Preparation of Vanadium Nitride Using a Thermally Processed ... - MDPI

metals Article

Preparation of Vanadium Nitride Using a Thermally Processed Precursor with Coating Structure Jingli Han 1,2,3 , Yimin Zhang 1,2,3,4, *, Tao Liu 1,2,3 , Jing Huang 1,2,3 , Nannan Xue 1,3 and Pengcheng Hu 1,2,3 1

2 3 4

*

College of Resource and Environment Engineering, Wuhan University of Science and Technology, Wuhan 430081, China; [email protected] (J.H.); [email protected] (T.L.); [email protected] (J.H.); [email protected] (N.X.); [email protected] (P.H.) Hubei Collaborative Innovation Center of High Efficient Utilization for Vanadium Resources, Wuhan 430081, China Hubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Wuhan 430081, China College of Resource and Environment Engineering, Wuhan University of Technology, Wuhan 430070, China Correspondence: [email protected]; Tel.: +86-027-6886-2057

Received: 30 July 2017; Accepted: 5 September 2017; Published: 11 September 2017

Abstract: A new effective method is proposed to prepare vanadium nitride (VN) via carbothermal reduction–nitridation (CRN) of the precursor, obtained by adding carbon black (C) to the stripping solution during the vanadium recovery from black shale. VN was successfully prepared at a low temperature of 1150 ◦ C for only 1 h with a C/V2 O5 mass ratio of 0.30 in N2 atmosphere, but a temperature of 1300–1500 ◦ C is required for several hours in the traditional CRN method. The low synthesis temperature and short period for the preparation of VN was due to the vanadium-coated carbon structure of the precursor, which enlarged the contact area between reactants significantly and provided more homogeneous chemical composition. In addition, the simultaneous direct reduction and indirect reduction of the interphase caused by the coating structure obviously accelerated the reaction. The phase evolution of the precursor was as follows: (NH4 )2 V6 O16 ·1.5H2 O → V2 O5 → V6 O13 → VO2 → V4 O7 → V2 O3 → VC → VN. The precursor converted to V6 O13 and VO2 completely after being calcined at 550 ◦ C, indicating that the pre-reduction of V2 O5 in the traditional CRN method can be omitted. This method combined the synthesis of VN with the vanadium extraction creatively, having the advantages of simple reaction conditions, low cost and short processing time. Keywords: vanadium nitride; precursor; phase evolution; nitrogen content; black shale

1. Introduction Vanadium nitride (VN) has received increasing attention in recent years due to its typical properties including extreme hardness, high melting point, wear and corrosion resistance, good electric and thermal conductivity, high-temperature stability, as well as good catalytic activity. It can be widely used as iron and steel additive [1,2], electrode [3,4], catalyst [5,6], superconductor [7], and coating [8,9]. The frequent application of VN, as an important steel additive, can be ascribed to its beneficial effect on fine grain and precipitation strengthening, which can improve the comprehensive performance of steel and reduce the cost of the smelting process by using nitrogen to reduce the vanadium content of steel [10]. The traditional procedure for the synthesis of VN is the process of carbothermal reduction– nitridation (CRN) of V2 O5 in N2 at a high temperature of 1500 ◦ C for 3 h with a N2 flow rate of 13.3 × 10−6 m3 · s−1 [11], and at 1400 ◦ C with a N2 flow rate of 50 L·h−1 , using a microwave method [12]. The low melting point (670 ◦ C) and the high saturation vapor pressure of V2 O5 will result in a volatile loss of V2 O5 near its Metals 2017, 7, 360; doi:10.3390/met7090360

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melting point. Hence, the pre-reduction during transformation from V2 O5 to a high melting point and low-valent vanadium oxides below the melting point of V2 O5 is necessary [13], resulting inevitably in a long reaction period. However, using the low-valent vanadium oxides will increase the production cost due to the higher price compared to V2 O5 . In addition to the traditional CRN method, VN was prepared by thermal liquid-solid reaction [14]: Ammonolysis of precursor compounds of metal [7], self-propagating high-temperature synthesis [15], sol-gel method [16], chemical vapor deposition [17], mechanical alloying [18] and other methods [19,20]. However, most methods are plagued by the presence of air-sensitive or toxic reagents, expensive equipment and raw materials, elevated temperatures and long reaction time, which limit its large-scale production and application. Therefore, there is a need for a simple synthesis route of VN. Thermal processing precursor, as a novel method to prepare metal nitrides, has gradually become a research hotspot. Zhao et al. [21] synthesized VN nanopowders by thermal nitridation of the precursor obtained by physically drying an aqueous solution of ammonium vanadate (NH4 VO3 ) and nanometer carbon black. (Cr,V)2 (C,N) solid solution powders were synthesized by CRN of the precursor, prepared by heating admixtures of ammonium dichromate ((NH4 )2 Cr2 O7 ), NH4 VO3 , and carbon black in distilled water with continuous stirring [22]. These methods can reduce the synthesizing temperature greatly, but the production cost remained high due to the use of de-ionized water, NH4 VO3 and nanometer carbon black, indicating the strong demand of a simple, low-cost and low-temperature approach to synthesize VN. Moreover, the formation process and microstructure of the precursor and its promoting effect on the preparation of metal nitrides were not discussed. In China, most of the vanadium resources exist in the form of black shale, so the utilization and exploitation of black shale is an essential way to recover vanadium [23]. The vanadium recovery from black shale was investigated using a pyro-hydrometallurgical process specifically including roasting, acid leaching, purification, and chemical precipitation [24–26]. Solvent extraction was the most commonly adopted process to purify the acid leaching solution for better selectivity and economy [27,28]. After solvent extraction and stripping, vanadium can be precipitated from the stripping solution with acidic ammonium salt [29]. In view of the preparation method of precursor [7,21,22] in the synthesis of VN, and the application of the precipitation method [30] to synthesize Pb(Mg1/3 Nb2/3 )O3 ceramic, it may be feasible to prepare the precursor during the vanadium precipitation from the stripping solution in the process of vanadium extraction from black shale, to synthesize VN. This method revealed the merits of a low reaction temperature, short period, and a widely available, cheap vanadium source, which make it more practical, economical and efficient in industrial applications. In this study, we combined the synthesis of VN with vanadium extraction from black shale for the first time by adding carbon black (C) to the stripping solution. First, the precursor containing the vanadium source and reducing material formed gradually during the vanadium precipitation, then the precursor was reduced and nitrided in the N2 atmosphere to yield VN. The effects of the main reaction conditions on the nitrogen content (N content) in VN product were investigated. In order to explain the lower temperature and shorter time for the preparation of VN in comparison with the conventional CRN method: The phase and microstructure of the precursor were analyzed. In addition, the mechanism of preparing VN from the precursor was discussed through phase analysis, thermodynamic calculation, and thermogravimetric (TG) experiment. 2. Experimental Section 2.1. Materials The stripping solution was obtained by blank roasting, acid leaching, solvent extraction and stripping of black shale obtained from Tongshan, China. The main chemical composition of the stripping solution is given in Table 1, the concentration of V in the stripping solution is 20.63 g·L−1 , while the major impurity ion is Al with the content of 5.78 g·L−1 . The pH of the stripping solution is

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0.03. Carbon black containing 94.80% of carbon with the particle size of less than 0.074 mm was used as the reducing agent. Metals 2017, 7, 360 3 of 12 Table 1. Main chemical composition of the stripping solution. Table 1. Main chemical composition of the stripping solution.

Element Element V −1 −1·)L ) 20.63 Concentration Concentration (g·L(g

VAl 20.63 5.78

Al Fe 5.78 0.06

Fe K 0.06 0.34

K Na 0.340.42

Na P 0.42 0.1

P 0.1

2.2. Typical Typical Procedure Procedure for for the the Preparation PreparationofofVN VN 2.2. The process process flow flow sheet sheet to to prepare prepare VN VN is is shown shown in in Figure sodium chlorate chlorate (NaClO (NaClO3,, The Figure 1. 1. Typically, Typically, sodium 3 AR (analytical (analytical reagent), reagent), 2.95 2.95 g) g) was was added added to to the the stripping stripping solution solution (200 (200 mL) mL) to to oxidize oxidize vanadium vanadium AR (IV) to to vanadium vanadium (V) (V) in magnetic stirrer Keer Instrument Instrument (IV) in aa temperature-controlled temperature-controlled magnetic stirrer (SZCL-2A, (SZCL-2A, Wuhan Wuhan Keer Co., Ltd., Wuhan, China) at room temperature. After complete oxidation, a certain amount of carbon carbon Co., Ltd., Wuhan, China) at room temperature. After complete oxidation, a certain amount of black was pH of of thethe solution was adjusted to 1.8 black was added added to to the thesolution solutionand anddispersed dispersedby bystirring. stirring.The The pH solution was adjusted to with ammonia (AR), and then the solution was stirred to precipitate ammonium polyvanadate (APV) 1.8 with ammonia (AR), and then the solution was stirred to precipitate ammonium polyvanadate at 90 °C for◦ C 1 for h. After solid-liquid separation by by vacuum filtration Keer (APV) at 90 1 h. After solid-liquid separation vacuum filtration(model (modelSHB-III, SHB-III,Wuhan Wuhan Keer Instrument Co., Ltd., Wuhan, China), the precursor was washed by water, dried in an oven, and then Instrument Co., Ltd., Wuhan, China), the precursor was washed by water, dried in an oven, and then compressed into cylindrical blocks of 30 mm diameter at a pressure of 14 MPa. The cylindrical block compressed into cylindrical blocks of 30 mm diameter at a pressure of 14 MPa. The cylindrical block was placed placed into into the the tubular tubular atmosphere atmosphere furnace furnace (SGL-1700, (SGL-1700, Shanghai Shanghai Institute Institute of of Optics Optics and and Fine Fine was Mechanics, Shanghai, China) with a certain flow rate of nitrogen (99.999%). The sample was calcined Mechanics, Shanghai, China) with a certain flow rate of nitrogen (99.999%). The sample was calcined to deaminize deaminize at at 550 550 ◦°C for 20 20 min min and and pre-reduced pre-reduced at at 650 650 ◦°C for 2.5 2.5 h, h, and and then then heated heated to to the the desired desired to C for C for temperature for for reduction reduction and and nitridation nitridation for for aa certain certain of of time. time. Subsequently, the product product was was cooled cooled temperature Subsequently, the to room temperature in nitrogen atmosphere. to room temperature in nitrogen atmosphere.

Figure Process flow Figure 1. 1. Process flow sheet sheet for for preparation preparation of of vanadium vanadium nitride nitride (VN). (VN).

2.3. 2.3. Material Material Characterization Characterization The The nitrogen nitrogen content content (N (N content) content) was was determined determined employing employing distillation-acid distillation-acid base base titration titration analysis. The X-ray diffraction (XRD) patterns were obtained by using a Rigaku D/MAX-RB analysis. The X-ray diffraction (XRD) patterns were obtained by using a Rigaku D/MAX-RB X-ray x-ray diffractometer (Rigaku, Akishima, Akishima,Japan) Japan)with withCu CuKα Kαradiation radiationtotoanalyze analyze the phase compositions diffractometer (Rigaku, the phase compositions in in products. Microscopicobservation observationand andelemental elementalanalysis analysis(SEM–EDS; (SEM–EDS; SEM: SEM: Scanning Scanning electron thethe products. Microscopic electron

microscope; EDS: Energy-dispersive X-Ray spectroscope) was conducted using a JSM-IT300 scanning electron microscope (JEOL, Tokyo, Japan), equipped with an X-ACT energy dispersive spectrometer (Oxford Instruments, Oxford, UK). The thermogravimetric (TG) experiment was carried out by utilizing a STA449C analyzer (Netzsch, Selb, Germany) to change from room temperature to 1400 °C with a linear heating rate of 10 °C·min−1 under N2, and a gas flow rate of 50 mL·min−1.

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microscope; EDS: Energy-dispersive X-Ray spectroscope) was conducted using a JSM-IT300 scanning electron microscope (JEOL, Tokyo, Japan), equipped with an X-ACT energy dispersive spectrometer (Oxford Instruments, Oxford, UK). The thermogravimetric (TG) experiment was carried out by utilizing a STA449C analyzer (Netzsch, Selb, Germany) to change from room temperature to 1400 ◦ C with a Metals 2017, 7, 360 4 of 12 linear heating rate of 10 ◦ C·min−1 under N2 , and a gas flow rate of 50 mL·min−1 .

3. Results and Discussion 3. Results and Discussion 3.1. Effect Effect of of the the Main Main Reaction Reaction Conditions Conditions on on the the N N Content Content in in VN VN Product Product 3.1. 3.1.1. Effect of C/V C/V22OO5 5Mass MassRatio Ratio The effect of 2OO ratio (m(C:V 2O5))Oon content in VNinproduct is shown in Figure of C/V C/V ratio (m(C:V on N the N content VN product is shown in 2 5 mass 5 mass 2 5 ))the ◦ 2, obtained at a reaction temperature of 1250 a reaction time of 3 h, a N2aflow rate rate of 300 Figure 2, obtained at a reaction temperature of°C, 1250 C, a reaction time of and 3 h, and N2 flow of mL·min . −1 . 300 mL· −1min

2. (a) Effect Effect of of C/V C/V22O Figure 2. O55mass massratio ratioon onthe theN Ncontent contentin in VN VN product product with with a reaction temperature flow rate rate of of300 300mL mL·min of 1250 1250 ◦°C, reaction time time of of 33hhand andN N22 flow of C, reaction ·min−−11;; (b) (b) XRD XRD (X-ray (X-ray diffraction) diffraction) patterns the products products obtained obtained at at different different C/V C/V22O of the O55 mass mass ratios ratios with with aa reaction reaction temperature temperature of 1250 ◦°C, C, − 1 −1 reaction time of 3 h and N 2 flow rate of 300 mL·min . reaction time of 3 h and N2 mL·min .

As evident evidentfrom fromFigure Figure2a, 2a,the them(C:V m(C:VO 2O5) has a pronounced effect on the N content in VN, as the As 2 5 ) has a pronounced effect on the N content in VN, as the N N content increased from 15.37% to 17.03% whenm(C:V m(C:VO 2O5) increased from 0.27 to 0.30. Beyond 0.30, content increased from 15.37% to 17.03% when 2 5 ) increased from 0.27 to 0.30. Beyond 0.30, the N N content content decreased decreased from from 17.03% 17.03% to to 10.17%. 10.17%. The content was was aa possible possible result result of of the the the The low low N N content incomplete reduction, due to the shortage of carbon. This may be confirmed by the appearance incomplete reduction, due to the shortage of carbon. This may be confirmed by the appearance of V2 Oof 3 V2O3 (Figure at an m(C:V 2O5) value of 0.27. Conversely, when the m(C:V2O5) exceeded 0.30, the (Figure 2b) at 2b) an m(C:V O ) value of 0.27. Conversely, when the m(C:V O ) exceeded 0.30, the excess 2 5 2 5 excess carbon the formation of vanadium carbonitride solid solution led to the decrease the N carbon and theand formation of vanadium carbonitride solid solution led to the decrease of the N of content. content. However, there were no peaks that could be indexed to C or VC in Figure 2b, which was However, there were no peaks that could be indexed to C or VC in Figure 2b, which was due to due the to the amorphous ofblack carbon very similar diffraction of VC VN [31], amorphous state of state carbon andblack very and similar diffraction patterns ofpatterns VC and VN [31],and respectively. respectively. Therefore, 0.30 was as the most 2O5 mass ratio for the reaction. Therefore, 0.30 was considered asconsidered the most suitable C/V suitable O massC/V ratio for the reaction. 2

5

3.1.2. Effect Effect of of Reaction Reaction Temperature Temperature 3.1.2. To investigate investigate the the effect effect of of the the reaction reaction temperature temperature on on the the N N content content in in VN VN product, product, several several To experiments were performed at a C/V 2 O 5 mass ratio of 0.30, a reaction time of 3 h and a N 2 flow rate experiments were performed at a C/V2 O5 mass ratio of 0.30, a reaction time of 3 h and a N2 flow rate −1 − 1 of 300 300 mL mL·min of ·min . .The Theexperimental experimentalresults resultsare aredepicted depictedin inFigure Figure3.3.

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Figure onon thethe NN content in VN product with a C/V 22O55 mass ratio Figure 3. 3. (a) (a)Effect Effectofofreaction reactiontemperature temperature content in VN product with a C/V 2 O5 mass −1 −1− 1 2 flow rate of 300 mL·min ; (b) XRD patterns of of 0.30, reaction time of 3 h and N 2 ratio of 0.30, reaction time of 3 h and N2 flow rate of 300 mL·min ; (b) XRD patterns ofthe the products products obtained different temperatures temperatures with with aa C/V C/V222O obtained at at different O555mass massratio ratioof of0.30, 0.30, reaction reaction time time of of 33 h h and and N N222 flow flow −1 −1 rate rate of of 300 300 mL·min mL·min−.1 .

As shown in Figure 3a, the N content in VN product increased rapidly from 4.69% to 16.70% as As shown in Figure 3a, the N content in VN product increased rapidly from 4.69% to 16.70% as the the reaction temperature increased from 950 °C to 1150 °C; the N content then stayed at a constant reaction temperature increased from 950 ◦ C to 1150 ◦ C; the N content then stayed at a constant level over level over the elevated reaction temperature. Figure 3b exhibits XRD patterns of the products the elevated reaction temperature. Figure 3b exhibits XRD patterns of the products obtained at different obtained at different temperatures. At 950 °C, the peaks were identified as V22O33 and VN, indicating temperatures. At 950 ◦ C, the peaks were identified as V2 O3 and VN, indicating that the reduction and that the reduction and nitridation of the precursor were not complete at a temperature as low as 950 nitridation of the precursor were not complete at a temperature as low as 950 ◦ C. The XRD patterns °C. The XRD patterns were identical and all diffraction peaks corresponded to VN at 1150 °C and were identical and all diffraction peaks corresponded to VN at 1150 ◦ C and 1250 ◦ C, revealing that the 1250 °C, revealing that the precursor was reduced and nitrided almost completely. Hence, a reaction precursor was reduced and nitrided almost completely. Hence, a reaction temperature of 1150 ◦ C was temperature of 1150 °C was chosen as the optimal condition in this experiment. chosen as the optimal condition in this experiment. 3.1.3. 3.1.3. Effect Effect of of Reaction Reaction Time Time The The effect effect of of reaction reaction time time on on the the N N content content in in VN VN product product was was investigated investigated under under the the conditions conditions −1 −1 of a C/V 22O55 mass ratio of 0.30, a reaction temperature of 1150 ◦°C and a N22 flow rate of 300 mL·min− of a C/V2 O5 mass ratio of 0.30, a reaction temperature of 1150 C and a N2 flow rate of 300 mL·min 1.. The are shown shown in in Figure Figure 4. 4. The results results are

Figure 4. Effect of of reaction O555 mass mass ratio ratio of of 0.30, 0.30, Figure 4. Effect reaction time time on on the the N N content content in in VN VN product product with with aaC/V C/V22O ◦ C and N flow rate of 300 mL·min−1 −1 . −1 reaction temperature of 1150 reaction temperature of 1150 °C and N222 flow rate of 300 mL·min .

It can be observed that the N content in VN product increased strongly from 13.80% to 16.38% as the reaction time increased from 0.5 h to 1 h and then remained almost constant beyond 1 h, which

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can7,be MetalsIt 2017, 360observed

that the N content in VN product increased strongly from 13.80% to 16.38% 6 of 12 as the reaction time increased from 0.5 h to 1 h and then remained almost constant beyond 1 h, indicated that thethat reduction and nitridation of the precursor was almost complete at 1 h. Thus, which indicated the reduction and nitridation of the precursor was almost complete at 1the h. optimum reaction time was determined to be 1 h. to be 1 h. Thus, the optimum reaction time was determined Flow Rate Rate 3.1.4. Effect of N22 Flow flow rate rate on on the the N N content content in in VN VN product product was was investigated investigated under the conditions The effect of N22 flow ◦ C and a reaction time of 1 h. The results of a C/V C/V22OO5 5mass massratio ratioofof0.30, 0.30,aareaction reactiontemperature temperatureof of 1150 1150 °C and a reaction time The results are shown in Figure 5.

Figure 5. Effect N2 flow rate on the N content in VN product with a C/V2O5 mass ratio of 0.30, Figure 5. Effect of of N 2 flow rate on the N content in VN product with a C/V2 O5 mass ratio of 0.30, reaction temperature of 1150 time of of 11 h. h. reaction temperature of 1150 ◦°C C and and reaction reaction time

Figure 5 indicates that the N2 flow rate substantially influenced the N content in VN product. Figure 5 indicates that the N2 flow rate substantially influenced the N content in VN product. The N content first rose from 13.73% to 16.38% and then decreased to 15.43% with an increasing N2 The N content first rose from 13.73% to 16.38% and then decreased to 15.43% with an increasing flow rate. There was an optimum N2 flow rate of 300 mL·min−1, at which a maximum N content was N2 flow rate. There was an optimum N2 flow rate of 300 mL·min−1 , at which a maximum N achieved. The flowing nitrogen can increase N2 partial pressure and decrease the CO partial pressure, content was achieved. The flowing nitrogen can increase N2 partial pressure and decrease the CO which might accelerate the reduction and nitridation reactions. However, the rapid flowing nitrogen partial pressure, which might accelerate the reduction and nitridation reactions. However, the rapid could not be sufficiently heated, and the contact time with vanadium oxides was too short to react flowing nitrogen could not be sufficiently heated, and the contact time with vanadium oxides was adequately, resulting in decreased N content as the flow rate increased further. Therefore, 300 too short to react adequately, resulting in decreased N content as the flow rate increased further. mL·min−1 was the optimal N2 flow rate. Therefore, 300 mL·min−1 was the optimal N2 flow rate. 3.2. Phase and Microstructure Analyses of the Precursor 3.2. Phase and Microstructure Analyses of the Precursor Although an appropriate flow rate of nitrogen can promote the synthesis of VN, high reaction Although an appropriate flow rate of nitrogen can promote the synthesis of VN, high reaction temperature and long reaction time were still required in the synthesis of VN with the flow of temperature and long reaction time were still required in the synthesis of VN with the flow of nitrogen nitrogen in the tubular furnace [11,12]. Compared with a temperature of 1300–1500 °C required to in the tubular furnace [11,12]. Compared with a temperature of 1300–1500 ◦ C required to prepare prepare VN by the traditional CRN method, this study permitted a lower temperature and a shorter VN by the traditional CRN method, this study permitted a lower temperature and a shorter time by time by as much as several hundred degrees centigrade and several hours, respectively. The as much as several hundred degrees centigrade and several hours, respectively. The traditional raw traditional raw materials were a mixture of V2O5 and C, while a precursor with V-coated C structure materials were a mixture of V2 O5 and C, while a precursor with V-coated C structure was applied in was applied in our research, a fact we consider an essential difference. Hence, we had to investigate our research, a fact we consider an essential difference. Hence, we had to investigate the precursor. the precursor. To understand the phase compositions of the precursor, XRD analysis was conducted. To understand the phase compositions of the precursor, XRD analysis was conducted. The result The result shown in Figure 6 revealed that the precursor consists of (NH4)2V6O16·1.5H2O with a small shown in Figure 6 revealed that the precursor consists of (NH4 )2 V6 O16 ·1.5H2 O with a small amount of amount of NH4Al(SO4)2·12H2O. No peaks can be indexed to C, due to the amorphous state of carbon NH4 Al(SO4 )2 ·12H2 O. No peaks can be indexed to C, due to the amorphous state of carbon black. black.

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Figure 6. The XRD pattern of the precursor. Figure 6. 6. The The XRD XRD pattern pattern of of the the precursor. precursor. Figure

To have a visualized comprehension of the distribution of (NH4)2V6O16·1.5H2O and carbon black have a visualized comprehension the distribution of results (NH4)2Vare 6O16 ·1.5H2O and carbon 7. black in theToprecursor, SEM–EDS analysis was of conducted and the presented in Figure As To have a visualized comprehension of the distribution of (NH4 )2 V6 O16 ·1.5H2 O and carbon in the precursor, SEM–EDS conducted and the results areCpresented Figurewhich 7. As shown in Figure 7a, there wasanalysis C in the was center and V was plentiful around in a singlein particle, black in the precursor, SEM–EDS analysis was conducted and the results are presented in Figure 7. shown in Figure 7a,carbon there was C inwas the center and V was plentiful around in aprocess single particle, which indicated that the black surrounded by vanadium duringCthe of vanadium As shown in Figure 7a, there was C in the center and V was plentiful around C in a single particle, indicated thatFigure the carbon wasenlarged surrounded vanadium during the shown processinofFigure vanadium precipitation. 7b is ablack partially view by of the edge of the particle 7a. It which indicated that the carbon black was surrounded by vanadium during the process of vanadium precipitation. Figure 7bVisisalocated partially view the edge of C the particle shown in Figure 7a. It can be clearly seen that at enlarged the edge of the of particle, with being in the interior. There were precipitation. Figure 7b is a partially enlarged view of the edge of the particle shown in Figure 7a. It can can be vanadium-coated clearly seen that V carbon is located at the edge of the particle, with C being in thein interior. were many structures (V-coated C structures), as shown the redThere circles in be clearly seen that V is located at the edge of the particle, with C being in the interior. There were many many vanadium-coated carbon structures (V-coated C structures), as shown in the red circles in Figure 7c. vanadium-coated carbon structures (V-coated C structures), as shown in the red circles in Figure 7c. Figure 7c.

Figure 7. SEM (scanning electron microscope) images with EDS (energy-dispersive X-ray spectroscope) Figure 7. SEM (scanning electron microscope) images with EDS (energy-dispersive X-ray element mapping of: (a) The precursor; (b) the partial enlarged view of the edge of the particle; (c) SEM spectroscope) element mapping of: (a) The precursor;images (b) the partial view of the edge of the Figure 7. SEM (scanning electron microscope) with enlarged EDS (energy-dispersive X-ray image of the overall distribution of the precursor. spectroscope) element of: (a)distribution The precursor; (b)precursor. the partial enlarged view of the edge of the particle; (c) SEM imagemapping of the overall of the particle; (c) SEM image of the overall distribution of the precursor.

The CC structure is schematically illustrated in Figure 8, where the The formation formationprocess processofofthe theV-coated V-coated structure is schematically illustrated in Figure 8, where black parts andand orange parts carbon black and APV, respectively. Due to presence of The formation process ofrepresent therepresent V-coated C structure isand schematically illustrated in the Figure 8, where the black parts orange parts carbon black APV, respectively. Due to the presence the dispered carbon black, the nuclei ofofthe vanadium precipitates on the surface thethe black parts and orange parts represent carbon black and APV, in respectively. Due the of dispered carbon black, the nuclei the vanadium precipitates inthe theslurry slurrygrow growto on thepresence of carbon black, whichblack, served the heterogeneous instead forming of the dispered carbon the as nuclei the vanadiumnucleation precipitatessite, in the slurryof grow on thediscrete surface carbon black, the of heterogeneous nucleation forming discrete precipitates. As the precipitation proceeded, APV particles grew gradually and coated carbon black of carbon black, which served as the heterogeneous nucleation site, instead of forming discrete completely, coated formed with the and the layer was carbon black precipitates.the Astypical the precipitation proceeded, APV particles grew coated completely, the typical coated structure structure formed with the core core and gradually the coatingand and APV, carbon black located around TheThe overall distribution the completely, the typicalthe coated structure formed with the core andAPV. the coating layer was carbonof black APV,respectively; respectively; theuncoated uncoated carbon black located around APV. overall distribution of precursor the V-coated structure which was was surrounded by the uncoated carbon black. andprecursor APV,was respectively; the C uncoated carbon black located around APV. overall distribution the was the V-coated C structure which surrounded by theThe uncoated carbon black. of the precursor was the V-coated C structure which was surrounded by the uncoated carbon black.

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Figure 8. 8. Formation Formation process process sketch sketch of of the the precursor. precursor. (APV: (APV: Ammonium Ammonium polyvanadate.) polyvanadate.) Figure polyvanadate.)

The V-coated V-coated C C structure structure evidently evidently promoted promoted the the reaction reaction between between vanadium vanadium oxide oxide and and carbon carbon The black due onon thethe internal interphase of the V-coated C structure, CO black due to to the thefollowing followingreasons. reasons.First, First, internal interphase of the V-coated C structure, produced by direct reduction of vanadium oxides by carbon black could not escape due to the coated CO produced by direct reduction of vanadium oxides by carbon black could not escape due structure; hence, it continued to react with vanadium indirect reduction. Subsequently, CO22 to the coated structure; hence, it continued to reactoxide with by vanadium oxide by indirect reduction. produced by indirect reduction will react with carbon black to produce CO at high temperature. That Subsequently, CO2 produced by indirect reduction will react with carbon black to produce CO at high is to say, inside vanadium oxides, the oxides, direct the reduction and indirect reduction occur temperature. That isthe to say, inside the vanadium direct reduction and indirect reduction simultaneously at theatinternal interphase. Secondly, outside the the vanadium oxides, thethe uncoated C occur simultaneously the internal interphase. Secondly, outside vanadium oxides, uncoated reacted with vanadium oxides simultaneously on the external interphase. Lastly, the V-coated C C reacted with vanadium oxides simultaneously on the external interphase. Lastly, the V-coated structure increased thethe contact areaarea between carbon black and and vanadium oxides remarkably and C structure increased contact between carbon black vanadium oxides remarkably provided a more homogeneous chemical composition than than traditional mechanical blending. APV and provided a more homogeneous chemical composition traditional mechanical blending. decomposed into V 22O55 and V22O55 was reduced and nitrided gradually during the synthesis of VN. The APV decomposed into V2 O5 and V2 O5 was reduced and nitrided gradually during the synthesis of coating always existed with the coating layer changing APV from to lower-valent vanadium VN. Thestructure coating structure always existed with the coating layer from changing APV to lower-valent oxides, vanadium carbide and vanadium nitride, until the precursor converted into VN completely. vanadium oxides, vanadium carbide and vanadium nitride, until the precursor converted into VN Hence, the V-coated C structure obtained byobtained adding carbon black to theblack stripping before the completely. Hence, the V-coated C structure by adding carbon to thesolution stripping solution precipitation of vanadium significantly reduced the reaction and shortened the reaction before the precipitation of vanadium significantly reduced thetemperature reaction temperature and shortened the time. reaction time. 3.3. Phase Preparing VN VN from the Precursor Precursor 3.3. Phase Evolution Evolution of of Preparing from the The progress progress of of the the phase phase formation The formation during during the the preparation preparation of of VN VN was was monitored monitored by by interrupting interrupting the reaction reaction intermittently intermittently at different temperatures temperatures and then the the samples samples were were analyzed analyzed by by XRD XRD the at different and then analysis. Figure 9 shows the XRD patterns of the precursor and products at different temperatures. analysis. Figure 9 shows the XRD patterns of the precursor and products at different temperatures.

Figure Figure 9. 9. XRD XRD patterns patterns of ofthe theprecursor precursorand andproducts productsatat atdifferent differenttemperatures temperatureswith withaaaC/V C/V222O O555 mass mass of the precursor and products different temperatures with C/V O ◦ ◦ ◦ ◦ ratio (a) The The C; (c) (c) 650 650 °C; C; (d) (d) 950 950 °C; C; (e) C. ratio of of 0.30: 0.30: (a) The precursor; precursor; (b) (b) 550 550 °C; °C; °C; °C; (e) 1150 1150 °C. °C. ratio of 0.30: precursor; (b) 550 16·1.5H22O and a small amount of NH44Al(SO44)22·12H22O The precursor precursorwas wascomposed composedofof(NH (NH 44)22V66O16 The 4 )2 V6 O16 ·1.5H2 O and a small amount of NH4 Al(SO4 )2 ·12H2 O 6O13 13 and VO22 [32–35] in curve (b), as shown as shown in in curve curve (a). (a). The The diffraction diffractionpeaks peakswere wereidentified identifiedasasV6V 6 O13 and VO2 [32–35] in curve 4 2 6 16 2 2 5 attributed to the decomposition of (NH 4 ) 2 V 6 O 16 ·1.5H 2 O into V 2 O 5 and NH the3V; 22then, O55 was initially (b), attributed to the decomposition of (NH4 )2 V6 O16 ·1.5H2 O into V2 O353; then, and NH the V2 O5 13 and VO22 by NH33 under 550 °C. As shown in curve (c), all peaks corresponded to reduced to V66O13

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was initially reduced to V6 O13 and VO2 by NH3 under 550 ◦ C. As shown in curve (c), all peaks corresponded to V4 O7 , which indicated that the further reduction of V6 O13 and VO2 to V4 O7 may be by the progressive reduction of V6 O13 to VO2 and VO2 to V4 O7 below 650 ◦ C. The appearance of V2 O3 and VN in curve (d) illustrates that V4 O7 was reduced to V2 O3 continuously and the VN phase was formed from V2 O3 below 950 ◦ C. There was only a single phase of VN at 1150 ◦ C. Based on the above recorded phase changes, the possible mechanism of preparing VN could be formulated as follows:

(NH4 )2 V6 O16 · 1.5H2 O = 3V2 O5 + 2NH3 ↑ +2.5H2 O

(1)

3V2 O5 + 2NH3 = V6 O13 + N2 ↑ +2H2 O + H2 ↑

(2)

3V2 O5 + 2NH3 = 6VO2 + N2 ↑ +3H2 O

(3)

2V6 O13 + 5C = 3V4 O7 + 5CO ↑

(4)

V6 O13 + C = 6VO2 + CO ↑

(5)

4VO2 + C = V4 O7 + CO ↑

(6)

V4 O7 + C = 2V2 O3 + CO ↑

(7)

2V2 O3 + 2N2 = 4VN + 3O2 ↑, ∆Gθ = 1371222 − 82.73T, J·mol−1

(8)

V2 O3 + 5C = 2VC + 3CO ↑, ∆Gθ = 535438.31 − 490.18T, J·mol−1

(9)

2VC + N2 = 2VN + 2C,∆Gθ = −185495.60 + 147.56T, J·mol−1

(10)

According to the standard Gibbs free energy changes (∆Gθ (T)) of Reactions (8)–(10), calculated by the “Reaction” module of Factsage 7.1 software (Factsage 7.1, Thermfact/CRCT, Montreal, QC, Canada; GTT-Technologies, Aachen, Germany), the VN phase can be formed at 16,575 ◦ C via V2 O3 reacting directly with nitrogen under standard state conditions, while the VC phase can be formed at a lower temperature of about 1092 ◦ C. The transformation of VC to VN was spontaneous below 1257 ◦ C. This means that intermediate formation of VC can occur and subsequently convert to VN when the temperature varies between 1092 ◦ C and 1257 ◦ C under standard state conditions and in an extended range of temperatures in flowing nitrogen atmosphere, indicating that 1150 ◦ C satisfies the thermodynamic conditions. Therefore, the phase evolution of (NH4 )2 V6 O16 ·1.5H2 O to VN took place in the following sequential order: (NH4 )2 V6 O16 ·1.5H2 O → V2 O5 → V6 O13 → VO2 → V4 O7 → V2 O3 → VC → VN It is worth noting that after being heated at 550 ◦ C for 20 min, the precursor converted completely to low-valent vanadium oxides V6 O13 and VO2 without the appearance of residual unreacted V2 O5, which has a low melting point (670 ◦ C) and high saturation vapor pressure. V6 O13 can decompose to VO2 at 700 ◦ C, and VO2 has a high melting point of 1542 ◦ C [36]. Thus, the pre-reduction at 650 ◦ C for 2.5 h can be eliminated in this process. TG experiments were carried out under N2 atmosphere to reflect the physical and chemical phenomena in the preparation process of VN. TG and DTG (differential thermogravimetric) curves of the precursor are shown in Figure 10. There were four major weight losses indicated by four DTG peaks in the DTG curve. The first major weight loss between room temperature and 136 ◦ C was attributed to the evaporation of physically absorbed and crystalline water. The second major weight loss from 380 ◦ C to 447 ◦ C was due to the decomposition of APV and the initial reduction of V2 O5 to V6 O13 and VO2 , which corresponded to the Reactions (1)–(3). The third weight loss corresponded to the further reduction of V6 O13 and VO2 to V4 O7 according to Reactions (4)–(6). In this stage, the absolute value of weight loss rates first increased, then decreased and eventually increased again at the temperatures of 600 ◦ C–653 ◦ C, 653 ◦ C–660 ◦ C and 660 ◦ C–700 ◦ C, respectively. The absolute value of weight loss rate

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with increasing temperature because the reduction reaction was endothermic and increasing Metalsescalated 2017, 7, 360 10 of 12

the temperature promoted the reduction. The absolute value of weight loss rate decreased as the reaction progressed. The increase of the weight loss rate was due to the decomposition of V6 O13 to corresponded to the decomposition temperature of V 6O13. The last weight loss was assigned to the VO2 ; the maximum, absolute value of the weight loss rate at 700 ◦ C corresponded to the decomposition phase transformation from V4O7 to V2O3 and V2O3 to the desired phase VN, which corresponded to temperature of V O13 . The last weight loss was assigned to the phase transformation from V4 O7 to Reactions (7), (9) and6 (10). V2 O3 and V2 O3 to the desired phase VN, which corresponded to Reactions (7), (9) and (10).

Figure 10. (thermogravimetric) TG (thermogravimetric)and andDTG DTG(differential (differential thermogravimetric) curves of the precursor Figure 10. TG thermogravimetric) curves of the precursor from 30 ◦ C to 1400 ◦ C with a linear heating rate of 10 ◦ C·min−−11 and N2 flow rate of 50 mL·min−1 −1 . from 30 °C to 1400 °C with a linear heating rate of 10 °C·min and N2 flow rate of 50 mL·min .

4. Conclusions

4. Conclusions

A novel synthesis method of a (NH4 )2 V6 O16 ·1.5H2 O precursor to prepare VN was proposed,

A novel synthesis method of a (NH4)2V6O16·1.5H2O precursor to prepare VN was proposed, characterized by adding carbon black to the stripping solution during the vanadium recovery from characterized adding carbon black the stripping solution duringbythe vanadium recovery black shale.byVN with an N content of to 16.38% was successfully prepared thermal processing of thefrom 1 the blackprecursor shale. VN N content 16.38% was successfully prepared by thermal processing−of at with 1150 ◦an C for 1 h withof a C/V 2 O5 mass ratio of 0.30 and a N2 flow rate of 300 mL·min . −1 precursor at 1150 °C for 1 h with a C/V 2Operiod 5 mass for ratio 0.30 and a of N2VN flow rate of to 300the mL·min . Low Low synthesis temperature and short theofpreparation was due V-coated synthesis temperature and shortwhich periodenlarged for the preparation of VN was due to the V-coated C structure C structure of the precursor, the contact area between reactants significantly and provided a more homogeneous composition. In addition, the simultaneous direct reduction of the precursor, which enlargedchemical the contact area between reactants significantly and provided a and indirect reduction at the interphase caused by the coating structure evidently accelerated the and more homogeneous chemical composition. In addition, the simultaneous direct reduction reaction. XRD at patterns and thermodynamic analysis indicated that the phase evolution from the indirect reduction the interphase caused by the coating structure evidently accelerated the reaction. precursor to VN took place in the following order (NH4 )2 V6 O16 ·1.5H2 O → V2 O5 → V6 O13 → VO2 XRD patterns and thermodynamic analysis indicated that the phase evolution from the precursor to → V O7 → V2 O3 → VC → VN. The precursor converted to V6 O13 and VO2 completely after being VN took 4place in the following order (NH4)2V6O16·1.5H2O → V2O5 → V6O13 → VO2 → V4O7 → V2O3 → calcined at 550 ◦ C, indicating that the pre-reduction of V2 O5 in the traditional CRN method can be VC → VN. The precursor converted to V6O13 and VO2 completely after being calcined at 550 °C, dispensed with. Therefore, this method is an effective, simple and low-cost route to prepare VN. indicating that the pre-reduction of V2O5 in the traditional CRN method can be dispensed with. Acknowledgments: wassimple fundedand by low-cost the National Natural ScienceVN. Foundation of China Therefore, this methodThis is anresearch effective, route to prepare (No. 51404174 and No. 51474162) and the Project in the National Science & Technology Pillar Program of China (No. 2015BAB18B01).

Acknowledgments: This research was funded by the National Natural Science Foundation of China (No. Author Contributions: Jingli Han and Yimin Zhang conceived and designed the experiments; Jingli Han 51404174 and No. and Projectthe in data; the National Science Technology Pillarcontributed Program of China (No. performed the 51474162) experiments andthe analyzed Yimin Zhang, Tao& Liu and Jing Huang reagents, 2015BAB18B01). materials and analysis tools; Nannan Xue and Pengcheng Hu provided valuable scientific advice for the study; Jingli Han wrote the paper.

Author Contributions: Jingli Han and Yimin Zhang conceived and designed the experiments; Jingli Han Conflicts of Interest: The authors declare no conflict of interest. performed the experiments and analyzed the data; Yimin Zhang, Tao Liu and Jing Huang contributed reagents, materials and analysis tools; Nannan Xue and Pengcheng Hu provided valuable scientific advice for the study; Jingli Han wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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