propagated in a stable mode. The corresponding combustion temperature was 980 °C, and the front velocity 1.4 mm/s. Increasing the flow rate from 8 to 12 L/min did not change the combustion temperature and front velocity. Figure 6 shows the temporal local temperature at the center of the sample generated by carbon combustion synthesis of Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 ferrites using reactant mixtures containing 12 wt % carbon at two different oxygen flow rates. These experiments showed that the variation of the FO2 may affect the combustion temperature, front velocity, and the duration of reaction time. Decreasing the oxygen flow rate from 12 to 5 L/min doubles the average reaction time, from 20 to 40 s. The longer reaction time at a low oxygen flow rate probably resulted from the limiting rate of oxygen infiltration in the reaction zone which limited the carbon combustion rate. The gas composition formed during the synthesis of Ni-Zn and Mn-Zn ferrites was determined by a mass spectroscopy analyzer for reactant mixtures containing different carbon concentrations and at least twice the required stoichiometric oxygen flow rate. The experiments revealed that the effluent gas contained only oxygen and carbon dioxide, indicating that the conversion of combusted carbon to CO2 was complete. Phase Transformations and Product Structure. The quenched front method was used to follow the phase and structural transformations occurring during the synthesis of
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Figure 4. IR thermal images obtained during the carbon combustion synthesis of Mn0.25Zn0.75Fe2O4 ferrite at 14 wt % carbon concentration in mixture: (a) macro- and (b) microlevel (the images were recorded every 16.6 ms).
Figure 5. Influence of the oxygen flow rate on the maximum combustion temperature and average front velocity during the carbon combustion synthesis of Ni-Zn ferrite at 12 wt % carbon in the reactant mixture.
Figure 7. X-ray powder diffraction patterns at different distances, from the tip of the cone in the quenched front experiment of the carbon combustion synthesis of Ni0.35Zn0.65Fe2O4 ferrite. Key: (b) Fe2O3; (+) ZnO; (-) NiO; (/) Ni0.35Zn0.65Fe2O4.
Figure 6. Temporal local temperature at the center of the sample generated by carbon combustion synthesis of Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 ferrites at 12 wt % carbon in the reactants mixture with 5 and 12 L/min oxygen flow rates.
Ni0.35Zn0.65Fe2O4 by CCSO from a reactant mixture containing 12 wt % carbon. The combustion front moved inside the copper cone at a velocity of about 0.9 mm/s and was arrested at a
distance of ∼3 mm from the tip of the cone. The combustion temperature in the middle of the sample attained its maximum value at about 27 s ()0.9 × 27). The average length of the preheating and reaction zone was estimated as ∼24 mm. X-ray patterns of the quenched samples at different distances from the tip of the cone are shown in Figure 7. We define the beginning of the prefront zone as the line at which the color of the green mixture started to change, indicating an interaction among the components. XRD analysis of the powder in the prefront zone shows formation of a face-centered cubic (FCC) crystalline spinel structure of Ni0.35Zn0.65Fe2O4 (peak [113] at 2θ ) 34.99°) at a distance of 3 mm from the prefront line or 6 mm from the tip of the cone. The main species in this region
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Figure 8. X-ray powder diffraction patterns of as-synthesized Mn-Zn ferrite powder at 25 and 12 wt % carbon concentration in the reactant mixture.
were the reactants ZnO, NiO, and R-modified Fe2O3. Almost complete conversion of the reactants to the cubic Ni0.35Zn0.65Fe2O4 ferrite was achieved at a distance of ∼40 mm from the tip of cone, indicating complete formation of the spinel phase in the postcombustion zone. Intermediate phases of iron oxides FeO and Fe3O4, which usually form during the conventional SHS of ferrites, did not form during the CCSO of ferrites. XRD patterns of as-synthesized powders (Figure 8) showed that reaction 4 using mixtures containing 12 and 25 wt % carbon produced almost pure spinel ferrites. Pure crystalline Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 ferrites formed from mixtures containing at least 11 wt % carbon and did not require further calcinations to complete the reactants conversion. Carbon analysis of these ferrites indicated that the concentration of the residual carbon was less than 0.1 wt %. Low-angle XRD patterns of the synthesized powders did not include any “amorphous hump” and had a flat background, indicating that no amorphous phases were present. The lattice parameters of the Mn-Zn and Ni-Zn ferrites were 8.423 and 8.405 Å, respectively. These values are in close agreement with published values.34-36 When the reactant mixture contained less than 11 wt % carbon, the conversion of the reactants to the ferrites was not complete and the products included some unconverted reactants (NiO, ZnO, R-Fe2O3) or (MnCO3, ZnO, R-Fe2O3). This is probably caused by the relatively low reaction temperature (∼700 °C) that limited the rate of formation of the ferrites. Under low oxygen flow rate the conversion of products was also incomplete, and some oxides and carbon were present in the combustion products. Complete conversion for both reactions (3 and 4) using mixtures containing 12 wt % C was achieved for FO2 > 5 L/min. All combustion products were friable and had a spongy porous structure (porosity of 60-70%). The morphology of the as-synthesized Ni-Zn and Mn-Zn ferrites prepared via reactions 3 and 4 at different carbon concentration is shown in Figure 9. The combustion products were predominantly spherical particles with an average particle size of about 0.1-0.8 µm at low carbon concentration of 12-14 wt % (Figure 9a,c). The
particles shape became more irregular as the carbon concentration was increased. The particle size was about 2-5 µm at 25 wt % C (Figure 9b,d). As expected, the average particle size increased with increasing combustion temperature. Increasing the amount of carbon decreased the fraction of the submicrometer particles and increased the pore size. On the other hand, increasing the oxygen flow rate from 5 to 12 L/min increased the particle size from 0.1 to 0.8 µm. The specific surface area of the Mn-Zn ferrites produced by reaction 4, following milling for 2 h, decreased monotonically from 8.6 to 1.6 m2/g as the carbon concentration increased from 12 to 25 wt % (Figure 10). The combustion temperature is a monotonic increasing function of the carbon concentration. Thus, a decrease of the carbon concentration minimized particle sintering and increased the surface area. Electromagnetic Characteristics. The as-synthesized NiZn and Mn-Zn ferrites powders produced at different carbon concentrations have soft magnetic properties that are described in Figure 11. A higher carbon concentration increases the saturation magnetization (Bmax) and decreased the remanent magnetization (Br) and coercitive field (Hc). Increasing the carbon concentration in the reactant mixture increases the reaction temperature and consequently the product grain size. The ac conductivity (σ) of the as-synthesized Ni-Zn ferrites increases with increasing applied frequency and carbon concentration in the mixture (Figure 12). The powders have low ac conductivity of the order of 10-4-10-5 S/m at 20 MHz, as is needed in high-frequency applications. The magnetic properties of the sintered Ni-Zn ferrites at three different temperatures of 1050, 1150, and 1250 °C were determined from their hysteresis loops. The results are presented in Table 2. After sintering of Ni-Zn ferrites at 1250 °C, the sample density of 5.0 g/cm3 is 96% of the theoretical value. The sintering of as-synthesized powder shows that increasing the annealing temperature from 1050 to 1250 °C considerably increased the saturation magnetization and reduced the values of Hc and Br. The optimal magnetic properties were obtained by annealing at a temperature of 1150 °C. Discussion CCSO can be used to synthesize submicrometer (0.1-0.8 µm), soft magnetic Ni-Zn and Mn-Zn ferrite particles. The process is significantly faster (reaction time of about a minute) than the common calcination method and cheaper than the SHS process. In addition CCSO produces highly crystalline ferrite powder. However, as in SHS a stable self-sustaining reaction can be produced in CCSO only above a critical fuel concentration in the reaction zone. The temperature rise and front propagation during the carbon combustion synthesis of ferrites was similar to that in traditional SHS of ferrites (Figures 2-6). The local hot spots that formed ahead of the temperature front are similar to those observed in previous studies of other systems.31-33 The ignition occurs locally as the hot spots propagate ahead of the boundary between the initial mixture and products (Figure 4). There are some important differences between CCSO and SHS of ferrites. For example, the maximum combustion temperature in CCSO of Ni-Zn ferrite with a reactant mixture containing up to 40 wt % C was ∼1300 °C. In SHS of the same ferrites the maximum reaction temperature is much higher and may exceed the melting temperature of the products ∼1600 °C.17,36,37 The length of the postreaction zone during the carbon combustion of ferrites is 30-50 mm, which is more than five times longer than that in conventional SHS, which is about 6 mm. Moreover, the intermediate phases such
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Figure 9. SEM images of as-synthesized Ni-Zn and Mn-Zn ferrites powders produced by carbon combustion synthesis via (a, b) reaction 3 and (c, d) reaction 4 at 12 and 25 wt % of carbon, respectively, FO2 )10 L/m.
Figure 10. Influence of the carbon concentration in the reactant mixture on the specific surface area of the grinding powder after carbon combustion synthesis of Ni-Zn and Mn-Zn ferrites.
as FeO and Fe3O4, which usually form when ferrites are produced via SHS, did not form during CCSO of ferrites. This indicates that the reaction pathway during ferrite formation via CCSO differs from that during SHS. In addition, the emission of CO2 during CCSO creates products that are more friable than in SHS. CCSO of ferrites uses a less expensive reactants mixture than SHS as it replaces the metal fuel by carbon, as well as decreasing the fuel concentration in mixture. Decreasing the heat generation and increasing the heat loss to the surroundings may destabilize the reaction leading to a nonconstant velocity of the reaction front or even extinction of the reaction. The stability of the combustion front propagation affects the quality of the CCSO products. A layer-by-layer combustion is needed to produce a uniform product. Oxygen flow rate affects the stability of the process, because its transports the oxidizer and heat to the reaction zone. When CCSO is conducted under low oxygen flow rates (Figure 5), the combustion temperature and front velocity are low because of the oxygen limitations to the reaction zone. Increasing the FO2 from 2 to 12 L/min increases the maximum temperature by
Figure 11. Dependence of the carbon concentration in the reactant mixture on (a) the maximum of the saturation magnetization and (b) the residual magnetization and coercivity of Ni-Zn and Mn-Zn ferrites after the carbon combustion synthesis.
about 300 °C (Figure 5), which increased the product particle size. At least twice the minimum (stechiometric) oxygen flow rate was needed to create a stable combustion front and to form a complete cubic spinel structure. The oxygen flow rate is an additional tool for controlling the maximum temperature, reaction time, and subsequent product structure in CCSO.
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Figure 12. ac conductivity of Ni-Zn ferrites (sintered at the temperature 1150 °C) as a function of frequency, measured at room temperature.
Heat for these endothermic (20-40 kJ/mol) reactions is provided by the carbon combustion. The quenched front experiments show that reactions 9 and 10 occur in the early period of the combustion during which the carbon was not completely combusted (Figure 7). The formation of the high porous crystalline spinel ferrite structure was completed in the postcombustion zone (Figures 7 and 8). The wide reaction zone in CCSO (about 50 mm) probably was caused by the slow rate of carbon oxidation. The Ni-Zn and Mn-Zn ferrites prepared by the CCSO had soft magnetic properties as illustrated by Figures 11 and 12. Increasing the combustion temperature of the mixture increased Bmax and σ and decreased Br and Hc. This behavior is attributed mainly to an increase in the particles grain size at the higher temperatures. The magnetic properties of the sintered samples (Table 2) compare well with the commercial ferrites produced by other methods.
Table 2. Magnetic Properties and Bulk Density of Sintered Ni0.35Zn0.65Fe2O4 Samples at Different Temperatures
Conclusions
sintering temp (°C) Bmax (T) Br (T) Hc (A/m) σ (S/m; 10 MHz) µ (100 kHz) bulk density (g/cm3)
1050 0.28 0.18 95 2.8 × 10-5 340 4.83
1150 0.30 0.14 62 3.5 × 10-5 540 4.94
1250 0.32 0.11 46 4.6 × 10-5 620 5.0
The overall rate of the ferrite formation by CCSO is rather complex, as it is influenced by the several simultaneous physical and chemical rate processes: carbon heating, oxygen infiltration, absorption/desorption, decomposition of some reactants, and solid-solid reactions. CCSO include several main reaction steps. The first is carbon combustion:
C(s) + 1/2O2(g) f CO(g) C(s) + O2(g) f CO2(g)
∆H298 ) -110.5 kJ/mol (6) ∆H298 ) -394.5 kJ/mol (7)
The second is the oxidation of CO as it diffuses to the sample surface by the counter diffusing oxygen:
CO(g) + 1/2O2(g) f CO2(g)
∆H298 ) -283.1 kJ/mol (8)
The equilibrium level of carbon monoxide by reaction 6 depends on the temperature and oxygen concentration. Increasing the oxygen concentration and temperature in the reaction zone decreases the CO concentration.38-42 Consequently, reaction 7 is more realistic and probably dominates during CCSO conditions (high temperature and oxygen concentration). This is proved by the gas spectroscopy experiments which did not detect any carbon monoxide, indicating that if any carbon monoxide was formed in the initial stage of the reaction, it was converted to CO2. Some of the present oxides such as NiO, ZnO, or Fe2O3, may have catalyzed this process. Reaction 7 is more exothermic than reactions 6 and 8 and significantly increases the precursor particles temperature, leading to a front propagation and rapid interaction between the solid reactants by the following reactions:
0.35NiO + 0.65ZnO + Fe2O3 f Ni0.35Zn0.65Fe2O4 (9) 0.25MnCO3 + 0.75ZnO + Fe2O3 f Mn0.25Zn0.75Fe2O4 (10)
Pure, crystalline Ni-Zn and Mn-Zn ferrite powders with submicrometer particle size (0.1-0.8 µm) can be synthesized by CCSO at a lower price than other processes. The major parameters affecting the process and product properties are the carbon concentration in the reactant mixture and the oxygen flow rate. A stable self-sustaining reaction front can be obtained only at carbon concentrations exceeding a critical value of about 9 wt % in the mixture. The formation of a complete spinel structure of Ni0.35Zn0.65Fe2O4 and Mn0.25Zn0.75Fe2O4 was obtained using reactant mixtures containing at least 11 wt % carbon and at least double the amount of stoichiometric oxygen. Increasing the carbon concentration increased the maximum temperature and front velocity to asymptotic values of 1300 °C and of 3.4 mm/s, respectively. A Ni-Zn and Mn-Zn ferrites formed during the early stage of the combustion, before the carbon was entirely combusted. The reaction was completed in the postcombustion zone. XRD patterns of as-synthesized ferrites showed the formation of single-phase cubic spinel particles crystallizes in a FCC structure. Increasing the carbon concentration from 12 to 25 wt % decreased the fraction of the submicrometer ferrite particles and decreased the particles surface area from 8.7 to 1.5 m2/g. The emission of CO2 increased the friability of the product and particle porosity up to 70%. Ni-Zn and Mn-Zn ferrites prepared by the CCSO have soft magnetic properties (Hc ) 46-95 A/m, Br ) 0.11-0.18 T, Bm ) 0.28-0.32 T, and µ ) 340-620) comparable with ferrites produced by the other methods. CCSO can enable an economic and energy efficient production of Ni-Zn and Mn-Zn ferrites, which have wide industrial applications. The process may be employed to synthesize other analogous magnetic materials. Acknowledgment This work was supported in part by the Texas Air Research Center (TARC), Texas Center for Superconductivity and a GEAR grant by UH. Literature Cited (1) Snelling, E. C.; Giles, A. D. Ferrites for Inductors and Transformers; Wiley: New York, 1983. (2) Goldman, A. Modern Ferrite Technology, 2nd ed.; Springer: New York, 2005. (3) Smit, J. Ferrites: Physical Properties of Ferrimagnetic Oxides in Relation to Their Technical Applications; Wiley: New York, 1959.
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ReceiVed for reView May 8, 2006 Accepted November 20, 2006 IE060571L
