Journal
J. Am. Ceram. Soc., 90 [10] 3085–3090 (2007) DOI: 10.1111/j.1551-2916.2007.01898.x r 2007 The American Ceramic Society
Synthesis of Nanocrystalline Chromium Nitride Powder by Mechanical Processing Concepcio´n Real,w Manuel A. Rolda´n, Marı´ a D. Alcala´, and Andre´s Ortega Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-US, Av. Ame´rico Vespucio No. 49, 41092-Sevilla, Spain
nitrogen or in dry ammonia atmospheres.19,28–33 These milling processes were performed with sealed vials filled with the corresponding gas at known pressures or under a nitrogen gas flow. To our knowledge, only two works have dealt with the preparation of CrN through ball milling of Cr in a nitrogen atmosphere. Calka and Williams28 achieved a partial nitridation of metallic chromium after milling for 300 h in a planar-type ball mill and, after annealing at 8001C for 1 h, produced a composite Cr1Cr2N1CrN, while Ogino et al.30 obtained the metal nitride after a milling time of 180 h in a vibrational-type ball mill. On the other hand, Ren et al.,31 using a modified high-energy milling process called mechanical activation synthesis, prepared CrN under an NH3 atmosphere. In the present study, we have synthesized pure CrN by mechanical treatment of metallic chromium under nitrogen pressure in a planetary mill with a milling time of 50 h, without any post-heating treatment. The CrN has been obtained at three nitrogen pressures and the characterization of the final product is also presented.
The transition metal nitrides such as chromium nitride (CrN) show interesting properties that make them suitable for many technological uses. In the present work, pure nanocrystalline CrN has been directly prepared by mechanical alloying, from chromium metal under a pressurized nitrogen atmosphere, in a short milling time and without post-heating treatment. The characterization of the final product by X-ray diffraction, scanning electron microscopy, electron energy loss , and X-ray photoelectron spectroscopy is presented.
I. Introduction
T
RANSITION metal nitrides have attracted considerable attention in the past few years due to their technologically interesting properties: extreme hardness, abrasive resistance, electrical conductivity, inertness, reflectance, diffusion resistance, wear, corrosion resistance, etc. These compounds are suitable for coating materials, used in industrial catalysts and in composites with other materials, which can improve their properties and even reveal novel applications.1,2 Traditionally, metal nitrides can be prepared at high temperatures through several methods, such as carbothermal reduction and nitridation of metal oxides with graphite in a nitrogen atmosphere, by heating metal powders in an N2 or NH3 flow,3 and by ammonolysis of metal chlorides, oxides, or sulfides.4–8 Some of the methods recently used to synthesize them are: solid-state metathesis,9,10 self-propagating high-temperature synthesis,11 benzene thermal synthesis,12 electrochemical synthesis, plasma nitridation, sputtering, and ion beam deposition.13–15 However, most of these methods require rigorous synthetic conditions and are not economically profitable, because of the use of high temperatures, air-sensitive or toxic reagents, or because they are expensive. On the other hand, particular emphasis should be placed on the use of methods that permit us to obtain chromium nitride (CrN) as a nanocrystalline powder. Previous investigations16–20 had shown that mechanochemical processing can be applied to the synthesis of a wide range of nanoparticulate materials. In recent decades, mechanical milling has received growing interest because it has been shown to be a powerful technique for the synthesis of a great number of novel materials such as amorphous phases, supersaturated solid solutions, metastable phases, intermetallic compounds, composites, and ceramic materials.21–27 Mechanical milling has generally been applied to solid–solid reactions, but it can be also used for gas–solid reactions. The small number of publications concerning solid–gas reactions promoted by grinding is probably due to the difficulty in developing high-energy grinding mills for operating under strictly controlled atmospheres. Metal nitride phases have been obtained by ball milling of elemental powders in
II. Experimental Procedure Chromium powder supplied by Aldrich (Steinheim, Germany) with a purity of 99.5% was milled under pressures of 3, 6, and 11 bars of high-purity nitrogen gas (H2O and O2o3 ppm) using a modified planetary ball mill (Model Pulverisette 7, Fritsch, Idaroberstein, Germany). A steel vial of 45 cm3 was used, with six steel balls and 5 g of Cr powder. The powder-to-ball mass ratio was 1:16. The vial was purged with nitrogen gas several times, and afterward the desired nitrogen pressure was selected before milling. The vial and the gas cylinder were connected through a rotary valve and a flexible polyamide tube, which allows working pressures up to 27 bars. The rotary valve can operate up to 25 000 rpm under pressures ranging from vacuum to 70 bars. In this way, the vial is permanently connected to the gas cylinder that supplies the gas at the desired pressure during the whole process. A spinning rate of 700 rpm for both the rotation of the supporting disc and the superimposed rotation in the opposite direction of the vial was always used. X-ray powder diffraction patterns were collected with a Siemens (Karlsruhe, Germany) D501 instrument equipped with a scintillation counter using CuKa radiation and a primary graphite monochromator; the scanning rate of the goniometer was 0.41/min. The full-width at half-maximum of (200) diffraction peaks of CrN were used to calculate the average diameter of the coherent diffraction domain using the Scherrer method.34 The lattice parameter refinement corresponding to the final product was also calculated from the whole set of peaks of the X-ray diffraction (XRD) diagram using the Lapods computer program35 and assuming a cubic symmetry for CrN. Microstructural observations were conducted using scanning electron microscopy (SEM, model JSM 5400, JEOL, Tokyo, Japan). Photographs were recorded at 30 kV from samples that were dispersed in ethanol, supported on a metallic grid. The observation in high resolution and microanalysis was performed
D. Butt—contributing editor
Manuscript No. 22941. Received March 16, 2007; approved May 22, 2007. w Author to whom correspondence should be addressed. e-mail:
[email protected]
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in a transmission electron microscope (Philips CM200, Eindhoven, the Netherlands) with a super twin objective lens, working at 200 kV with an LaB6 filament. The instrument is equipped with a heating holder, an energy-dispersive X-ray (EDX) detector, and a parallel electron energy loss (EELS) spectrometer (Model 766-2K, Gatan, Mu¨nchen, Germany). In order to record the EELS spectra at the N–K and Cr–L2,3 edges, the illuminated area was ca. 100–150 nm in diameter. Spectra were recorded in the diffraction mode with a camera length of 470 mm, a 2 mm (Cr) and 3 mm (N2) spectrometer entrance aperture, and a collection angle of ca. 1.45 mrad. The energy resolution measured at the zero-loss peak of the coupled microscope/spectrometer system was ca. 1.4 eV. Spectra were corrected for dark current and channel-to-channel gain variation. A low-loss spectrum was also recorded with each edge in the same illuminated area and retaining the same experimental conditions. After the subtraction of the background with a standard power-law function, the spectra were deconvoluted for plural scattering with the Fourierratio method. All these treatments were performed within the EL/P program (Gatan). X-ray photoelectron spectroscopy (XPS) spectra were recorded in a VG-Escalab (Manchester, UK) 210 spectrometer working in the constant analyzer energy mode, with a pass energy of 50 eV and using MgKa radiation as the excitation source. An estimated error of 70.1 eV can be assumed for all measurements.
III. Results and Discussion Figures 1(a)–(c) show the XRD patterns of the chromium samples milled for various times under three nitrogen pressures of 3, 6, and 11 bars, respectively, together with the one corresponding to the starting metal powder. The analysis of the XRD patterns reveals in all cases the formation of CrN from elemental chromium by milling under nitrogen. XRD diagrams from 0 to 7 h of milling show a broadening of the Cr peaks increasing with milling time due to the refinement of the crystallite size, the formation of defects, and microstrains. After milling from 7 to 20 h, the structure of the powder could not be well identified because of broadening in the diffraction peaks. Further milling up to 50 h led to a continuous increase in the intensities of the CrN diffraction peaks. The three samples showed a similar behavior. The considerable broadening of the CrN XRD peaks suggests that this phase was obtained with a very refined microstructure. The crystallite size of the final product was estimated using the Scherrer equation, and the values have been included in Table I. Solid-state reactions induced by ball milling took place during collisions between balls and the powder. The reaction progressed by multiple events with a very short reaction time. Under these conditions, the crystal growth of the new phase was hindered, leading generally to products with a nanometric microstructure. The dimension of the CrN cubic unit cell was determined from the whole set of peaks of the XRD diagram, recorded for 2Y values ranging from 101 to 901, for the samples obtained at different pressures. A least squares fitting of the XRD peaks was carried out with the Lapoud program.35 A value of lattice parameter a 5 4.138 A˚ was obtained for the length of the axis of the unit cell of the whole set of CrN samples obtained by milling under a nitrogen atmosphere of 3 bars, a value quite similar to that of the stoichiometric compound (4.1400 A˚). The sample milled under 6 bar of nitrogen had a value of lattice parameter a 5 4.124 A˚, and the sample milled at 11 bar showed a lower value a 5 4.1137 A˚, suggesting that a nonstoichiometric CrNx compound was obtained on increasing the pressure of nitrogen. Studies of the synthesis of CrN using different methods4,6,12–14,36,37 and for several transition metals of composition MNx9,13,38 have reported that the ‘‘a’’ parameter is very sensitive to the nitridation level. In order to analyze whether the reaction was completed after milling, we have used an indirect method of analysis (thermogravimetry (TG)). TG diagrams for the different chromium ground
Fig. 1. X-ray diffraction diagrams of the chromium milled under nitrogen (a) at 3 bar, (b) 6 bar, and (c) 11 bar for different grinding times.
samples were recorded at a heating rate of 5 K/min, from room temperature to 9501C, using a nitrogen flow of 150 cm3/min, at a pressure of 1 bar. Once 9501C was reached, the temperature was held until the maximum uptake of nitrogen was achieved. The
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Table I. Experimental Conditions and Results of the Characterization for the Three Samples of Chromium Nitride Obtained Sample
Cr 49 h Cr 50 h Cr 50 h
Pressure N2 (bar)
D (nm)
Lattice a (A˚)
N/Cr XPS
N/Cr EELS
11 6 3
5.4 4.9 6.0
4.1137 4.1239 4.1382
0.97 — 1.02
1.05 0.99 0.96
The error of the measurement by XPS and EELS is about 5%. XPS, X-ray photoelectron spectroscopy; EELS, electron energy loss.
conversion of Cr into CrN during the milling process was calculated from the mass gain, assuming a stoichiometric nitride as the final product. The validity of this indirect method was checked with an ungrounded Cr sample. In all cases, after milling for 49 and 50 h, the reaction of nitridation was completed, with no observed differences between them, at least within the detection limit of this method.
Fig. 2. (a) Electron energy loss (EELS) spectra N–K corresponding to the three final products of chromium nitride (CrN) obtained by milling and one standard for comparison. (b) EELS spectra Cr–L2–3 corresponding to the three final products of CrN obtained by milling and one standard for comparison.
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On the other hand, the content of nitrogen in CrN has been calculated from the microscopy analysis, EELS, and in addition to a bulk analysis technique, we have used a surface-sensitive technique like XPS for two of the final products. The EELS technique in transmission electron microscopy is very appropriate for the characterization of nanostructured samples, by recording core-level absorption edges at a microscopic level. Figures 2(a) and (b) show the N–K and Cr–L2–3 edges for the three samples prepared in the present work, and for other standards for comparison purposes, recorded in different areas. The N–K edges were very similar in the three samples obtained at different pressures, exhibiting the same shape, and thereby showing that the samples had the same structure. At the microscopic level, the samples were homogeneous, showing zones with similar atomic ratios of the elements. The composition of the material can be determined by integrating the number of counts under the edges and using relevant cross sections provided by the Gatan EL/P software. Representative results from the quantitative analysis performed from Figs. 2(a) and (b) are shown in Table I. These results are in good agreement with the results of the lattice parameter calculated from XRD. Figures 3(a) and (b) show the higher resolution XPS spectra of CrN (3 and 11 bars) in the N1s and Cr2p regions. The peak
Fig. 3. (a) X-ray photoelectron spectra (XPS) of the nitride N1s corresponding to the samples of chromium nitride (CrN) obtained by milling at 3 and 11 bars of nitrogen pressure. (b) XPS spectra of the nitride Cr2p corresponding to the samples of CrN obtained by milling at 3 and 11 bars of nitrogen pressure.
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Fig. 4. Scanning electron micrographs of the final products of chromium nitride obtained by milling in nitrogen at: (a) 3 bar, (b) 6 bar, and (c) 11 bar.
cores at about 396.8 eV were attributed to N1s of N3 , being in good agreement with the data reported in the literature.4–6,15 The peak cores at 575.7 eV were assigned to Cr2p3; the position of the main peak was compared with the energy corresponding to photoelectrons in metallic chromium (574 eV). The difference observed clearly indicated the formation of nitride on the surface. By measuring the peak areas of the Cr2p and N1s cores, the ratio of N/Cr was estimated; the results have been included in Table I. The resulting composition was in good agreement with that calculated by the EELS technique. Iron contamination arising from the grinding media was not clearly observed in the XRD patterns of the ground samples, but a weak peak, at approximately 44.51 2Y, which would correspond to the most intense diffraction peak of iron, seemed to be found in the samples ground more severely. The chemical analysis of the total iron content showed that contamination with
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this metal was approximately 20% in the sample milled under 11 bars and 6% in the samples milled under 3 and 6 bars of nitrogen, respectively. This contamination was completely removed by rinsing with diluted hydrochloric acid. Morphologic studies of the final products were performed after rinsing. Figures 4(a)–(c) show SEM micrographs illustrating the morphology of the final three products. These figures show that in all cases, the particles are aggregates of 1–5 mm formed themselves by smaller spherical particles of 5–10 nm, as can be seen in Fig. 5(a). The particle size distribution is shown in Fig. 5(b) for the three samples. This crystallite size is in good agreement with the diffracting domain size value found by XRD. TG measurements in air flow showed that the CrN products were stable up to 7001C. Then, they begin to oxidize, being completely oxidized at 10001C. XRD showed that the products consist of Cr2O3 and unreacted CrN. It can be seen that although the samples obtained by ball milling show in general more reactivity, their behavior is in good agreement with other reports in the literature for CrN obtained by thermal4,39 and sputtering methods.40–42 In a previous paper,19 we have reported that milling titanium under a nitrogen atmosphere enhances reactivity toward SHC (self-propagating high-temperature synthesis) processes. This process consists of a short-duration high-energy ball-milling step, followed by a self-sustaining reaction. The adiabatic combustion temperature for the formation of TiN is 46271C. The adiabatic combustion temperature for the formation of CrN is 20631C, and with this value it is also possible to synthesize the CrN using the SHS method, but no combustion reaction was observed in our study. The above results show that the formation of CrN by milling chromium metal under a nitrogen pressure occurs gradually by a diffusion mechanism. First, the particle size of the metal decreases and elastic and plastic deformations repeatedly occur in the powder as the milling time increases. The milling process generates a large number of new rough and presumably highly reactive surfaces that are freshly exposed to molecular nitrogen. This can favor the sorption and enhance the reaction kinetics, making possible the nitridation of the metal. Moreover, high-energy milling produces crystallite refinement and grain boundaries, reduces the length of ordered stacking in some crystallographic directions, and introduces a considerable amount of defects and internal strains. These effects could enhance the diffusion rates and at the same time increase the reaction driving force by increasing the free energy of Cr. Our results show that between 7 and 20 h, the sorption of nitrogen is enough to produce the Cr2N phase. This phase has previously been referred to in the literature13,28,31,43 as an intermediate phase in the nitridation of metal chromium (CrN). After 25 h of milling, the CrN phase is detected by X-ray and increases with the milling time until 50 h, when the formation of CrN is completed. The process is slow because of the diffusion mechanism and is not influenced by the pressure of nitrogen. In summary, we can conclude that mechanical alloying is a complex process and involves optimization of a number of variables that allows achieving both the desired product and microstructure. The planetary ball mill used in the present work produces considerable friction and impact, which are different from those applied in the previous work; this could explain the reduced time necessary for completing the synthesis, in comparison with the results of Ogino et al.,30 and without any posterior heating in contrast to that observed by Calka and Williams.28
IV. Conclusion CrN is completely formed by milling metal chromium under a nitrogen atmosphere for a short time, without the need for any posterior heating treatment. In spite of the particle nanosize (6 nm) the resistance to oxidation is similar to that reported in other studies. This procedure has the advantage of a low cost, small particle sizes, and narrow size distribution, as compared with other methods used in the preparation of this type of
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Fig. 5. (a) Transmission electron micrographs (TEM) obtained showing the particles that constituted the agglomerate obtained at 3 bar, (b) distribution of particle sizes from measurements of TEM micrographs.
compound. Also, the EELS technique is suggested to be a good tool to study the evolution of the formation of metal nitride, in those intermediate milling times when the X-ray diagrams cannot show the structure of the samples because of broadening of the peaks.
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