- Email: [email protected]
The role of non-crystalline structure in materials science
]OURNA h OF
Journal of Non-CrystallineSolids 192& 193 (1995) 1-8
ELSEVIER
Introductory lecture
The role of non-crystalline structure in materials science Kenji Suzuki * Institute for Materials Research, Tohoku Unit,ersity, Katahira 2-1-1, Aoba-ku, Sendai 980, Japan
Abstract Non-crystalline structures are demonstrated to function as promising precursors for controlling the nano-scale structure of materials: by controlling the local structure around a minority atom added to an amorphous alloy, nanocrystalline grains are optimized for a soft magnet alloy. An organic polymer, as a precursor, is converted into an inorganic nano-hybrid structure of crystalline clusters embedded in an amorphous matrix, by controlling the pyrolyzing conditions. Fast ionic conduction in a glass is controlled by the hierarchy of dynamic structures existing inherently in disordered materials.
1. Introduction Materials design is mostly based on nano-scale structure control, particularly in the processing of advanced high-tech materials. Non-crystalline structures often function as promising precursors for controlling the nano-scale structure of materials, because a high degree of freedom for topological and chemical structure persists in amorphous materials. Currently, minority atoms added in amorphous alloy precursors are found to provide a key effect on the improvement of material properties. The mechanism of forming nanocrystalline grains in Fe73.5Si13.sB9Cu1Nb 3 SOft magnet alloy is investigated by measuring the fluorescence X-ray absorption fine structure (XAFS) detected from only 1 at.% Cu and 3 at.% Nb. The origin of the very high tensile strength of S i - C - T i - O amorphous fibers, converted from an
* Corresponding author. Tel: + 81-22 215 2000. Telefax: + 8122 215 2002. E-mail: [email protected].
organic precursor, is shown to be a uniform dispersion of f3-SiC crystalline nanoclusters in an amorphous matrix, by small-angle X-ray scattering measurements. CuI-Cu2MoO 4 solid glass shows very fast conduction due to the Cu ÷ ions, comparable with that in the liquid state. The dynamic behavior of the Cu ÷ ions is observed by inelastic neutron scattering, to explore the correlation between the low-energy excitations in the medium-range structure and the translational diffusion of the Cu ÷ ions within the glass structure.
2. Nanocrystallization from amorphous alloys A soft magnet nanocrystalline alloy having high magnetic flux density and high magnetic permeability, the 'Finemet' named by Hitachi Metals Ltd., is obtained by crystallizing an Fe73.sSi13.sB9CutNb3 amorphous alloy precursor. Many people have still shown interest in the Finemet, because of a new approach away from conventional bulk crystalline alloy magnets.
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3093(95)00323-1
2
K. Suzuki/Journal of Non-Crystalline Solids 192& 193 (1995) 1-8
Yoshizawa et al. [1] demonstrated that nanocrystalline bcc Fe with dissolved Si is precipitated by annealing the Fe73.sSi13.sB9CulNb3amorphous alloy around 800 K and F e - B intermetallic compounds, such as Fe3B, Fe23B 6 and Fe2B, are formed above 870 K. Only 1 at.% Cu and 3 at.% Nb additives are necessary to form the optimized nanocrystalline grains of the bcc Fe(Si) precipitated at 823 K from the amorphous alloy precursor. What is the role of such minority Cu or Nb additives for the formation of bcc Fe(Si) nanocrystaUine grains? Fig. 1 shows the Cu K-edge extended X-ray absorption fine structure (EXAFS) function, k3x(k), for Fe73.sSit3.sBgCulNb3 amorphous alloy as a function of annealing temperature, together with those for fcc Cu pure metal and an Fe-1 at.% Cu quenched alloy [2]. The K-edge EXAFS of the Cu and Nb minority additives were selectively measured by the fluorescence detection method at room temperature, using the synchrotron radiation source at KEK, Japan. The Cu and Fe K-edge EXAFS are very similar to each other in the as-quenched amorphous alloy. However, the Cu K-edge EXAFS of the amorphous alloy is obviously modified towards that of fcc Cu far below the onset temperature (783 K) of crystallization to bcc Fe(Si). This suggests that the Cu atoms distributed uniformly in the amorphous alloy
Cu K-edge 160
Pure Fe
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Fig. 1. Cu K-edge EXAFS for Fe73.sSi13.sB9Cu1Nb3 alloys, F e - 1 at.% Cu quenched alloy and fcc Cu metal [2].
18~
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Fig. 2. Nb K-edge EXAFS for Fe73.sSi13.sB9CulNb3 alloys and
bcc Nb metal [2].
selectively aggregate to form fcc Cu nanoclusters before the precipitation of bcc Fe(Si). If it is assumed that each fcc Cu nanocluster functions as a nucleating center for the bcc Fe(Si) and is embedded in a bcc Fe(Si) grain, we can roughly estimate that each fcc Cu cluster consists of several hundred Cu atoms, which leads to a sphere of about 1-2 nm in diameter, based on the average diameter (10 nm) of the bcc Fe(Si) grains observed by transmission electron microscopy (TEM) [1]. On the other hand, the Nb K-edge EXAFS does not show any significant change between the asquenched amorphous alloy and the Finemet obtained by annealing at 823 K, as shown in Fig. 2 [2]. However, a completely different Nb K-edge EXAFS appears when the annealing temperature is high enough above 870 K to precipitate F e - B intermetallic compounds. This means that the local structure around Nb atoms in the as-quenched amorphous alloy is well preserved even in the Finemet. Nb atoms are excluded from the bcc Fe(Si) grains and enriched into the remaining amorphous alloy matrix, when the amorphous alloy precursor is annealed above 800 K. The Nb-enriched amorphous alloy exists at the boundary of the bcc Fe(Si) grains to prevent their growth. Metallic Cu and Fe are insoluble as well as Nb and Fe in the equilibrium crystalline state. However,
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
Amorphous state
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High-strength continuous S i - C - T i - O fibers synthesized by Yajima's method [4], the Tirano fiber
3
named by Ube Industries Ltd., are an interesting material with respect to controlling the nanostructure during the organic-to-inorganic conversion process. The Tirano fibers are prepared from the organic polymer precursor polytitanocarbosilane (PTC). The melt is spun into fibers, either while curing in oxygen gas or under electron irradiation, and finally pyrolyzed in an inert gas at a temperature of about 1200°C and converted into an inorganic ceramic. A typical chemical composition for the fibers is approximately Si30C50TilO20. In a previous paper [5], we reported that a single S i - C - T i - O fiber a few txm in diameter pyrolyzed at 1200°C comprises bundles of thousands o of fine filaments with a diameter of about 100 A, in which B-SiC nanocrystalline clusters are uniformly dispersed, as illustrated in Fig. 4. Therefore, the smallangle X-ray scattering (SAXS) of the fiber includes two contributions. One comes from the isotropic scattering caused by the B-SiC nanocrystalline cluso 1 ters, in the scattering vector range Q > 0.06 A , while the other is the anisotropic scattering originating from a bundle of fine filaments and appearing in the range Q < 0.05 .~-~. The anisotropic SAXS has a very high intensity in the Q direction perpendicular to the long axis of the fiber, which is one order of magnitude or more higher than the SAXS intensity parallel to the long axis of the fiber. The tensile strength of the S i - C - T i - O fibers increases drastically up to about 300 k g / m m 2 at the pyrolyzing temperature of 800-1000°C [6]. A prob-
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\~L~ 3O Fig. 4. The nano-scalestructuremodelof a Si-C-Ti-O fiber[5].
4
K. Suzuki / Journal of Non-Crystalline Solids 192 & 193 (1995) 1-8
lem is whether such a high strength should be ascribed to the isotropic or anisotropic structure of the fiber. Therefore, the SAXS intensities parallel and perpendicular to the long axis of a fiber, which was cured in oxygen gas, were measured as a function of pyrolyzing temperature using a point beam of Cu K s X-rays at room temperature. As shown in Fi~. 5, the anisotropic SAXS in the range Q < 0.04 A -] depends only slightly on the pyrolyzing temperature, while the isotropic SAXS, having a broad maximum o 1 in the range Q > 0.05 A - , changes drastically with the pyrolyzing temperature as shown in Fig. 6 [7]. The SAXS intensity of the fiber pyrolyzed above 1000°C follows the Porod law in the high Q range. This means that the precipitates have a well-defined boundary with the matrix when the pyrolyzing temperature is above 1000°C. The nanoimage and nanodiffraction observed by the cold cathode field-emission-type high-resolution TEM (Hitachi HF2000) clearly demonstrates that the precipitates are 13-SIC
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Fig. 6. Isotropic SAXS intensities as a function of pyrolyzing temperature for a Si-C-Ti-O fiber with the scattering vector parallel to the long axis of the fiber [7]. The straight line follows the Porod law (Q-4).
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Fig. 5. Anisotropic SAXS intensities as a function of pyrolyzing temperature for a Si-C-Ti-O fiber with the scattering vector perpendicular to the long axis of the fiber [7].
nanoclusters embedded in an S i - C - T i - O amorphous matrix, as shown in Fig. 7. The radius of gyration, Rg, for the 13-SIC clusters is estimated from a Guinier plot of the SAXS intensity in a low Q range (Q << 1/Rg). The growth of the 13-SIC clusters is examined by monitoring the integrated intensity of the SAXS. The value of Rg increases continually from about 4 A at the pyrolyzing temperature, 1000°C, to 10 A at 1400°C, while the integrated SAXS intensity decreases drastically beyond the pyrolyzing temperature, 1000°C. We conclude that the high strength of the Tirano fiber is realized by optimizing the size, crystallinity, boundary and quantity of the 13-SIC nanoclusters, dispersed in the S i - C - T i - O amorphous matrix, during the conversion process from organic polymer to inorganic ceramic.
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
4. Fast ionic conduction in glasses The crystalline compounds c~-AgI and CuI are known to show fast conduction by the Ag + and Cu ÷ ions, similar to that in the liquid state. Minami and
5
Machida [8] found that the ionic conductivity of a glass including these compounds, such as AgIAg2MoO4, is one order of magnitude or more higher than that of its crystalline counterpart. This behavior is opposite to that of the electronic conduction in
Fig. 7. Nanoimages and nanodiffraction patterns for a S i - C - T i - O fiber pyrolyzed at 1400°C: (A) cured in oxygen gas and (B) cured by electron irradiation.
6
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
amorphous metals. An interesting problem is what types of ionic motion appear due to the disorder in the atomic arrangement of glasses• The dotted lines in Fig. 8 show the dynamic structure factor, S(Q, to), for (CuI)49.2(Cu2MoO4)50.8 glass, measured as a function of temperature in the Q range corresponding to the main peak in the total structure factor, S(Q), using the inelastic neutron scattering spectrometer LAM40 (energy resolution = 150 ixeV) installed at the pulsed neutron source at KEK. This glass was prepared by quenching the corresponding liquid, encapsulated in a quartz vessel, into water• The solid lines in Fig. 8 indicate the contribution of the Boson peak at each temperature, calculated based on the S(Q, to) measured at 90 K. A significant difference between the dotted and solid lines is found in the energy range 1-5 meV at the temperature above 200 K. The high-resolution profile of S(Q, to) in a narrow energy range below 100 IxeV is shown as a function of temperature in Fig. 9. This observation was made using the inelastic neutron scattering spectrometer LAM80ET (energy resolution = 15 txeV) at KEK. Quasielastic-type broadening obviously appears above 373 K. Since the corrections for multiple scattering, resolution of the spectrometer and absorp-
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tion can to a first approximation be neglected in the range of (Q, to) scanned in this work, the energyintegrated intensity and energy width of S(Q, to) o 1 at Q = 1.21 A - , which is approximated by a Lorentzian function, are obtained as a function of temperature, as shown in Fig. 10. The energy-integrated intensity approximately corresponds to the number of translationally diffusing ions, n ot e x p ( - E , / k T ) , while the energy width is roughly proportional to the diffusion constant of the ions, D ct e x p ( - E D / k T ) , where E, and E D are the activation energy for formation and translational motion of a diffusing ion, respectively• Based on the relationship for the ionic conductivity, trot nD/kT, we obtain E c t E . + E D from a plot of orTat exp(-E/kT) versus 1/T. The values E n = 11 kJ/mol and E D = 14 k J / m o l are obtained from Fig. 10. The value E = 26.5 kJ/mol, measured by ionic conductivity, is close to the value E, + E D = 25 k J / m o l obtained in the inelastic neutron scattering experiment. The low-energy excitation in the range of 1-5 meV, observed in S(Q, to) below 200 K, corresponds to the Boson peak which describes the excess quasiharmonic vibrations inherently existing in the
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
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disordered structure of the glass. With increasing temperature above 200 K, the local diffusion of the Cu + ions becomes pronounced, but is still dynamically confined within the medium-range structure. The long-range translational diffusion of the Cu ÷ ions is found directly above 373 K by observing the quasielastic neutron scattering peak appearing in the energy range below 100 IxeV.
5. Conclusion
Non-crystalline structures, particularly amorphous solid structures in the non-equilibrium state, have a high degree of freedom in the topological and chemical arrangement of their constituent atoms. There-
7
fore, the artificial control of nano-scale structures is often made by using amorphous solids as a promising precursor. In this report, three cases for nanostructure control, starting from the amorphous state, have been reviewed. First, the structural mechanism for the formation of bcc Fe(Si) nanograins in an Fe73.sSil3.sB9CulNb3 amorphous alloy, by adding 1 at.% Cu and 3 at.% Nb, is discussed. Cu atoms form fcc nanoclusters uniformly distributed as nucleation centers, far below the onset temperature of the bcc Fe(Si) phase precipitation, while the Nb atoms remain in the amorphous boundary between the bcc Fe(Si) nanograins after completing the precipitation of the bcc Fe(Si) phase to prevent the growth of these grains. Second, the very high tensile strength of S i - C Ti-O fibers is shown to originate from the fact that the hybrid structure of the 13-SIC nanoclusters, embedded uniformly in the amorphous matrix, is optimized by controlling the pyrolyzing conditions for the conversion process from the organic precursor to the inorganic fiber. Finally, fast ionic conduction by (CuI)49.2(CueMoO4)50.8 glass, which is one order of magnitude higher than that of its crystalline counterpart, is directly demonstrated to arise from the long-range translational diffusion of the Cu ÷ ions, by measuring the high-resolution quasielastic neutron scattering in the energy range below 100 )xeV. The motion of the Cu ÷ ions in the glass is classified into three categories, depending of the energy range: (i) quasiharmonic and anharmonic vibration in short-range structure, (ii) local diffusion confined in the medium-range structure, and (iii) long-range translational diffusion in a liquid-like state. The dynamical structure of amorphous solids will become increasingly important in materials science for investigating the control of nano-structure.
The author would like to thank T. Kamiyama, M. Matsuura, K. Shibata, M. Sakurai, K. Wakoh, S.-H. Kim, Y. Wang and S. Hirokami for their collaboration in this work, which was supported in part by the New Frontier Program Grant-in-Aid for Scientific Research (No. 05NP0501) of the Ministry of Education, Science and Culture, Japan.
8
K. Suzuki/Journal of Non-Crystalline Solids 192& 193 (1995) 1-8
References [1] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] S.-H. Kim, M. Matsuura, M. Sakurai and K. Suzuki, Jpn. J. Appl. Phys. 32 (1993) 679. [3] S.-H. Kim, doctoral thesis, Tohoku University (1994). [4] S. Yajima, Philos. Trans. R. Soc. London A294 (1980) 419.
[5] K. Suzuya, T. Kamiyama, T. Yamamura, K. Okamura and K. Suzuki, J. Non-Cryst. Solids 150 (1992) 167. [6] T. Yamamura, T. Ishikawa, M. Shibuya, T. Hisayuki and K. Okamura, J. Mater. Sci. 23 (1988) 2589. [7] T. Kamiyama, Y. Wang, K. Suzuki, M. Shibuya and T. Yamamura, J. Jpn. Soc. Powder Powder Metall. 41 (1994) 795. [8] T. Minani and N. Machida, Mater. Sci. Eng. B13 (1992) 203.
Journal of Non-CrystallineSolids 192& 193 (1995) 1-8
ELSEVIER
Introductory lecture
The role of non-crystalline structure in materials science Kenji Suzuki * Institute for Materials Research, Tohoku Unit,ersity, Katahira 2-1-1, Aoba-ku, Sendai 980, Japan
Abstract Non-crystalline structures are demonstrated to function as promising precursors for controlling the nano-scale structure of materials: by controlling the local structure around a minority atom added to an amorphous alloy, nanocrystalline grains are optimized for a soft magnet alloy. An organic polymer, as a precursor, is converted into an inorganic nano-hybrid structure of crystalline clusters embedded in an amorphous matrix, by controlling the pyrolyzing conditions. Fast ionic conduction in a glass is controlled by the hierarchy of dynamic structures existing inherently in disordered materials.
1. Introduction Materials design is mostly based on nano-scale structure control, particularly in the processing of advanced high-tech materials. Non-crystalline structures often function as promising precursors for controlling the nano-scale structure of materials, because a high degree of freedom for topological and chemical structure persists in amorphous materials. Currently, minority atoms added in amorphous alloy precursors are found to provide a key effect on the improvement of material properties. The mechanism of forming nanocrystalline grains in Fe73.5Si13.sB9Cu1Nb 3 SOft magnet alloy is investigated by measuring the fluorescence X-ray absorption fine structure (XAFS) detected from only 1 at.% Cu and 3 at.% Nb. The origin of the very high tensile strength of S i - C - T i - O amorphous fibers, converted from an
* Corresponding author. Tel: + 81-22 215 2000. Telefax: + 8122 215 2002. E-mail: [email protected].
organic precursor, is shown to be a uniform dispersion of f3-SiC crystalline nanoclusters in an amorphous matrix, by small-angle X-ray scattering measurements. CuI-Cu2MoO 4 solid glass shows very fast conduction due to the Cu ÷ ions, comparable with that in the liquid state. The dynamic behavior of the Cu ÷ ions is observed by inelastic neutron scattering, to explore the correlation between the low-energy excitations in the medium-range structure and the translational diffusion of the Cu ÷ ions within the glass structure.
2. Nanocrystallization from amorphous alloys A soft magnet nanocrystalline alloy having high magnetic flux density and high magnetic permeability, the 'Finemet' named by Hitachi Metals Ltd., is obtained by crystallizing an Fe73.sSi13.sB9CutNb3 amorphous alloy precursor. Many people have still shown interest in the Finemet, because of a new approach away from conventional bulk crystalline alloy magnets.
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3093(95)00323-1
2
K. Suzuki/Journal of Non-Crystalline Solids 192& 193 (1995) 1-8
Yoshizawa et al. [1] demonstrated that nanocrystalline bcc Fe with dissolved Si is precipitated by annealing the Fe73.sSi13.sB9CulNb3amorphous alloy around 800 K and F e - B intermetallic compounds, such as Fe3B, Fe23B 6 and Fe2B, are formed above 870 K. Only 1 at.% Cu and 3 at.% Nb additives are necessary to form the optimized nanocrystalline grains of the bcc Fe(Si) precipitated at 823 K from the amorphous alloy precursor. What is the role of such minority Cu or Nb additives for the formation of bcc Fe(Si) nanocrystaUine grains? Fig. 1 shows the Cu K-edge extended X-ray absorption fine structure (EXAFS) function, k3x(k), for Fe73.sSit3.sBgCulNb3 amorphous alloy as a function of annealing temperature, together with those for fcc Cu pure metal and an Fe-1 at.% Cu quenched alloy [2]. The K-edge EXAFS of the Cu and Nb minority additives were selectively measured by the fluorescence detection method at room temperature, using the synchrotron radiation source at KEK, Japan. The Cu and Fe K-edge EXAFS are very similar to each other in the as-quenched amorphous alloy. However, the Cu K-edge EXAFS of the amorphous alloy is obviously modified towards that of fcc Cu far below the onset temperature (783 K) of crystallization to bcc Fe(Si). This suggests that the Cu atoms distributed uniformly in the amorphous alloy
Cu K-edge 160
Pure Fe
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,
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Fig. 1. Cu K-edge EXAFS for Fe73.sSi13.sB9Cu1Nb3 alloys, F e - 1 at.% Cu quenched alloy and fcc Cu metal [2].
18~
Nb K-edge /~
A
,~
Pure Nb (xl/5)
12
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8 10 W a v e V e c t o r k / ~,-1
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14
Fig. 2. Nb K-edge EXAFS for Fe73.sSi13.sB9CulNb3 alloys and
bcc Nb metal [2].
selectively aggregate to form fcc Cu nanoclusters before the precipitation of bcc Fe(Si). If it is assumed that each fcc Cu nanocluster functions as a nucleating center for the bcc Fe(Si) and is embedded in a bcc Fe(Si) grain, we can roughly estimate that each fcc Cu cluster consists of several hundred Cu atoms, which leads to a sphere of about 1-2 nm in diameter, based on the average diameter (10 nm) of the bcc Fe(Si) grains observed by transmission electron microscopy (TEM) [1]. On the other hand, the Nb K-edge EXAFS does not show any significant change between the asquenched amorphous alloy and the Finemet obtained by annealing at 823 K, as shown in Fig. 2 [2]. However, a completely different Nb K-edge EXAFS appears when the annealing temperature is high enough above 870 K to precipitate F e - B intermetallic compounds. This means that the local structure around Nb atoms in the as-quenched amorphous alloy is well preserved even in the Finemet. Nb atoms are excluded from the bcc Fe(Si) grains and enriched into the remaining amorphous alloy matrix, when the amorphous alloy precursor is annealed above 800 K. The Nb-enriched amorphous alloy exists at the boundary of the bcc Fe(Si) grains to prevent their growth. Metallic Cu and Fe are insoluble as well as Nb and Fe in the equilibrium crystalline state. However,
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
Amorphous state
T,
Zx~Za
The starting of the crystallization of Fe-Si
T~
High-strength continuous S i - C - T i - O fibers synthesized by Yajima's method [4], the Tirano fiber
3
named by Ube Industries Ltd., are an interesting material with respect to controlling the nanostructure during the organic-to-inorganic conversion process. The Tirano fibers are prepared from the organic polymer precursor polytitanocarbosilane (PTC). The melt is spun into fibers, either while curing in oxygen gas or under electron irradiation, and finally pyrolyzed in an inert gas at a temperature of about 1200°C and converted into an inorganic ceramic. A typical chemical composition for the fibers is approximately Si30C50TilO20. In a previous paper [5], we reported that a single S i - C - T i - O fiber a few txm in diameter pyrolyzed at 1200°C comprises bundles of thousands o of fine filaments with a diameter of about 100 A, in which B-SiC nanocrystalline clusters are uniformly dispersed, as illustrated in Fig. 4. Therefore, the smallangle X-ray scattering (SAXS) of the fiber includes two contributions. One comes from the isotropic scattering caused by the B-SiC nanocrystalline cluso 1 ters, in the scattering vector range Q > 0.06 A , while the other is the anisotropic scattering originating from a bundle of fine filaments and appearing in the range Q < 0.05 .~-~. The anisotropic SAXS has a very high intensity in the Q direction perpendicular to the long axis of the fiber, which is one order of magnitude or more higher than the SAXS intensity parallel to the long axis of the fiber. The tensile strength of the S i - C - T i - O fibers increases drastically up to about 300 k g / m m 2 at the pyrolyzing temperature of 800-1000°C [6]. A prob-
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Diameter of a filament
~loo- lOOOi
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~1[[ ~
'
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amorphous Si-Ti-C-O matrix '\,
\~L~ 3O Fig. 4. The nano-scalestructuremodelof a Si-C-Ti-O fiber[5].
4
K. Suzuki / Journal of Non-Crystalline Solids 192 & 193 (1995) 1-8
lem is whether such a high strength should be ascribed to the isotropic or anisotropic structure of the fiber. Therefore, the SAXS intensities parallel and perpendicular to the long axis of a fiber, which was cured in oxygen gas, were measured as a function of pyrolyzing temperature using a point beam of Cu K s X-rays at room temperature. As shown in Fi~. 5, the anisotropic SAXS in the range Q < 0.04 A -] depends only slightly on the pyrolyzing temperature, while the isotropic SAXS, having a broad maximum o 1 in the range Q > 0.05 A - , changes drastically with the pyrolyzing temperature as shown in Fig. 6 [7]. The SAXS intensity of the fiber pyrolyzed above 1000°C follows the Porod law in the high Q range. This means that the precipitates have a well-defined boundary with the matrix when the pyrolyzing temperature is above 1000°C. The nanoimage and nanodiffraction observed by the cold cathode field-emission-type high-resolution TEM (Hitachi HF2000) clearly demonstrates that the precipitates are 13-SIC
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Fig. 6. Isotropic SAXS intensities as a function of pyrolyzing temperature for a Si-C-Ti-O fiber with the scattering vector parallel to the long axis of the fiber [7]. The straight line follows the Porod law (Q-4).
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r~< 0.1
Fig. 5. Anisotropic SAXS intensities as a function of pyrolyzing temperature for a Si-C-Ti-O fiber with the scattering vector perpendicular to the long axis of the fiber [7].
nanoclusters embedded in an S i - C - T i - O amorphous matrix, as shown in Fig. 7. The radius of gyration, Rg, for the 13-SIC clusters is estimated from a Guinier plot of the SAXS intensity in a low Q range (Q << 1/Rg). The growth of the 13-SIC clusters is examined by monitoring the integrated intensity of the SAXS. The value of Rg increases continually from about 4 A at the pyrolyzing temperature, 1000°C, to 10 A at 1400°C, while the integrated SAXS intensity decreases drastically beyond the pyrolyzing temperature, 1000°C. We conclude that the high strength of the Tirano fiber is realized by optimizing the size, crystallinity, boundary and quantity of the 13-SIC nanoclusters, dispersed in the S i - C - T i - O amorphous matrix, during the conversion process from organic polymer to inorganic ceramic.
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
4. Fast ionic conduction in glasses The crystalline compounds c~-AgI and CuI are known to show fast conduction by the Ag + and Cu ÷ ions, similar to that in the liquid state. Minami and
5
Machida [8] found that the ionic conductivity of a glass including these compounds, such as AgIAg2MoO4, is one order of magnitude or more higher than that of its crystalline counterpart. This behavior is opposite to that of the electronic conduction in
Fig. 7. Nanoimages and nanodiffraction patterns for a S i - C - T i - O fiber pyrolyzed at 1400°C: (A) cured in oxygen gas and (B) cured by electron irradiation.
6
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
amorphous metals. An interesting problem is what types of ionic motion appear due to the disorder in the atomic arrangement of glasses• The dotted lines in Fig. 8 show the dynamic structure factor, S(Q, to), for (CuI)49.2(Cu2MoO4)50.8 glass, measured as a function of temperature in the Q range corresponding to the main peak in the total structure factor, S(Q), using the inelastic neutron scattering spectrometer LAM40 (energy resolution = 150 ixeV) installed at the pulsed neutron source at KEK. This glass was prepared by quenching the corresponding liquid, encapsulated in a quartz vessel, into water• The solid lines in Fig. 8 indicate the contribution of the Boson peak at each temperature, calculated based on the S(Q, to) measured at 90 K. A significant difference between the dotted and solid lines is found in the energy range 1-5 meV at the temperature above 200 K. The high-resolution profile of S(Q, to) in a narrow energy range below 100 IxeV is shown as a function of temperature in Fig. 9. This observation was made using the inelastic neutron scattering spectrometer LAM80ET (energy resolution = 15 txeV) at KEK. Quasielastic-type broadening obviously appears above 373 K. Since the corrections for multiple scattering, resolution of the spectrometer and absorp-
[xl 0 -2] 1 O[ 9I '
(OUl)49'2- (Cu2M°O4)50'8
8~ .
Q / ,&, 1 =2.0~2.5
• "J~.{.
,
E J
3ooK
(~_ 4 ~ "...~...... . •
. • ~ 5 . . . K .
....
....
/
t 0
2
4 6 8 Energy [ meV ]
10
Fig. 8. Dynamic structure factor, S(Q, to), for (CuI)49.2(Cu2MoO4)50.8 glass, measured as a function of temperature using the LAM40 inelastic neutron scattering spectrometer.
10 (Cul)49 2- ( O u 2 M o 0 4 ) 5 o 8
.... O F •
E f3 5-.
'-'5
4>
Vanadium 413K 373K 295K
d v
09
I I
+ • .-o
I
0
r
I
I
._1
I
I
I
0.05
I
0.
Energy T r a n s f e r [ m e V ] Fig. 9. Dynamic structure factor, S(Q, to), for (CuI)49.2(Cu2MoO4)50.8 glass, measured as a function of temperature using the LAM80ET inelastic neutron scattering spectrometer.
tion can to a first approximation be neglected in the range of (Q, to) scanned in this work, the energyintegrated intensity and energy width of S(Q, to) o 1 at Q = 1.21 A - , which is approximated by a Lorentzian function, are obtained as a function of temperature, as shown in Fig. 10. The energy-integrated intensity approximately corresponds to the number of translationally diffusing ions, n ot e x p ( - E , / k T ) , while the energy width is roughly proportional to the diffusion constant of the ions, D ct e x p ( - E D / k T ) , where E, and E D are the activation energy for formation and translational motion of a diffusing ion, respectively• Based on the relationship for the ionic conductivity, trot nD/kT, we obtain E c t E . + E D from a plot of orTat exp(-E/kT) versus 1/T. The values E n = 11 kJ/mol and E D = 14 k J / m o l are obtained from Fig. 10. The value E = 26.5 kJ/mol, measured by ionic conductivity, is close to the value E, + E D = 25 k J / m o l obtained in the inelastic neutron scattering experiment. The low-energy excitation in the range of 1-5 meV, observed in S(Q, to) below 200 K, corresponds to the Boson peak which describes the excess quasiharmonic vibrations inherently existing in the
K. Suzuki~Journal of Non-Crystalline Solids 192&193 (1995) 1-8
I'
I
+
i
+''
0.04
S-"
0.03
E :s
0.02
!
i
T -'r-
0.01 0.008 0.006 0.0022
+00
+°or °
0.0026
0.0028
1/-I- [K 1]
0.0030 0.0032
o0+ i i
/7 >- 0.05 _~
0.0024
O=12 A-1
i
i
0.04 En=11 iJrnol+ 0.03
1 ~
0.0022
L,
0.0024
,
IZ,
0.0026
i \i I
J U
0.0028
+ , ~,
0+0030 0.0032
1/T [K11 Fig. 10. Arrhenius plot of energy integration intensity and energy width for S(Q, to), shown in Fig. 9, versus reciprocal temperature (l/T).
disordered structure of the glass. With increasing temperature above 200 K, the local diffusion of the Cu + ions becomes pronounced, but is still dynamically confined within the medium-range structure. The long-range translational diffusion of the Cu ÷ ions is found directly above 373 K by observing the quasielastic neutron scattering peak appearing in the energy range below 100 IxeV.
5. Conclusion
Non-crystalline structures, particularly amorphous solid structures in the non-equilibrium state, have a high degree of freedom in the topological and chemical arrangement of their constituent atoms. There-
7
fore, the artificial control of nano-scale structures is often made by using amorphous solids as a promising precursor. In this report, three cases for nanostructure control, starting from the amorphous state, have been reviewed. First, the structural mechanism for the formation of bcc Fe(Si) nanograins in an Fe73.sSil3.sB9CulNb3 amorphous alloy, by adding 1 at.% Cu and 3 at.% Nb, is discussed. Cu atoms form fcc nanoclusters uniformly distributed as nucleation centers, far below the onset temperature of the bcc Fe(Si) phase precipitation, while the Nb atoms remain in the amorphous boundary between the bcc Fe(Si) nanograins after completing the precipitation of the bcc Fe(Si) phase to prevent the growth of these grains. Second, the very high tensile strength of S i - C Ti-O fibers is shown to originate from the fact that the hybrid structure of the 13-SIC nanoclusters, embedded uniformly in the amorphous matrix, is optimized by controlling the pyrolyzing conditions for the conversion process from the organic precursor to the inorganic fiber. Finally, fast ionic conduction by (CuI)49.2(CueMoO4)50.8 glass, which is one order of magnitude higher than that of its crystalline counterpart, is directly demonstrated to arise from the long-range translational diffusion of the Cu ÷ ions, by measuring the high-resolution quasielastic neutron scattering in the energy range below 100 )xeV. The motion of the Cu ÷ ions in the glass is classified into three categories, depending of the energy range: (i) quasiharmonic and anharmonic vibration in short-range structure, (ii) local diffusion confined in the medium-range structure, and (iii) long-range translational diffusion in a liquid-like state. The dynamical structure of amorphous solids will become increasingly important in materials science for investigating the control of nano-structure.
The author would like to thank T. Kamiyama, M. Matsuura, K. Shibata, M. Sakurai, K. Wakoh, S.-H. Kim, Y. Wang and S. Hirokami for their collaboration in this work, which was supported in part by the New Frontier Program Grant-in-Aid for Scientific Research (No. 05NP0501) of the Ministry of Education, Science and Culture, Japan.
8
K. Suzuki/Journal of Non-Crystalline Solids 192& 193 (1995) 1-8
References [1] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] S.-H. Kim, M. Matsuura, M. Sakurai and K. Suzuki, Jpn. J. Appl. Phys. 32 (1993) 679. [3] S.-H. Kim, doctoral thesis, Tohoku University (1994). [4] S. Yajima, Philos. Trans. R. Soc. London A294 (1980) 419.
[5] K. Suzuya, T. Kamiyama, T. Yamamura, K. Okamura and K. Suzuki, J. Non-Cryst. Solids 150 (1992) 167. [6] T. Yamamura, T. Ishikawa, M. Shibuya, T. Hisayuki and K. Okamura, J. Mater. Sci. 23 (1988) 2589. [7] T. Kamiyama, Y. Wang, K. Suzuki, M. Shibuya and T. Yamamura, J. Jpn. Soc. Powder Powder Metall. 41 (1994) 795. [8] T. Minani and N. Machida, Mater. Sci. Eng. B13 (1992) 203.