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(Ni/sub m/Co/sub 1-m/)/sub 1-d/O多晶體因氧化分解促成之布朗轉動及相互擴散造成之奈米顆粒(I)Oxidation-Decomposition Facilitated Brownian Rotation and Interdiffusion-Induced Nanoparticles in (Ni/sub m/Co/sub 1-m/)/sub 1-d/O Polycrystals (I)

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行政院國家科學委員會專題研究計畫 期中進度報告

(NimCo1-m)1-dO 多晶體因氧化分解促成之布朗轉動及相互

擴散造成之奈米顆粒(1/2)

計畫類別: 個別型計畫 計畫編號: NSC91-2216-E-110-015-執行期間: 91 年 08 月 01 日至 92 年 07 月 31 日 執行單位: 國立中山大學材料科學研究所 計畫主持人: 沈博彥 計畫參與人員: 沈博彥 研究生李名言等人 報告類型: 精簡報告 處理方式: 本計畫涉及專利或其他智慧財產權,1 年後可公開查詢

國 92 年 5 月 14 日

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行政院國家科學委員會專題研究計畫期中成果報告 (1/2)

(NimCo1-m)1-δO 多晶體因氧化分解促成之布朗轉動及相互擴散造成之奈米顆粒

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Oxidation-decomposition facilitated Brownian rotation and interdiffusion-induced nanoparticles in (NimCo1-m)1-δO polycrystals (1/2)

計畫編號 NSC91-2216-E-110-015 執行期限 : 民國九十一年八月至九十二年七月 主持人 : 沈博彥, 國立中山大學材料科學研究所 中文摘要 第 一 年 度 , 如 期 完 成 (Ni0.33Co0.67)1-δO 多晶燒結體,於低特 性溫度 (homologous temperature, T/Tm ~ 0.45) ,因氧化分解促成之顆粒布朗 轉動。將 1000oC 反應燒結,具雙尺寸 分佈之岩鹽結構多晶體,於大氧中進 行 720oC,長達 72 小時退火,確發生 界面鬆動,引起奈米級尖晶石微晶顆 粒之布朗轉動,調整晶向關係。過程 中,氧化分解與奈米尺寸效應,同樣 重要。 關鍵詞:晶向改變;布朗轉動;Co1-xO; Ni1-xO;氧化分解;原始氧化物; 尖晶石 Abstr act

(Ni0.33Co0.67)1-δO polycrystals with

rock salt structure and a bimodal size distribution due to reactive sintering at 1000oC were subject to annealing at 720oC for 2 to 72 h in air and studied by analytical electron microscopy with regard to the effect of oxidation decomposition on the reorientation of nanoparticles in host grains. Upon annealing, the nanoparticles rapidly

oxidized as spinel structure

progressively Co-richer, whereas the host protoxide grains with rock-salt-type structure progressively Ni-richer. The spinel particles less than 100 nm in size readily detached from grain boundaries and fell into parallel epitaxial relationship with respect to the host protoxide grains sharing a coherent interface. Such a Brownian-type reorientation process, in terms of anchorage release at interphase interface and driven by epitaxy energy cusp, at a

rather low apparent homologous

temperature (T/Tm=0.45) was facilitated

by oxidation decomposition process and nanometer-size effect.

.

Key words: orientation change; Brownian-type rotation; Co1-xO;

Ni1-xO; oxidation decomposition;

protoxide; spinel

1. Intr oduction

Annealing of ceramic composites prepared via a solid-state sintering route, e.g. Ni1-xO/yttria partially stabilized

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[2], and Co1-xO/Y-PSZ [3] was found to

cause orientation change of intragranular particles until they reached epitaxial relationships with respect to the host grains. A relatively high homologous

temperature (T/Tm) for the

Ni1-xO/NiAl2O4 [2] and Co1-xO/Y-PSZ

composites [3] resulted in a faster orientation change and more significant coalescence of the particles than the Ni1-xO/Y-PSZ composite at a specified

annealing temperature of 1600oC [1]. Reorientation of the intragranular particles in these composites has little to do with sintering [4], diffusion induced recrystallization [5] or dynamic recrystallization [6], but can be reasonably explained by rotation of the particles above a critical temperature for anchorage release at interface with respect to the host grain [1-3].

In such a thermally activated rotation process of the intragranular particles, Brownian motion of the particles in terms of interfacial diffusion of atoms was suggested to happen [1-3] as for the case of f.c.c. metal crystallites migrating and rotating on single crystal substrate, KCl(100) with or without steps [7-11]. In this case, the size and temperature dependence of diffusivity of the crystallites has been measured over KCl(100) [7] and found to be in accordance with Brownian-type motion of the crystallites in terms of interfacial diffusion of atoms from leading edge to trailing edge of the crystallites. Einstein's molecular theory of heat [12],

Eyring's transition-state model [13] and frictional force at a viscous interface were thus adopted to formulate the diffusivity equation of the crystallite over the single crystal substrate [8, 9].

Here we report annealing-induced

oxidation decomposition and

reorientation of nanoparticles in

bimodal-sized, yet homogenized,

(Ni0.33Co0.67)1-δO protoxide polycrystals

with rock salt-type structure. We focused on size-dependent oxidation and

decomposition that accompanied

reorientation of nanoparticles in a two-phase field at a surprising low homologous temperature. Besides a previously proposed reorientation

mechanism, i.e. size-dependent

Brownian type rotation of particles in terms of anchorage release at the interface, we suggest that oxidation decomposition facilitated reorientation of intragranular particles in this case

2. Exper imental

Co1-xO (Cerac, 325 mesh) and

Ni1-xO (Cerac, 100 mesh) powders in 2:1

molar ratio were ball milled in alcohol, the resultant size being 0.5 to 2.5 µm and 0.2 to 0.5 µm, respectively. The powdery mixture was dry pressed at 650 MPa to form pellets. The pellets were fired at 1000oC for 72 h in an open-air furnace and quenched in air or cooled in the furnace. Those quenched in air were subject to further annealing at 720oC for 2, 20 and 72 h, followed by cooling in the furnace.

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X-ray diffraction (XRD) was used to identify the phases of the fired pellets. Scanning electron microscopy (SEM, JSM-6400, 20 kV) was used to study chemically etched surface of the fired pellets. Thin sections of the samples were

Ar-ion milled to electron

transparency for analytical

electron microscopy (AEM, JEOL

3010 coupled with energy

dispersive x-ray (EDX) analysis at 300 kV).

3. Results and discussion

XRD indicated the (Ni0.33Co0.67)1-δO

sample reaction-sintered and

homogenized at 1000oC for 72 h and air quenched to room temperature contained a singe phase of rock salt structure having a room temperature lattice parameter of 0.4236 ± 0.0001 nm. This cell parameter coincides with the protoxide of (Ni0.33Co0.67)1-δO

composition interpolated linearly from 0.4177 nm for Ni1-xO and 0.4260 nm for

Co1-xO.

When the homogenized

(Ni0.33Co0.67)1-δO was subject to further

annealing at 720oC for 2, 20, and 72 h, the lattice parameters of protoxide decreased to 0.4220 ± 0.0002 nm, 0.4212 ± 0.0003 nm, and 0.4206 ± 0.0003 nm, respectively, due to substitution of a smaller cation Ni2+ (0.069 nm) for Co2+ (0.0745 nm, high spin in coordination number (C.N.) 6 [14]. Meanwhile, the spinel phase

emerged with decreasing lattice

parameters 0.8091 ± 0.0003 nm, 0.8089 ± 0.0002 nm, and 0.8084 ± 0.0001 nm, respectively. This trend was a result of substitution of smaller low-spin cation Co3+ (0.0545 nm) for Ni3+ (0.056 nm) in octahedral site [15], yet never reached normal spinel Co3O4

with Co2+ and Co3+ in tetrahedral and octahedral site, respectively according to magnetic measurement [16], which typically requires several weeks of dwelling in this temperature range [17]. It is noteworthy that the spinel content increased with increasing annealing time at 720oC as indicated by the peak height ratio of spinel (440) to rock salt (200). No further appreciable phase content change beyond 20 h of annealing although the composition of the co-existent phases kept modified following the tie lines in the two-phase field as discussed later.

SEM showed the as-sintered and homogenized sample has protoxide grains about 10 µm in size (Fig. 1a). Further annealing at 720oC for 2, 20, and 72 h (Figs. 1b to 1d) caused a rather limited grain growth, but a significant oxidation decomposition to form spinel phase (Fig. 1d). The area in Fig. 1d is magnified as Fig. 1e to show more

clearly the intergranular and

intragranular spinel particles more or less coalesced. EDX analysis indicated the spinel is relatively Co-rich and Ni-poor.

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TEM indicated the homogenized sample air-quenched to room temperature indeed contained only protoxide with rock salt-type structure, yet with a bimodal size distribution (Fig. 2a). As shown by corresponding SAED pattern in Fig. 3b, the nanometer-size particles were randomly oriented with respect to an attached large grain in [001] zone axis (Fig. 2c). It is noteworthy that the nanoparticles originally on top of the large grains were largely removed by Ar-ion milling.

When the homogenized protoxide sample was subject to annealing at 720oC for 2 h, the intragranular spinel particles ca. 100 nm in size showed up as shown in the BFI and DFI in Fig. 3a and 3b, respectively. These particles were apparently derived from small protoxide particles and detached from grain boundaries of relatively large protoxide particles. The corresponding SAED pattern (Fig. 3c) indicated the intragranular spinel particles of this size

have reached parallel epitaxy

relationship with the host protoxide

grain, whereas relic protoxide

nanoparticles were randomly oriented at the grain boundary as indicated by ring diffractions. The spinel particles coalesced to ca. 500 nm in size of this

sample remained nonepitaxy with

respect to the host protoxide grain (Fig. 3d). The representative lattice image of an intragranular and epitaxial spinel particle (Fig. 4) showed a coherent interphase interface. Further annealing at 720oC for 20 h and 72 h has activated

reorientation of the coalesced

intragranular spinel particles toward parallel epitaxial relationship with the host protoxide grain.

Point-count EDX analysis indicated the sample reaction-sintered at 1000oC for 72 h was homogenized to a specific composition (Ni0.33Co0.67)1-δO. Further

annealing at 720oC for 72 h caused decomposition into nickel-rich, i.e. (Ni0.65Co0.35)1-δO, protoxide host grains

and cobalt-rich, i.e. (Ni0.04Co0.96)3O4,

spinel particles. In fact, decomposition already occurred for the sample annealed at 720oC for 2 h, and the resultant composition profile across the protoxide grain and the spinel-particle rim indicated a slight outward diffusion of Co and inward diffusion of Ni for the protoxide grain. In this connection, the spinel phase has a room temperature lattice parameter of 0.8091 ± 0.0003 nm, 0.8089 ± 0.0002 nm, and 0.8084 ± 0.0001 nm upon dwelling at 720oC for 2, 20 and 72 h, respectively. These cell volumes correspond to Ni0.42Co2.58O4,

Ni0.38Co2.62O4, Ni0.28Co2.72O4

respectively, according to composition dependence of lattice parameter for NixCo3-xO4 [15].

The compositional and structural evidences of the samples annealed at 720oC in air indicated the following kinetics in the order of decreasing rate: (1) cooling-induced oxidation, (2) decomposition in terms of Ni/Co interdiffusion, (3) Brownian motion of intragranular particles. The underlying factors for these competitive events are

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as following.

XRD lattice parameters and EDX analysis indicated a specific diffusion path for the annealed samples in the Ni-Co-O ternary system, i.e. rapid oxidation to form spinel phase with composition progressively Co-richer. In this connection, Co1-xO was known to

oxidize spontaneously to Co3O4 spinel

upon cooling below 900oC [18]. This oxidation process was even more rapid for nanoparticles with a high specific surface area per volume and a beneficial short-circuit diffusion. By contrast, the

decomposition of protoxide

(Ni0.33Co0.67)1-δO involved a much

slower solid-state interdiffusion between large sintered grain and nanoparticles at grain boundaries as indicated by a gradual lattice parameter change at 720oC and EDX composition profiles across the diffusion couples. The final compositions of the couples after 72 h annealing are (Ni0.65Co0.35)1-δO for

protoxide grain and (Ni0.05Co0.95)3O4 for

spinel particles, basically following the tie lines determined at a specified oxygen partial pressure of 0.21 atm [17].

Our previous study indicated that a high annealing temperature was required to activate anchorage release, and hence the orientation change of intragranular particles in matrix grain [3]. Size-dependent orientation change is in accordance with the mechanism of Brownian-type rotation of particles in terms of anchorage release at the interphase interface. The diffusivity (D)

of spherical particles confined in a grain

was formulated to decrease

exponentially with the increase of number of atoms in anchorage at the

interface; whereas increase

exponentially with ∆T above a critical temperature (To) for anchorage release

[3].

The To as well as free energy is in

fact a function of particle size due to capillarity effect. Thus the reasons that a smaller particle has a higher diffusivity are two fold, firstly a less number of atoms in anchorage at the interface, and secondly a lower To. A range of To

corresponding to a specific size range of the particles is thus expected [3]. Above

To, the migration-rotation of crystallite is

determined by the “viscous” diffusion along interphase interface rather than debonding at the interface. The activation energy of such a “viscous” motion is independent of particle size, but To must be reached first so that the

particle can move under a frictional force related to interfacial viscosity [8, 9, 11].

In our previous study of rock-salt type protoxide composites, the Ni1-xO particles

smaller than 2 µm in size were capable of Brownian rotation within CaO grains at 1400oC, i.e. at a homologous temperature of 0.85, but such a reorientation process was suppressed at 1200oC (T/Tm=0.74) regardless of particle size [19]. By contrast, in the two-phase field of the present NiO-CoO system, the Brownian-type reorientation process was activated at a rather low homologous temperature (T/Tm=0.45) for

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intragranular particles. Possible underlying causes for a surprisingly low To, mainly

nanometer-size effect on lowering de-bonding temperature and accompanied oxidation decomposition process in two-phase field, are as follows.

A relative low temperature for ductility and de-bonding at the interphase interface is expected for a bulk material with constituent particles in nanometer regime [20, 21]. Diffusional creep and grain boundary sliding due to atomic diffusion along grain boundaries of the nanophase material resulted in enhanced ductility [20]. In such a terrain, capillarity effect prevails and surface tension may raise considerably the surface energy, rendering it easier for debonding at interface or for surface premelting [22]. Thus for the case of nanoparticles attached to the grain boundaries of the rock salt-type protoxide grains, Tm could be

considerably lowered, and hence homologous temperature raised, to activate Brownian-type rotation of intragranular particles. This is supported by a supplement observation of the furnace-cooled specimens, showing nanoparticles already transformed to spinel and became parallel epitaxy with the attached host protoxide grain, in drastic contrast to the homogenized and air-quenched sample without appreciable transformation and reorientation of the nanoparticles. In this sample, internal oxidation already caused the formation of elongate spinel precipitates at dislocations (not shown), which can be readily distinguished from the particles.

The thermomechanical properties of materials, generally expressed as a specific equation of state in terms of

temperature, pressure or composition, can be significantly altered at transformation invariant point. Such a discontinuity has been experimentally proved for transformation superplasticity at polymorphic transition temperature [6, 23, 24] and eutectoid decomposition temperature [25] for metals and ceramics systems. In general for a rigid ideal plastic material undergoing a phase transition under a small tensile stress σ, the accommodation strain per cycle follows ε = (5/3)(∆V/V)(σ/Y) assuming no shape change, where ∆V/V is volume misfit and Y the yield strength of the weaker phase [24]. As to the present case of oxidation decomposition of (Ni0.33Co0.67)1-δO in the two-phase

field without an applied stress, transformation strain may still cause considerable softening, and thereby facilitating de-bonding of those nanoparticles. The volume mismatch, ∆V/V, of double protoxide cell with

respect to spinel of specific

compositions for the sample annealed at 720oC for 2, 20, and 72 h are 0.119, 0.115 and 0.112, respectively, based on room temperature lattice parameter and assuming thermal expansion coefficient of the phases are not much different from that (14x10-6 K-1) for Co1-xO [26].

Thus, the effect of transformation strain on interface de-bonding became less important, although the driving force toward epitaxial orientation became larger, as the rock salt-structured and spinel phases became Ni- and Co-rich,

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respectively with increasing annealing time.

References

[1] J. Chen and P. Shen, Scripta Mater. 37 (1997) 1287.

[2] S.R. Wang and P. Shen, Mater. Sci. Eng. A 251 (1998) 106.

[3] K.T. Lin and P. Shen, Mater. Sci. Eng. A 270 (1999) 125.

[4] J. Rankin and B.W. Sheldon, Mat. Sci. Eng. A 204 (1995) 48.

[5] V.Y. Doo and R.W. Balluffi, Acta Metall. 6 (1958) 428.

[6] J.P. Poirier, "Creep of Crystals," pp. 260, Cambridge University Press, Cambridge (1985).

[7] A. Masson, J.J. Métois and R. Kern, Surface Science 27 (1971) 463. [8] R. Kern, A. Masson and J.J. Métois,

Surface Science 27 (1971) 483. [9] J.J. Métois, M. Gauch, A. Masson

and R. Kern, Surface Science 30 (1972) 43.

[10] J.J. Métois, Surface Science 36 (1973) 269.

[11] L.Y. Kuo and P. Shen, Surface Science 373 (1997) L350.

[12] A. Einstein, "Investigations of the

Theory of the Brownian

Movement," Methuen & Co. London, edited with notes by R. Furth and translated by A.D. Cowper (1926).

[13] S. Gladstone, K. Laidler and H. Eyring, "The Theory of Rate Processes," McGraw-Hill, New York, 1941.

[14] R.D. Shannon, Acta Crystallogr. A32 (1976) 751.

[15] E. Ríos, H. Nguyen-Cong, J.F. Marco, J.R. Ganvedo, P. Chartier and J.L. Gautier, Electrochimica Acta 45 (2000) 4431.

[16] P. Cossee, Rec. Trav. Chim. Pays-Bas, 75 (1956) 1089.

[17] R.J. Moore and J. White, J. Mater. Sci. 9 (1974) 1393.

[18] M. Oku and Y. Sato, Applied Surface Sci. 55 (1992) 37.

[19] M.L. Jeng and P. Shen Mater. Sci. Eng. (A). 287 (2000) 1.

[20] J. Karach, R. Birringer and H. Gleiter, Nature 330 (1987) 556.

[21] R.W. Siegel, Physics Today, October, (1993) p. 64-68.

[22] J.J. Métois and J.C. Heyraud, Ultramicroscopy 31 (1989) 73.

[23] J.P. Poirier, J. Geophys. Res. 87 (1982) 6791.

[24] G.W. Greenwood and R.H. Johnson, Proc. Royal Soc. Lond. A 283 (1965) 403.

[25] J.R. Smyth, R.C. Bradt, and J.H. Hoke, J. Am. Ceram. Soc. 58 (1975) 381.

[26] Y.S. Touloukian, R.K. Kirby, R.E. Tayor and T.Y.R. Lee (eds.), Thermal Expansion - Nonmetallic Solids, Thermophysical Properties of Matter Vol. 13, Plenum Pub. New York (1977) p. 221.

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Figure 1. SEM image for fired (Ni0.33Co0.67)1-δO samples etched by 30%

HCl solution: (a) reaction-sintered and homogenized at 1000oC for 72 h and air quenched to room temperature, (b), (c) and (d) subject to further annealing at 720oC for 2, 20, and 72 h, respectively and cooled in the furnace, showing rather limited grain growth of matrix grains, but significant oxidation decomposition to form spinel (dark) upon prolonged annealing in air. (e) further magnified SEM image (SEI) from arrowed area in Fig. 1d, showing relatively acid-resistant clusters of intra- and intergranular spinel particles enriched in Co according to EDX analysis.

Figure 2. TEM image for

(Ni0.33Co0.67)1-δO sample

reaction-sintered and homogenized at 1000oC for 72 h and air quenched to room temperature: (a) BFI showing a bimodal size distribution of rock-salt

protoxide, (b) SAED pattern of

randomly oriented and nanometer-sized particles, (c) SAED pattern of the relatively large grain tilted to [001] zone axis.

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Figure 3. TEM image for (Ni0.33Co0.67)1-δO sample subject to

post-sintering annealing at 720oC for 2 h and cooled in the furnace showing the appearance of intragranular spinel particles in rock salt-type grain at expense of rock salt-type nanoparticles at grain boundary: (a) BFI, (b) DFI imaged with 111 spot of spinel, (c) SAED pattern corresponding to (a) and (b) showing intragranular spinel (denoted as S) particles have reached parallel epitaxy with rock salt-type (denoted as R) host grain in [011] zone

axis, whereas randomly oriented

rock-salt nanoparticles are still present at grain boundary as indicated by ring diffractions, (d) BFI for larger and coalesced intragranular spinel particles still nonepitaxy with respect to host protoxide grain.

Figure 4.Lattice image of a

parallel-epitaxial and intragranular spinel particle (denoted as S) in a host grain of rock salt structure (denoted as R) having coherent (200) and {111} lattice planes across the interface. The same sample as Fig. 3.

數據

Figure  1.  SEM  image  for  fired  (Ni 0.33 Co 0.67 ) 1- δ O samples  etched  by  30%
Figure  4.Lattice  image  of  a  parallel-epitaxial  and  intragranular  spinel  particle  (denoted  as  S)  in  a  host  grain of rock salt structure (denoted as R)  having  coherent  (200)  and  {111}  lattice  planes  across  the  interface

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