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Study on the reaction mechanism of synthesis of Zr2CN by carbothermal reduction
Ceramics International 46 (2020) 1111–1118
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Study on the reaction mechanism of synthesis of Zr2CN by carbothermal reduction
T
G.Y. Zhang∗, R.L. Wang, L.R. Wang, Y.F. Liu Hebei Provincial Key Laboratory of Inorganic Nonmetallic Materials, College of Materials Science and Engineering, North China University of Science and Technology, Tangshan, Hebei, 063210, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zr2CN Carbothermal reduction method Synthesis
In this study, the Zr2CN powder was fabricated using a carbothermal reduction method under a nitrogen (N2) atmosphere with nanometer m-ZrO2, micron-sized Ca-PSZ and nano-carbon black as raw materials. The product was characterized by X-ray diffractometry (XRD) and energy dispersive spectroscopy (EDS) under a scanning electron microscopy (SEM). The Zr2CN synthesis mechanism was analyzed. The results showed that the formation of Zr2CN phase was attributed to C, N diffusion, C–CO2–CO system equilibrium variation and ZrO2 decomposition; its formation primarily includes four stages of cyclical intersection (solid-gas diffusion and formation of high-temperature stable ZrO2, surface Zr2CN formation, ZrO2 partial decomposition and vapor deposition, inner layer Zr2CN formation and surface layer Zr2CN Oxidation). The four stages were cycled until the end of the ZrO2 formation of Zr2CN.
1. Introduction Zr(C,N) exhibits many excellent properties (e.g., chemical stability, high hardness and high melting point) [1]. Besides, it exhibits strong corrosion resistance and friction resistance in its applications. The nonoxides primarily include ZrC, ZrN and Zr2CN. Zirconium non-oxide ceramics, as a novel advanced material, are extensively favored for their unique and excellent properties. For instance, the processing of high hardness materials (e.g., corundum, glass and hard metal) can draw upon ultra-high hardness and corrosion resistance of Zr (C, N) non-oxide. Moreover, Zr (C, N) non-oxide can also be adopted to produce wear-resistant and corrosion-resistant tools, and it can serve as an additive to enhance the strength and corrosion resistance of cemented carbide [2]. Zirconium non-oxide ceramics are also extensively adopted in industries (e.g., electronic and electrical products, aerospace materials, nuclear fuels and specialty refractories). ZrC is a refractory metal carbide of which the crystal is a NaCl-type face-centered cubic structure. The Zr atoms form a close cubic lattice, and the C atoms are located in the lattice octahedral gap, thus forming a simple gap phase (C/Zr atomic radius ratio of 0.148, less than 0.59). The Zr–C bond exhibits a high bond energy of 83.6257 kJ/mol, and its symmetric distribution leads to the formation of a ZrC crystal. Due to the mixture of ionic, covalent, and metallic bonding. ZrC presents excellent properties such as ultra-high hardness (25.5GPa) and melting point (3400°C~3540°C) [3,4]. ∗
ZrN exhibits a crystal structure of a NaCl-type face-centered cubic structure with a lattice constant a = 4.56 Å [5]. The crystal lattice is preferentially oriented along the < 111 > direction and the (111) plane, and it displays a golden metallic luster. ZrN refers to a mixture of metal bonds and covalent bonds. The atoms are connected by Zr^N, exhibiting the characteristics of both metal crystals and covalent crystals. They exhibit high melting point (2980°C), high hardness (9800MPa-19600MPa), abrasion resistance, good corrosion and oxidation resistance (700 °C), excellent thermal conductivity, electrical conductivity and metal reflectance, as well as high superconducting critical temperature (about 10 K) [6–11]. The non-metallic C and N elements can occupy the interstitial sites of the transition metal Zr lattice at the same time to form Zr–C–N ternary compounds, whereas the synthesis and performance studies on Zr–C–N have been limited. The Zr2CN phase existed in the Zr–C–N composite film prepared in the literature [12,13], and the hardness and wear resistance of the film layer were enhanced. The literature reported that the Zr–C–N composite membrane was biocompatible with human body, and coating the composite membrane on invasive surgical instruments can optimize the fatigue life of the component and enhance its mechanical properties and decorative functions [14]. The study on Zr–C–N ternary system largely focused on the membrane layer, while the synthesis of Zr2CN powder is rare. In this present work, Zr2CN powder was prepared using carbothermal reduction method. Also, the synthesis reaction mechanism is
Corresponding author. E-mail address: [email protected] (G.Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.09.079 Received 3 July 2019; Received in revised form 8 September 2019; Accepted 8 September 2019 Available online 12 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 46 (2020) 1111–1118
G.Y. Zhang, et al.
Table 1 Chemical composition of raw materials in the experiment. Name
m-ZrO2 Ca-PSZ Nano- carbon black
Chemical composition /% ZrO2+HfO2
CaO
C
Impurity oxide
99.9% 94.2% –
– 4.2% –
– – –
– MgO+Al2O3+SiO2+Fe2O3+Nb2O5 = 1.6% SO3+CaO+Fe2O3+ZnO+PbO+MgO+SiO2+Al2O3 = 96.8%
scale m-ZrO2 is expressed as “m”, and the micro-scale Ca-PSZ as “P”; the “CR” indicates that the proportion of the compound is 1 mol of zirconia /CR/10 mol Carbon black; “TEM” refers to the synthesis temperature/ 10; “H” is the holding time. For instance: the sample number is m201604, implying that the zirconia raw material is m-ZrO2, the compounding ratio is 1 mol zirconia /2.0 mol carbon black, the synthesis temperature is 1600 °C, and the holding time is 4 h.
discussed in detail. 2. Experimental 2.1. Raw materials The raw materials employed in this study included nano carbon black (median diameter d50 = 50 nm), nanometer m-ZrO2 (median diameter d50 = 50 nm), and micron-sized Ca-PSZ powder (particle size < 74 μm). The chemical composition of the experimental materials (fluorescence X-ray analysis results) is listed in Table 1, and the SEM topography is illustrated in Fig. 1. The atmosphere required for this study was high purity N2 gas (purity N2 > 99.99%).
2.3. Testing and characterization XRD and SEM-EDS analysis were conducted on the samples after being burnt. Besides, the reactant phase in the sample was identified, and the relative percentage of Zr2CN in the product was obtained. The formation of Zr2CN reaction was discussed, and the synthesis reaction mechanism was studied.
2.2. Experimental process
3. Results and discussion
The ingredients are proportioned with nanometer m-ZrO2, micronsized Ca-PSZ and nano-carbon black as raw materials and according to n(ZrO2): n(C) = 1:2, 1:2.5, 1:3, 1:3.5 and 1:4. The batch materials were mixed using the wet method for 4 h with anhydrous ethanol as solvent, and the slurry was dried at 110 °C for 24 h. The dried powder was repeatedly sieved through a 74μm (200 mesh) standard sieve 5 times to produce the ingredients to fabricate Zr2CN. The mixed powder was placed in a mortar of appropriate size, and 12% to 15% of a PVA binder at a mass concentration of 2% was added for grinding. Then, the powder was uniformly placed in a pellet for 2 h. The batch mixed powder was pressed into a blank sample with a size of ϕ 30 mm × 10 mm under a hydraulic press and a molding pressure of 65 KN (92MPa). The green body was dried in an oven at 35 °C for firing. The green body was buried and evacuated, and then it underwent carbothermal reduction synthesis of Zr2CN powder in a box atmosphere furnace at the flow rate of nitrogen about 100 ml/min. The heating rate in the furnace was 5 °C /min below 1000 °C, and 3 °C /min above 1000 °C. The reaction temperatures were 1500 °C, 1550 °C, 1600 °C and 1650 °C, respectively, and the holding time was 3 h, 4 h and 5 h in graphite powder. The experiment unified the numbering of each sample as a set of alphanumeric characters in the format of MCRTEMH. Among them, the initial letter “M” denotes the type of zirconia raw material; the nano-
With nanometer m-ZrO2, micron-sized Ca-PSZ and nano-scale carbon black as raw materials, at ZrO2/C molar ratio of 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4 for the proportion of ingredients, the blank samples of each batch were placed in a high-temperature atmosphere electric furnace after being batched, mixed, formed and dried in a dynamic high-purity N2 atmosphere under the 1500 °C × 4 h, 1550 °C × 3 ~ 5 h, 1600 °C × 3 h ~ 5 h, 1650 °C × 3 h ~ 5 h of the fire. Given the results of high temperature reaction of ZrO2–C–N2 system, the formation of Zr2CN phase was discussed by XRD, SEM and EDS. The results showed that the formation of the Zr2CN phase was attributed to the following four stages. 3.1. Solid-gas diffusion and high temperature stable ZrO2 formation Taking P251555 as an example, the results of SEM-EDS analysis of typical variations in surface characteristics during Ca-PSZ particle reaction are shown in Fig. 2. The SEM-EDS analysis results (Fig. 2) suggest that there are two adjacent regions of “depression” and “bump” in the microscopic portion of the surface of the Ca-PSZ particles not fully reacted to form granular crystals. The EDS analysis results (Fig. 2(b)) suggested that the “depression” area contained more C, O and less N, while the “bump” area
Fig. 1. SEM pictures of raw materials morphology. 1112
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Fig. 2. SEM-EDS analysis of Ca-PSZ typical particle surface in the reaction process.
had C, O, N content opposite to the “depression” area. The analysis revealed that the formation of the “depression” and “bump” regions might be due to three reasons: first, the crystal transformation of ZrO2, i.e., the volumetric shrinkage effect of ZrO2 from low-density to high-density conversion; second, the surface of ZrO2 particles in different microscopic regions had different degrees of contact with C and N2; third, the difference in the difficulty of diffusion of C and N in different microscopic regions. The comprehensive results determined the degree and the speed of ZrO2 crystallization stabilization. The ZrO2 part with high degree of stabilization and high speed formed a “depression” zone, and the “bumping” area was often formed between two “depression” zones with different shrinkage centers. O in the displaced ZrO2 led to the formation of O2 and reacted with C in the environment to form CO2, which is expressed as:
ZrO2(s) + C (s)+N2(g) = ZrO2 − x−y C x Ny □ x+y(s)+ O2(g)
C(s)+ O2(g) = CO2(g)
Fig. 3. XRD analysis results of sample m251555.
the ZrO2 particles. In the experiment of high temperature reaction of ZrO2–C–N2 system, raw materials employed were nanometer m-ZrO2 powder, micron-sized Ca-PSZ powder and nano carbon black, m-ZrO2+C and CaPSZ+C were mixed in different ratios of ZrO2/C molar ratio of 1:1.5 and 1:2.25, and PVA was adopted as a binder, which was mixed, shaped and dried to prepare a batch blank sample. Given the results of TG-DSCMS analysis of the mixture of m-ZrO2+C and Ca-PSZ+C in high purity N2 atmosphere, the temperature condition of 1350 °C × 4 h was taken as the ZrO2–C–N2 system batch. The temperature system of the sample was burnt, and the blank samples of each batch were burnt in high-
3.2. Formation phase of surface Zr2CN phase The results of XRD analysis of sample m251555 are shown in Fig. 3; Fig. 4 shows the results of SEM-EDS analysis of crystals and regions covering only Zr, C, and N elements on the surface of samples m251555 and P251555. The XRD analysis results (Fig. 3) and the SEM-EDS analysis results (Fig. 4) revealed that the phase in Fig. 4(a) was Zr2CN, and the rib portion on the surface of the particle shown in Fig. 4(b) was a Zr–C–N polymer which has not yet led to the formation of a certain crystal topography. In other words, Zr2CN could be formed on the surface of 1113
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Fig. 4. SEM-EDS analysis of m251555 and P251555 surface crystal area containing only Zr, C, N.
Fig. 5. XRD analysis of m-ZrO2+C and Ca-PSZ+C batch sample sintered at 1350 °C × 4 h
The results of the XRD analysis (Fig. 5) and the SEM-EDS analysis (Fig. 2) suggest that the high temperature reaction of the ZrO2–C–N2 system began with the diffusion of C and N, and the study by Sondhi et al. [15] showed that the diffusion depth of C in ZrO2 material did not exceed 50 μm, so the formation of Zr2CN phase should primarily be prioritized over the surface of ZrO2 particles.
purity N2 atmosphere; The results of XRD analysis of m-ZrO2+C and Ca-PSZ+C batch samples after being burnt at 1350 °C × 4 h are shown in Fig. 5. The XRD analysis results of the m-ZrO2+C batch sample (Fig. 5(a)) suggested that the new phase tetragonal t-ZrO2 was formed in the sample after being burnt at 1350 °C × 4 h, and the intensity of the diffraction peak increased with the rise in the amount of carbon black. This implied that the reaction of ZrO2 with C and N2 in the ZrO2–C–N2 system was not an interfacial contact reaction, whereas it was achieved by interdiffusion. According to the analysis, the C and N in the environment were diffused internally to ZrO2, and the O in ZrO2 diffused to the external environment. The position of part O in ZrO2 was replaced by C and N, resulting in lattice distortion of ZrO2, making the monoclinic part m-ZrO2 converted into tetragonal t-ZrO2; O was internally diffused by ZrO2 to react with C to form CO2 and release heat. The XRD analysis results of the Ca-PSZ+C batch sample shown in Fig. 5(b) suggested that the sample phase formed a full cubic c-ZrO2 after being burnt at 1350 °C × 4 h. Therefore, it can be further explained that the diffusion of C and N into ZrO2 could not only convert m-ZrO2 into t-ZrO2, but also convert t-ZrO2 into a c-ZrO2.
3.3. ZrO2 partial decomposition and vapor deposition formation stage 3.3.1. ZrO2 partial decomposition Fig. 6 shows the step topography resulting from the decomposition of ZrO2 on the surface of the sample particles. Due to the existence of C and the displacement of O2 attributed to the diffusion of C and N into ZrO2 at high temperature, C was oxidized to CO2. At high temperature, there must be a balance problem of C–CO2–CO system in ZrO2–C–N2 system, i.e., there was a reversible chemical reaction:
C+ CO2 ⇄ 2CO
(1)
According to David et al. [16] in the study of the mechanism of ZrC synthesis, the presence of CO can lead to ZrO2 decomposition: 1114
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Fig. 6. SEM analysis of step morphology produced by the decomposition of ZrO2 in m301655.
ZrO2(s) + CO(g) = ZrO(g) + CO2(g)
(2)
and form a spherical aggregate containing Zr, O, C, N and other elements. Since the m-series sample is a nanometer m-ZrO2+C batch sample, ZrO2 exhibited a large specific surface area and high activity, and ZrO2 was easy to decompose to produce more ZrO(g), which would form more whiskers and globular aggregates containing elements (e.g., O, C, and N).
The results of the SEM-EDS analysis (Fig. 2) revealed that cracks are often formed between the two “recessed” regions due to volume shrinkage in the formation of the high-temperature stable ZrO2. The presence of this crack allowed CO to enter the interior of the ZrO2 particle. When CO entered the interior of ZrO2 particles, the reverse reaction of formula (1) could occur due to the imbalance of C–CO2–CO system and lead to carbon deposition, laying a material foundation for the internal circulation of C and N to ZrO2. Besides, decomposition reaction of ZrO2 as shown in formula (2) might occur, resulting in decomposition of ZrO2 and formation of a ZrO(g) gas phase product. The decomposition reaction of ZrO2 must occur at its surface or at the crack, i.e., the region in contact with the CO gas phase will leave a disassembly step opposite to the ZrO2 crystallization. The SEM analysis results of the step shown in Fig. 6 demonstrated the decomposition reaction of ZrO2 in the ZrO2–C–N2 system at high temperature.
3.4. Partial oxidation of surface layer Zr2CN and formation stage of inner layer Zr2CN In the synthesis of Zr2CN, the results of XRD analysis suggested that with the rise in temperature, the holding time and the appropriate amount of C were prolonged and increased, the overall trend of macroscopic statistics was that Zr2CN synthesis rate was improved, whereas there were often irregular variations. SEM-EDS analysis suggested that the Zr2CN phase formed on the surface of the raw material particles had a secondary oxidation phenomenon with the rise in temperature or the prolongation of the holding time. This is probably because as the synthesis of Zr2CN proceeded in the ZrO2 particles, the O2 released by the inner layer reaction partially oxidized the Zr2CN in the surface layer.
3.3.2. Vapor deposit formation Taking burned samples m251555, P251605, P301653, and P401603 as an example, Fig. 7 and Fig. 8 respectively show SEM-EDS analysis results of whiskers and globular aggregates present in the sample. It was found in the experiment that there were a certain number of whiskers (Fig. 7) and spherical aggregates (Fig. 8) in the microstructure of each sample; among them, the number of spheroids and whiskers in the m-series sample more than the P-series samples. From the results of EDS analysis shown in Fig. 7 (c) and Fig. 8 (c), it is known that the whiskers and spherical aggregates formed in the sample were composed of elements e.g., Zr, O, C, and N. The results of the SEM-EDS test (Fig. 7 and Fig. 8) revealed that the formation of whiskers and globular aggregates in the sample originated from the gas phase reaction, which was attributed to ZrO2 decomposition to produce ZrO(g). On the one hand, ZrO(g) and CO, N2 and other gas phase materials could accumulate into the appropriate concentration at the edge of c-ZrO2 or Zr–O–C–N solid solution, react nucleation and grow along the one-dimensional direction to form a kind whisker containing Zr, O, C, N. On the other hand, ZrO(g) and CO, N2 and other gas phase substances could accumulate into a suitable concentration between carbon materials, react nucleation and aggregate
3.4.1. Typical Zr2CN phase morphology A typical SEM topography of Zr2CN in samples m251655, m301655 and P401604 is shown in Fig. 9. The synthesis ratios of Zr2CN in the samples m251655, m301655 and P401604 (Fig. 9) were nearly 99%, 100% and 97%, respectively, and the product purity was high. The SEM analysis results shown in the figure suggest that the Zr2CN grain size in the m-series sample was about 0.5~2μm on average, and the Zr2CN grain size in the P-series sample was about 0.5~3μm on average; the Zr2CN grain shape was mostly spherical and evenly distributed in the sample. 3.4.2. Analysis of anomalous phenomena in the synthesis of Zr2CN According to the XRD analysis and calculation in the synthesis of Zr2CN, the relative content of Zr2CN in sample m301654 after being burnt at 1650 °C for 4 h was about 71%, 15% less than that of Zr2CN in sample m301653 after being burnt at 1650 °C for 3 h; the relative content of Zr2CN in the sample P401605 after being burnt at 1600 °C for 1115
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Fig. 7. SEM-EDS analysis of the whiskers in sample.
Fig. 8. SEM-EDS analysis of globular aggregate in the sample. 1116
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Fig. 9. SEM analysis of typical morphology of Zr2CN and sample particle surface characteristics.
heated at 1650 °C for 5 h. Due to the presence of CO in the synthesis of Zr2CN, ZrO2 was decomposed into ZrO(g) and O2. It was found that as the synthesis reaction of Zr2CN took place deep into the ZrO2 particles, the O2 released by the inner layer reaction could partially oxidize the Zr2CN in the surface layer besides the reaction with C or CO. The variation of the morphology of the sample's surface layer (Fig. 10) should be the result of the continuous oxidation of Zr2CN, and O2 generated by the inner layer reaction caused the surface layer Zr2CN to be oxidized twice in the discharge process. This analysis can better interpret the abnormal variation of Zr2CN content in the sample with the extension of synthesis temperature and holding time.
5 h (about 71%) was 9% less than that of Zr2CN (about 97%) in the sample P401604 after being incubated at 1600 °C for 4 h. Taking m301653 and m301654, as well as P401604 and P401605, as examples, Fig. 10 shows the anomalous phenomena and morphological variations of phase morphology during the Zr2CN synthesis. The SEM analysis results (Fig. 10) suggest that the morphology of the sample m301653 after 3 h insulation at 1650 °C was mostly a spheroid with a smaller size and a higher dispersion, and that of the phase in the sample m301654 after undergoing 4 h insulation at 1650 °C was mostly the spreading of particles; the phase morphology of the sample P401604 after undergoing 1600 °C heat preservation for 4 h was mostly spherical or spheroidal and closely arranged, and the morphology of the sample in P401605 was fragmented after being
Fig. 10. SEM analysis of abnormal phenomenon with the extension of holding time and phase morphology change in the synthesis process of Zr2CN. 1117
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4. Conclusion Zr2CN formation-oxidation in the synthesis reaction was cycled. Zr2CN was first formed on the surface of high-temperature stable ZrO2 particles, oxidized by O2 from ZrO2 partial decomposition and then released. In the meantime, ZrO(g) and CO or C and N2 were re-deposited to form Zr2CN. The formation of Zr2CN took place gradually from the surface of ZrO2 to the inner layer. After all ZrO2 reacted to form Zr2CN, the reaction was achieved.
[5]
[6] [7]
[8]
Acknowledgements [9]
This work was supported by Hebei Iron and Steel Joint Fund, China (No. E2014209273).
[10] [11]
Appendix A. Supplementary data [12]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.079.
[13]
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Study on the reaction mechanism of synthesis of Zr2CN by carbothermal reduction
T
G.Y. Zhang∗, R.L. Wang, L.R. Wang, Y.F. Liu Hebei Provincial Key Laboratory of Inorganic Nonmetallic Materials, College of Materials Science and Engineering, North China University of Science and Technology, Tangshan, Hebei, 063210, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zr2CN Carbothermal reduction method Synthesis
In this study, the Zr2CN powder was fabricated using a carbothermal reduction method under a nitrogen (N2) atmosphere with nanometer m-ZrO2, micron-sized Ca-PSZ and nano-carbon black as raw materials. The product was characterized by X-ray diffractometry (XRD) and energy dispersive spectroscopy (EDS) under a scanning electron microscopy (SEM). The Zr2CN synthesis mechanism was analyzed. The results showed that the formation of Zr2CN phase was attributed to C, N diffusion, C–CO2–CO system equilibrium variation and ZrO2 decomposition; its formation primarily includes four stages of cyclical intersection (solid-gas diffusion and formation of high-temperature stable ZrO2, surface Zr2CN formation, ZrO2 partial decomposition and vapor deposition, inner layer Zr2CN formation and surface layer Zr2CN Oxidation). The four stages were cycled until the end of the ZrO2 formation of Zr2CN.
1. Introduction Zr(C,N) exhibits many excellent properties (e.g., chemical stability, high hardness and high melting point) [1]. Besides, it exhibits strong corrosion resistance and friction resistance in its applications. The nonoxides primarily include ZrC, ZrN and Zr2CN. Zirconium non-oxide ceramics, as a novel advanced material, are extensively favored for their unique and excellent properties. For instance, the processing of high hardness materials (e.g., corundum, glass and hard metal) can draw upon ultra-high hardness and corrosion resistance of Zr (C, N) non-oxide. Moreover, Zr (C, N) non-oxide can also be adopted to produce wear-resistant and corrosion-resistant tools, and it can serve as an additive to enhance the strength and corrosion resistance of cemented carbide [2]. Zirconium non-oxide ceramics are also extensively adopted in industries (e.g., electronic and electrical products, aerospace materials, nuclear fuels and specialty refractories). ZrC is a refractory metal carbide of which the crystal is a NaCl-type face-centered cubic structure. The Zr atoms form a close cubic lattice, and the C atoms are located in the lattice octahedral gap, thus forming a simple gap phase (C/Zr atomic radius ratio of 0.148, less than 0.59). The Zr–C bond exhibits a high bond energy of 83.6257 kJ/mol, and its symmetric distribution leads to the formation of a ZrC crystal. Due to the mixture of ionic, covalent, and metallic bonding. ZrC presents excellent properties such as ultra-high hardness (25.5GPa) and melting point (3400°C~3540°C) [3,4]. ∗
ZrN exhibits a crystal structure of a NaCl-type face-centered cubic structure with a lattice constant a = 4.56 Å [5]. The crystal lattice is preferentially oriented along the < 111 > direction and the (111) plane, and it displays a golden metallic luster. ZrN refers to a mixture of metal bonds and covalent bonds. The atoms are connected by Zr^N, exhibiting the characteristics of both metal crystals and covalent crystals. They exhibit high melting point (2980°C), high hardness (9800MPa-19600MPa), abrasion resistance, good corrosion and oxidation resistance (700 °C), excellent thermal conductivity, electrical conductivity and metal reflectance, as well as high superconducting critical temperature (about 10 K) [6–11]. The non-metallic C and N elements can occupy the interstitial sites of the transition metal Zr lattice at the same time to form Zr–C–N ternary compounds, whereas the synthesis and performance studies on Zr–C–N have been limited. The Zr2CN phase existed in the Zr–C–N composite film prepared in the literature [12,13], and the hardness and wear resistance of the film layer were enhanced. The literature reported that the Zr–C–N composite membrane was biocompatible with human body, and coating the composite membrane on invasive surgical instruments can optimize the fatigue life of the component and enhance its mechanical properties and decorative functions [14]. The study on Zr–C–N ternary system largely focused on the membrane layer, while the synthesis of Zr2CN powder is rare. In this present work, Zr2CN powder was prepared using carbothermal reduction method. Also, the synthesis reaction mechanism is
Corresponding author. E-mail address: [email protected] (G.Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.09.079 Received 3 July 2019; Received in revised form 8 September 2019; Accepted 8 September 2019 Available online 12 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Table 1 Chemical composition of raw materials in the experiment. Name
m-ZrO2 Ca-PSZ Nano- carbon black
Chemical composition /% ZrO2+HfO2
CaO
C
Impurity oxide
99.9% 94.2% –
– 4.2% –
– – –
– MgO+Al2O3+SiO2+Fe2O3+Nb2O5 = 1.6% SO3+CaO+Fe2O3+ZnO+PbO+MgO+SiO2+Al2O3 = 96.8%
scale m-ZrO2 is expressed as “m”, and the micro-scale Ca-PSZ as “P”; the “CR” indicates that the proportion of the compound is 1 mol of zirconia /CR/10 mol Carbon black; “TEM” refers to the synthesis temperature/ 10; “H” is the holding time. For instance: the sample number is m201604, implying that the zirconia raw material is m-ZrO2, the compounding ratio is 1 mol zirconia /2.0 mol carbon black, the synthesis temperature is 1600 °C, and the holding time is 4 h.
discussed in detail. 2. Experimental 2.1. Raw materials The raw materials employed in this study included nano carbon black (median diameter d50 = 50 nm), nanometer m-ZrO2 (median diameter d50 = 50 nm), and micron-sized Ca-PSZ powder (particle size < 74 μm). The chemical composition of the experimental materials (fluorescence X-ray analysis results) is listed in Table 1, and the SEM topography is illustrated in Fig. 1. The atmosphere required for this study was high purity N2 gas (purity N2 > 99.99%).
2.3. Testing and characterization XRD and SEM-EDS analysis were conducted on the samples after being burnt. Besides, the reactant phase in the sample was identified, and the relative percentage of Zr2CN in the product was obtained. The formation of Zr2CN reaction was discussed, and the synthesis reaction mechanism was studied.
2.2. Experimental process
3. Results and discussion
The ingredients are proportioned with nanometer m-ZrO2, micronsized Ca-PSZ and nano-carbon black as raw materials and according to n(ZrO2): n(C) = 1:2, 1:2.5, 1:3, 1:3.5 and 1:4. The batch materials were mixed using the wet method for 4 h with anhydrous ethanol as solvent, and the slurry was dried at 110 °C for 24 h. The dried powder was repeatedly sieved through a 74μm (200 mesh) standard sieve 5 times to produce the ingredients to fabricate Zr2CN. The mixed powder was placed in a mortar of appropriate size, and 12% to 15% of a PVA binder at a mass concentration of 2% was added for grinding. Then, the powder was uniformly placed in a pellet for 2 h. The batch mixed powder was pressed into a blank sample with a size of ϕ 30 mm × 10 mm under a hydraulic press and a molding pressure of 65 KN (92MPa). The green body was dried in an oven at 35 °C for firing. The green body was buried and evacuated, and then it underwent carbothermal reduction synthesis of Zr2CN powder in a box atmosphere furnace at the flow rate of nitrogen about 100 ml/min. The heating rate in the furnace was 5 °C /min below 1000 °C, and 3 °C /min above 1000 °C. The reaction temperatures were 1500 °C, 1550 °C, 1600 °C and 1650 °C, respectively, and the holding time was 3 h, 4 h and 5 h in graphite powder. The experiment unified the numbering of each sample as a set of alphanumeric characters in the format of MCRTEMH. Among them, the initial letter “M” denotes the type of zirconia raw material; the nano-
With nanometer m-ZrO2, micron-sized Ca-PSZ and nano-scale carbon black as raw materials, at ZrO2/C molar ratio of 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4 for the proportion of ingredients, the blank samples of each batch were placed in a high-temperature atmosphere electric furnace after being batched, mixed, formed and dried in a dynamic high-purity N2 atmosphere under the 1500 °C × 4 h, 1550 °C × 3 ~ 5 h, 1600 °C × 3 h ~ 5 h, 1650 °C × 3 h ~ 5 h of the fire. Given the results of high temperature reaction of ZrO2–C–N2 system, the formation of Zr2CN phase was discussed by XRD, SEM and EDS. The results showed that the formation of the Zr2CN phase was attributed to the following four stages. 3.1. Solid-gas diffusion and high temperature stable ZrO2 formation Taking P251555 as an example, the results of SEM-EDS analysis of typical variations in surface characteristics during Ca-PSZ particle reaction are shown in Fig. 2. The SEM-EDS analysis results (Fig. 2) suggest that there are two adjacent regions of “depression” and “bump” in the microscopic portion of the surface of the Ca-PSZ particles not fully reacted to form granular crystals. The EDS analysis results (Fig. 2(b)) suggested that the “depression” area contained more C, O and less N, while the “bump” area
Fig. 1. SEM pictures of raw materials morphology. 1112
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Fig. 2. SEM-EDS analysis of Ca-PSZ typical particle surface in the reaction process.
had C, O, N content opposite to the “depression” area. The analysis revealed that the formation of the “depression” and “bump” regions might be due to three reasons: first, the crystal transformation of ZrO2, i.e., the volumetric shrinkage effect of ZrO2 from low-density to high-density conversion; second, the surface of ZrO2 particles in different microscopic regions had different degrees of contact with C and N2; third, the difference in the difficulty of diffusion of C and N in different microscopic regions. The comprehensive results determined the degree and the speed of ZrO2 crystallization stabilization. The ZrO2 part with high degree of stabilization and high speed formed a “depression” zone, and the “bumping” area was often formed between two “depression” zones with different shrinkage centers. O in the displaced ZrO2 led to the formation of O2 and reacted with C in the environment to form CO2, which is expressed as:
ZrO2(s) + C (s)+N2(g) = ZrO2 − x−y C x Ny □ x+y(s)+ O2(g)
C(s)+ O2(g) = CO2(g)
Fig. 3. XRD analysis results of sample m251555.
the ZrO2 particles. In the experiment of high temperature reaction of ZrO2–C–N2 system, raw materials employed were nanometer m-ZrO2 powder, micron-sized Ca-PSZ powder and nano carbon black, m-ZrO2+C and CaPSZ+C were mixed in different ratios of ZrO2/C molar ratio of 1:1.5 and 1:2.25, and PVA was adopted as a binder, which was mixed, shaped and dried to prepare a batch blank sample. Given the results of TG-DSCMS analysis of the mixture of m-ZrO2+C and Ca-PSZ+C in high purity N2 atmosphere, the temperature condition of 1350 °C × 4 h was taken as the ZrO2–C–N2 system batch. The temperature system of the sample was burnt, and the blank samples of each batch were burnt in high-
3.2. Formation phase of surface Zr2CN phase The results of XRD analysis of sample m251555 are shown in Fig. 3; Fig. 4 shows the results of SEM-EDS analysis of crystals and regions covering only Zr, C, and N elements on the surface of samples m251555 and P251555. The XRD analysis results (Fig. 3) and the SEM-EDS analysis results (Fig. 4) revealed that the phase in Fig. 4(a) was Zr2CN, and the rib portion on the surface of the particle shown in Fig. 4(b) was a Zr–C–N polymer which has not yet led to the formation of a certain crystal topography. In other words, Zr2CN could be formed on the surface of 1113
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Fig. 4. SEM-EDS analysis of m251555 and P251555 surface crystal area containing only Zr, C, N.
Fig. 5. XRD analysis of m-ZrO2+C and Ca-PSZ+C batch sample sintered at 1350 °C × 4 h
The results of the XRD analysis (Fig. 5) and the SEM-EDS analysis (Fig. 2) suggest that the high temperature reaction of the ZrO2–C–N2 system began with the diffusion of C and N, and the study by Sondhi et al. [15] showed that the diffusion depth of C in ZrO2 material did not exceed 50 μm, so the formation of Zr2CN phase should primarily be prioritized over the surface of ZrO2 particles.
purity N2 atmosphere; The results of XRD analysis of m-ZrO2+C and Ca-PSZ+C batch samples after being burnt at 1350 °C × 4 h are shown in Fig. 5. The XRD analysis results of the m-ZrO2+C batch sample (Fig. 5(a)) suggested that the new phase tetragonal t-ZrO2 was formed in the sample after being burnt at 1350 °C × 4 h, and the intensity of the diffraction peak increased with the rise in the amount of carbon black. This implied that the reaction of ZrO2 with C and N2 in the ZrO2–C–N2 system was not an interfacial contact reaction, whereas it was achieved by interdiffusion. According to the analysis, the C and N in the environment were diffused internally to ZrO2, and the O in ZrO2 diffused to the external environment. The position of part O in ZrO2 was replaced by C and N, resulting in lattice distortion of ZrO2, making the monoclinic part m-ZrO2 converted into tetragonal t-ZrO2; O was internally diffused by ZrO2 to react with C to form CO2 and release heat. The XRD analysis results of the Ca-PSZ+C batch sample shown in Fig. 5(b) suggested that the sample phase formed a full cubic c-ZrO2 after being burnt at 1350 °C × 4 h. Therefore, it can be further explained that the diffusion of C and N into ZrO2 could not only convert m-ZrO2 into t-ZrO2, but also convert t-ZrO2 into a c-ZrO2.
3.3. ZrO2 partial decomposition and vapor deposition formation stage 3.3.1. ZrO2 partial decomposition Fig. 6 shows the step topography resulting from the decomposition of ZrO2 on the surface of the sample particles. Due to the existence of C and the displacement of O2 attributed to the diffusion of C and N into ZrO2 at high temperature, C was oxidized to CO2. At high temperature, there must be a balance problem of C–CO2–CO system in ZrO2–C–N2 system, i.e., there was a reversible chemical reaction:
C+ CO2 ⇄ 2CO
(1)
According to David et al. [16] in the study of the mechanism of ZrC synthesis, the presence of CO can lead to ZrO2 decomposition: 1114
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Fig. 6. SEM analysis of step morphology produced by the decomposition of ZrO2 in m301655.
ZrO2(s) + CO(g) = ZrO(g) + CO2(g)
(2)
and form a spherical aggregate containing Zr, O, C, N and other elements. Since the m-series sample is a nanometer m-ZrO2+C batch sample, ZrO2 exhibited a large specific surface area and high activity, and ZrO2 was easy to decompose to produce more ZrO(g), which would form more whiskers and globular aggregates containing elements (e.g., O, C, and N).
The results of the SEM-EDS analysis (Fig. 2) revealed that cracks are often formed between the two “recessed” regions due to volume shrinkage in the formation of the high-temperature stable ZrO2. The presence of this crack allowed CO to enter the interior of the ZrO2 particle. When CO entered the interior of ZrO2 particles, the reverse reaction of formula (1) could occur due to the imbalance of C–CO2–CO system and lead to carbon deposition, laying a material foundation for the internal circulation of C and N to ZrO2. Besides, decomposition reaction of ZrO2 as shown in formula (2) might occur, resulting in decomposition of ZrO2 and formation of a ZrO(g) gas phase product. The decomposition reaction of ZrO2 must occur at its surface or at the crack, i.e., the region in contact with the CO gas phase will leave a disassembly step opposite to the ZrO2 crystallization. The SEM analysis results of the step shown in Fig. 6 demonstrated the decomposition reaction of ZrO2 in the ZrO2–C–N2 system at high temperature.
3.4. Partial oxidation of surface layer Zr2CN and formation stage of inner layer Zr2CN In the synthesis of Zr2CN, the results of XRD analysis suggested that with the rise in temperature, the holding time and the appropriate amount of C were prolonged and increased, the overall trend of macroscopic statistics was that Zr2CN synthesis rate was improved, whereas there were often irregular variations. SEM-EDS analysis suggested that the Zr2CN phase formed on the surface of the raw material particles had a secondary oxidation phenomenon with the rise in temperature or the prolongation of the holding time. This is probably because as the synthesis of Zr2CN proceeded in the ZrO2 particles, the O2 released by the inner layer reaction partially oxidized the Zr2CN in the surface layer.
3.3.2. Vapor deposit formation Taking burned samples m251555, P251605, P301653, and P401603 as an example, Fig. 7 and Fig. 8 respectively show SEM-EDS analysis results of whiskers and globular aggregates present in the sample. It was found in the experiment that there were a certain number of whiskers (Fig. 7) and spherical aggregates (Fig. 8) in the microstructure of each sample; among them, the number of spheroids and whiskers in the m-series sample more than the P-series samples. From the results of EDS analysis shown in Fig. 7 (c) and Fig. 8 (c), it is known that the whiskers and spherical aggregates formed in the sample were composed of elements e.g., Zr, O, C, and N. The results of the SEM-EDS test (Fig. 7 and Fig. 8) revealed that the formation of whiskers and globular aggregates in the sample originated from the gas phase reaction, which was attributed to ZrO2 decomposition to produce ZrO(g). On the one hand, ZrO(g) and CO, N2 and other gas phase materials could accumulate into the appropriate concentration at the edge of c-ZrO2 or Zr–O–C–N solid solution, react nucleation and grow along the one-dimensional direction to form a kind whisker containing Zr, O, C, N. On the other hand, ZrO(g) and CO, N2 and other gas phase substances could accumulate into a suitable concentration between carbon materials, react nucleation and aggregate
3.4.1. Typical Zr2CN phase morphology A typical SEM topography of Zr2CN in samples m251655, m301655 and P401604 is shown in Fig. 9. The synthesis ratios of Zr2CN in the samples m251655, m301655 and P401604 (Fig. 9) were nearly 99%, 100% and 97%, respectively, and the product purity was high. The SEM analysis results shown in the figure suggest that the Zr2CN grain size in the m-series sample was about 0.5~2μm on average, and the Zr2CN grain size in the P-series sample was about 0.5~3μm on average; the Zr2CN grain shape was mostly spherical and evenly distributed in the sample. 3.4.2. Analysis of anomalous phenomena in the synthesis of Zr2CN According to the XRD analysis and calculation in the synthesis of Zr2CN, the relative content of Zr2CN in sample m301654 after being burnt at 1650 °C for 4 h was about 71%, 15% less than that of Zr2CN in sample m301653 after being burnt at 1650 °C for 3 h; the relative content of Zr2CN in the sample P401605 after being burnt at 1600 °C for 1115
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Fig. 7. SEM-EDS analysis of the whiskers in sample.
Fig. 8. SEM-EDS analysis of globular aggregate in the sample. 1116
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Fig. 9. SEM analysis of typical morphology of Zr2CN and sample particle surface characteristics.
heated at 1650 °C for 5 h. Due to the presence of CO in the synthesis of Zr2CN, ZrO2 was decomposed into ZrO(g) and O2. It was found that as the synthesis reaction of Zr2CN took place deep into the ZrO2 particles, the O2 released by the inner layer reaction could partially oxidize the Zr2CN in the surface layer besides the reaction with C or CO. The variation of the morphology of the sample's surface layer (Fig. 10) should be the result of the continuous oxidation of Zr2CN, and O2 generated by the inner layer reaction caused the surface layer Zr2CN to be oxidized twice in the discharge process. This analysis can better interpret the abnormal variation of Zr2CN content in the sample with the extension of synthesis temperature and holding time.
5 h (about 71%) was 9% less than that of Zr2CN (about 97%) in the sample P401604 after being incubated at 1600 °C for 4 h. Taking m301653 and m301654, as well as P401604 and P401605, as examples, Fig. 10 shows the anomalous phenomena and morphological variations of phase morphology during the Zr2CN synthesis. The SEM analysis results (Fig. 10) suggest that the morphology of the sample m301653 after 3 h insulation at 1650 °C was mostly a spheroid with a smaller size and a higher dispersion, and that of the phase in the sample m301654 after undergoing 4 h insulation at 1650 °C was mostly the spreading of particles; the phase morphology of the sample P401604 after undergoing 1600 °C heat preservation for 4 h was mostly spherical or spheroidal and closely arranged, and the morphology of the sample in P401605 was fragmented after being
Fig. 10. SEM analysis of abnormal phenomenon with the extension of holding time and phase morphology change in the synthesis process of Zr2CN. 1117
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4. Conclusion Zr2CN formation-oxidation in the synthesis reaction was cycled. Zr2CN was first formed on the surface of high-temperature stable ZrO2 particles, oxidized by O2 from ZrO2 partial decomposition and then released. In the meantime, ZrO(g) and CO or C and N2 were re-deposited to form Zr2CN. The formation of Zr2CN took place gradually from the surface of ZrO2 to the inner layer. After all ZrO2 reacted to form Zr2CN, the reaction was achieved.
[5]
[6] [7]
[8]
Acknowledgements [9]
This work was supported by Hebei Iron and Steel Joint Fund, China (No. E2014209273).
[10] [11]
Appendix A. Supplementary data [12]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.079.
[13]
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