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Synthesis of zinc ferrite under high-pressure shock loading
Volume
3, number
9,lO
SYNTHESIS
MATERIALS
OF ZINC FERRITE UNDER
E.L. VENTURINI, Sm&
Natmnal
Received
B. MOROSIN
L.&oratones.
Ahquerque,
July
LETTERS
HIGH-PRESSURE
SHOCK
1985
LOADING
and R.A. GRAHAM NM 87185.
USA
X April 1985
Mechanically blended stoichiometric mixtures of zinc oxide and ferric oxide powders have been subjected to high-pressure shock-wave loading and preserved for post-shock analysis. Partial conversion to a non-stoichiometric spine1 structure zinc ferrtte was found by X-ray diffraction. The ferrite is strongly magnetic as shown by static magnetization measurements.
1. Introduction The synthesis of inorganic compounds by highpressure shock-wave loading with preservation of the sample for post-shock analysis is a rapidly expanding area of materials science [ 1,2]. Such shock modification has been shown to introduce extraordinarily large concentrations of defects into many refractory solids and cause greatly enhanced solid-state reactivity [3,4]. Hence, the physical and chemical properties of shocksynthesized solids may differ significantly from those for the same materials prepared by more conventional processing. Prior work [5] has shown that it is possible to synthesize a zinc ferrite from a mechanical mixture of zinc oxide and ferric oxide by shock loading. The solid-state reaction of ZnO and Fe,O, powders at elevated temperatures appears to be complex, and has been investigated under various conditions [6-l 11. The reaction begins near 4OO”C, and a substantial evolution of oxygen is observed between 400 and 500°C [9]. Below 1100°C the rate of formation of the spine1 phase is determined by zinc diffusion [&lo]. When heated in an inert-gas atmosphere to 12OO”C, the product of the reaction between stoichiometric amounts of ZnO and Fe203 powders ballmilled to 1 pm diameter contained excess ZnO plus the mixed spine1 ferrite Zno.85 Fe2.15 0, [9]. The mixed ferrite was unstable in air above 515”C, re” Work supported Contract
by the U.S. Department No. DE-AC04-76DPO0789.
of Energy
under
0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
oxidizing to form stoichiometric zinc ferrite (ZnFe,O,) plus hematite (Fe203). Stoichiometric zinc ferrite can be obtained from the reaction of zinc oxide and ferric oxide powders by using excess zinc oxide [ 121 or an oxygen atmosphere [6,13,14]. Stoichiometric zinc ferrite ZnFe,04 is paramag netic [ 12,15,16] at room temperature, and antiferromagnetic [17] below an ordering temperature of 10 K. It crystallizes in the normal spine1 lattice where the tetrahedral or A sites are mainly occupied by divalent zinc ions while the octahedral or B sites contain trivalent iron ions [ 18,191. The slight site disorder can be described writing the composition as ZnI,Fe, [ZnXFeZ_X ] 0, where the B site occupation is enclosed in brackets and x is less than 0.05 [19]. The presence of iron ions on isolated A sites leads to superparamagnetic clusters which can explain the anomalous magnetic properties of the stoichiometric compound [ 121. The magnetic properties of nonstoichiometric zinc ferrite Zn,Fe3,04 where the A sites contain both Fe and Zn have been extensively studied forx from 0 to 1 [6,9,14,15,20,21]. In the present paper we discuss the structural and magnetic properties of the shock-synthesized product of ZnO and Fe,O, powders. This product contains a mixed zinc-iron spine1 ferrite plus excess zinc oxide, similar to that obtained in the furnace reaction described above. In the shock-loading process, however, the few microsecond duration of the event and the retention of the powder in a gas-tight copper fixture until the experiment has returned to room temperature 349
Volume 3, number 9,lO
MATERIALS LE’ITERS
prevent the substantial oxygen loss [9] observed in the furnace preparation. In addition the peak shock temperature is below 6OO”C,and substantial zinc diffusion at this temperature in the short timescale of the shock experiment requires extraordinarily rapid diffusion controlled by defects and mechanically driven mass motion. Hence the zinc ferrite synthesized by highpressure shock-wave loading from the starting oxide powders may differ substantially from material prepared by more conventional processing.
2. Experimentaldetails The ZnO and Fez03 powders used in these experiments were commercial grade (99.9% purity, CEFLAC, Inc., Milwaukee, WI). A stoichiometric batch for all experiments was prepared by ball milling the powders together for 24 h. Scanning electron microscopy showed the dispersal at grain ievel with typical particle size of 0.3 (urn [22]. The stoichiometric batch powder was pressed into compacts in place within the copper capsules to various initial packing densities. The capsules were then subjected to controlled, quantifiable explosive loading. The shock-loading conditions for the three experiments discussed in this paper are included in table 1. The copper capsules are designed to seal the compacts in place in the capsule so that after explosive loading there is confidence that the loading conditions did not cause any extr~eous, ~~aracte~~d effects. Quantification of pressure and temperature conditions within the compact is achieved with an extensive program of numerical simulation described elsewhere [23]. An important feature of these capsules is the gas-tight seal of the powder compact which is maintained throughout the shock-wave loading and unloading. Hence there is no path for oxygen evolution during the shock synthesis in contrast to heating in an inert atmosphere where rapid evolution was observed above 400°C [9]. Following the shock experiment the copper capsule rapidly cools to room temperature before the gas-tight seal is cut open and the product removed. The recovered powder is lightly ground to ensure complete dispersion of any reacted material within an X-ray or magnetization sample. X-ray diffraction studies employed Fe Ktu radiation. A series of standards prepared by mixing the unreacted 350
July 1985
stoichiometric powder with that of ZnFe,O, spine1 powder served to quantify our compositional determination. It should be noted that shock-modi~ed powders typically show broadened X-ray diffraction profiles due to residual lattice strain and to reduced coherent crystal domain size so that the detailed diffraction profiles of the X-ray pattern of our standards are not identical to the resultant dock-modi~ed powders. Magnetization measurements were made on a SQUID susceptometer (S.H.E. Corporation, San Diego, CA) for temperatures between 7 and 400 K and mag netic field strengths from 5 to 40 kOe, A right circular cylinder of pure Pt was used for calibration. Measurements on the various samples were made by placing 20 to 100 mg of loose powder in a plastic cylindrical bucket (a small correction was used for the diamagnetic moment of this bucket).
3. Results and discussion The most immediate feature upon opening of the copper recovery capsules is the dramatic color’change of the powder compact for the higher shock pressures. The extent of the darkening depends on the pressure and temperature. The powder around the outer edge of the disk is typically a darker red-brown than in the bulk of the major portion, The numerical simulations [23] identify this region as one with higher temperatures than in the interior. X-ray diffraction and mag netic measurement samples were usually selected from the bulk of the recovered material (i.e. the major portion of the compact defined by numerical simulation to have been subjected to a similar loading history); however, both on these and other materials, edge and center samples were occasionally monitored. X-ray diffraction shows formation of a spinel-structure type as indicated in table 1. In addition, the unreacted, recovered Fe,O, and ZnO powder shows broadened line profiles resulting from reduction in crystallite size and/or residual lattice strain. As the data in table 1 show, both shock-loading pressure and temperature appear to influence formation of the spine1 material. Shock pulse signature may also be important. In addition, on samples with larger amounts of ferrite formed, a greater amount of Fe203 is consumed relative to that of ZnO, suggesting that a defective, iron-rich spine1 is being formed. The spine1 is
Volume 3, number 9,lO Table 1 Schedule and results of shock-loaded ZnO/FezOa Experiment
July 1985
MATERIALS LETTERS
Compact density (Mg/m3)
mixtures a)
Peak shock pressure
Peak shock temperature
Sample moment at 7 K, 40 kOe
Ferrite formed from X-ray
(GPa)
(“C)
(emu/g)
(%I
unshocked b,
-
_
ZnFeaO4 ‘) 286846
-
_
_ _
2.75
24G846
3.03
16 22
400
11.55
8
25G846
2.48
22
550
26.31
25
0.48
0 100 N.D. dI
18.21 0.70
400
a) The peak shock pressure and temperature produced in the powder compact are based on realistic two-dimensional simulations [ 231. All capsules are the “Momma Bear A” design, The explosives used are baratol for 16 GPa and Composition B for 22 GPa. b, Ball-milled stoichiometric mixture of ZnO and Fe901 uowders used in the shock experiments. c) Zinc ferrite X-ray reference which contained 5 weight% MnO. d, N.D. = none detected.
probably Zn l_xFexFe204 rather than stoichiometric ZnFe204. This is particularly the case for a sample shock-loaded to 27 GPa in which consumption of Fe203 is complete while some ZnO remains. Static magnetization versus applied field measurements for the samples show a positive moment rising with increasing field at all temperatures from 7 to 400 K in fields from 5 to 40 kOe. There was negligible field hysteresis observed in these samples over the range studied, and no change with time in the magnetization at 400 K, i.e. no annealing of the shock-synthesized material. The magnetization of the unshocked powder did show some temperature hysteresis between 200 and 300 K due to the Morin transition [24] of Fe203 near 250 K. The recovered powders from the two shock experiments at 22 GPa (table 1) have a moment which is nearly saturated at fields above 10 kOe, although there is a small linear increase with increasing field from 20 to 40 kOe. This behavior may result from strong domain wall pinning at extended defects, although magnetization versus field curves [21] at 4.2 K for non-stoichiometric Zn,Feg_,04 show similar behavior for x < 0.6. It precludes a simple determination of the “saturation” magnetization in these samples; thus, table 1 compares the total sample moment at 7 K and 40 kOe. The two powders shock-loaded to 22 GPa have a dominant ferromagnetic (or ferrimagnetic) constituent as determined from the shape of their Arrott plots [25] at all temperatures. Fig. 1 shows the temperature dependence of the
I 0
?
2
0 0 0 0
20
cl
0
5
$10 E 2
.”
0
vvv
0 0
.
v
n
v
Temperature
.
v
.
.
.’
v
v
v_
(K)
Fig. 1. Magnetic moment at 40 kOe versus temperature for powders recovered from three shock-compression experiments: open circles for 25G846, closed squares for 24G846, and open triangles for 28G846 (see table 1).
moment at 40 kOe for the three shock-loaded powders. The open circles and closed squares compare powders shocked to the same peak pressure of 22 GPa, but with peak temperatures of 550 and 4OO”C, respectively. These data show that the higher shock temperature results in a doubling of the moment at 7 K, and a 50% increase at 400 K. The closed squares and open triangles compare powders shocked to the same peak temperature of 4OO’C but with peak pressures of 22 and 16 GPa, respectively. These data show very little magnetic spine1 phase in the latter powder, indicating 351
Volume 3, number 9,lO
MATERIALS LETTERS
the necessity of higher pressures to produce significant synthesis product at this relatively low peak temperature. The magnetization data at 7 K and 40 kOe are summarized in the fourth column of table 1. The unshocked ZnO/Fe203 mixture has a small moment which agrees with the expected value [24]. Aithough stoichiomet~~ zinc ferrite ZnFe,O, is antiferromag netic [ 171 with a NCeltemperature of 10 K, the reference powder has a substantial moment at high fields at 7 K. The reported [12] susceptibility of 4.2 X 10m4 emu/g at this temperature for ZnFe204 would produce a moment of 17 emu~g compared to the measured 18.2 emu/g. The small increase may be due to the 5 weight% MnO in the reference sample used here. The shock experiment at 16 GPa and 400°C yields a powder with a moment somewhat greater than that for the starting mixture, but consistent with the X-ray result of no detectable spine1 phase formation. In contrast the two powders shocked to 22 GPa have moments an order of magnitude larger. The product of experiment 25G846 contains 25% spine1 phase from X-ray diffraction, and a moment of 26.3 emu~g. This moment arises almost entirely from the spine1 component, which implies an effective magnetization near 100 emu/g for this constituent since it is only l/4 of the measured powder. (Similar shockloadings of Fe203 powders produce moments less than 1 emu~g,) This effective moment is double the saturation ma~etization at low temperature for the ~zn0.85Fe2.1504 I’reduced by furnace reaction of the stoichiometric powders [9]. It agrees with the values [2 11 at 4.2 K and 40 kOe for either of the two nonstoicbiometric compounds Zn,Feg_xG4 withx = 0 and 0.8. The ma~eti~tion [20] of the latter material falls rapidly with increasing temperature to less than 1 emu/g at 330 K, while fig. 1 shows that the shocksynthesized material has an effective magnetization near 50 emu/g at the same temperature. Hence the spine1 zinc ferrite produced by shook-load~g differs substantially from that made by more conventional processing. The chemical conversion of mechanical mixtures of zinc and ferric oxide powders under the few microsecond duration of the shock-compression event requires greatly ebbed solid-state reactivity. This enharmed reactivity can result from the extraordinarily large concentration of extended and point defects 352
July 1985
produced by the shock, mechanical abrasion of surfaces and opening of fresh surfaces. Fur~ermore, shock compression conditions of forced relative mass motion, local stress and temperature gradients and rapid quenching from high-pressure, high-temperature states act to produce highly non-equilibrium conditions in the powders. The unique nature of the shock process and the demonstrated unique nature of shockmodified or shock-synthesized materials appears to open new opportunities in materials technology.
References [ 11 B. Morosin and R.A. Graham, in: Shock waves in condensed matter - 1981, Menlo Park, eds. W.J. Nellis, L. Seaman and R.A. Graham (American Institute of Physics, New York, 1982) p. 4. ]ZJ S.S. Batsanov, in: Shock waves in condensed matter 1981, Menlo Park, eds. W.J. Nellis, L. Seaman and R.A. Graham (American Institute of Physics, New York, 1982) p. 1. [ 31 E.L. Venturini and R.A. Graham, in: Defect properties of processing of high-technology nonmetallic materials, eds. J.H. Crawford Jr., Y. Chen and W.A. Sibley (NorthHolland, Amsterdam, 1984) p. 383. [4] B. Morosin and R.A. Graham, in: Shock waves in condensed matter - 1983, Santa Fe, eds. J.R. Asay, R.A. Graham and G.K. Straub (North-Holland, Amsterdam, 1984) p. 355. IS] Y. Kimura, Japan. J. Appl. Phys. 2 (1963) 312. 16J E.W. Garter, Philips Res. Rept. 9 (1954) 295, and references therein. [ 71 J.F. Duncan and D.J. Stewart, Trans. Faraday Sot. 63 (1967) 1031. [8] J. Beretka and M.J. Ridge, Nature 216 (1967) 473. 191 R. Parker, C.J. Rigden and C.J. Tinsley, Trans. Faraday Sot. 65 (1969) 219. [lo] G.C. Kuczynski, in: Proceedings of the Internation~ Conference on Ferrites, eds. Y. Hoshino, S. Iida and M. Sugimoto (University Park Press, Tokyo, 1971) p. 87. [ II] G.A. Kolta, S.Z. El-Tawil, A.A. Ibrahim and N.S. Felix, Thermochim. Acta 36 (1980) 359, f 121 F.K. Lotgering, J. Phys. Chem. Solids 27 (1966) 139. [ 131 C.J. Rigden, J. Mat. Sci. 4 (1969) 1084. [ 141 A.L. Stnijts, D. Veeneman and A. Broese van Groenou, in: Proceedings of the International Conference on Ferrites, eds. Y. Hoshino, S. Iida and M. Sugimoto (University Park Press, Tokyo, 1971) p. 236. [ 1.51 E.W. Gorter, Nature 165 (1950) 798. [ 161 A. Arrott and J.E. Goldman, Phys. Rev. 98 (1955) 1201. [ 171 J.M. Hastings and L.M. Co&s, Phys. Rev. 102 (1956) 1460.
Volume 3, number 9,lO
MATERIALS LETTERS
[IS] E.J.W. Verwey and E.L. Heilmann, J. Chem. Phys. 15 (1947) 174. [ 19] J.M. Hastings and L.M. Corliss, Rev. Mod. Phys. 25 (1953) 114. [20] C.M. Srivastava, S.N. Shringi, R.G. Srivastava and N.G. Nanadikar, Phys. Rev. B14 (1976) 2032. [21] P.A. Dickof, P.J. Schurer and A.H. Morrish, Phys. Rev. B22 (1980) 115.
July 1985
[22J M.J. Carr, private communication. 1231 R.A. Graham and D.M. Webb, in: Shock waves in condensed matter - 1983, Santa Fe, eds. J.R. Asay, R.A. Graham and G.K. Straub (North-Holland, Amsterdam, 1984) p. 211. [ 241 F.J. Morin, Phys. Rev, 78 (1950) 819. [25] A. Arrott, Phys. Rev. 108 (1957) 1394.
353
3, number
9,lO
SYNTHESIS
MATERIALS
OF ZINC FERRITE UNDER
E.L. VENTURINI, Sm&
Natmnal
Received
B. MOROSIN
L.&oratones.
Ahquerque,
July
LETTERS
HIGH-PRESSURE
SHOCK
1985
LOADING
and R.A. GRAHAM NM 87185.
USA
X April 1985
Mechanically blended stoichiometric mixtures of zinc oxide and ferric oxide powders have been subjected to high-pressure shock-wave loading and preserved for post-shock analysis. Partial conversion to a non-stoichiometric spine1 structure zinc ferrtte was found by X-ray diffraction. The ferrite is strongly magnetic as shown by static magnetization measurements.
1. Introduction The synthesis of inorganic compounds by highpressure shock-wave loading with preservation of the sample for post-shock analysis is a rapidly expanding area of materials science [ 1,2]. Such shock modification has been shown to introduce extraordinarily large concentrations of defects into many refractory solids and cause greatly enhanced solid-state reactivity [3,4]. Hence, the physical and chemical properties of shocksynthesized solids may differ significantly from those for the same materials prepared by more conventional processing. Prior work [5] has shown that it is possible to synthesize a zinc ferrite from a mechanical mixture of zinc oxide and ferric oxide by shock loading. The solid-state reaction of ZnO and Fe,O, powders at elevated temperatures appears to be complex, and has been investigated under various conditions [6-l 11. The reaction begins near 4OO”C, and a substantial evolution of oxygen is observed between 400 and 500°C [9]. Below 1100°C the rate of formation of the spine1 phase is determined by zinc diffusion [&lo]. When heated in an inert-gas atmosphere to 12OO”C, the product of the reaction between stoichiometric amounts of ZnO and Fe203 powders ballmilled to 1 pm diameter contained excess ZnO plus the mixed spine1 ferrite Zno.85 Fe2.15 0, [9]. The mixed ferrite was unstable in air above 515”C, re” Work supported Contract
by the U.S. Department No. DE-AC04-76DPO0789.
of Energy
under
0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
oxidizing to form stoichiometric zinc ferrite (ZnFe,O,) plus hematite (Fe203). Stoichiometric zinc ferrite can be obtained from the reaction of zinc oxide and ferric oxide powders by using excess zinc oxide [ 121 or an oxygen atmosphere [6,13,14]. Stoichiometric zinc ferrite ZnFe,04 is paramag netic [ 12,15,16] at room temperature, and antiferromagnetic [17] below an ordering temperature of 10 K. It crystallizes in the normal spine1 lattice where the tetrahedral or A sites are mainly occupied by divalent zinc ions while the octahedral or B sites contain trivalent iron ions [ 18,191. The slight site disorder can be described writing the composition as ZnI,Fe, [ZnXFeZ_X ] 0, where the B site occupation is enclosed in brackets and x is less than 0.05 [19]. The presence of iron ions on isolated A sites leads to superparamagnetic clusters which can explain the anomalous magnetic properties of the stoichiometric compound [ 121. The magnetic properties of nonstoichiometric zinc ferrite Zn,Fe3,04 where the A sites contain both Fe and Zn have been extensively studied forx from 0 to 1 [6,9,14,15,20,21]. In the present paper we discuss the structural and magnetic properties of the shock-synthesized product of ZnO and Fe,O, powders. This product contains a mixed zinc-iron spine1 ferrite plus excess zinc oxide, similar to that obtained in the furnace reaction described above. In the shock-loading process, however, the few microsecond duration of the event and the retention of the powder in a gas-tight copper fixture until the experiment has returned to room temperature 349
Volume 3, number 9,lO
MATERIALS LE’ITERS
prevent the substantial oxygen loss [9] observed in the furnace preparation. In addition the peak shock temperature is below 6OO”C,and substantial zinc diffusion at this temperature in the short timescale of the shock experiment requires extraordinarily rapid diffusion controlled by defects and mechanically driven mass motion. Hence the zinc ferrite synthesized by highpressure shock-wave loading from the starting oxide powders may differ substantially from material prepared by more conventional processing.
2. Experimentaldetails The ZnO and Fez03 powders used in these experiments were commercial grade (99.9% purity, CEFLAC, Inc., Milwaukee, WI). A stoichiometric batch for all experiments was prepared by ball milling the powders together for 24 h. Scanning electron microscopy showed the dispersal at grain ievel with typical particle size of 0.3 (urn [22]. The stoichiometric batch powder was pressed into compacts in place within the copper capsules to various initial packing densities. The capsules were then subjected to controlled, quantifiable explosive loading. The shock-loading conditions for the three experiments discussed in this paper are included in table 1. The copper capsules are designed to seal the compacts in place in the capsule so that after explosive loading there is confidence that the loading conditions did not cause any extr~eous, ~~aracte~~d effects. Quantification of pressure and temperature conditions within the compact is achieved with an extensive program of numerical simulation described elsewhere [23]. An important feature of these capsules is the gas-tight seal of the powder compact which is maintained throughout the shock-wave loading and unloading. Hence there is no path for oxygen evolution during the shock synthesis in contrast to heating in an inert atmosphere where rapid evolution was observed above 400°C [9]. Following the shock experiment the copper capsule rapidly cools to room temperature before the gas-tight seal is cut open and the product removed. The recovered powder is lightly ground to ensure complete dispersion of any reacted material within an X-ray or magnetization sample. X-ray diffraction studies employed Fe Ktu radiation. A series of standards prepared by mixing the unreacted 350
July 1985
stoichiometric powder with that of ZnFe,O, spine1 powder served to quantify our compositional determination. It should be noted that shock-modi~ed powders typically show broadened X-ray diffraction profiles due to residual lattice strain and to reduced coherent crystal domain size so that the detailed diffraction profiles of the X-ray pattern of our standards are not identical to the resultant dock-modi~ed powders. Magnetization measurements were made on a SQUID susceptometer (S.H.E. Corporation, San Diego, CA) for temperatures between 7 and 400 K and mag netic field strengths from 5 to 40 kOe, A right circular cylinder of pure Pt was used for calibration. Measurements on the various samples were made by placing 20 to 100 mg of loose powder in a plastic cylindrical bucket (a small correction was used for the diamagnetic moment of this bucket).
3. Results and discussion The most immediate feature upon opening of the copper recovery capsules is the dramatic color’change of the powder compact for the higher shock pressures. The extent of the darkening depends on the pressure and temperature. The powder around the outer edge of the disk is typically a darker red-brown than in the bulk of the major portion, The numerical simulations [23] identify this region as one with higher temperatures than in the interior. X-ray diffraction and mag netic measurement samples were usually selected from the bulk of the recovered material (i.e. the major portion of the compact defined by numerical simulation to have been subjected to a similar loading history); however, both on these and other materials, edge and center samples were occasionally monitored. X-ray diffraction shows formation of a spinel-structure type as indicated in table 1. In addition, the unreacted, recovered Fe,O, and ZnO powder shows broadened line profiles resulting from reduction in crystallite size and/or residual lattice strain. As the data in table 1 show, both shock-loading pressure and temperature appear to influence formation of the spine1 material. Shock pulse signature may also be important. In addition, on samples with larger amounts of ferrite formed, a greater amount of Fe203 is consumed relative to that of ZnO, suggesting that a defective, iron-rich spine1 is being formed. The spine1 is
Volume 3, number 9,lO Table 1 Schedule and results of shock-loaded ZnO/FezOa Experiment
July 1985
MATERIALS LETTERS
Compact density (Mg/m3)
mixtures a)
Peak shock pressure
Peak shock temperature
Sample moment at 7 K, 40 kOe
Ferrite formed from X-ray
(GPa)
(“C)
(emu/g)
(%I
unshocked b,
-
_
ZnFeaO4 ‘) 286846
-
_
_ _
2.75
24G846
3.03
16 22
400
11.55
8
25G846
2.48
22
550
26.31
25
0.48
0 100 N.D. dI
18.21 0.70
400
a) The peak shock pressure and temperature produced in the powder compact are based on realistic two-dimensional simulations [ 231. All capsules are the “Momma Bear A” design, The explosives used are baratol for 16 GPa and Composition B for 22 GPa. b, Ball-milled stoichiometric mixture of ZnO and Fe901 uowders used in the shock experiments. c) Zinc ferrite X-ray reference which contained 5 weight% MnO. d, N.D. = none detected.
probably Zn l_xFexFe204 rather than stoichiometric ZnFe204. This is particularly the case for a sample shock-loaded to 27 GPa in which consumption of Fe203 is complete while some ZnO remains. Static magnetization versus applied field measurements for the samples show a positive moment rising with increasing field at all temperatures from 7 to 400 K in fields from 5 to 40 kOe. There was negligible field hysteresis observed in these samples over the range studied, and no change with time in the magnetization at 400 K, i.e. no annealing of the shock-synthesized material. The magnetization of the unshocked powder did show some temperature hysteresis between 200 and 300 K due to the Morin transition [24] of Fe203 near 250 K. The recovered powders from the two shock experiments at 22 GPa (table 1) have a moment which is nearly saturated at fields above 10 kOe, although there is a small linear increase with increasing field from 20 to 40 kOe. This behavior may result from strong domain wall pinning at extended defects, although magnetization versus field curves [21] at 4.2 K for non-stoichiometric Zn,Feg_,04 show similar behavior for x < 0.6. It precludes a simple determination of the “saturation” magnetization in these samples; thus, table 1 compares the total sample moment at 7 K and 40 kOe. The two powders shock-loaded to 22 GPa have a dominant ferromagnetic (or ferrimagnetic) constituent as determined from the shape of their Arrott plots [25] at all temperatures. Fig. 1 shows the temperature dependence of the
I 0
?
2
0 0 0 0
20
cl
0
5
$10 E 2
.”
0
vvv
0 0
.
v
n
v
Temperature
.
v
.
.
.’
v
v
v_
(K)
Fig. 1. Magnetic moment at 40 kOe versus temperature for powders recovered from three shock-compression experiments: open circles for 25G846, closed squares for 24G846, and open triangles for 28G846 (see table 1).
moment at 40 kOe for the three shock-loaded powders. The open circles and closed squares compare powders shocked to the same peak pressure of 22 GPa, but with peak temperatures of 550 and 4OO”C, respectively. These data show that the higher shock temperature results in a doubling of the moment at 7 K, and a 50% increase at 400 K. The closed squares and open triangles compare powders shocked to the same peak temperature of 4OO’C but with peak pressures of 22 and 16 GPa, respectively. These data show very little magnetic spine1 phase in the latter powder, indicating 351
Volume 3, number 9,lO
MATERIALS LETTERS
the necessity of higher pressures to produce significant synthesis product at this relatively low peak temperature. The magnetization data at 7 K and 40 kOe are summarized in the fourth column of table 1. The unshocked ZnO/Fe203 mixture has a small moment which agrees with the expected value [24]. Aithough stoichiomet~~ zinc ferrite ZnFe,O, is antiferromag netic [ 171 with a NCeltemperature of 10 K, the reference powder has a substantial moment at high fields at 7 K. The reported [12] susceptibility of 4.2 X 10m4 emu/g at this temperature for ZnFe204 would produce a moment of 17 emu~g compared to the measured 18.2 emu/g. The small increase may be due to the 5 weight% MnO in the reference sample used here. The shock experiment at 16 GPa and 400°C yields a powder with a moment somewhat greater than that for the starting mixture, but consistent with the X-ray result of no detectable spine1 phase formation. In contrast the two powders shocked to 22 GPa have moments an order of magnitude larger. The product of experiment 25G846 contains 25% spine1 phase from X-ray diffraction, and a moment of 26.3 emu~g. This moment arises almost entirely from the spine1 component, which implies an effective magnetization near 100 emu/g for this constituent since it is only l/4 of the measured powder. (Similar shockloadings of Fe203 powders produce moments less than 1 emu~g,) This effective moment is double the saturation ma~etization at low temperature for the ~zn0.85Fe2.1504 I’reduced by furnace reaction of the stoichiometric powders [9]. It agrees with the values [2 11 at 4.2 K and 40 kOe for either of the two nonstoicbiometric compounds Zn,Feg_xG4 withx = 0 and 0.8. The ma~eti~tion [20] of the latter material falls rapidly with increasing temperature to less than 1 emu/g at 330 K, while fig. 1 shows that the shocksynthesized material has an effective magnetization near 50 emu/g at the same temperature. Hence the spine1 zinc ferrite produced by shook-load~g differs substantially from that made by more conventional processing. The chemical conversion of mechanical mixtures of zinc and ferric oxide powders under the few microsecond duration of the shock-compression event requires greatly ebbed solid-state reactivity. This enharmed reactivity can result from the extraordinarily large concentration of extended and point defects 352
July 1985
produced by the shock, mechanical abrasion of surfaces and opening of fresh surfaces. Fur~ermore, shock compression conditions of forced relative mass motion, local stress and temperature gradients and rapid quenching from high-pressure, high-temperature states act to produce highly non-equilibrium conditions in the powders. The unique nature of the shock process and the demonstrated unique nature of shockmodified or shock-synthesized materials appears to open new opportunities in materials technology.
References [ 11 B. Morosin and R.A. Graham, in: Shock waves in condensed matter - 1981, Menlo Park, eds. W.J. Nellis, L. Seaman and R.A. Graham (American Institute of Physics, New York, 1982) p. 4. ]ZJ S.S. Batsanov, in: Shock waves in condensed matter 1981, Menlo Park, eds. W.J. Nellis, L. Seaman and R.A. Graham (American Institute of Physics, New York, 1982) p. 1. [ 31 E.L. Venturini and R.A. Graham, in: Defect properties of processing of high-technology nonmetallic materials, eds. J.H. Crawford Jr., Y. Chen and W.A. Sibley (NorthHolland, Amsterdam, 1984) p. 383. [4] B. Morosin and R.A. Graham, in: Shock waves in condensed matter - 1983, Santa Fe, eds. J.R. Asay, R.A. Graham and G.K. Straub (North-Holland, Amsterdam, 1984) p. 355. IS] Y. Kimura, Japan. J. Appl. Phys. 2 (1963) 312. 16J E.W. Garter, Philips Res. Rept. 9 (1954) 295, and references therein. [ 71 J.F. Duncan and D.J. Stewart, Trans. Faraday Sot. 63 (1967) 1031. [8] J. Beretka and M.J. Ridge, Nature 216 (1967) 473. 191 R. Parker, C.J. Rigden and C.J. Tinsley, Trans. Faraday Sot. 65 (1969) 219. [lo] G.C. Kuczynski, in: Proceedings of the Internation~ Conference on Ferrites, eds. Y. Hoshino, S. Iida and M. Sugimoto (University Park Press, Tokyo, 1971) p. 87. [ II] G.A. Kolta, S.Z. El-Tawil, A.A. Ibrahim and N.S. Felix, Thermochim. Acta 36 (1980) 359, f 121 F.K. Lotgering, J. Phys. Chem. Solids 27 (1966) 139. [ 131 C.J. Rigden, J. Mat. Sci. 4 (1969) 1084. [ 141 A.L. Stnijts, D. Veeneman and A. Broese van Groenou, in: Proceedings of the International Conference on Ferrites, eds. Y. Hoshino, S. Iida and M. Sugimoto (University Park Press, Tokyo, 1971) p. 236. [ 1.51 E.W. Gorter, Nature 165 (1950) 798. [ 161 A. Arrott and J.E. Goldman, Phys. Rev. 98 (1955) 1201. [ 171 J.M. Hastings and L.M. Co&s, Phys. Rev. 102 (1956) 1460.
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MATERIALS LETTERS
[IS] E.J.W. Verwey and E.L. Heilmann, J. Chem. Phys. 15 (1947) 174. [ 19] J.M. Hastings and L.M. Corliss, Rev. Mod. Phys. 25 (1953) 114. [20] C.M. Srivastava, S.N. Shringi, R.G. Srivastava and N.G. Nanadikar, Phys. Rev. B14 (1976) 2032. [21] P.A. Dickof, P.J. Schurer and A.H. Morrish, Phys. Rev. B22 (1980) 115.
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[22J M.J. Carr, private communication. 1231 R.A. Graham and D.M. Webb, in: Shock waves in condensed matter - 1983, Santa Fe, eds. J.R. Asay, R.A. Graham and G.K. Straub (North-Holland, Amsterdam, 1984) p. 211. [ 241 F.J. Morin, Phys. Rev, 78 (1950) 819. [25] A. Arrott, Phys. Rev. 108 (1957) 1394.
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