Mixed metal oxalate hydrazinates as compound precursors to spinel ferrites

Materials ~~e~~stry and Physics, 9 (1983) 423438 423 MIXED METAL OXAUTE RYDRAZINATES AS COMPOUND PRECURSORS TO SPINRL FURIES D.GAJAPATRY and K.C.PATIL Depaxtment of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012 (India) Received 6 April 1983; accepted 25 April 1983 ABSTRACT Eixed metal oxalate hydrazinates Ml%,(C204)3(R2R4)6 where M= !&-I, Co, Ni, Zn have been prepared , characterised and investi- gated as compound precursors to spine1 ferrites, IWe204. These precursors decompose exothermically at very low temperatures (130'~250°C) to yield ferrites of large surface area. These compound precursors exhfbit autocatalytic behaviour, ie. once ignited combustion is self sustained. Characterisation of the precursors and the combustion products has been done using chemical analysis, infrared and MBssbauer spectra and X-ray diffraction. Mixed metal oxides are of interest from both practical and fundamental points of view. Complex oxides are able to stabilise unusual oxidation states, have significant non-stoiohiometry and certain unique structural arrangements. Of the compounds belonging to this Class, spine1 ferrites are of importance 0254-0584/83/$3.00 0 Elsevier Sequoia/Printed in The Netherlands 424 because of their use as magnetic materials and catalysts. Ferrite5 are widely used as magnetic materials in high frequency transformer cores, antenna rods, induction tuners etc. To be used in high frequency applications, the ferrite should have a low value of saturation magnetisation and low eaay current loss. Losses associated with domain wall resonance will be minimised when the ferrite is porous. The ferrites find application as catalysts in debydrogenation reactions. Needless to mention ferrites should have a large surface area to function effectively as catalysts. Thus, ferrites with large surface area are needed in either application. The traditional ceramic approaches to synthesise ferrites involve repeated high temperature firing of the component oxides with frequent regrindings. These harsh conditions are required to overcome the slow reaction kinetics that occur when two solids are brought together. The severity of the reaction conditions necessary to overcome the diffusional limitations of solid state reactions lead to crystalline but low surface area material, limiting its applications as a catalyst and high frequency core material. To obtain ferrites of large surface area, the spine1 forma- tion temperature ought to be reduced considerably. A number of attempts have been made to bring down the spine1 formation tempe- rature D-43. In order to achieve complete reaction in the shortest amount of time and at the lowest possible temperature, the mixing of the component cations has to be on an atomic scale. Compound precursors 15,63 or solid solution precursors 171 have been reported to achieve this goal. Compound precursors are known to achieve excellent stoichiometry, low trace impurity content and homogeneity approaching the maximum theoretically 425 possible. Many precursors such as pyridinates [8] and ammo- niates @J have been reported for the synthesis of spine1 ferri- tes. However the temperatures of formation of ferrites from these precursors are as high as 1000°C. We have recently repor- ted the formation of magnesium ferrite and cobaltite at low tempesatures using compou~ precursors, ~~e~(~*O~~3(N*H~)~ [IO] and N~CO,(C~O~)~(N~H~)~ [ll] respectively. In continuation of this study we now report a low temperature route for the aynthe- sis of a number of ferrites using mixed metal oxalate hydrazi- nates as compound precursors. The results of the magnesium precursor have also been included here for comparison. EXPERIMENTAL Mixed metal oxalate hydrazinates MFe2(C204)3(N2H4)6, where 11 = Mn, Co, Ni, Zn, were prepared by either or both of the following methods. (i) From metal powders Stoichiometric quantities of the respective metal powders were dissolved in a solution of ammonia oxalate in hydrazine hydrate. JU"+2Feo+30JH&Cp04.H20+6N2H4.H20 -3 ~e2@204)3(N2H4)6 The reactions were carried out in a nitrogen atmosphere in order to prevent absorption of atmospheric carbon dioxide by hydrazine. After the metal powders had completely dissolved, alcohol was added to the mixture to complete the precipitation of the product. The product was washed with alcohol and ether. 426 (ii) Prom mixed metal oxalate hydrates Axed metal oxalate hydrates MPe2(C204)3(H20)6 were pre- pared by the method reported in the literature [12] . Treatment of these hydrates with excess of hydrazine hydrate yielded the corresponding hydrazinates. The compositions of the complexes were fixed by chemical analysis. The hydrazine content was determined by titration with 0.05M KI03 solution and the metal content by hlYl!A titration. Infrared spectra of the samples were recorded as nujol null using a Perkin Elmer PE 599 spectrophotometer. Differential thermal analysis (DT.4) of the samples was recorded on an instrument described elsewhere, cl3) fitted with an omniscribe strip chart recorder. Thermogravimetric analysis (TG) was carried out on a Stanton Redcroft TG-750 thermobalance. Both the thermal experiments were done in air with platinum cups as sample holders and a heating rate of 10°C min-'. X-ray powder diffraction of the combustion/decomposition residues of these precursors were obtained on a Philips PW 1050/70 diffractometer using CoK, radiation with iron filter. Surface area measurements on the combustion/decomposition products of the precursors were obtained using a single point BET apparatus (mantachrome Corporation). Nitrogen was used as adsorbate. M8ssbauer spectra of the ferrites were recorded on an ECIL MBS 35 spectrometer, operating on multiscalar mode. The 427 radioactive source ( v' ray emitter) was 57 Co in palladium matrix. The MZissbauer spectrometer was calibrated by recording the spectra of natural iron foil (0.06 mm thick containing 99% 57Fe). The values of isomer shift reported herein are with respect to stainless steel. The deconvolution of the M8ssbauer spectrum was carried out by a least squares technique on a DIE 1090 computer. RESmTS ARD DISCUSSION (a) Reaction of metal powders M" and Fe0 (?d" = AQ, i%n, Co, Ni, Zn> in the ratio 1:2 with a solution of ammonium oxalate in hydrazine hydrate, or (b) Reaction of mixed metal oxalate hydrates MIM2(C204)3 (H20)6 with excess of hydrazine hydrate yielded the mixed metal oxalate hydrazinates ~2(C*04)~(N2H4~x (BB = 3&g, b& Co, Ni, Zn and X ii 5 or 6). The complexes are crystalline solids with characteristic @010x.53. They are unstable and decompose on storage losing hydrazine. Iron in the complexes is present as Fe'*, since the reactions are carried out under reducing conditions. The presence of Ps2+ in the complexes makes them rather susceptible to atmospheric oxidation and therefore they need to be stored in a nitrogen atmosphere. The reaults of chemical analyses (Table 1) are in good agreement with the proposed formulae. Further characterisation of these complexes has been done by infrared spectra and the thermal properties have been investiga- ted by TG and DTA. (i) Infrared spectra Infrared spectral data of all the complexes are summarized in Table 2 and assigned based on earlier work [74,153 . The 420 T a b le 2 . In fr a re d S p e ct ra l D a ta o f M Fe 2 (C 2 0 4 )3 (N 2 H 4 )x , b f = M g , b k, C O , p i, Z n & M n co N i 2 % A ss ig n m e n t 33 20 33 10 32 70 32 30 3f 50 31 60 16 50 13 30 16 50 13 20 13 20 13 05 12 20 12 00 11 30 9 9 0 9 8 0 9 8 0 9 6 0 9 6 0 9 5 s 83 5 78 5 8 9 5 8 9 5 78 0 77 5 65 0 65 0 57 0 55 0 50 0 50 0 3 6 0 34 0 E Z o ” 11 30 ;: :: 31 50 16 50 12 10 11 90 1 1 2 0 65 0 65 0 56 0 54 0 51 5 35 0 33 00 33 00 32 40 32 40 31 40 31 40 16 50 16 40 13 20 13 05 12 10 11 90 11 30 4 9 0 35 0 12 10 11 90 11 30 11 20 9 8 0 9 5 5 8 9 5 77 5 65 0 53 5 4 9 5 35 0 N H s tr e tc h in g O C O a sy m . s tr e tc h in g O C O sy m st re tc h in g N H 2 tw is ti n g & w a g g in g N -N s tr e tc h in g O C O b e n d in g q ro ck in g M -O s tr e tc h in g O C O b e n d in g + C C 0 b e n d in g M -N s tr e tc h in g 430 spectra show characteristic absorptions of bridging bide&ate oxalate cl43 at 1640, 1320 and 780 cm -'and _3 N-N of bridged -1 hydrasine cl53 at 960-990 cm . A typical infrared spectrum of magnesium iron complex is shown in Fig.1. (ii) Thermal analysis and reactivity The results of thermal analysis (TG and DTA) of the pre- cursors are summarised in Table 3. Table 3. Thermal Data of MFe2(C204 j3(N2H4 jx , M = Mg, Mn, Co, Ni, Zn;x = 5 or 6 Compound TG wt. loaa $ DTA Temp.range peak temp % Found Cal0 Oc ~~2(c*O4~3(N~H4)5 700-230 64 64.28 125 (exe> 349 (exe) me2 @go4 I3 CN2f14 4j Ito-204 62 62.90 128 (exe) coFe2(C204)3(N2H4)6 145-230 61 62.55 152 (exo) NiFe2(C204)3(N2H4)6 149-240 62 62.57 164 (exe> Zr@'e2(C204)3(N2H4)6 147-250 63 61 90 157 (exe) All the complexes undergo a single step decomposition forming the corresponding ferrites (MFe204). The observed weight losses in TG show good agreement for the formation of ferrites. DTA shows the decomposition of precursors to ferrites to be exo- thermic. All the complexes show a single exothervnic peak corresponding to the TG step. Only in the case of the magnesium complex is a second exotherm observed which has been attributed to the recrystallisation of the kgFe204 formed. This has been 3500 2500 1800 1400 1000 600 200 WAVENUMBER (CM-‘) FIG. 1. Infrared Spectrum of hlgFe2(C20& (N2H& in Nujol Mull L (a) -___--- ._*---------- (b) , I I 0 100 200 300 400 5( TEMPERATURE. l C 3 FIG. 2. DTA Curves of (a) COFe2K2Q)36H20 b) COFe2K+O& ($H&. +6 .4 42 0 -2 -4 -6 -8 VELOCITY (mm /seC) FIC.3. MGssbauer spectrum of MgFe204. 432 confirmed by comparing the X-ray powder patterns of the residue before and after the second exotherm. Though the patterns are identical, a narrowing of lines has been observed in the X-ray pattern of the residue obtained after the second exotherm. It should be mentionedthat mixed metal oxalate hydrates do not yield ferrite8 at such low temperatures. The DTA of magnesium diiron oxalate hexahydrate, Mgl%2(C204)5(H20)6 pre- pared according to the procedure reported [12] shows the dehyd- ration peak (endotherm) at 21O'C followed by the exothermic decom- position of the double oxalate at 240°C to give a mixture of Oc-l?e205 and a mixed oxide containing excess of MgO. Magnesium ferrite (~Fe204) is reported cl23 to form only above 1000°C. Similarly in the case of CoFe2(C204)5(H20)6, dehydration occurs at 215'C (Fig.21 and the anhydrous oxalate decomposes at 260°C but no ferrite (CoFe204) is formed around this temperature. Complexation of these oxalates with hydrazine makes it possible to obtain the ferrites from these precursors at low temperatures c150°C (Fig.2). This shows that the exothermic decomposition of hydrazine plays a vital role in the formation of these spinels at low temperatures. The mixed metal oxalate hydrazinates exhibit autocatalytic behaviour, ie. once ignited combustion is self sustained and the product of combustion is MFe204. It is of interest to note that the precursors ignite while undergoing suction filtering if allowed to dry. Similar observations have been made by Anagnostopoulos et al 16 in the case of Fe2+ -- hydrazine complexes. 433 The residues obtained by thermal decomposition of the precursors were characterised by chemical analysis, X-ray diff- raction, infxaxed and Mossbauer spectra. CHARACTERIZATION OF THE RESIDUES (i) Chemical analysis The final residues obtained by the thermal decomposition of mixed metal oxalate hydrazinate precursors were characterized by conventional chemical analysis (Table 4). In all cases the Fe/M ratio was found to be almost 2 proving that the ferrites obtained were stoichiometric in composition. (ii.1 X-ray diffraction X-ray powder difrraction patterns of the samples corres- pond to those of the ferrites. The d values calculated match very well with the literature cl73 values. The a, values of the ferrites formed are given in Table 4. (iii) Infrared spectra Infrared spectra of the residues show two absorptions -1 "400 and 550 cm corresponding to metal oxygen stretching from tetrahedral and octahedral sites respectively which are characteristic of ferrites cl83 . (iv> MNssbauer spectra Further chaxacterisation of the residues was done by recording the MCissbauer spectra. A six finger pattern charac- teristic of fsrrites was obtained in all cases. A typical spectrum of MgFe204 is shown in Fig.?. The isomer shift values and internal fields observed (Table 4) are in good agreement with literature values [19J . Zinc ferrite obtained by the decomposition of Z~e2(C204)~(N2H4~~ was found to be T a b le 4 . A n a W ic a l, X -r a y , M o ss b a u e r a n d S u rf a ce A re a D a ta o f M l? e 2 0 4 , ld o ! & I, co , p i, zn _ A n a ly ti ca l d a ta X -r a y d a ta M U ss b a u e r d a ta S u rf a ce a re a M % B k a c g Is o m e r sh if t fn te rn a l 2 -3 m ,E J -1 ff rt x a O b sd T h e cx O b sd T h e o r m m 8 e c m e x! x” q 3 4 1 2 .7 6 1 2 .1 6 5 5 .7 1 5 5 .8 4 8 .4 2 0 .3 0 0 2 4 9 7 .7 7 6 m B 2 0 4 2 3 .4 0 2 3 ,8 U 4 7 .2 2 4 8 .4 3 8 .. 5 1 U ,3 8 5 0 5 2 4 .2 3 7 C o Fe 2 0 4 2 5 .0 2 2 5 .1 0 4 7 .1 4 4 7 .6 0 0 .3 7 0 .4 7 9 0 5 0 5 1 6 4 7 N iF e 2 0 4 2 4 .7 2 2 5 .0 0 4 6 ,9 4 4 7 .6 0 8 .3 5 0 .4 6 9 0 51 1. 0 - Z n Fe 2 0 4 2 6 .8 7 2 7 .7 0 4 6 .0 8 4 6 .3 3 8 .4 4 0 .4 7 0 0 5 0 8 ,4 2 2 435 ferrimagnetic in nature as shown by the hyperfine splitting of the Bossbauer spectrum. It may be mentioned that ZriEk204 is known to be paramagnetic at room temperature. However if quenched from high temperature, some of the 2+ Zn ions may occupy some of the octahedral sites or eventually some of the Zn2+ may volatilize; yielding a solid solution between ZnFe204 and q-Fe203 giving rise to a ferrimagnetic compound [20] . (v> Surface area measurements The surface areas of the ferrites (Table 4) obtained by the decomposition of these mixed metal oxalate hydrazinate precursors are very large compared to the ferrites prepared by the ceramic methods. Those prepared by a conventional ceramic method are reported C213 to have surface areas ranging from 0.5 to 7.7 m2.g -1 , whereas those obtained in the present investigation have surface areas ranging from 22 to 76 m2.g -1 . The variation in the surface area values could be due to the variation of ferrite formation temperatures as well as the exothermicity of decompositions. (vi> Magnetio properties Sinoe ferrites are widely used as magnetic materials, a study of magnetic properties of the ferrites synthesized would indicate how useful they could be as magnetic materials. !l%e magnetic hysteresis curve of magnesium ferrite was recorded on an instrument constructed at the National Aero- nautical Laboratory, Pangalore and compared with that of standard CrO2 (Du Pont, He = 477 Ce, bR = 35.8 emu g4 and Ss = 75.9 emu g-'1. The approximate values of the magnetic properties of magnesium ferrite thus obtained are: 436 Saturatron magnetisation Magnetic remanance Ratio of Coercive force Hc Magnetic moment /uR fG3 = 22.7 emu g -1 cr = 7.67 emu g-l d-r/ 6s = 0.34 = 75.0e = 0.9669 BM Conductivity measurements of this sample of magnesium ferrite showed a resistivity of the order of 10' ohm. cm. The low value of the saturation magnetization and high value of the resistivity suggest that this soft ferrite could be used in high frequency applicatiOns where the use of a meta- llic core is ruled out because of the prohibitive eddy current losses. It is well known that when the resistivity is high the eddy current and relaxation losses are very low. The saturation magnetization (and hence magnetic moment) value observed for magnesium ferrite is lower than the reported [22) value of 27 emu.g -1 . This difference is due to the fact that saturation magnetization depends strongly on heat treatment. CONCLUSION Mixed metal oxalate hydrazinates MFe2(C204)3(N2H4)6, where M= Mn, Co, Ni, Zn, have been prepared, characterised and investigated as precursors to spine1 ferritea. These precursors decompose exothermically at very low temperatures (130-25O'C) to yield ferritea. 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