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Preparation of ultrafine tungsten powders by in-situ hydrogen reduction of nano-needle violet tungsten oxide
Int. Journal of Refractory Metals and Hard Materials 29 (2011) 686–691
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Preparation of ultrafine tungsten powders by in-situ hydrogen reduction of nano-needle violet tungsten oxide Chonghu Wu ⁎ China National R&D Center for Tungsten Technology, Xiamen Tungsten Co., Ltd. Technology Center, Xiamen 361009, China Xiamen Golden Egret Special Alloy Co., Ltd. Xiamen 361006, China
a r t i c l e
i n f o
Article history: Received 24 February 2011 Accepted 11 May 2011 Keywords: In-situ Hydrogen reduction Violet tungsten oxide Ultrafine Tungsten powder
a b s t r a c t Ultrafine tungsten powders with a grain size below 0.5 μm are key raw materials for fabricating ultrafine cemented carbides. Conventional hydrogen reduction technique has been utilized to prepare the ultrafine tungsten powders. In the present work, highly pure nano-needles of violet tungsten oxide (WO2.72) were reduced by dry hydrogen. Nucleation and growth of the metallic tungsten in the early stage of hydrogen reduction have been studied by XRD, FESEM and HRTEM. Mechanism of formation of nano-size tungsten powders is proposed and a concept of in-situ hydrogen of the nano-needle WO2.72 is presented. Empirical relations between an average diameter of nano-needle WO2.72 and an average particle size of the resultant tungsten powders in both stage of nucleation and industrial conduction have been established. These empirical relations could be a reasonable guidance for suitably choosing the raw materials of nano-needle WO2.72 to prepare ultrafine tungsten powders. It has been determined that the BET special surface areas of the in-situ hydrogen-reduced tungsten powders with the average particle size of 0.2 μm and 0.3 μm, which were produced from the raw nano-needle WO2.72 powders with the average diameter of 60 nm and 80 nm, are 6.03 m 2/g and 4.65 m 2/g, and the oxygen contents are 0.35% and 0.29%, respectively. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ultrafine metallic tungsten powders are key raw materials for producing ultrafine cemented carbide powders and cemented carbides [1,2]. Hydrogen reduction of tungsten oxides is a traditional technique to produce tungsten powders in mass production. Since the 1990s, critical processing parameters and reduction mechanism for producing submicron grade tungsten powders (mean particle size b1.0 μm, typically 0.6 to 1.0 μm) and ultrafine grade tungsten powders (particle size ≦ 0.5 μm) have been intensively investigated [3,4]. Critical effects of various tungsten oxides used in hydrogen reduction on fineness, homogeneity and looseness of reduced ultrafine tungsten powders were reported [5]. It was pointed out that owing to a special particle morphology structure, WO2.72 could produce ultrafine, homogeneous and loose metal tungsten powders under ‘dry’ hydrogen reduction [5]. The special particle morphology structure of the WO2.72 was considered to be a wedgeshape pore structure among the powder particles, leading to H2O out of and H2 into the pore most easily, so the metal tungsten particle reduced from WO2.72 was very fine, homogeneous and loose [5]. However, when one considers the influence of morphological ⁎ China National R&D Center for Tungsten Technology, Xiamen Tungsten Co., Ltd. Technology Center, Xiamen 361009, China. Tel.: + 86 592 5766508. E-mail address: [email protected]. 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.05.002
characteristics of the WO2.72 particles themselves instead of the pore structure, there exist certain detail technical aspects that are worth exploring. For example, the WO2.72 powders are a mixture of needle, coarse bar and granule (see Fig. 1(c) in Ref. [5]). If the WO2.72 powders are pure needles, what influence of the morphology on nucleation and growth of the metallic tungsten is there? And, is there any quantitative relationship between the fineness of the resultant tungsten powders and the morphology of the WO2.72 to be reduced? Correct answers of these questions are extremely important for choosing suitable raw materials of the WO2.72 to prepare ultrafine tungsten powders with high quality. In the spirit of finding the above answers via the most efficient way, we choose highly pure nano-needle WO2.72 instead of the mixture of needle, coarse bar and granule as the starting materials for hydrogen reduction in the present work. Nucleation and growth of the metallic tungsten in the early stage of hydrogen reduction have been experimentally studied. Mechanism of formation of nano-size tungsten powders is proposed and a concept of in-situ hydrogen reduction of the nano-needle WO2.72 is presented. After finding an empirical relation between an average diameter of nano-needle WO2.72 and an average particle size of the resultant tungsten powders, industrial trials of hydrogen reduction of nano-needle WO2.72 have been carried out to modify that the empirical relation for giving better prediction of the particle size of the ultrafine tungsten powder obtained in the industrial condition.
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2. Experimental The highly pure WO2.72 nano-needles used were produced by Xiamen Golden Egret Special Alloy Co., Ltd. China P. R. (GESAC) [6]. The WO2.72 powders are needles, having the high aspect ratios (the length /the diameter) ranged from 20 to 40 and nano-scale sizes in diameter. We call these needles as nano-needles. They are the highaspect-ratio phases. One might call this kind of needles as 1-D (onedimensional) material. Hydrogen reductions of the WO2.72 nanoneedles were carried out in a ceramic corundum tubular electrical resistance furnace. The corundum tube was 600 mm in length, 150 mm in length of high temperature zone and 23 mm in inner diameter. The experimental conditions used are shown in Table 1. A small amount of water vapor came out at an outlet of the furnace tube at 630 °C while the water vapor was not observed at below 630 °C in the experiments, indicating that the hydrogen reduction had taken place well at 630 °C. Thus to study the nucleation and grain growth of the tungsten in the early stage of the reduction, another three experiments were conducted. In the first, sample 1 was heated in a high temperature zone of the furnace and when temperature reached 630 °C, heating was stopped and then cooled in the furnace with protection of hydrogen. In the second, sample 2 was heated up to 640 °C and then heating was stopped. In the third, sample 3 was heated up to 640 °C and soaked for 5 min, and then heating was stopped. Sample 2 and sample 3 were cooled in the same manner as those of sample 1. The crystal structure and phases determination were identified by X-ray diffraction (XRD) (PANalytical X'pert PRO) with Cu Kα (λ = 0.154056 nm) radiation. The scan range was 20° to 50° (2θ) with the constant step width of 0.033° and a count time is 10 s per step. The composition and morphology of the specimens were observed by field-emission scanning electron microscope (FESEM) (Hitachi S-4800II). Some specimens were investigated by high-resolution transmission electron microscope (HRTEM, JEOL JEM2100) with selected-area electron diffraction (SAED) method. The particle size of specimens was characterized by the BET method (Quantachrame NOVA 2000e) and oxygen content of them was determined by the Oxygen/Nitrogen analyzer (Horiba EMGA 620 W).
Fig. 1. XRD patterns of phases (a) raw material nano-needle WO2.72 , (b) sample 1, heated to 630 °C in hydrogen without soaking, (c) sample 2 heated to 640 °C in hydrogen without soaking , and (d) the samples at 640 °C, hydrogen-reduced for 5 min.
nucleation temperature is not determined yet. For sample 2, which was without soaking at 640 °C, the diffraction peaks of α-W (110) become stronger significantly (see Fig. 1 pattern (c)). For sample 3 reducing for 5 min at 640 °C, the diffraction peak of α-W (110) becomes the strongest. Meanwhile, there is a weak diffraction peak of β-W (211). No diffraction peaks of WO2 are identified. These results are consistent with data determined by high temperature XRD , which claimed that WO2.72 is directly reduced into α-W and there is a small amount of β-W, but any phase of WO2[7]. The single phase of α-W exists only when the reduction temperature is higher than 750 °C [7]. Fig. 2 shows the hydrogen reduction equilibrium diagram of WO3 pH2 O [8–10]. We can find in Fig. 2 that when ≤ 0.15 at above 620 °C, pH2 the stable phase is tungsten phase. The XRD results in Fig. 1 are in perfect agreement with the thermodynamic prediction.
3.2. Micro-morphological changes
3. Results and discussion 3.1. Changes in the chemistry Fig. 1 shows the XRD patterns of the raw nano-needle WO2.72 and three reduced powders. The pattern (a) of the raw powders in Fig. 1 demonstrates that only WO2.72 diffraction peaks exist without any other peaks, indicating that the starting powders are purer WO2.72 phase. The diffraction peak at 33.4° of the WO2.90 is its characteristic one, but it is hardly found in Fig. 1, so there is no WO2.90that was detected by XRD in the raw materials. Percentage of WO2.72 phase in the raw powders is very important. According to our experience, only in such WO2.72 powders that there is no other peak in XRD pattern which can be used for producing the ultrafine tungsten powders. For sample 1 heated to 630 °C without any soaking, the diffraction peaks of α-W (110) crystal emerge (see Fig. 1pattern (b)). This means that the nuclei of α-W had been formed during heating to 630 °C. Exact
Fig. 3 is the FESEM images of several nano-needles. The raw nanoneedles, as shown as Fig. 3(a), have diameters ranging from 20 to 50 nm with 500 to 700 nm in length. It appears that there are some coarsely rectangular rods also. With careful observation, it can be found that they are many very small nano-needles, bonded with the
Table 1 Experimental conditions of in-situ hydrogen reduction of the nano-needle WO2.72. Operations
Conditions
Boat load/g Powder layer height/mm H2 purity/% H2 dew point/ °C H2 flow condition H2 amount/m3 h− 1 Heating up rate/°C min− 1
2 1 99.99 − 60 to − 70 Counter-current 0.1 10
Fig. 2. The log (p H2O/p H2) vs. 1/T curves in the hydrogen reduction equilibrium diagram of WO3[10].
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Fig. 3. FESEM micro-morphology evolution of nano-needle WO2.72 (a) WO2.72 before hydrogen reduction, (b) sample 1, heated to 630 °C in hydrogen without soaking, (c) sample 2 heated to 640 °C in hydrogen without soaking , and (d) the samples at 640 °C, hydrogen-reduced for 5 min.
same long axis orientation, and interfaces among them can be observed clearly. One has been interested in the WO2.72 needles for long time. In the 1970s to 1980s, the existence of fine acicular WO2.72 crystals was reported [11,12]. It was reported that there are two types of micromorphology of WO2.72, the short columns with random orientation and the bundles with a certain orientation [13]. The formation of the bundles was attributed to CVT (chemical vapor transport) of tungsten atoms via the WO2(OH)2 vapor phase, while the random-orientated short columns were formed by vapor–solid reaction [13–15]. Fig. 3(b) shows the micro-morphology of early growth of a few tungsten crystals. Further nucleation and growth result in the formation of the strings of tungsten particle, as shown in Fig.3(c) and (d). A formation mechanism of this string can be explained as follows. The nano-needle WO2.72 is the high-aspect-ratio phase and this phase has morphology instability. There is spontaneously thermodynamic tendency that the needle will disintegrate into particles. This is the well known Rayleigh instability. The theory modeled instability of fluid jets first and extended to solids in the 1960s [16]. Solid fibers were predicted to evolve into strings of particles and particle size is sensitive to the mass transport mechanism dominating the morphologic evolution. 3.2.1. In-situ nucleation The nucleation of tungsten crystal at a certain site of the nanoneedle surface is a type of in-situ nucleation. This nucleation needs the energy much lower than those in 3-D solid due to the fact that nanoneedle is a 1-D solid. Since the nucleation energy is lower, the number of nuclei will be larger. Moreover, growth of the nuclei has no resistance in the other
two dimensions, thus the nuclei will grow easily. These factors enable the metallic tungsten crystal to grow in-situ and to form the string easily. Also, one can find evidence of the in-situ nucleation. In the middle of Fig. 3(b), we can see clearly a small round-like tungsten crystal between the needle with a size larger than the diameter of the needle by 1.5 to 2.5 times.
3.2.2. Short-range CVT The string consists of uniform-size tungsten grains. The mechanism of grain growth can be considered to be short-range CVT of WO2 (OH)2, which is different from long-range CVT dominated during the production of coarser powders. Chemically speaking, the oxygen content of WO2.72 is lower than those of other oxides (WO3 or WO2.9) so that water vapor formed by hydrogen reduction will be less. Moreover, in the present experiment, the larger hydrogen amount (0.1 m 3/h) for the small amount of WO2.72 (2 g) can dilute concentration of water vapor in the furnace tube further. The concentration of the water vapor can be saturated only in short range around the nano-needle where in-situ hydrogen reduction just takes place. The concentration of WO2(OH)2 produced through the reaction between the water and the oxides can be higher. Thus, the condensation of the tungsten atoms provided through the short-range CVT allows the neighbor nuclei to grow. One of the reasons why the particle size produced hydrogen reduction of WO2.72 is more uniform may attributed to the short-rang CVT and absence of long-rang CVT. We can see in Fig. 3(b) again, there are two shrinkage necks contacting to the round-like crystal. The original atoms in this place have been carried by short-range CVT out and deposited on this neighbor crystal. This analysis is consistent with some literatures, indicating that the vapor phase of WO2(OH)2 is able to transfer tungsten atoms rapidly [13,14].
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Besides, we thought that this small round-like crystal is one particle, which is proved by TEM characterization later on. 3.3. TEM observation Fig. 4 shows TEM and HRTEM images of the samples. Fig. 4(a) shows HRTEM images of sample 1. These nano-needles are 20 to
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50 nm in diameter and 300 to 400 nm in length. The aspect ratio is about 8 to 15. Fig. 4(b) shows the lattice structure of a tungsten nucleus formed in the side of the nano-needle. The plane spacing has been determined to be about 0.225 nm, consisting the value (0.224 nm) of the plane spacing along α-W (110) well. Its Fourier transform (lower right corner of Fig. 4(b)) also indicates that this crystal face should be the crystal face (110). It can also be seen from Fig. 4(b) that there are lattice distortion zones on the interface between α-W atoms and nano-needle atoms and that multi-layers of tungsten atoms arranged disorderly on the exterior surface of α-W nucleus. The disorder arrangement of tungsten atoms may be attributed to that the condensation is the short-range CVT rapid process, during which there is no time to allow the atoms to adjust their crystal orientations. If diffusion mechanism of solid phase reaction dominates, the atoms can adjust their orientation during the slow process, resulting in the disappearance of disorder arrangement. Fig. 4(c) shows the TEM image of sample 3. The tungsten crystals exhibit a higher degree of crystallization. Particle coalescence can be also observed. The inset of Fig. 4(c) shows the result of SAED analysis. The corresponding diffraction pattern of the tungsten crystal is shown in the grey frame of Fig. 4(c), indicating a single crystal structure. Basically, that one single particle is one single crystal is an important characteristic of in-situ hydrogen reduction of WO2.72 nano-needles. 3.4. Dependence of the diameter and aspect ratio Size grades of the ultrafine tungsten powders depend upon qualities of the nano-needle WO2.72. The most important qualities are the diameter and the aspect ratio of the nano-needles. The diameter should be in the range of ultrafine dimensions. In Section 3.3, we explained Fig. 3(b) and it seems that there is a quantitative relation between the particle size of tungsten powder and the diameter of the nano-needle. To find this relation experimentally, we measured and calculated a large amount of data of the sizes and diameters and inferred an empirical relation of 1.5 to 2.5 times as follows, Dparticle = 1:5e2:5 dneedle
ð1Þ
where Dparticle is the average particle size of the tungsten powders, dneedle is the diameter of the needle of WO2.72. This relationship in Eq. (1) can be considered to be a guide, which helps us to understand that the nano-needle is needed to produce nanometer sized and ultrafine hydrogen-reduction tungsten powders. A question is what role the aspect ratio of the needle plays. Generally speaking, the aspect ratio will determine the amount of particles in the string. In other words, the amount of particles (Nparticle) in the string is closely related to the aspect ratio (Lneedle/dneedle, Lneedle: length of the needle and dneedle: diameter of the needle). Now we define a ratio that the length of the nano-needle is divided by the diameter of tungsten particle as the Nparticle, as follows, Nparticle = Lneedle = Dparticle
ð2Þ
With an assumption of Lneedle/dneedles N 1 and substituting Dparticle by Eq. (1) into Eq. (2) a simple estimation can be obtained, Nparticle = 0:67e0:40 ðLneedle = dneedles Þ
Fig. 4. TEM and HRTEM images of specimen, (a) TEM image of sample 1, (b) HRTEM image of sample 1 heated to 630 °C in hydrogen without soaking, nano-needle WO2.72 with a tungsten microcrystal on the surface and constituency Fourier Transform of the microcrystal. (c) TEM image of tungsten crystals reduced at 640 °C, reduced for 5 min, with inset showing the SAED pattern of single crystal.
ð3Þ
where Lneedle/dneedles is the aspect ratio. With the high aspect ratio, ultrafine particles can be yielded and the morphology of such nano-needles is shown as in Fig. 5(a). The diameter of the nano-needle is about 25 nm, the aspect ratio is about 20. This needle can be broken into about 10 particles with the particle size of about 50 nm, estimated by Eq. (1) and Eq. (2). This type of nano-needles WO2.72 is suitable to be raw material for producing
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hydrogen reduction process, how does the coefficient of the Eq. (1) change? An industrial trial to prepare ultrafine tungsten with target particle size of 0.2 μm was conducted. Because of an industrial furnace used and complete reduction and crystallization needed instead of an observation on the nucleus growth in the early stage of reduction, reduction parameters should be properly adjusted. Thus, a larger amount of dry hydrogen of 60m 3/h and a higher reduction temperature of 700 ± 5 °C were used. A total operation time took 150 min, instead of a couple of minutes, including pre-heating, hydrogen reduction and furnace cooling. To prepare the tungsten powder with the average particle size of 0.2 μm, the raw nano-needle WO2.72 powders with the average diameter of 60 nm were used. Fig. 6(a) demonstrates the FESEM micro-morphology of the unground reduced tungsten powders with the average particle size of 0.2 μm, showing the average particle size of 200 nm, the particle size range of 90% tungsten powders are 32 to 280 nm with the largest particles of 430 nm. Using these data, a new empirical relationship can be established as follows, Dparticle = 3:33dneedle
ð4Þ
To verify Eq. (4) is helpful to choose raw nano-needle of the WO2.72 for preparing ultrafine tungsten powder. The tungsten powders with the average particle size of 0.3 μm were prepared in the same industrial condition. The nano-needles of 80 nm were used as the starting material. Fig. 6(b) is the FESEM micro-morphology of the reduced tungsten powders with the average particle size of 0.3 μm, showing the average
Fig. 5. FESEM images of two typical needle-shaped WO2.72.
tungsten nano-powders. Contrasting with this, the needles shown in Fig. 5(b) have the diameter of 200 nm. Using this needle it is impossible to prepare tungsten nano-powders, however, it is possible to make ultrafine tungsten powder with the particle size below 0.5 μm. It should be noticed that the Eq. (1) and Eq. (2) are established from the bench-scale experiments, in which the reduction time was very short, taking 5 min only. For industrial production, however, the reduction time will be much longer, for example, taking 150 min even more. The coefficients of 1.5–2.5 in Eq. (1) will change to larger value because of grain growth during the longer reduction period. And then the coefficient of Eq. (1) will vary correspondingly. 3.5. Discussion 3.5.1. In-situ hydrogen reduction of nano-needle WO2.72 It can be clearly observed from Fig. 3(c) and Fig. 4 that the nanosize particles of the metallic tungsten are formed along the nanoneedles. This results from the nucleation and growth of tungsten atoms through hydrogen reduction. This kind of reduction process might be considered to be a sort of ‘in-situ’ hydrogen reduction process, in which the morphology of the nano-needle plays an important role. Due to morphological instability and the hydrogen reduction, the high-aspect-ratio nano-needles of WO2.72 are broken into a string of tungsten particles. This is the main feature of the in-situ reduction. 3.5.2. Modification of Eq. (1) Eq. (1) indicates the dependence of the particle size of the tungsten powder on the average diameter of the nano-needle used. Actually this is the case of tungsten nuclei in the early stage of hydrogen reduction at 630 °C or 640 °C. However, for an industrial condition of complete
Fig. 6. FESEM images of (a) 200 nm and (b) 300 nm tungsten powders, reduced in the industrial furnace, raw materials: nano-needle WO2.72 powders with the average diameters of 60 nm for (a) or 80 nm for (b), reduce temperature: 700 ± 5 °C, hydrogen amount: 60 m3/h.
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particle size of 300 nm, the particle size range of 90% tungsten powders are 40 to 380 nm with the largest particles of 520 nm. It can be calculated that the coefficient of the Eq. (4) is 3.75, indicating a deviation is 12.6%, obtained through comparing with the coefficient of Eq. (4) for the tungsten powders of 0.2 μm. Therefore, it might be said that Eq. (4) is a reasonable guidance for choosing the raw materials to prepare ultrafine tungsten powders.
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Acknowledgements The National Key Technologies R & D Program of China under Contract No. 2007BAE05B 01 is gratefully acknowledged for the financial support of this work.
References 3.5.3. Properties of the in-situ hydrogen-reduced ultrafine tungsten powders It can be seen from Fig. 6 that these powders are the equiaxied particles and have perfect looseness and homogeneity. No abnormal grown powders are observed. It has been determined that the BET special surface areas of the unground tungsten powders with the average particle size of 0.2 μm and 0.3 μm are 6.03 m 2/g and 4.65 m2/g, and the oxygen contents are 0.35% and 0.29%, respectively. These ultrafine tungsten powders prepared by in-situ hydrogen reduction should be perfect raw materials for manufacturing ultrafine cemented carbide powders [1,17]. 4. Conclusions (1) Ultrafine tungsten powders can be prepared by in-situ hydrogen reduction of the nano-needles of violet tungsten oxide. (2) Due to the morphology instability and hydrogen reduction, the high-aspect-ratio nano-needles of violet tungsten oxide are broken into a string of tungsten particles. This is the main feature of the in-situ reduction. (3) In the stage of nucleation and early grain growth, the dependence of the average particle size of the small tungsten grains on the average diameter of the raw nano-needles of the WO2.72 can be expressed as Dparticle = (1.5–2.5) dneedle. (4) In the case of industrial hydrogen reduction, the dependence of the average particle size of the completely reduced tungsten powders on the average diameter of the raw nano-needles of the WO2.72 can be expressed as Dparticle = 3.33dneedle. This is a reasonable guidance for choosing the raw materials to prepare ultrafine tungsten powders using the nano-needle WO2.72 through in-situ hydrogen reduction. (5) The ultrafine tungsten powders produced by in-situ reduction are the equiaxied particles and have perfect looseness and homogeneity.
[1] Wu C, Wu Q, Lin G. Process for preparing high-performance ultra-fine tungsten carbide powder; 2003-09-10. CN 1594078. [2] Fang ZZ, Wang X, Ryu T, Hwang KS, Sohn HY. Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide — a review. Int J Refract Met Hard Mater 2009;27:288–99. [3] Schubert WD, Lassner E. Production and characterization of hydrogen- reduced submicron tungsten powders. Part I: State of the art in research, production and characterization of raw materials and tungsten powders. Int J Refract Met Hard Mater 1991;10:133–41. [4] Schubert WD, Lassner E. Production and characterization of hydrogen- reduced submicron tungsten powders. Part II: controlled decomposition of APT and hydrogen reduction of the oxides. Int J Refract Met Hard Mater 1991;17:171–83. [5] Liao J, Chen S, Zou Z, Lu H, Huang B. Influence of tungsten oxides’ characteristics on fineness, homogeneity and looseness of reduced ultrafine tungsten powder. Int J Refract Met Hard Mater 1999;10:423–9. [6] Wu C, Wu Q, Chen J, Lin G, Zhang Q. Method and apparatus for preparing blue tungsten oxide by ammonium paratungstate self-reduction; 2003-09-10. CN 1594107. [7] Chen S. The preparation of tungsten powder from violet W18O49. J Central South insti mine metall 1994;25:607–11. [8] Barin I. Thermochemical data of pure substances. 3rd ed. VCH Verlagsgesellschaft mbH: Weinheim; 1995. [9] Rizzo FE, Bidwell LR, Frank DF. Thermodynamics of the tungsten–oxygen system. Trans Met Soc AIME 1967;239:1901–5. [10] Hasegawa R, Kurosawa T, Yagihashi T, Yagihashi T. Hydrogen reduction of tungsten trioxide and ammonium para-tungstate under high pressure. Trans JIM 1974;15:75–82. [11] Zhao Q, Bohr M, Kruger J. Dynamic study on the morphology of hydrogen reduced tungsten oxide. Chin J Rare Metals 1987;2:81–7. [12] Sarin VK. Morphological changes occurring during reduction of WO3. J Mater Sci 1975;10:593–8. [13] Zhao Q, Bohr M, Kruger J. Morphology evolution of the product prepared by hydrogen reduction of Tungsten oxides. Chin J Rare Metals 1987;1:1–7. [14] Haubner R, Schubert WD, Hellmer H, Lux B. Mechanism of technical reduction of tungsten: Part 2. Hydrogen reduction of tungsten blue oxide to tungsten powder. Int J Refract Met Hard Mater 1983;2:156–63. [15] Pfeifer J, Badaljan E, Tekula-Buxbaum P, Kovács T, Geszti O, Tóth AL, et al. Growth and morphology of W18O49 crystals produced by microwave decomposition of ammonium paratungstate. J Cryst Growth 1996;169:727–33. [16] Santala MK, Glaeser AM. Surface-energy-anisotropy-induced orientation effects on Rayleigh instabilities in sapphire. Surf Sci 2006;600:782–92. [17] Wu C, Wu Q, Lin G, Zhang Q. Preparation method of high performance nanometer sized and superfine tungsten powder; 2003-09-10. CN1593822.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Preparation of ultrafine tungsten powders by in-situ hydrogen reduction of nano-needle violet tungsten oxide Chonghu Wu ⁎ China National R&D Center for Tungsten Technology, Xiamen Tungsten Co., Ltd. Technology Center, Xiamen 361009, China Xiamen Golden Egret Special Alloy Co., Ltd. Xiamen 361006, China
a r t i c l e
i n f o
Article history: Received 24 February 2011 Accepted 11 May 2011 Keywords: In-situ Hydrogen reduction Violet tungsten oxide Ultrafine Tungsten powder
a b s t r a c t Ultrafine tungsten powders with a grain size below 0.5 μm are key raw materials for fabricating ultrafine cemented carbides. Conventional hydrogen reduction technique has been utilized to prepare the ultrafine tungsten powders. In the present work, highly pure nano-needles of violet tungsten oxide (WO2.72) were reduced by dry hydrogen. Nucleation and growth of the metallic tungsten in the early stage of hydrogen reduction have been studied by XRD, FESEM and HRTEM. Mechanism of formation of nano-size tungsten powders is proposed and a concept of in-situ hydrogen of the nano-needle WO2.72 is presented. Empirical relations between an average diameter of nano-needle WO2.72 and an average particle size of the resultant tungsten powders in both stage of nucleation and industrial conduction have been established. These empirical relations could be a reasonable guidance for suitably choosing the raw materials of nano-needle WO2.72 to prepare ultrafine tungsten powders. It has been determined that the BET special surface areas of the in-situ hydrogen-reduced tungsten powders with the average particle size of 0.2 μm and 0.3 μm, which were produced from the raw nano-needle WO2.72 powders with the average diameter of 60 nm and 80 nm, are 6.03 m 2/g and 4.65 m 2/g, and the oxygen contents are 0.35% and 0.29%, respectively. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ultrafine metallic tungsten powders are key raw materials for producing ultrafine cemented carbide powders and cemented carbides [1,2]. Hydrogen reduction of tungsten oxides is a traditional technique to produce tungsten powders in mass production. Since the 1990s, critical processing parameters and reduction mechanism for producing submicron grade tungsten powders (mean particle size b1.0 μm, typically 0.6 to 1.0 μm) and ultrafine grade tungsten powders (particle size ≦ 0.5 μm) have been intensively investigated [3,4]. Critical effects of various tungsten oxides used in hydrogen reduction on fineness, homogeneity and looseness of reduced ultrafine tungsten powders were reported [5]. It was pointed out that owing to a special particle morphology structure, WO2.72 could produce ultrafine, homogeneous and loose metal tungsten powders under ‘dry’ hydrogen reduction [5]. The special particle morphology structure of the WO2.72 was considered to be a wedgeshape pore structure among the powder particles, leading to H2O out of and H2 into the pore most easily, so the metal tungsten particle reduced from WO2.72 was very fine, homogeneous and loose [5]. However, when one considers the influence of morphological ⁎ China National R&D Center for Tungsten Technology, Xiamen Tungsten Co., Ltd. Technology Center, Xiamen 361009, China. Tel.: + 86 592 5766508. E-mail address: [email protected]. 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.05.002
characteristics of the WO2.72 particles themselves instead of the pore structure, there exist certain detail technical aspects that are worth exploring. For example, the WO2.72 powders are a mixture of needle, coarse bar and granule (see Fig. 1(c) in Ref. [5]). If the WO2.72 powders are pure needles, what influence of the morphology on nucleation and growth of the metallic tungsten is there? And, is there any quantitative relationship between the fineness of the resultant tungsten powders and the morphology of the WO2.72 to be reduced? Correct answers of these questions are extremely important for choosing suitable raw materials of the WO2.72 to prepare ultrafine tungsten powders with high quality. In the spirit of finding the above answers via the most efficient way, we choose highly pure nano-needle WO2.72 instead of the mixture of needle, coarse bar and granule as the starting materials for hydrogen reduction in the present work. Nucleation and growth of the metallic tungsten in the early stage of hydrogen reduction have been experimentally studied. Mechanism of formation of nano-size tungsten powders is proposed and a concept of in-situ hydrogen reduction of the nano-needle WO2.72 is presented. After finding an empirical relation between an average diameter of nano-needle WO2.72 and an average particle size of the resultant tungsten powders, industrial trials of hydrogen reduction of nano-needle WO2.72 have been carried out to modify that the empirical relation for giving better prediction of the particle size of the ultrafine tungsten powder obtained in the industrial condition.
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2. Experimental The highly pure WO2.72 nano-needles used were produced by Xiamen Golden Egret Special Alloy Co., Ltd. China P. R. (GESAC) [6]. The WO2.72 powders are needles, having the high aspect ratios (the length /the diameter) ranged from 20 to 40 and nano-scale sizes in diameter. We call these needles as nano-needles. They are the highaspect-ratio phases. One might call this kind of needles as 1-D (onedimensional) material. Hydrogen reductions of the WO2.72 nanoneedles were carried out in a ceramic corundum tubular electrical resistance furnace. The corundum tube was 600 mm in length, 150 mm in length of high temperature zone and 23 mm in inner diameter. The experimental conditions used are shown in Table 1. A small amount of water vapor came out at an outlet of the furnace tube at 630 °C while the water vapor was not observed at below 630 °C in the experiments, indicating that the hydrogen reduction had taken place well at 630 °C. Thus to study the nucleation and grain growth of the tungsten in the early stage of the reduction, another three experiments were conducted. In the first, sample 1 was heated in a high temperature zone of the furnace and when temperature reached 630 °C, heating was stopped and then cooled in the furnace with protection of hydrogen. In the second, sample 2 was heated up to 640 °C and then heating was stopped. In the third, sample 3 was heated up to 640 °C and soaked for 5 min, and then heating was stopped. Sample 2 and sample 3 were cooled in the same manner as those of sample 1. The crystal structure and phases determination were identified by X-ray diffraction (XRD) (PANalytical X'pert PRO) with Cu Kα (λ = 0.154056 nm) radiation. The scan range was 20° to 50° (2θ) with the constant step width of 0.033° and a count time is 10 s per step. The composition and morphology of the specimens were observed by field-emission scanning electron microscope (FESEM) (Hitachi S-4800II). Some specimens were investigated by high-resolution transmission electron microscope (HRTEM, JEOL JEM2100) with selected-area electron diffraction (SAED) method. The particle size of specimens was characterized by the BET method (Quantachrame NOVA 2000e) and oxygen content of them was determined by the Oxygen/Nitrogen analyzer (Horiba EMGA 620 W).
Fig. 1. XRD patterns of phases (a) raw material nano-needle WO2.72 , (b) sample 1, heated to 630 °C in hydrogen without soaking, (c) sample 2 heated to 640 °C in hydrogen without soaking , and (d) the samples at 640 °C, hydrogen-reduced for 5 min.
nucleation temperature is not determined yet. For sample 2, which was without soaking at 640 °C, the diffraction peaks of α-W (110) become stronger significantly (see Fig. 1 pattern (c)). For sample 3 reducing for 5 min at 640 °C, the diffraction peak of α-W (110) becomes the strongest. Meanwhile, there is a weak diffraction peak of β-W (211). No diffraction peaks of WO2 are identified. These results are consistent with data determined by high temperature XRD , which claimed that WO2.72 is directly reduced into α-W and there is a small amount of β-W, but any phase of WO2[7]. The single phase of α-W exists only when the reduction temperature is higher than 750 °C [7]. Fig. 2 shows the hydrogen reduction equilibrium diagram of WO3 pH2 O [8–10]. We can find in Fig. 2 that when ≤ 0.15 at above 620 °C, pH2 the stable phase is tungsten phase. The XRD results in Fig. 1 are in perfect agreement with the thermodynamic prediction.
3.2. Micro-morphological changes
3. Results and discussion 3.1. Changes in the chemistry Fig. 1 shows the XRD patterns of the raw nano-needle WO2.72 and three reduced powders. The pattern (a) of the raw powders in Fig. 1 demonstrates that only WO2.72 diffraction peaks exist without any other peaks, indicating that the starting powders are purer WO2.72 phase. The diffraction peak at 33.4° of the WO2.90 is its characteristic one, but it is hardly found in Fig. 1, so there is no WO2.90that was detected by XRD in the raw materials. Percentage of WO2.72 phase in the raw powders is very important. According to our experience, only in such WO2.72 powders that there is no other peak in XRD pattern which can be used for producing the ultrafine tungsten powders. For sample 1 heated to 630 °C without any soaking, the diffraction peaks of α-W (110) crystal emerge (see Fig. 1pattern (b)). This means that the nuclei of α-W had been formed during heating to 630 °C. Exact
Fig. 3 is the FESEM images of several nano-needles. The raw nanoneedles, as shown as Fig. 3(a), have diameters ranging from 20 to 50 nm with 500 to 700 nm in length. It appears that there are some coarsely rectangular rods also. With careful observation, it can be found that they are many very small nano-needles, bonded with the
Table 1 Experimental conditions of in-situ hydrogen reduction of the nano-needle WO2.72. Operations
Conditions
Boat load/g Powder layer height/mm H2 purity/% H2 dew point/ °C H2 flow condition H2 amount/m3 h− 1 Heating up rate/°C min− 1
2 1 99.99 − 60 to − 70 Counter-current 0.1 10
Fig. 2. The log (p H2O/p H2) vs. 1/T curves in the hydrogen reduction equilibrium diagram of WO3[10].
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Fig. 3. FESEM micro-morphology evolution of nano-needle WO2.72 (a) WO2.72 before hydrogen reduction, (b) sample 1, heated to 630 °C in hydrogen without soaking, (c) sample 2 heated to 640 °C in hydrogen without soaking , and (d) the samples at 640 °C, hydrogen-reduced for 5 min.
same long axis orientation, and interfaces among them can be observed clearly. One has been interested in the WO2.72 needles for long time. In the 1970s to 1980s, the existence of fine acicular WO2.72 crystals was reported [11,12]. It was reported that there are two types of micromorphology of WO2.72, the short columns with random orientation and the bundles with a certain orientation [13]. The formation of the bundles was attributed to CVT (chemical vapor transport) of tungsten atoms via the WO2(OH)2 vapor phase, while the random-orientated short columns were formed by vapor–solid reaction [13–15]. Fig. 3(b) shows the micro-morphology of early growth of a few tungsten crystals. Further nucleation and growth result in the formation of the strings of tungsten particle, as shown in Fig.3(c) and (d). A formation mechanism of this string can be explained as follows. The nano-needle WO2.72 is the high-aspect-ratio phase and this phase has morphology instability. There is spontaneously thermodynamic tendency that the needle will disintegrate into particles. This is the well known Rayleigh instability. The theory modeled instability of fluid jets first and extended to solids in the 1960s [16]. Solid fibers were predicted to evolve into strings of particles and particle size is sensitive to the mass transport mechanism dominating the morphologic evolution. 3.2.1. In-situ nucleation The nucleation of tungsten crystal at a certain site of the nanoneedle surface is a type of in-situ nucleation. This nucleation needs the energy much lower than those in 3-D solid due to the fact that nanoneedle is a 1-D solid. Since the nucleation energy is lower, the number of nuclei will be larger. Moreover, growth of the nuclei has no resistance in the other
two dimensions, thus the nuclei will grow easily. These factors enable the metallic tungsten crystal to grow in-situ and to form the string easily. Also, one can find evidence of the in-situ nucleation. In the middle of Fig. 3(b), we can see clearly a small round-like tungsten crystal between the needle with a size larger than the diameter of the needle by 1.5 to 2.5 times.
3.2.2. Short-range CVT The string consists of uniform-size tungsten grains. The mechanism of grain growth can be considered to be short-range CVT of WO2 (OH)2, which is different from long-range CVT dominated during the production of coarser powders. Chemically speaking, the oxygen content of WO2.72 is lower than those of other oxides (WO3 or WO2.9) so that water vapor formed by hydrogen reduction will be less. Moreover, in the present experiment, the larger hydrogen amount (0.1 m 3/h) for the small amount of WO2.72 (2 g) can dilute concentration of water vapor in the furnace tube further. The concentration of the water vapor can be saturated only in short range around the nano-needle where in-situ hydrogen reduction just takes place. The concentration of WO2(OH)2 produced through the reaction between the water and the oxides can be higher. Thus, the condensation of the tungsten atoms provided through the short-range CVT allows the neighbor nuclei to grow. One of the reasons why the particle size produced hydrogen reduction of WO2.72 is more uniform may attributed to the short-rang CVT and absence of long-rang CVT. We can see in Fig. 3(b) again, there are two shrinkage necks contacting to the round-like crystal. The original atoms in this place have been carried by short-range CVT out and deposited on this neighbor crystal. This analysis is consistent with some literatures, indicating that the vapor phase of WO2(OH)2 is able to transfer tungsten atoms rapidly [13,14].
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Besides, we thought that this small round-like crystal is one particle, which is proved by TEM characterization later on. 3.3. TEM observation Fig. 4 shows TEM and HRTEM images of the samples. Fig. 4(a) shows HRTEM images of sample 1. These nano-needles are 20 to
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50 nm in diameter and 300 to 400 nm in length. The aspect ratio is about 8 to 15. Fig. 4(b) shows the lattice structure of a tungsten nucleus formed in the side of the nano-needle. The plane spacing has been determined to be about 0.225 nm, consisting the value (0.224 nm) of the plane spacing along α-W (110) well. Its Fourier transform (lower right corner of Fig. 4(b)) also indicates that this crystal face should be the crystal face (110). It can also be seen from Fig. 4(b) that there are lattice distortion zones on the interface between α-W atoms and nano-needle atoms and that multi-layers of tungsten atoms arranged disorderly on the exterior surface of α-W nucleus. The disorder arrangement of tungsten atoms may be attributed to that the condensation is the short-range CVT rapid process, during which there is no time to allow the atoms to adjust their crystal orientations. If diffusion mechanism of solid phase reaction dominates, the atoms can adjust their orientation during the slow process, resulting in the disappearance of disorder arrangement. Fig. 4(c) shows the TEM image of sample 3. The tungsten crystals exhibit a higher degree of crystallization. Particle coalescence can be also observed. The inset of Fig. 4(c) shows the result of SAED analysis. The corresponding diffraction pattern of the tungsten crystal is shown in the grey frame of Fig. 4(c), indicating a single crystal structure. Basically, that one single particle is one single crystal is an important characteristic of in-situ hydrogen reduction of WO2.72 nano-needles. 3.4. Dependence of the diameter and aspect ratio Size grades of the ultrafine tungsten powders depend upon qualities of the nano-needle WO2.72. The most important qualities are the diameter and the aspect ratio of the nano-needles. The diameter should be in the range of ultrafine dimensions. In Section 3.3, we explained Fig. 3(b) and it seems that there is a quantitative relation between the particle size of tungsten powder and the diameter of the nano-needle. To find this relation experimentally, we measured and calculated a large amount of data of the sizes and diameters and inferred an empirical relation of 1.5 to 2.5 times as follows, Dparticle = 1:5e2:5 dneedle
ð1Þ
where Dparticle is the average particle size of the tungsten powders, dneedle is the diameter of the needle of WO2.72. This relationship in Eq. (1) can be considered to be a guide, which helps us to understand that the nano-needle is needed to produce nanometer sized and ultrafine hydrogen-reduction tungsten powders. A question is what role the aspect ratio of the needle plays. Generally speaking, the aspect ratio will determine the amount of particles in the string. In other words, the amount of particles (Nparticle) in the string is closely related to the aspect ratio (Lneedle/dneedle, Lneedle: length of the needle and dneedle: diameter of the needle). Now we define a ratio that the length of the nano-needle is divided by the diameter of tungsten particle as the Nparticle, as follows, Nparticle = Lneedle = Dparticle
ð2Þ
With an assumption of Lneedle/dneedles N 1 and substituting Dparticle by Eq. (1) into Eq. (2) a simple estimation can be obtained, Nparticle = 0:67e0:40 ðLneedle = dneedles Þ
Fig. 4. TEM and HRTEM images of specimen, (a) TEM image of sample 1, (b) HRTEM image of sample 1 heated to 630 °C in hydrogen without soaking, nano-needle WO2.72 with a tungsten microcrystal on the surface and constituency Fourier Transform of the microcrystal. (c) TEM image of tungsten crystals reduced at 640 °C, reduced for 5 min, with inset showing the SAED pattern of single crystal.
ð3Þ
where Lneedle/dneedles is the aspect ratio. With the high aspect ratio, ultrafine particles can be yielded and the morphology of such nano-needles is shown as in Fig. 5(a). The diameter of the nano-needle is about 25 nm, the aspect ratio is about 20. This needle can be broken into about 10 particles with the particle size of about 50 nm, estimated by Eq. (1) and Eq. (2). This type of nano-needles WO2.72 is suitable to be raw material for producing
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hydrogen reduction process, how does the coefficient of the Eq. (1) change? An industrial trial to prepare ultrafine tungsten with target particle size of 0.2 μm was conducted. Because of an industrial furnace used and complete reduction and crystallization needed instead of an observation on the nucleus growth in the early stage of reduction, reduction parameters should be properly adjusted. Thus, a larger amount of dry hydrogen of 60m 3/h and a higher reduction temperature of 700 ± 5 °C were used. A total operation time took 150 min, instead of a couple of minutes, including pre-heating, hydrogen reduction and furnace cooling. To prepare the tungsten powder with the average particle size of 0.2 μm, the raw nano-needle WO2.72 powders with the average diameter of 60 nm were used. Fig. 6(a) demonstrates the FESEM micro-morphology of the unground reduced tungsten powders with the average particle size of 0.2 μm, showing the average particle size of 200 nm, the particle size range of 90% tungsten powders are 32 to 280 nm with the largest particles of 430 nm. Using these data, a new empirical relationship can be established as follows, Dparticle = 3:33dneedle
ð4Þ
To verify Eq. (4) is helpful to choose raw nano-needle of the WO2.72 for preparing ultrafine tungsten powder. The tungsten powders with the average particle size of 0.3 μm were prepared in the same industrial condition. The nano-needles of 80 nm were used as the starting material. Fig. 6(b) is the FESEM micro-morphology of the reduced tungsten powders with the average particle size of 0.3 μm, showing the average
Fig. 5. FESEM images of two typical needle-shaped WO2.72.
tungsten nano-powders. Contrasting with this, the needles shown in Fig. 5(b) have the diameter of 200 nm. Using this needle it is impossible to prepare tungsten nano-powders, however, it is possible to make ultrafine tungsten powder with the particle size below 0.5 μm. It should be noticed that the Eq. (1) and Eq. (2) are established from the bench-scale experiments, in which the reduction time was very short, taking 5 min only. For industrial production, however, the reduction time will be much longer, for example, taking 150 min even more. The coefficients of 1.5–2.5 in Eq. (1) will change to larger value because of grain growth during the longer reduction period. And then the coefficient of Eq. (1) will vary correspondingly. 3.5. Discussion 3.5.1. In-situ hydrogen reduction of nano-needle WO2.72 It can be clearly observed from Fig. 3(c) and Fig. 4 that the nanosize particles of the metallic tungsten are formed along the nanoneedles. This results from the nucleation and growth of tungsten atoms through hydrogen reduction. This kind of reduction process might be considered to be a sort of ‘in-situ’ hydrogen reduction process, in which the morphology of the nano-needle plays an important role. Due to morphological instability and the hydrogen reduction, the high-aspect-ratio nano-needles of WO2.72 are broken into a string of tungsten particles. This is the main feature of the in-situ reduction. 3.5.2. Modification of Eq. (1) Eq. (1) indicates the dependence of the particle size of the tungsten powder on the average diameter of the nano-needle used. Actually this is the case of tungsten nuclei in the early stage of hydrogen reduction at 630 °C or 640 °C. However, for an industrial condition of complete
Fig. 6. FESEM images of (a) 200 nm and (b) 300 nm tungsten powders, reduced in the industrial furnace, raw materials: nano-needle WO2.72 powders with the average diameters of 60 nm for (a) or 80 nm for (b), reduce temperature: 700 ± 5 °C, hydrogen amount: 60 m3/h.
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particle size of 300 nm, the particle size range of 90% tungsten powders are 40 to 380 nm with the largest particles of 520 nm. It can be calculated that the coefficient of the Eq. (4) is 3.75, indicating a deviation is 12.6%, obtained through comparing with the coefficient of Eq. (4) for the tungsten powders of 0.2 μm. Therefore, it might be said that Eq. (4) is a reasonable guidance for choosing the raw materials to prepare ultrafine tungsten powders.
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Acknowledgements The National Key Technologies R & D Program of China under Contract No. 2007BAE05B 01 is gratefully acknowledged for the financial support of this work.
References 3.5.3. Properties of the in-situ hydrogen-reduced ultrafine tungsten powders It can be seen from Fig. 6 that these powders are the equiaxied particles and have perfect looseness and homogeneity. No abnormal grown powders are observed. It has been determined that the BET special surface areas of the unground tungsten powders with the average particle size of 0.2 μm and 0.3 μm are 6.03 m 2/g and 4.65 m2/g, and the oxygen contents are 0.35% and 0.29%, respectively. These ultrafine tungsten powders prepared by in-situ hydrogen reduction should be perfect raw materials for manufacturing ultrafine cemented carbide powders [1,17]. 4. Conclusions (1) Ultrafine tungsten powders can be prepared by in-situ hydrogen reduction of the nano-needles of violet tungsten oxide. (2) Due to the morphology instability and hydrogen reduction, the high-aspect-ratio nano-needles of violet tungsten oxide are broken into a string of tungsten particles. This is the main feature of the in-situ reduction. (3) In the stage of nucleation and early grain growth, the dependence of the average particle size of the small tungsten grains on the average diameter of the raw nano-needles of the WO2.72 can be expressed as Dparticle = (1.5–2.5) dneedle. (4) In the case of industrial hydrogen reduction, the dependence of the average particle size of the completely reduced tungsten powders on the average diameter of the raw nano-needles of the WO2.72 can be expressed as Dparticle = 3.33dneedle. This is a reasonable guidance for choosing the raw materials to prepare ultrafine tungsten powders using the nano-needle WO2.72 through in-situ hydrogen reduction. (5) The ultrafine tungsten powders produced by in-situ reduction are the equiaxied particles and have perfect looseness and homogeneity.
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