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Preparation of zirconium-based ceramic and composite fine-grained powders
Int. Journal of Refractory Metals and Hard Materials 30 (2012) 133–138
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 zirconium-based ceramic and composite fine-grained powders Hong Youl Ryu a, H.H. Nersisyan b, Jong Hyeon Lee a, c,⁎ a b c
Graduate School of Green Energy Technology, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Rapidly Solidified Materials Research Institute, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of NanoMaterials Engineering, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 July 2011 Accepted 25 July 2011 Keywords: Zirconium composite Particle size Temperature distribution Fine powder
a b s t r a c t Zirconium-based ceramics and composites such as ZrC, ZrB2, ZrC–SiC, ZrB2–SiC–ZrC, and ZrB2–SiC–ZrC–ZrSi were synthesized in fine powder form via combustion synthesis (CS) using ZrSiO4, Mg, C, B, and NaCl as raw materials. Temperature distributions in the combustion wave were measured by thermocouples and used to estimate the combustion temperature and wave propagation velocity. The influence of the NaCl mole fraction on the combustion parameters, phase composition, and particle size of the composite powders was investigated. The experimental results revealed that the combustion temperature and particle size of the composites have a stable decreasing tendency with increase in the NaCl mole fraction in the starting mixture. It was found that near the combustion limit (1.5 mol NaCl), the combustion temperature drops below 1500 °C and the particle size reaches the nanometer scale. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Zirconium-based ceramics and composites such as ZrC, ZrB2, ZrC– SiC, ZrB2–SiC, and ZrB2–ZrC–SiC have a combination of several excellent properties such as a high melting point, high hardness, high thermal and electrical conductivity, and good resistance to oxidation and chemical attack. Hence, they have been proposed as potential candidates for applications such as furnace elements, plasma-arc electrodes, rocket engines, and thermal protection structures for space vehicles [1–4]. ZrB2–SiC ceramic composites of suitable composition are known to have better strength and oxidation resistance than monolithic ZrB2 materials [2,5]. Generally, zirconiumbased ceramics are fabricated from conventional powders by one of the known sintering techniques: hot pressing, SPS, microwave sintering, etc. Consequently, the characteristics of raw powders have a direct impact on the physical–mechanical properties of bulk materials. Currently, zirconium-based ceramic powders are mainly synthesized by carbothermal reduction [6,7]. This processing route involves relatively high synthesis temperatures (N1500 °C) and long processing times and affords coarse powders with poor sinterability. Improved sinterability is obtained through the use of a number of sintering aids such as MoSi2, Ni, TiB2, B4C, Si3N4, Al2O3, and Y2O3 [8– 10]. However, introduction of these sintering aids leads to the formation of unstable second phases that will always remain at the grain boundaries, leading to deterioration of the high-temperature ⁎ Corresponding author at: Graduate School of Green Energy Technology, Chungnam National University, Daejon 305-764, Republic of Korea. Tel.: + 82 42 821 6596; fax: + 82 42 822 5850. E-mail address: [email protected] (J.H. Lee). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.07.015
mechanical properties. Thus, the development of an alternative method to produce ultra-fine powders of zirconium composites is required. Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, can be considered an alternative technique for the production of zirconium-based composites [11–13]. Khanra et al. reported the synthesis of ZrB2 fine powder through double combustion of a ZrO2–Mg–H3BO3 mixture [11]. In this process, NaCl is used as a diluent to control the particle size of the product. The average particle size of an SHS-produced ZrB2 powder was reported to be 75–125 nm. Camurlu et al. [12] also reported preparation of nanosized ZrB2 powder by SHS starting from a Zr–B–NaCl mixture. It was revealed that addition of 30 wt.% NaCl afforded ZrB2 powder contained particles mostly smaller than 200 nm. Tsuchida et al. reported [13], the synthesis of fine powders of ZrB2 and ZrC from a mechanically activated mixture of Zr/B/C in equal proportions. The mechanically activated mixture self-ignited spontaneously when exposed to air, and combustion synthesis of ZrC and ZrB2 simultaneously occurred. The method afforded polydispersed particles ranging from a few micrometers to few tens micrometers in diameter. Licheriet et al. [14] obtained a fully dense ZrB2–ZrC–SiC composite by combining SHS with spark plasma sintering (SPS). A starting mixture of Zr–B4C–Si–C (graphite) was ignited and subsequently consolidated by SPS at 1800 °C for 10 min. A fully dense material (relative density higher than 99.5% of the theoretical value) was reportedly obtained when the applied pressure was 20 MPa. An analysis of the literature reveals that the combustion method is particularly rapid and convenient compared with other possible techniques for the preparation of zirconium-based composite powders. Herein we are report the synthesis of ultrafine powders of zirconium-based ceramics and composites (ZrC, ZrB2, ZrC–SiC, and
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ZrB2–SiC) using zirconium silicate (ZrSiO4) as a cost-effective precursor material. ZrSiO4 was reduced with Mg under Ar using NaCl as a flux. The other components of the starting mixture were black soot (C) and boron (B). 2. Experimental The following materials were commercially sourced: ZrSiO4 powder (64% ZrO2; particle size, ≤45 μm; Junsei Chemicals Co., Ltd., Japan), Mg powder (99% purity; particle size, 50–150 μm; Daejung Chemical and Metals Co., Ltd., Republic of Korea), NaCl powder (99.5% pure, particle size after the grinding ≤50 μm, Daejung Chemicals and Metals Co., Ltd., Korea), boron powder (95% purity; particle size, ≤1.0 μm; Grand Chemical and Materials Co., Ltd., Republic of Korea), and black soot (99% purity, Grand Chemical and Materials Co., Ltd., Republic of Korea). In the proposed process, zirconium composite powders were produced by combustion of a ZrSiO4–Mg–C–B–NaCl mixture under an Ar atmosphere. Subsequently, the reaction products were acid leached to remove any Mg-containing reaction by-products. For each sample, the starting materials were combined in stoichiometric ratios corresponding to the target compositions: ZrSi, ZrC, ZrB2, ZrC–SiC, and ZrB2– ZrC–SiC. The precursor powders were mixed using a ball-mill system in a polymer bottle with zirconia balls. In a typical experiment, a cylindrical sample (diameter, 4 cm; height, 10–12 cm) was prepared by filling the ball-mixed powder into a metallic cup. The sample was then placed in a high-pressure reactor, and combusted under an Ar atmosphere at 2.5 MPa. A thin layer of a Ti + C + (−C2F4–)n mixture was placed below the filament system to facilitate ignition of the reaction pellet. After the local ignition of the sample by a resistivity-heated nickel–chromium wire, a combustion wave formed and propagated steadily from the top to the bottom of the sample, converting the initial mixture into the final product. The temperature–time profiles were measured using WR-20/ WR-5 thermocouples inserted in the reaction pellet, and used to calculate the combustion temperature (Tc) and wave velocity (Uc). These thermocouples were preliminary coated with thin layer of Al2O3 in order to avoid chemical interaction between reaction components and thermocouples. After the completion of the combustion process, the sample was cooled to room temperature and was then removed from the combustion chamber. After combustion, the burned sample was ground into a powder for acid leaching. Dilute HCl (10%–15%) was used for all triturations. After treatment with acid, the remaining solids were always rinsed with distilled water and dried under vacuum. Powder diffraction patterns were recorded using an X-ray diffractometer with Cu-K α radiation (Siemens D5000, Germany). The powder morphology was studied using a scanning electron microscope (SEM; JSM 5410, JEOL, Japan).
ZrSiO4 + 4Mg + 2B + 1:3NaCl→ZrB2 + Si + MgO + 1:3NaCl:
ð3Þ
The characteristic thermal profiles in the combustion wave are shown in Fig. 1. In this figure, temperature–time history is converted to a temperature-coordinate profile that provides useful information regarding the flame structure and flame propagation mechanism. The profiles measured for the above stated systems are broadly similar to one other and they always begin with a relatively sharp initial peak. Here, T is the current temperature, x is the coordinate along which propagation of the combustion wave occurs, T* is the reaction ignition temperature. The boundary between the pre-flame (left side of coordinate) and reaction zones (right side of coordinate) relates to the partial melting of Mg (650 °C), which makes stronger and faster the combustion process between reactant particles, and a large amount of heat is generated within milliseconds. A sharp increase in temperature therefore occurs at the boundary between the pre-flame and reaction zones. This temperature was regarded as the starting temperature of the reaction (T*). A closer examination of the temperature profiles shows a similar increasing tendency in delivered temperature (from To to Tc =1400–1450 °C), possibly indicating a similar ignition mechanism under different synthesis conditions. A slight decrease in the temperature after the initial sharp increase may be related to the partially evaporation of Mg (1200 °C) and NaCl (1450 °C). The temperature drop is then retarded and a subsequent increase follows due to high combustion heat evaluation. A small drop in temperature at 1500 °C may indicate the start of carburization or boronization, which increases the system temperature to 1550 °C. The average size of the reaction zone is about 20 mm, where the reaction process undergoes completion. More results are shown in Table 1. The aforementioned combustion products were purified by acid leaching to eliminate MgO and NaCl, followed by washing with distilled water to eliminate surplus acid. The silicon-containing reaction products (see reactions (2) and (3)) were additionally treated with an alkali solution to convert Si into a water-soluble Na2SiO3 phase. The XRD patterns of the as-purified products, shown in Fig. 2, can be attributed to ZrSi, ZrC and ZrB2 phases. Other phases, such as Zr5Si4, Zr5Si3, Zr3Si2, and ZrB, were not detected even though their enthalpies of formation are sufficiently high. XRD peaks of zirconium compounds are neatly expressed and the purified products did not show any peaks that could be assigned to MgO, NaCl or to any by-product containing Si. The morphology, size distribution, and microstructure of the assynthesized ZrSi, ZrC and ZrB2 powders were further studied using the SEM. Fig. 3 shows a representative SEM image of the samples at 20,000 magnification. All powders have fine-sized structure due to flux addition to the reactant mixture. Analysis of the particle size
3. Experimental results and discussion 3.1. Synthesis of ZrSi, ZrC, and ZrB2 fine powders The first goal of the experiment was to synthesize zirconium compounds such as ZrSi, ZrC, and ZrB2. In order to do so, the combustion process of the ZrSiO4 + 4Mg, ZrSiO4 + 4Mg + C. and ZrSiO4 + 4Mg + 2B mixtures diluted with an optimum quantity of NaCl was investigated by the thermocouple technique. The optimum quantity of NaCl was experimentally found to be ~ 1.3 mol with respect to the combustion temperature and the final powder composition. The combustion process in the given systems can be described by the following equations:
ZrSiO4 + 4Mg + 1:3NaCl→ZrSi + MgO + 1:3NaCl
ð1Þ
ZrSiO4 + 4Mg + C + 1:3NaCl→ZrC + Si + MgO + 1:3NaCl
ð2Þ
Fig. 1. Temperature distributions in the combustion wave of ZrSiO4 + 4Mg + NaCl + 2B (C) system.
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0.2 μm in diameter Fig. 3(b). The ZrB2 particles can be characterized as round-shaped particles of size 0.1–1.0 μm Fig. 3(c).
Table 1 Combustion parameters and phase composition. Initial mixture
Tad, °C
Tc, °C
Uc, cm/s
Phase composition
ZrSiO4 + 4Mg ZrSiO4 + 4Mg + 1.0NaCl ZrSiO4 + 4Mg + C + 1.0NaCl ZrSiO4 + 4Mg + 2B + 1.0NaCl ZrSiO4 + 4Mg + 1.5NaCl ZrSiO4 + 4Mg + 2C ZrSiO4 + 4Mg + 2C + 1.0NaCl ZrSiO4 + 4Mg + 2 C + 1.5NaCl ZrSiO4 + 4Mg + 2B + C ZrSiO4 + 4Mg + 2B + C + 1.0NaCl ZrSiO4 + 4Mg + 2B + C + 1.5NaCl
2030 1600 1657 1720 1446 2120 1720 1550 2260 1930 1770
1830 1750 1550 1550 1480 1780 1630 1400 1940 1670 1490
0.13 0.11 0.1 0.1 0.05 0.18 0.1 0.065 0.16 0.1 0.07
ZrSi ZrSi ZrC, Si ZrB2, Si ZrSi ZrC, SiC ZrC, SiC ZrC, SiC, ZrSi ZrB2, SiC, ZrC ZrB2, SiC, ZrC ZrB2, SiC, ZrC, ZrSi
3.2. Synthesis of ZrC–SiC composite powders The synthesis of ZrC–SiC composite powders was carried out from the ZrSiO4 + 4Mg + 2C+ kNaCl mixture for the three values of k: 0, 1.0, and 1.5. The corresponding thermal profiles are shown in Fig. 4. The thermal profile for k = 0 (without flux) has single-stage character with a combustion temperature of about 1780 °C. Addition of 1 mol of NaCl lowers the combustion temperature to 1630 °C and a small quasiisothermal field appears on the profile at 1200 °C. With 1.5 mol of NaCl, the combustion temperature drops to 1460 °C, which is close to the boiling point of NaCl (1413 °C). Additionally, the length of the quasiisothermal field at 1200 °C increased to 15 mm. As mentioned above, this extension of the quasi-isothermal field at 1200 °C may be related to the evaporation of Mg; the low “screening” effect of the maximum temperature extends the size of the quasi-isothermal field. Incidentally, the reduction process of ZrSiO4 is characterized by a low combustion velocity: thermocouple measurement shows that the combustion velocity for the undiluted system is about 0.11 cm/s, whereas the velocity of system diluted with 1.5 mol of NaCl drops to 0.065 cm/s. The overall reaction in the designed system can be described as follows: ZrSiO4 + 4Mg + 2C + 1:5NaCl→ZrC + SiC + MgO + 1:5NaCl
Fig. 2. XRD patterns of zirconium pure-phase compounds synthesized from ZrSiO4 + 4Mg + NaCl + 2B(C) reactive mixture.
distribution indicated that the average diameter of ZrSi particles was around 0.2–0.7 μm, as shown in the micrograph of Fig. 3(a). ZrC powder consisted of uniform and well dispersed spherical particles
ð4Þ
From Eq. (4), it follows that after acid purification of the combustion products, a composite of ZrC/SiC composition can be expected. XRD patterns show very strong diffraction peaks of two carbide phases: monoclinic ZrC and hexagonal SiC (Moissanite-2H; Fig. 5, (a)). Adding 1.0 mol of NaCl to the mixture did not result in visible changes in phase composition [Fig. 5, (b)]. However, when NaCl concentration reached 1.5 mol, a noticeable amount of ZrSi phase appeared in the final product (Fig. 5, (c)). The-observed incompleteness of the carburization process was most likely due to
Fig. 3. SEM micrographs of acid- and alkali-leached combustion products prepared from ZrSiO4 + 4Mg + NaCl + 2B(C) mixture.
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the low combustion temperature (1460 °C) and short reaction time (from several to tens of seconds). Additional experiments made with large diameter samples (d N 7.0 cm) produced a ZrSi free carbide composite consisting of ZrC and SiC phases. Fig. 6 shows the SEM images of ZrC/SiC composite powders obtained when using starting mixtures with different NaCl content. The composite synthesized without NaCl (k = 0) is contained two type of particles: small particles of ZrC (0.1–0.3 μm) and relatively large particles of SiC (0.5–1.0 μm; Fig. 6(a)). At k = 1.0, the particle size become noticeably smaller than at k = 0 (Fig. 6(b)). The average particle size in this composition was less than 0.3 μm. A further increase in NaCl concentration (k = 1.5) resulted in a very fine composite powder, where most of the particles had a size less than 100 nm (Fig. 6(c)). However, some particles larger than 100 nm were also detected. Fig. 4. Temperature distributions in the combustion wave of ZrSiO4 + 4Mg + 2C + kNaCl system versus k: 1, k = 0; 2, k = 1.0; 3, k = 1.5.
Fig. 5. XRD patterns of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2C+ kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
3.3. Synthesis of ZrB2–SiC–ZrC composite powders We also conducted experimental investigations on the use of the ZrSiO4 + 4Mg+ 2B+ C + kNaCl multicomponent system to produce ZrB2–SiC composite powders. Partial replacement of carbon with boron served to increase the exothermic nature of the combustion process, as the heat of formation of ZrB2 (ΔH=−77.7 kcal/mole) is higher than that of ZrC (ΔH=−48.5 kcal/mole). The thermal profiles measured for k=0, 1.0, and 1.5 are shown in Fig. 7. Almost identical single-stage distributions can be observed, regardless of the NaCl content employed. The experimental results also show that Tc decreased from 1940 to 1490 °C when the NaCl concentration was changed from 0 to 1.5 mol. The XRD results shown in Fig. 8(a), imply that in addition to the formation of ZrB2 and SiC phases, a noticeable amount of ZrC was also produced during the combustion of the undiluted system. No change in phase composition could be detected after the addition 1.0 mol of NaCl (Fig. 8(b)). However, close to the combustion limit (k = 1.5), the intermediate ZrSi phase also remained, since the combustion temperature dropped below 1500 °C (Fig. 8(c)). Typical SEM images of ZrB2–ZrC–SiC particles formed from varying NaCl concentrations (0.5, 1.0, and 1.5 mol) are shown in Fig. 9. As seen, the sizes of the composite particles obtained from the
Fig. 6. SEM micrographs of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2C + kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
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undiluted mixture (k = 0) were 0.5–3.0 μm (Fig. 9(a)). Increasing the diluent concentration to 1.0 mol affords a composite powder with a larger portion of submicron sized particles (Fig. 9(b)). It is believed that the round-shaped particles are ZrB2, (similar to Fig. 3), whereas the unshaped particles are SiC. A further increase in NaCl content produced a fine powder with an average particle size of 0.1–0.2 μm. Some relatively large particles (~ 0.3 μm) can be also seen in Fig. 9(c), which most likely consist of the ZrSi phase. 3.4. Mechanism of dilution
Fig.7. Temperature distributions in the combustion wave of ZrSiO4 + 4Mg + 2B + C + kNaCl system versus k: 1, k = 0; 2, k = 1.0; 3, k = 1.5.
Fig. 8. XRD patterns of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2B+ C + kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
The experimental investigations carried out on the ZrSiO4–4Mg–2B– C–kNaCl system clearly demonstrate that fine powders of zirconium compounds such as ZrSi, ZrC, ZrB2 and composites such as ZrC–SiC, ZrC– SiC–ZrB2, and ZrC–SiC–ZrB2–ZrSi can be prepared if ZrSiO4 is reduced in the presence of large amounts of NaCl. It is clear that dilution of the starting systems with NaCl decreases the overall level of energy generated and the reaction temperature and reaction rate during the combustion process. This effect was also observed in our previous studies, where nanopowders of transition metals were prepared by the NaCl dilution technique [15–17]. Thus, the combustion reaction is impeded by the addition of a diluent phase to the starting materials due to the following reasons: (a) the combustion heat is reduced due to the presence of the diluent phase “absorbing” some of the energy for preheating, melting, and partial evaporation; and (b) the molten diluent phase tends to cover the surface of the reactant particles and reduce the reaction rate. In addition, the molten diluent phase increases the degree of conversion by increasing physical contact between reactant particles and prohibits particle growth by forming a thin layer that isolates the product particles. Despite the effect of dilution, NaCl is a favorable reaction medium for zirconium composites as it affords spherical-shaped and uniform-sized particles. Spherical particles of zirconium composites were apparently formed because of the interfacial tension of the liquid diluent in which the composite particles are distributed. SEM micrographs (Figs. 3, 6, and 9) demonstrate that the proposed technique can produce composite powders consisting of nano- and submicron-sized particles when the correct amount of diluent phase (1.3–1.5 mol) is employed.
Fig. 9. SEM micrographs of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2B + C + kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
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4. Conclusions The combustion process in the ZrSiO4–4Mg–C–2B–kNaCl system was investigated by a thermocouple techniqueto identify the optimum diluent concentration (k). One-step in situ production of powders of ZrC, ZrB2, ZrC–SiC, ZrB2–SiC–ZrC, and ZrB2–SiC–ZrC–ZrSi by means of combustion has been successfully demonstrated. The characteristic thermal profiles in the combustion wave were recorded and used to estimate the values of the combustion parameters (temperature Tc and velocity Uc). The experimental results revealed that Tc and Uc and the particle size have a stable decreasing tendency with increasing NaCl mole fraction in the starting mixture. Near the combustion limit (1.5 mol NaCl), the combustion temperature drops below 1500 °C and the composite particles reach the nanometer scale. Acknowledgments This work was supported by the Power Generation & Electricity Delivery project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy of the Korean Government (no. 2010T100100392). References [1] Cutler RA. Engineering properties of borides. In: Schneider SJ, editor. Ceramics and glasses, engineered materials handbook, vol 4. Ohio, Materials Park: ASM International; 1992. p. 787–803. [2] Chamberlain AL, Fahrenholtz WG, Hilmas GE, Ellerby DT. High strength zirconium diboride-based ceramics. J Am Ceram Soc 2004;87:1170–2. [3] Samsonov GV. Refractory transition metal compounds: high temperature cermets. New York: Academic Press; 1964.
[4] Upadhya K, Yang J, Hoffman W. Materials for ultrahigh temperature structural applications. Am Ceram Soc Bull 1997;76:51–6. [5] Tripp WC, Davis HH, Graham HC. Effect of a silicon carbide addition on the oxidation of zirconium diboride. Am Ceram Soc Bull 1973;52:612–6. [6] Blumenthal H. Production of transition metal diborides and their solid solutions from metal oxides and boron oxide. Powder Met Bull 1956;7:79–81. [7] Karasev AI. Preparation of zirconium diboride by the carbothermic reduction of mixtures of zirconium and boron oxides. Poroshkovaya Met 1973;11:80–4. [8] Monteverde F, Bellosi A. Effect of the addition of silicon nitride on sintering behaviour and microstructure of zirconium diboride. Scr Mater 2002;46:223–8. [9] Andrievskii RA, Korolev LA, Klimenko VV, Lanin AG, Spivak II, Taubin IL. Effect of zirconium carbide and carbon additions on some physicomechanical properties of zirconium diboride. Powder Metall Met Ceram 1980;19:93–4. [10] Zhang SC, Hilmas GE, Fahrenholtz WG. Pressureless densification of zirconium diboride with boron carbide additions. J Am Ceram Soc 2006;89:544–50. [11] Khanra AK, Pathak LC, Godkhindi MM. Double SHS of ZrB2 powder. J Mater Process Tech 2008;202:386–90. [12] Camurlu HE, Maglia F. Preparation of nano-size ZrB2 powder by self-propagating high-temperature synthesis. J Eur Ceram Soc 2009;29:1501–6. [13] Tsuchida T, Yamamoto S. Mechanical activation assisted self-propagating hightemperature synthesis of ZrC and ZrB2 in air from Zr/B/C powder mixtures. J Eur Ceram Soc 2004;24:45–51. [14] Licheri R, Orrù R, Musa C, Cao G. Combination of SHS and SPS techniques for fabrication of fully dense ZrB2–ZrC–SiC composites. Mater Lett 2008;62:432–5. [15] Nersisyan HH, Won HI, Won CW. Combustion synthesis of molybdenum disilicide (MoSi2) fine powders. J Am Ceram Soc 2008;91:2802–7. [16] Nersisyan HH, Lee JH, Won CW. Self-propagating high-temperature synthesis of nano-sized titanium carbide powder. J Mater Res 2002;17:2859–64. [17] Nersisyan HH, Lee JH, Won CW. A study of tungsten nanopowders formation under self-propagating high-temperature synthesis mode. Combust Flame 2005;142:241–8.
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 zirconium-based ceramic and composite fine-grained powders Hong Youl Ryu a, H.H. Nersisyan b, Jong Hyeon Lee a, c,⁎ a b c
Graduate School of Green Energy Technology, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Rapidly Solidified Materials Research Institute, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of NanoMaterials Engineering, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 July 2011 Accepted 25 July 2011 Keywords: Zirconium composite Particle size Temperature distribution Fine powder
a b s t r a c t Zirconium-based ceramics and composites such as ZrC, ZrB2, ZrC–SiC, ZrB2–SiC–ZrC, and ZrB2–SiC–ZrC–ZrSi were synthesized in fine powder form via combustion synthesis (CS) using ZrSiO4, Mg, C, B, and NaCl as raw materials. Temperature distributions in the combustion wave were measured by thermocouples and used to estimate the combustion temperature and wave propagation velocity. The influence of the NaCl mole fraction on the combustion parameters, phase composition, and particle size of the composite powders was investigated. The experimental results revealed that the combustion temperature and particle size of the composites have a stable decreasing tendency with increase in the NaCl mole fraction in the starting mixture. It was found that near the combustion limit (1.5 mol NaCl), the combustion temperature drops below 1500 °C and the particle size reaches the nanometer scale. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Zirconium-based ceramics and composites such as ZrC, ZrB2, ZrC– SiC, ZrB2–SiC, and ZrB2–ZrC–SiC have a combination of several excellent properties such as a high melting point, high hardness, high thermal and electrical conductivity, and good resistance to oxidation and chemical attack. Hence, they have been proposed as potential candidates for applications such as furnace elements, plasma-arc electrodes, rocket engines, and thermal protection structures for space vehicles [1–4]. ZrB2–SiC ceramic composites of suitable composition are known to have better strength and oxidation resistance than monolithic ZrB2 materials [2,5]. Generally, zirconiumbased ceramics are fabricated from conventional powders by one of the known sintering techniques: hot pressing, SPS, microwave sintering, etc. Consequently, the characteristics of raw powders have a direct impact on the physical–mechanical properties of bulk materials. Currently, zirconium-based ceramic powders are mainly synthesized by carbothermal reduction [6,7]. This processing route involves relatively high synthesis temperatures (N1500 °C) and long processing times and affords coarse powders with poor sinterability. Improved sinterability is obtained through the use of a number of sintering aids such as MoSi2, Ni, TiB2, B4C, Si3N4, Al2O3, and Y2O3 [8– 10]. However, introduction of these sintering aids leads to the formation of unstable second phases that will always remain at the grain boundaries, leading to deterioration of the high-temperature ⁎ Corresponding author at: Graduate School of Green Energy Technology, Chungnam National University, Daejon 305-764, Republic of Korea. Tel.: + 82 42 821 6596; fax: + 82 42 822 5850. E-mail address: [email protected] (J.H. Lee). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.07.015
mechanical properties. Thus, the development of an alternative method to produce ultra-fine powders of zirconium composites is required. Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, can be considered an alternative technique for the production of zirconium-based composites [11–13]. Khanra et al. reported the synthesis of ZrB2 fine powder through double combustion of a ZrO2–Mg–H3BO3 mixture [11]. In this process, NaCl is used as a diluent to control the particle size of the product. The average particle size of an SHS-produced ZrB2 powder was reported to be 75–125 nm. Camurlu et al. [12] also reported preparation of nanosized ZrB2 powder by SHS starting from a Zr–B–NaCl mixture. It was revealed that addition of 30 wt.% NaCl afforded ZrB2 powder contained particles mostly smaller than 200 nm. Tsuchida et al. reported [13], the synthesis of fine powders of ZrB2 and ZrC from a mechanically activated mixture of Zr/B/C in equal proportions. The mechanically activated mixture self-ignited spontaneously when exposed to air, and combustion synthesis of ZrC and ZrB2 simultaneously occurred. The method afforded polydispersed particles ranging from a few micrometers to few tens micrometers in diameter. Licheriet et al. [14] obtained a fully dense ZrB2–ZrC–SiC composite by combining SHS with spark plasma sintering (SPS). A starting mixture of Zr–B4C–Si–C (graphite) was ignited and subsequently consolidated by SPS at 1800 °C for 10 min. A fully dense material (relative density higher than 99.5% of the theoretical value) was reportedly obtained when the applied pressure was 20 MPa. An analysis of the literature reveals that the combustion method is particularly rapid and convenient compared with other possible techniques for the preparation of zirconium-based composite powders. Herein we are report the synthesis of ultrafine powders of zirconium-based ceramics and composites (ZrC, ZrB2, ZrC–SiC, and
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ZrB2–SiC) using zirconium silicate (ZrSiO4) as a cost-effective precursor material. ZrSiO4 was reduced with Mg under Ar using NaCl as a flux. The other components of the starting mixture were black soot (C) and boron (B). 2. Experimental The following materials were commercially sourced: ZrSiO4 powder (64% ZrO2; particle size, ≤45 μm; Junsei Chemicals Co., Ltd., Japan), Mg powder (99% purity; particle size, 50–150 μm; Daejung Chemical and Metals Co., Ltd., Republic of Korea), NaCl powder (99.5% pure, particle size after the grinding ≤50 μm, Daejung Chemicals and Metals Co., Ltd., Korea), boron powder (95% purity; particle size, ≤1.0 μm; Grand Chemical and Materials Co., Ltd., Republic of Korea), and black soot (99% purity, Grand Chemical and Materials Co., Ltd., Republic of Korea). In the proposed process, zirconium composite powders were produced by combustion of a ZrSiO4–Mg–C–B–NaCl mixture under an Ar atmosphere. Subsequently, the reaction products were acid leached to remove any Mg-containing reaction by-products. For each sample, the starting materials were combined in stoichiometric ratios corresponding to the target compositions: ZrSi, ZrC, ZrB2, ZrC–SiC, and ZrB2– ZrC–SiC. The precursor powders were mixed using a ball-mill system in a polymer bottle with zirconia balls. In a typical experiment, a cylindrical sample (diameter, 4 cm; height, 10–12 cm) was prepared by filling the ball-mixed powder into a metallic cup. The sample was then placed in a high-pressure reactor, and combusted under an Ar atmosphere at 2.5 MPa. A thin layer of a Ti + C + (−C2F4–)n mixture was placed below the filament system to facilitate ignition of the reaction pellet. After the local ignition of the sample by a resistivity-heated nickel–chromium wire, a combustion wave formed and propagated steadily from the top to the bottom of the sample, converting the initial mixture into the final product. The temperature–time profiles were measured using WR-20/ WR-5 thermocouples inserted in the reaction pellet, and used to calculate the combustion temperature (Tc) and wave velocity (Uc). These thermocouples were preliminary coated with thin layer of Al2O3 in order to avoid chemical interaction between reaction components and thermocouples. After the completion of the combustion process, the sample was cooled to room temperature and was then removed from the combustion chamber. After combustion, the burned sample was ground into a powder for acid leaching. Dilute HCl (10%–15%) was used for all triturations. After treatment with acid, the remaining solids were always rinsed with distilled water and dried under vacuum. Powder diffraction patterns were recorded using an X-ray diffractometer with Cu-K α radiation (Siemens D5000, Germany). The powder morphology was studied using a scanning electron microscope (SEM; JSM 5410, JEOL, Japan).
ZrSiO4 + 4Mg + 2B + 1:3NaCl→ZrB2 + Si + MgO + 1:3NaCl:
ð3Þ
The characteristic thermal profiles in the combustion wave are shown in Fig. 1. In this figure, temperature–time history is converted to a temperature-coordinate profile that provides useful information regarding the flame structure and flame propagation mechanism. The profiles measured for the above stated systems are broadly similar to one other and they always begin with a relatively sharp initial peak. Here, T is the current temperature, x is the coordinate along which propagation of the combustion wave occurs, T* is the reaction ignition temperature. The boundary between the pre-flame (left side of coordinate) and reaction zones (right side of coordinate) relates to the partial melting of Mg (650 °C), which makes stronger and faster the combustion process between reactant particles, and a large amount of heat is generated within milliseconds. A sharp increase in temperature therefore occurs at the boundary between the pre-flame and reaction zones. This temperature was regarded as the starting temperature of the reaction (T*). A closer examination of the temperature profiles shows a similar increasing tendency in delivered temperature (from To to Tc =1400–1450 °C), possibly indicating a similar ignition mechanism under different synthesis conditions. A slight decrease in the temperature after the initial sharp increase may be related to the partially evaporation of Mg (1200 °C) and NaCl (1450 °C). The temperature drop is then retarded and a subsequent increase follows due to high combustion heat evaluation. A small drop in temperature at 1500 °C may indicate the start of carburization or boronization, which increases the system temperature to 1550 °C. The average size of the reaction zone is about 20 mm, where the reaction process undergoes completion. More results are shown in Table 1. The aforementioned combustion products were purified by acid leaching to eliminate MgO and NaCl, followed by washing with distilled water to eliminate surplus acid. The silicon-containing reaction products (see reactions (2) and (3)) were additionally treated with an alkali solution to convert Si into a water-soluble Na2SiO3 phase. The XRD patterns of the as-purified products, shown in Fig. 2, can be attributed to ZrSi, ZrC and ZrB2 phases. Other phases, such as Zr5Si4, Zr5Si3, Zr3Si2, and ZrB, were not detected even though their enthalpies of formation are sufficiently high. XRD peaks of zirconium compounds are neatly expressed and the purified products did not show any peaks that could be assigned to MgO, NaCl or to any by-product containing Si. The morphology, size distribution, and microstructure of the assynthesized ZrSi, ZrC and ZrB2 powders were further studied using the SEM. Fig. 3 shows a representative SEM image of the samples at 20,000 magnification. All powders have fine-sized structure due to flux addition to the reactant mixture. Analysis of the particle size
3. Experimental results and discussion 3.1. Synthesis of ZrSi, ZrC, and ZrB2 fine powders The first goal of the experiment was to synthesize zirconium compounds such as ZrSi, ZrC, and ZrB2. In order to do so, the combustion process of the ZrSiO4 + 4Mg, ZrSiO4 + 4Mg + C. and ZrSiO4 + 4Mg + 2B mixtures diluted with an optimum quantity of NaCl was investigated by the thermocouple technique. The optimum quantity of NaCl was experimentally found to be ~ 1.3 mol with respect to the combustion temperature and the final powder composition. The combustion process in the given systems can be described by the following equations:
ZrSiO4 + 4Mg + 1:3NaCl→ZrSi + MgO + 1:3NaCl
ð1Þ
ZrSiO4 + 4Mg + C + 1:3NaCl→ZrC + Si + MgO + 1:3NaCl
ð2Þ
Fig. 1. Temperature distributions in the combustion wave of ZrSiO4 + 4Mg + NaCl + 2B (C) system.
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0.2 μm in diameter Fig. 3(b). The ZrB2 particles can be characterized as round-shaped particles of size 0.1–1.0 μm Fig. 3(c).
Table 1 Combustion parameters and phase composition. Initial mixture
Tad, °C
Tc, °C
Uc, cm/s
Phase composition
ZrSiO4 + 4Mg ZrSiO4 + 4Mg + 1.0NaCl ZrSiO4 + 4Mg + C + 1.0NaCl ZrSiO4 + 4Mg + 2B + 1.0NaCl ZrSiO4 + 4Mg + 1.5NaCl ZrSiO4 + 4Mg + 2C ZrSiO4 + 4Mg + 2C + 1.0NaCl ZrSiO4 + 4Mg + 2 C + 1.5NaCl ZrSiO4 + 4Mg + 2B + C ZrSiO4 + 4Mg + 2B + C + 1.0NaCl ZrSiO4 + 4Mg + 2B + C + 1.5NaCl
2030 1600 1657 1720 1446 2120 1720 1550 2260 1930 1770
1830 1750 1550 1550 1480 1780 1630 1400 1940 1670 1490
0.13 0.11 0.1 0.1 0.05 0.18 0.1 0.065 0.16 0.1 0.07
ZrSi ZrSi ZrC, Si ZrB2, Si ZrSi ZrC, SiC ZrC, SiC ZrC, SiC, ZrSi ZrB2, SiC, ZrC ZrB2, SiC, ZrC ZrB2, SiC, ZrC, ZrSi
3.2. Synthesis of ZrC–SiC composite powders The synthesis of ZrC–SiC composite powders was carried out from the ZrSiO4 + 4Mg + 2C+ kNaCl mixture for the three values of k: 0, 1.0, and 1.5. The corresponding thermal profiles are shown in Fig. 4. The thermal profile for k = 0 (without flux) has single-stage character with a combustion temperature of about 1780 °C. Addition of 1 mol of NaCl lowers the combustion temperature to 1630 °C and a small quasiisothermal field appears on the profile at 1200 °C. With 1.5 mol of NaCl, the combustion temperature drops to 1460 °C, which is close to the boiling point of NaCl (1413 °C). Additionally, the length of the quasiisothermal field at 1200 °C increased to 15 mm. As mentioned above, this extension of the quasi-isothermal field at 1200 °C may be related to the evaporation of Mg; the low “screening” effect of the maximum temperature extends the size of the quasi-isothermal field. Incidentally, the reduction process of ZrSiO4 is characterized by a low combustion velocity: thermocouple measurement shows that the combustion velocity for the undiluted system is about 0.11 cm/s, whereas the velocity of system diluted with 1.5 mol of NaCl drops to 0.065 cm/s. The overall reaction in the designed system can be described as follows: ZrSiO4 + 4Mg + 2C + 1:5NaCl→ZrC + SiC + MgO + 1:5NaCl
Fig. 2. XRD patterns of zirconium pure-phase compounds synthesized from ZrSiO4 + 4Mg + NaCl + 2B(C) reactive mixture.
distribution indicated that the average diameter of ZrSi particles was around 0.2–0.7 μm, as shown in the micrograph of Fig. 3(a). ZrC powder consisted of uniform and well dispersed spherical particles
ð4Þ
From Eq. (4), it follows that after acid purification of the combustion products, a composite of ZrC/SiC composition can be expected. XRD patterns show very strong diffraction peaks of two carbide phases: monoclinic ZrC and hexagonal SiC (Moissanite-2H; Fig. 5, (a)). Adding 1.0 mol of NaCl to the mixture did not result in visible changes in phase composition [Fig. 5, (b)]. However, when NaCl concentration reached 1.5 mol, a noticeable amount of ZrSi phase appeared in the final product (Fig. 5, (c)). The-observed incompleteness of the carburization process was most likely due to
Fig. 3. SEM micrographs of acid- and alkali-leached combustion products prepared from ZrSiO4 + 4Mg + NaCl + 2B(C) mixture.
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the low combustion temperature (1460 °C) and short reaction time (from several to tens of seconds). Additional experiments made with large diameter samples (d N 7.0 cm) produced a ZrSi free carbide composite consisting of ZrC and SiC phases. Fig. 6 shows the SEM images of ZrC/SiC composite powders obtained when using starting mixtures with different NaCl content. The composite synthesized without NaCl (k = 0) is contained two type of particles: small particles of ZrC (0.1–0.3 μm) and relatively large particles of SiC (0.5–1.0 μm; Fig. 6(a)). At k = 1.0, the particle size become noticeably smaller than at k = 0 (Fig. 6(b)). The average particle size in this composition was less than 0.3 μm. A further increase in NaCl concentration (k = 1.5) resulted in a very fine composite powder, where most of the particles had a size less than 100 nm (Fig. 6(c)). However, some particles larger than 100 nm were also detected. Fig. 4. Temperature distributions in the combustion wave of ZrSiO4 + 4Mg + 2C + kNaCl system versus k: 1, k = 0; 2, k = 1.0; 3, k = 1.5.
Fig. 5. XRD patterns of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2C+ kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
3.3. Synthesis of ZrB2–SiC–ZrC composite powders We also conducted experimental investigations on the use of the ZrSiO4 + 4Mg+ 2B+ C + kNaCl multicomponent system to produce ZrB2–SiC composite powders. Partial replacement of carbon with boron served to increase the exothermic nature of the combustion process, as the heat of formation of ZrB2 (ΔH=−77.7 kcal/mole) is higher than that of ZrC (ΔH=−48.5 kcal/mole). The thermal profiles measured for k=0, 1.0, and 1.5 are shown in Fig. 7. Almost identical single-stage distributions can be observed, regardless of the NaCl content employed. The experimental results also show that Tc decreased from 1940 to 1490 °C when the NaCl concentration was changed from 0 to 1.5 mol. The XRD results shown in Fig. 8(a), imply that in addition to the formation of ZrB2 and SiC phases, a noticeable amount of ZrC was also produced during the combustion of the undiluted system. No change in phase composition could be detected after the addition 1.0 mol of NaCl (Fig. 8(b)). However, close to the combustion limit (k = 1.5), the intermediate ZrSi phase also remained, since the combustion temperature dropped below 1500 °C (Fig. 8(c)). Typical SEM images of ZrB2–ZrC–SiC particles formed from varying NaCl concentrations (0.5, 1.0, and 1.5 mol) are shown in Fig. 9. As seen, the sizes of the composite particles obtained from the
Fig. 6. SEM micrographs of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2C + kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
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undiluted mixture (k = 0) were 0.5–3.0 μm (Fig. 9(a)). Increasing the diluent concentration to 1.0 mol affords a composite powder with a larger portion of submicron sized particles (Fig. 9(b)). It is believed that the round-shaped particles are ZrB2, (similar to Fig. 3), whereas the unshaped particles are SiC. A further increase in NaCl content produced a fine powder with an average particle size of 0.1–0.2 μm. Some relatively large particles (~ 0.3 μm) can be also seen in Fig. 9(c), which most likely consist of the ZrSi phase. 3.4. Mechanism of dilution
Fig.7. Temperature distributions in the combustion wave of ZrSiO4 + 4Mg + 2B + C + kNaCl system versus k: 1, k = 0; 2, k = 1.0; 3, k = 1.5.
Fig. 8. XRD patterns of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2B+ C + kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
The experimental investigations carried out on the ZrSiO4–4Mg–2B– C–kNaCl system clearly demonstrate that fine powders of zirconium compounds such as ZrSi, ZrC, ZrB2 and composites such as ZrC–SiC, ZrC– SiC–ZrB2, and ZrC–SiC–ZrB2–ZrSi can be prepared if ZrSiO4 is reduced in the presence of large amounts of NaCl. It is clear that dilution of the starting systems with NaCl decreases the overall level of energy generated and the reaction temperature and reaction rate during the combustion process. This effect was also observed in our previous studies, where nanopowders of transition metals were prepared by the NaCl dilution technique [15–17]. Thus, the combustion reaction is impeded by the addition of a diluent phase to the starting materials due to the following reasons: (a) the combustion heat is reduced due to the presence of the diluent phase “absorbing” some of the energy for preheating, melting, and partial evaporation; and (b) the molten diluent phase tends to cover the surface of the reactant particles and reduce the reaction rate. In addition, the molten diluent phase increases the degree of conversion by increasing physical contact between reactant particles and prohibits particle growth by forming a thin layer that isolates the product particles. Despite the effect of dilution, NaCl is a favorable reaction medium for zirconium composites as it affords spherical-shaped and uniform-sized particles. Spherical particles of zirconium composites were apparently formed because of the interfacial tension of the liquid diluent in which the composite particles are distributed. SEM micrographs (Figs. 3, 6, and 9) demonstrate that the proposed technique can produce composite powders consisting of nano- and submicron-sized particles when the correct amount of diluent phase (1.3–1.5 mol) is employed.
Fig. 9. SEM micrographs of acid leached combustion products prepared from ZrSiO4 + 4Mg + 2B + C + kNaCl mixture: (a) k = 0; (b) k = 1.0; (c) k = 1.5.
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4. Conclusions The combustion process in the ZrSiO4–4Mg–C–2B–kNaCl system was investigated by a thermocouple techniqueto identify the optimum diluent concentration (k). One-step in situ production of powders of ZrC, ZrB2, ZrC–SiC, ZrB2–SiC–ZrC, and ZrB2–SiC–ZrC–ZrSi by means of combustion has been successfully demonstrated. The characteristic thermal profiles in the combustion wave were recorded and used to estimate the values of the combustion parameters (temperature Tc and velocity Uc). The experimental results revealed that Tc and Uc and the particle size have a stable decreasing tendency with increasing NaCl mole fraction in the starting mixture. Near the combustion limit (1.5 mol NaCl), the combustion temperature drops below 1500 °C and the composite particles reach the nanometer scale. Acknowledgments This work was supported by the Power Generation & Electricity Delivery project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy of the Korean Government (no. 2010T100100392). References [1] Cutler RA. Engineering properties of borides. In: Schneider SJ, editor. Ceramics and glasses, engineered materials handbook, vol 4. Ohio, Materials Park: ASM International; 1992. p. 787–803. [2] Chamberlain AL, Fahrenholtz WG, Hilmas GE, Ellerby DT. High strength zirconium diboride-based ceramics. J Am Ceram Soc 2004;87:1170–2. [3] Samsonov GV. Refractory transition metal compounds: high temperature cermets. New York: Academic Press; 1964.
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