Zirconium carbide, hafnium carbide and their ternary carbide nanoparticles by an in situ polymerization route

Abstract

An in situ polymerizable complex method to produce zirconium carbide, hafnium carbide and their ternary carbides at a relatively low temperature (1300 °C) using simple and mainly nontoxic starting reagents is presented. In this aqueous process, citric acid (CA) was used to chelate the metal ion and ethylene glycol (EG) to form a polymerized complex resin. We suggest that, based on the results of FT-IR and ¹³C NMR spectroscopies, a very stable metal–CA chelate complex formed in the starting solution, which was thermally stable upon gelation even up to 350 °C. Immobilization of the metal ion in a rigid polymer can largely guarantee the in situ charring, resulting in carbon adjacent to the metal oxide in the pyrolysed product. The contiguous carbon and metal oxide led to in situ reaction (1100 °C) with a minimum of diffusion, which involved the formation of large numbers of metastable phases. Afterwards, well-defined binary and ternary carbide nanoparticles (∼100 nm) were formed through localized particle coarsening by Ostwald ripening.

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Introduction

Metal carbides of the fourth to sixth groups of the periodic table are, in general, known for their chemically inertness, high melting points and hardness, excellent high-temperature corrosion resistance and strength, and weak damage sensitivity under irradiation.^1–4 As a result, these materials have numerous possible structural applications, which always involve hostile environments.^1,4 In addition; however, these carbides possess a wide range of other ‘added’ properties. For instance, the thermal and electronic conductivities can be at the metallic level.⁵ Moreover, these carbides with a high surface area present intriguing potential as promising candidates for heterogeneous catalysis or catalyst supports.^6,7 Among these carbides, zirconium carbide (ZrC) has a high melting point and excellent corrosion resistance, and in addition, low cost and broad availability, and its chemistry and characteristics have been greatly studied.^8–12 As an analogy to ZrC, HfC has the highest melting point (3890 °C), low oxygen diffusion coefficient, and high phase stability, which make HfC-based composites of high interest for aerospace applications.^2,13–15 However, its scarcity, high cost, and the disadvantage of high-density have limited the broad application of this compound. The use of ternary compounds would be a novel strategy to resolve the problems mentioned above. Moreover, these ternary carbides, rather like metal alloys, may show superior properties to either component alone.^6,16 Nevertheless, these carbides are expensive and not commercially available at nanoparticle size. Nanostructures, because of their unusual features, often cause unpredictable behavior.^17,18

Despite a large number of papers about the synthesis of metal carbides,^19–31 a general environmentally benign and competitive synthetic method to simplify and scale up their production as nanostructures is still urgently needed. In recent years, chemical solution processes such as the sol–gel process^19,20 and the organometallic method²¹ have been regarded as effective methods to synthesize the nanoparticles. The chemical solution process has potential advantages over the conventional solid-state route,^30–32 not only for achieving intimate mixing of the component on an atomic scale to obtain a low conversion temperature, but also for the ability to form fibers and thin films.^22,23 The sol–gel methods have been extensively studied because of their broad availability.^{2,19,20,22–27} However, in most cases of the sol–gel process, the metal alkoxide or modified metal alkoxide were used, which had the disadvantages of high sensitivity to moisture and relatively high costs.^2,20,22,23 Moreover, in some cases, the lack of data to confirm the interaction between the metal and carbon source indicates that evidence of atomic-scale mixing is always empirical.^26,27 Therefore, there is a general consensus today that an ideal liquid precursor for metal carbide formation is an organometallic polymer^21,28,29 containing a direct metal–carbon bond, rather like polycarbosilane and polysilazane. However, the need to use Schlenk techniques, the toxicity of the solvents and reagents, and high costs are key points which limit its further application. Drawing inspiration from the Pechini method,³³ which has been extensively used for the preparation of oxide particles and films,^34–36 an in situ polymerization route is put forward to circumvent the problems mentioned above. This route possesses an easy handling technique and, moreover, uses water as the solvent. Although the Pechini method could, in theory, be adapted to the synthesis of sulfides, carbides, or nitrides, to our knowledge, no such synthesis has ever been verified by experiment.

Herein, we use citric acid (CA) as a chelate ligand and ethylene glycol (EG) as a cross-linker to produce zirconium, hafnium and their ternary carbide precursors. We demonstrate a simple, safe, and environmentally benign method to prepare these precursors, bypassing the complicated organometallic method. Via this aqueous polymerized complex route, homogeneous nanostructured carbides could be produced. At the same time, formation of the in situ polymerizable complex, in situ charring, and in situ reaction were proved by using FT-IR, ¹³C NMR, TG-DSC, XRD, SEM, and TEM techniques.

Experimental section

Materials

Citric acid monohydrate (CA), ethylene glycol (EG), and zirconium oxychloride octahydrate (ZrOCl₂·8H₂O) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Hafnium oxychloride octahydrate (HfOCl₂·8H₂O) obtained from China New Metal Materials Technology Co., Ltd. was used as hafnium source. Distilled water was used as the solvent for all experiments, and no other reagents were needed for our experiments.

Precursor preparation and pyrolysis

A general procedure for precursor preparation was as follows. Appropriate quantities of metal salt (ZrOCl₂·8H₂O or HfOCl₂·8H₂O or both), CA, and EG were dissolved in distilled water under stirring at room temperature. Clear solutions were obtained in more than ten minutes, which were used as the metal carbide precursors. Before the main experiments, several trial and error experiments were conducted to optimize the molar ratios of metal salt, CA, and EG. Based on these results (Fig. S1†), we set the CA : EG : metal molar ratio to 0.5 : 1 : 1 in the following experiments. As far as the ternary carbide precursors were concerned, the metal source was comprised of zirconium and hafnium salts in the molar ratios 0.7Zr : 0.3Hf, 0.5Zr : 0.5Hf and 0.3Zr : 0.7Hf. For simplicity, abbreviations were used for naming the precursor. For example, CZ and CH denoted the ZrC and HfC precursors, respectively, and CZ_0.7H_0.3 means the ternary carbide precursor with a 0.7Zr : 0.3Hf atom ratio, in addition, the bare sample indicates the precursor only without addition of metal source.

All precursor solutions were placed in alumina crucibles, and then subjected to heat treatment at 150 °C to form dried gels. The gels were then put into a graphite furnace which was heated in flowing argon (purity 99.999%) to the desired temperatures at 6 °C min⁻¹ and then under vacuum (∼20 Pa) at the final temperature for 2 h. Upon cooling to room temperature, black powders were obtained.

Details of characterization techniques are provided in the ESI.†

Results and discussion

Solution preparation of carbide precursor

In our carbide precursor system, universal and mainly nontoxic chemical reagents were used. In addition, a very important practical and environmentally beneficial aspect of this method is connected with the use of water as an economical and safe solvent and the easy handling of the technique (no inert atmosphere is required). Furthermore, the precursors are highly stable toward hydrolysis and acidic environment in their chelated structure (confirmed by the following spectroscopy results) and therefore have potential applications for the inexpensive and convenient syntheses of a variety of carbide materials for industrially important products. A further advantage of this method is that the viscosity and polymer molecular weight can be tailored by varying the citric acid : ethylene glycol (CA : EG) ratio and the synthesis temperature, which allows the control of the rheological parameter of the solution. Thus, this precursor system could be applied to not only the synthesis of particles, but also to the preparation of fibers, coatings and films.

Zirconium carbide

To obtain better insight into the chemical behavior of the precursor and its transformation under heat treatment, FT-IR and NMR spectra were obtained for chemical reagents, the bare sample, ZrC precursor, and selected samples after heat treatment at selected temperatures. It is well known that Zr and Hf are very chemically similar elements, so here the ZrC precursor is selected as a typical sample.

Fig. 1 shows FT-IR spectra for chemical reagents, bare CZ, and CZ. Heat treatment at 105 °C for 6 h gives a white dried gel for CZ and a viscous liquid for bare CZ, which are all believed to be free of free water. The FT-IR spectra of ZrOCl₂·8H₂O, CA, and EG are in agreement with literature data.^9,37 For the bare CZ sample, the band for ν_as(C=O) of the carboxylate groups slightly shifts to 1732 cm⁻¹, compared to the 1720 cm⁻¹ for the as-received CA, which is attributed to ester formation between EG and CA. In addition, the bands at 1384 cm⁻¹ for ν_s(C=O), 1191, 1073 cm⁻¹ for ν_as(C–O) and ν_s(C–O) are all the characteristic bands of the ester group, which indicates that the heat treatment at 105 °C significantly promotes the esterification reaction. Compared with the bare CZ, the FT-IR spectrum of CZ at 105 °C presents significant differences, especially for the carboxylate carbonyls. Specifically, the antisymmetric stretching vibrations ν_as(C=O) for the carboxylate carbonyls present as doublet at 1628, 1559 cm⁻¹ (Fig. 1e). The symmetric vibration ν_s(C=O) for the same groups appears at ∼1394 cm⁻¹. The vibrations ν_as(C=O) shift to lower wavenumbers in comparison to the corresponding vibrations of bare CZ, indicating changes in the vibrational status of the citrate ligand upon coordination to the zirconium. The differences, Δ(ν_as(C=O) − ν_s(C=O)), are 234 cm⁻¹, which indicates monodentate coordination, and 165 cm⁻¹, indicating a bridging coordination mode of the carboxylate group to two zirconiums.^38–40 Therefore, the FTIR spectra indicate the presence of two kinds of differently coordinating carboxylate groups. However, note that a shoulder band still appears at 1732 cm⁻¹ with weak strength, indicating the existence of a small number of free carboxylate groups without involving coordination. Besides, the 1447 cm⁻¹ band corresponds to the bending of the CH₂ group, and the bands at 861 and 617 cm⁻¹ are believed to be related to the vibrations of Zr–O.⁴⁰ The two bands at 1224 and 1069 cm⁻¹, despite a slight shift for the coordinated feature compared with the bare sample, are assigned to the ν_as(C–O) and ν_s(C–O) of the ester group. However, the ν_s(C–O) band is stronger, going against common sense, wherein the ν(C–O) of the unreacted C–OH group in EG may account for the anomalous strength for ν_s(C–O). Based on the above results, it is believed that a chelated structure is formed; and the zirconium ion is thus immobilized in the rigid gel. However, whether or not the molecular-level mixing will be retained in the following heat treatments still remains to be investigated.

Fig. 1 FT-IR spectra of (a) ZrOCl₂·8H₂O, (b) EG, (c) CA, (d) bare ZrC precursor (bare CZ) and (e) CZ. The bare CZ and CZ for FT-IR spectra were heat treated at 105 °C for 6 h.

FT-IR spectra of CZ and bare CZ after heat treatment at selected temperatures are show in Fig. 2. The spectrum of CZ dried at 150 °C shows infrared bands similar to that dried at 105 °C. The vibration modes of Zr–O still remain in the spectrum, in addition, the bands related to vibrations of the carboxylate carbonyls become sharper without any shifts and can be distinguished very well. Moreover, the ν_s(C–O) band at 1069 cm⁻¹ is still stronger than ν_as(C–O) at 1224 cm⁻¹; here the ν(C–O) of unreacted terminal C–OH of the chelated oligomer may contribute to the strength of ν_s(C–O) partially. Thus, we can confirm that the chelated structure is maintained and stable at 150 °C for a long time. When the precursor is heated up to 250 °C, significant differences appear for the FT-IR spectrum compared to that dried at 150 °C, indicating great changes in the molecular structure. Specifically, the broad band located at ∼3350 cm⁻¹ assigned to ν(O–H) becomes very weak, thus the band at ∼2900 cm⁻¹ corresponding to the ν(C–H) becomes visible. In addition, the band at 1447 cm⁻¹ assigned to bending of the CH₂ group remains and is still strong. The weak shoulder located at ∼1600 cm⁻¹ indicates the occurrence of the alkenes,⁴¹ which needs to be further confirmed. Now, attention is paid to the carboxylate carbonyls. The doublet at 1628, 1559 cm⁻¹ for ν_as(C=O) after heat treatment at 105 °C becomes a singlet after heated at 250 °C. Only 1559 cm⁻¹ remains and is still very strong, indicating that the bonding between the zirconium ions and carboxylate groups changes from two coordinating modes to one mode (bridging) by heating at a higher temperature. The bands at 1224 and 1069 cm⁻¹ corresponding to the C–O vibrations of the ester group disappear, while the band at 617 cm⁻¹ attributed to vibration of Zr–O is retained. The changes of ν(O–H), ν_as(C=O), ν(C–O) and the appearance of alkenes suggest that dehydroxylation and decarboxylation take place, indicating the occurrence of in situ carbonization with maintenance of the coordinated structure. It is astonishing that the coordinated structure is even maintained up to 350 °C after 6 h heat treatment, which can be inferred from the retained weak bands located at 1559, 1394 cm⁻¹ for the bridging mode of the complex. Thus, it is believed that the coordinated structure is maintained while the in situ carbonization is proceeding, suggesting the in situ charring in the pyrolysis process. The FT-IR spectra of bare CZ dried at 105 and 250 °C are shown in Fig. 2e and f. The spectrum of 250 °C shows two noteworthy bands, i.e. 1716 and 1600 cm⁻¹. The former corresponding to carboxylate carbonyls is weak, while the latter assigned to alkene can be visibly observed, which correlates well with literature data.⁴² Therefore, the bare sample could show a different structure evolution during heat treatments.

Fig. 2 FT-IR spectra of (a–d) CZ and (e and f) bare CZ after heat treatment at (a) 105, (b) 150, (c) 250, (d) 350, (e) 105, (f) 250 °C for 6 h. The heat treatments at 250 and 350 °C are under argon atmosphere, while the rest are in air.

Fig. 3 shows the ¹³C NMR spectra of bare CZ and CZ solutions using D₂O as a solvent. For the sake of convenience, Scheme 1 shows the assignment of some NMR peaks to certain carbon centers. The expected ¹³C NMR signals for bare CZ in solution correspond well with the literature data.^43–45 However; some new aspects should be pointed out. Despite the limited reaction time, some extra peaks assigned to carbon centers of esterified derivatives between EG and CA (a₁, b₁, c₁, e₁, and e₂ as defined in Scheme 1) are observed during the dissolution process. It is noteworthy that three signals appear in the “carboxylic acid” carbon regions of ¹³C NMR spectra. The two resonance signals at 173.4 and 171.4 ppm are due to carbons of the two terminal carboxylic acid groups (c₁, c), while one signal located at the lower field region of 176.7 ppm is assigned to the carbon in the central carboxylic acid group (d). This means the preferential reaction of EG with the terminal carboxyl group because of its relatively high molar ratio. After the introduction of zirconium ion, the ¹³C NMR spectrum of CZ solution exhibits significant differences. What is confusing is that the signals of CA are very weak, while the signals of EG are still strong after introduction of zirconium. This may be caused by coordination of zirconium with CA; nevertheless, further details should be investigated. The signal (e) of EG is so strong that it is denoted as a dashed line, no other chemical shifts can be observed, indicating that it is not involved in the complexation. One of the most striking results is that the presence of zirconium ions in CA : EG solution gives rise to one new resonance at 83.2 ppm (b′) in addition to the weak resonance at 73.3 ppm (b) associated with the alcoholic carbons of CA. In many cases in the literature,^43–45 this resonance shifting to lower field region (91 ppm) was attributed to the carbon with a fully deprotonated alcoholic group of CA provided that there is the simultaneous presence of two metal ions. In our case, it is difficult for the ionization of the alcoholic group of CA to take place in acid solutions that are free of metals. However, the coordination of zirconium results in the sharing of the electron density of the oxygen of the alcoholic group, thus weakening O–H bond, and as a result, substantially lowers the pK_a of the alcoholic group of CA. Therefore, the partially deprotonated C–OH is responsible for the resonance at 83.2 ppm. Another striking result is that the fully resolved resonances of the carbons in the carboxylate groups for the metal free CA–EG system turn into one broad signal at 175.9 ppm, showing a small shift compared with the individual resonance of bare CZ in Fig. 3A. These two results have been maintained upon gelation up to 150 °C as verified by solid-state ¹³C NMR of CZ shown in Fig. 4A and B, further confirming the high stability of the molecular structure. These observations suggest that the zirconium ions are bridged by one partially deprotonated alkoxide oxygen atom, together with two terminal carboxylic acid groups in a monodentate coordinating fashion, thus building fused six-membered chelate rings as described in Scheme 1.⁴⁵ This inference also explains the splitting of the peak at ∼43 ppm related to methylene. The relatively broad structure of the resonances assigned to the carbons of carboxylate groups for coordinated citrate can be explained by an equilibrium of coordination isomers. Alternatively, the simultaneous presence of mono-, bis-, tri-, and tetra-Zr(IV)–citrate complexes is evoked as a more general explanation, which can also explain the two kinds of differently coordinating carboxylate groups in the FT-IR results. Thus, it is believed that all the –COOH groups involved in complexation can be regarded as the extreme case (Scheme 1). In the case of Kakihana,⁴⁵ they proposed that esterification occurred between EG and free CA not bonded to metals because of the excess CA in their system. However, in our case, despite a CA : metal molar ratio of 0.5, extra peaks (e′, e′′) assigned to carbon centers of esterificated EG can be visibly observed, which confirms the polymerization reaction of the Zr–CA complex with EG.

Fig. 3 Liquid-state ¹³C NMR spectra of (A) bare CZ and (B) CZ in D₂O.

Scheme 1 The probable molecular structure of CZ and bare CZ in solution.

Fig. 4 Solid-state ¹³C NMR spectra of (A–C) CZ and (D) bare CZ heat treated at (A) 80, (B) 150, (C) 250, (D) 250 °C for 6 h, the heat treatments of 250 °C are under argon atmosphere, while the rest are in air. For definition of symbols marked on peaks, see Schemes 1 and 2.

Solid-state ¹³C NMR spectra of CZ and bare CZ after heat treatment at selected temperatures are shown in Fig. 4. The spectra of CZ heat treated at 80 and 150 °C present similar patterns (Fig. 4A and B). The resonances of the alcoholic carbon (b′) and carbon of methylene (a′, a′′) in CA show little change compared with that of these carbons in solution. The resonances of the carbons of the carboxylate groups dried at 80 and 150 °C present one broad signal at ∼179 ppm, shifting to a lower field region compared with that of these carbons in solution. This can be explained by the decrease of interactions for the dried sample and the further esterification reaction. It is noteworthy that some new changes of the resonances for carbon centers of EG take place after heat treatment. Specifically, the resonances present two broad peaks and both of them are shifted to the lower field region. In addition, the peak located at ∼72 ppm appears stronger while the peak at ∼66 ppm gets weaker with increase of temperature from 80 to 150 °C, which confirms the esterification reaction between CA and EG to form a higher molecular structure. Note that two novel indistinct peaks at ∼20, ∼130 ppm appear for the CZ heat treated at 150 °C, which are attributed to CH₃ and unsaturated carbon (C=C).⁴¹ The coexistence of C=C and CH₃ for CZ heated at 150 °C indicates that the starting of decarboxylation of the aconitic acid leads to itaconic acid derivatives.⁴² After heat treatment at 250 °C, the ¹³C-NMR spectrum of CZ shows significant differences. The resonances of the alcoholic carbon (83.2 ppm) in CA and carbon centers (∼66, ∼72 ppm) of EG disappear, while two new strong peaks at ∼20, ∼130 ppm attributed to CH₃ and unsaturated carbon (C=C) can be visibly observed, which indicates the proceeding of the decarboxylation and dehydration. In addition to the peaks located at ∼20, ∼43, ∼130 ppm attributed to CH₃, –CH₂– and unsaturated carbon (C=C), the resonance of carbons for carboxylate groups are still retained in the ¹³C-NMR spectrum of CZ heated at 250 °C, while the resonances of the same carbons for the bare CZ are weak and not distinct. This means that the existence of the complex inhibits the decarboxylation of citric acid. The dehydration leads to an unsaturated citric acid derivative, which destroys the chelate ring and results in the disappearance of the monodentate coordination mode of carboxylate groups. Thus only the bridging mode remains, which corresponds with the FT-IR results at 250 °C. The decarboxylation, dehydration leading to CH₃ and unsaturated carbon (C=C) indicates the starting of carbonization while the complex structure is maintained. Therefore, the complex structure can be partially maintained at high temperature up to 250 °C, and even up to the temperature before the complete pyrolysis of the precursor inferred from the FT-IR results. Based on the FT-IR and ¹³C-NMR results, a mechanism for the structure evolution of CZ during heat treatment is proposed and shown in Scheme 2. In this scheme, it is proposed that the intermolecular dehydration (Fig. 4A and B) dominates at lower temperatures (<150 °C), resulting in an increase of the molecular weight, however, the existence of the chelate structure inhibits further increase of the molecular weight, which leads to the formation of the oligomer containing the chelated metal. At higher temperatures (>150 °C), intramolecular dehydration takes place, which leads to the disappearance of the resonance for the alcoholic carbon of CA. With the increase of temperature, the thermal elimination reaction of the ester occurs, resulting in the disappearance of resonance for the carbon centers of EG and appearance of new resonances attributed to CH₃ and unsaturated carbons, in agreement with Fig. 4C. It can be deduced from the existence of alkenes that the radical reaction will dominate the following cross-linking behavior, which is accompanied by the in situ carbonization.

Scheme 2 The possible structural evolution of CZ heated at different temperatures.

In the following heat treatments which lead to carbide formation, the thermal behavior, the phase, morphology and structural evolution of the precursor were studied using XRD, TG-DSC, SEM, and TEM techniques. Fig. 5 shows XRD patterns of CZ pyrolysed at various temperatures. The XRD patterns for CZ at 150 and 250 °C have no diffraction peaks, suggesting a completely amorphous state, which corresponds to the resin-like product. When the temperature increases to 350 °C, the XRD spectrum shows very broad lines due to t-ZrO₂, indicating the beginning of crystallization of the oxide phase. The XRD patterns of 650 and 1000 °C indicate a more crystalline state, in addition, at 1000 °C, the t-ZrO₂ has transformed into m-ZrO₂ partially without any carbide phase formation. The ZrC phase appears as a major phase in the products obtained at 1100 °C. After heat treatment at 1200 °C, the precursor has almost completely transformed into ZrC with a little of the ZrO₂ phase as remnants. In the following heat treatments at 1300 and 1400 °C, the XRD patterns of final products show only lines due to ZrC and provide no evidence of any oxide phase, according to a totally non-oxidic material, which has a visible decrease in synthesis temperature in comparison with other works.^{24,26,27,30,31}

Fig. 5 XRD patterns of CZ pyrolysed at varying temperatures.

Since the vacuum environment is difficult to achieve in the DSC-TG testing, we obtain the DSC-TG curves of CZ, graphite and ZrO₂ by simple physical mixing in flowing argon as shown in Fig. 6. The TG curve of CZ indicates three rapid weight loss regions, i.e. 80–160, 220–500 and 1200–1500 °C. The weight loss in the region of 80–160 °C is due to the evaporation of water, including physisorbed water and the water originating from the intermolecular dehydration corresponding to spectra results, which results in an endothermic peak at 105 °C in the DSC curve. A weight loss at 220–500 °C is observed as the intramolecular dehydration, decarboxylation, and finally the carbonization are proceeding in this temperature region. The high temperature weight loss at 1200–1500 °C is believed to be attributed to the carbothermal reduction reactions. The weight loss starts at ∼1200 °C for ZrC precursor, which is about 300 °C lower than for a graphite and ZrO₂ mixture by simple physical mixing (Fig. 6b), confirming more intimate mixing with shorter diffusion distance in the precursor system.

Fig. 6 DSC-TG curves of (a) CZ and (b) graphite and ZrO₂ mixture in flowing argon.

The morphology of powders obtained by pyrolysing at different temperatures is shown in Fig. 7. In particular, SEM allows the homogeneity of the samples to be judged on a larger scale, which is about perfect in our case for the homogeneity of the obtained products. Fig. 7a and b reveals typical coke pyrolysed at 1000 °C. The ZrO₂/carbon coke derived from the precursor is cemented in a form of agglomerated lumps with a large number of smaller particles distributed. Further increase in pyrolysing temperature completely changes the morphology of the product. Fig. 7c and d shows that the agglomerates at 1000 °C disappear after heat treatments at 1100 °C, and are replaced with much small particles (several to several tens of nanometers) without clear crystal forms. This change in morphology might be related to intensive transformation of ZrO₂ to ZrC by carbothermal reduction. Unlike powder annealed at 1100 °C, powders obtained at 1300 °C consist of well-defined, nearly equiaxed crystal particles with narrow size distribution. It can be roughly estimated that the particle size of a sample annealed at 1300 °C lies in the range of 50–150 nm.

Fig. 7 SEM images of CZ pyrolysed at (a and b) 1000, (c and d) 1100, (e and f) 1300 °C.

Given that in situ carbonization was obtained, whether or not the in situ reaction can be achieved still awaits further details on the microstructural evolution to be unveiled. Herein, transmission electron microscopy (TEM, Fig. 8) was used to investigate the morphology and microstructural development. Fig. 8a and b show a TEM picture of the ZrO₂/carbon coke obtained from CZ heated at 1000 °C. XRD measurements show that the zirconium is present as poorly crystalline t-ZrO₂ (major phase) and m-ZrO₂ (minor phase). The TEM image (Fig. 8a) shows that most of the ZrO₂ particles are homogeneously distributed in the carbon matrix. High-resolution transmission electron microscopy (HRTEM, Fig. 8b) shows part of a representative coke particle in detail and reveals that the coke consists of well-crystallized nanoparticles with size ranging from 10 to 20 nm, which are surrounded and isolated from one another by amorphous regions and whose size is consistent with the average size estimated by the Scherrer formula. The lattice spacing is 0.295 nm for (011) planes, giving evidence that the nanoparticles are t-ZrO₂. The coke particles are thus best described as ZrO₂/carbon nanocomposites consisting of metal oxide crystallites, about 10 nanometers in size, highly dispersed within an amorphous carbon matrix. Hence the objective of obtaining the metal oxide and carbon intimately mixed has been achieved by in situ carbonization, which results in shortened diffusion distances, leading to much improved reaction kinetics and hence lower reaction temperatures with corresponding benefits in product particle size. When the samples are heated at 1100 °C, XRD results show that the ZrC becomes the major phase with ZrO₂ as the minor phase. The TEM picture (Fig. 8c) shows a similarity to that heated at 1000 °C despite the heavily aggregated particles. However, the HRTEM image (Fig. 8d) presents significant differences compared to that heated at 1000 °C. The image reveals the boundary areas of several particles. The well-crystallized particle on the upper right corner shows significant grain growth and shares the (110) lattice fringe with interplanar spacing of 0.362 nm, in good agreement with the known value for monoclinic ZrO₂ (ICDD-PDF #65-1025). The left part of the image shows a poorly crystallized particle with many small nanocrystalline inclusions (the dashed areas in Fig. 8d). Moreover, a lattice spacing of d_[111] = 0.260–0.275 nm is observed for these inclusions in spite of more or less deviation from the known value for ZrC (ICDD-PDF #65-4932), giving convincing evidence that these inclusions are ZrC. These ZrC inclusions only show a size of several nanometers with poorly crystallized shape and are isolated by an amorphous phase, which corresponds with the broad peaks of ZrC in XRD pattern. It is notable that these inclusions have different orientations and parts of them involve dislocations as shown in the magnified inset of the image (Fig. 8d). It can be thus concluded that the in situ carbothermal reaction is achieved, which results in these highly disordered metastable intermediate inclusions. In addition, in the edge areas, a lattice spacing of d_[−111] = 0.315 nm assigned to m-ZrO₂ is observed, indicating that the carbothermal reaction is not preferred to take place in these edge areas. Reaction models known as “shrinking core” and “agglomeration and necking” were used to explain the mechanism for this solid-state reaction.⁴⁶ Here in our case, we come up with a new multi-reaction sites theory. Specifically, in the early stage of the reaction, in situ reaction leads to large numbers of highly disordered metastable intermediate phases with size of several nanometers, which have high surface energy and high surface contact. At increasing temperatures, larger well-defined particles are formed and stable phases are developed at the expense of these intermediates based on the Ostwald ripening.⁴⁷ After heat treatments at 1300 °C, the well-defined nanoparticles, just weakly aggregated, can be seen (Fig. 8e), which confirms the Ostwald ripening. The TEM image (Fig. 8e) confirms the absence of amorphous carbon in the final products, revealing that the content of carbon is appropriate. The ZrC produced by our precursor route has particle sizes ranging from 50 to 150 nm by TEM image, which is consistent with the particle size distribution results shown in Fig. S2,† and the surface-area measurements give a value of 29.8 m² g⁻¹. An HRTEM image of ZrC nanoparticles is shown in Fig. 8f. The well-defined, single-crystalline particle has an interplanar distance of 0.270 nm calculated from the picture, corresponding to the d_[111] plane of ZrC bulk. It is worth mentioning the turbostratic parts (dashed circle in Fig. 8f) adjacent to the well-crystallized regions, which have an interplanar distance of ∼0.270 nm assigned to d_[111] plane of ZrC bulk. It can be supposed that this turbostratic ZrC did not have enough time or energy to be arranged as an ordered part, further confirming the Ostwald ripening. The outmost region is surrounded by a thin amorphous carbon layer, which can act as a barrier layer inhibiting the particle growth.

Fig. 8 (a, c, e) TEM and (b, d, f) HRTEM images of CZ pyrolysed at (a and b) 1000, (c and d) 1100, (e and f) 1300 °C.

Hafnium carbide and ternary carbide

As discussed above, we expected to be able to prepare higher-order compositions by incorporating more than one metal in the precursor system. Zirconium oxychloride was thus employed together with hafnium oxychloride in the molar ratios 0.7Zr : 0.3Hf, 0.5Zr : 0.5Hf and 0.3Zr : 0.7Hf. The XRD patterns shown in Fig. 9A confirm that single-phase NaCl-type products have been obtained for all the above metal molar ratios. The inset of Fig. 9A shows the magnified peak located at 2θ = ∼33 degrees. We can see that this peak locates at a lower value for pure ZrC, and then shifts to a higher value with the increase of Hf content, finally showing the highest value for pure HfC. This result corresponds well with the lattice-parameter measurements. The actual measured values of the lattice parameters for pure ZrC, 0.7Zr : 0.3Hf, 0.5Zr : 0.5Hf, 0.3Zr : 0.7Hf ternary systems and pure HfC are 0.4688, 0.4664, 0.4659, 0.4645, 0.4634 nm, respectively. The lattice parameter for ternary (Zr, Hf)C is not very sensitive to the metal stoichiometry owing to the similarity in radius of the metal ions; however, the measured value for the ternary systems does fall between those pure ZrC and HfC and thus the observation of a single phase is indicative of a solid-solution ternary product. The SEM images of the pure HfC and ternary Zr_0.5Hf_0.5C products shown in Fig. 9B and C show well-defined particles with narrow size distribution. Both pure HfC and ternary Zr_0.5Hf_0.5C have particle size of ∼100 nm. The TEM image shown in Fig. 10a confirms the particle size of ∼100 nm for Zr_0.5Hf_0.5C. HRTEM image of Zr_0.5Hf_0.5C nanoparticles shown in Fig. 10b reveals well-defined, single-crystalline particles having an interplanar distance of 0.269 nm calculated from the picture, corresponding to the d_[111] plane of Zr_0.5Hf_0.5C. The coexistence of turbostratic structure and thin amorphous carbon layer show a similarity to ZrC product, confirming the Ostwald ripening mechanism for crystal growth.

Fig. 9 (A) XRD patterns of (a) CZ, (b) CZ_0.7H_0.3, (c) CZ_0.5H_0.5, (d) CZ_0.3H_0.7, (e) CH carbide precursors and SEM images of (B) CH, (C) CZ_0.5H_0.5 precursors pyrolysed at 1300 °C.

Fig. 10 (a) TEM and (b) HRTEM images of CZ_0.5H_0.5 pyrolysed at 1300 °C.

Conclusions

In the present article, a simple in situ polymerization route to produce zirconium carbide, hafnium carbide and their ternary carbides at relatively low temperature (1300 °C) is presented. The FT-IR and ¹³C NMR results indicate that a metal–CA chelate complex is formed in the starting aqueous solution and the metal ions are immobilized in the rigid polymer in the following heat treatments. Meanwhile, the occurrence of in situ carbonization, resulting in in situ charring for the polymer, is confirmed by decarboxylation, dehydration, and alkylation of the complex polymer. In situ reaction (1100 °C) with a minimum of diffusion has thus been achieved. On the basis of above observation, we have come up with a new multi-reaction sites theory, which firstly involves generation of huge numbers of highly disordered metastable phases with size of several nanometers. Afterwards, well-defined carbide nanoparticles are formed through localized particle coarsening by Ostwald ripening. What is more, the ternary carbides of zirconium and hafnium are prepared and show single NaCl-type phase with particle size of ∼100 nm.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 51102282) and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.

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Footnote

† Electronic supplementary information (ESI) available: Characterization techniques, XRD patterns of ZrC precursor with varying CA : metal molar ratios pyrolysed at 1400 °C, and particle size distributions for ZrC particles obtained at 1300 °C. See DOI: 10.1039/c4ra14996c

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