Preparation of Vanadium Nitride Using a Thermally Processed Precursor with Coating Structure

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 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.) 2 Hubei Collaborative Innovation Center of High Efﬁcient Utilization for Vanadium Resources, Wuhan 430081, China 3 Hubei Provincial Engineering Technology Research Center of High Efﬁcient Cleaning Utilization for Shale Vanadium Resource, Wuhan 430081, China 4 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/V2O5 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)2V6O16·1.5H2O → V2O5 → V6O13 → VO2 → V4O7 → V2O3 → VC → VN. The precursor converted to V6O13 and VO2 completely ◦ after being calcined at 550 C, indicating that the pre-reduction of V2O5 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 ◦ −6 3 −1 (CRN) of V2O5 in N2 at a high temperature of 1500 C for 3 h with a N2 flow rate of 13.3 × 10 m · s [11], ◦ −1 and at 1400 C with a N2 flow rate of 50 L·h , using a microwave method [12]. The low melting point ◦ (670 C) and the high saturation vapor pressure of V2O5 will result in a volatile loss of V2O5 near its

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melting point. Hence, the pre-reduction during transformation from V2O5 to a high melting point and low-valent vanadium oxides below the melting point of V2O5 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 V2O5. 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 (NH4VO3) 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)2Cr2O7), NH4VO3, 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, NH4VO3 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(Mg1/3Nb2/3)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 Table1, 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 Metals 2017, 7, 360 3 of 12

0.03. Carbon black containing 94.80% of carbon with the particle size of less than 0.074 mm was used asMetals the 2017 reducing, 7, 360 agent. 3 of 12

Table 1. Main chemical composition of the stripping solution. Table 1. Main chemical composition of the stripping solution.

ElementElement V V Al Al Fe Fe K K Na Na P P −1 ConcentrationConcentration (g·L (g−1·)L ) 20.6320.63 5.78 5.78 0.06 0.06 0.34 0.34 0.42 0.42 0.1 0.1

2.2. Typical ProcedureProcedure forfor thethe PreparationPreparation ofof VNVN

The process flow sheet to prepare VN is shown in Figure 1. Typically, sodium chlorate (NaClO3, The process flow sheet to prepare VN is shown in Figure1. Typically, sodium chlorate (NaClO 3, ARAR (analytical(analytical reagent),reagent), 2.95 g)g) waswas addedadded toto thethe strippingstripping solutionsolution (200(200 mL)mL) toto oxidizeoxidize vanadiumvanadium (IV)(IV) toto vanadium (V) in a temperature-controlled temperature-controlled magnetic stirrer (SZCL-2A, Wuhan Keer Instrument Co., Ltd., Wuhan, China)China) atat roomroom temperature.temperature. After complete oxidation, a certain amount of carbon black was added to to the the solution solution and and dispersed dispersed by by stir stirring.ring. The The pH pH of ofthe the solution solution was was adjusted adjusted to 1.8 to 1.8with with ammonia ammonia (AR), (AR), and andthen thenthe solution the solution was stirred was stirred to precipitate to precipitate ammonium ammonium polyvanadate polyvanadate (APV) (APV)at 90 °C at 90for◦ C1 forh. After 1 h. After solid-liquid solid-liquid separation separation by byvacuum vacuum filtration filtration (model (model SHB-III, SHB-III, Wuhan Wuhan Keer InstrumentInstrument Co.,Co., Ltd.,Ltd., Wuhan,Wuhan, China),China), thethe precursorprecursor waswas washedwashed byby water, drieddried inin anan oven,oven, and then compressed intointo cylindricalcylindrical blocksblocks ofof 3030 mm diameterdiameter at a pressure ofof 1414 MPa.MPa. The cylindrical block waswas placedplaced intointo thethe tubulartubular atmosphereatmosphere furnacefurnace (SGL-1700,(SGL-1700, Shanghai Institute of Optics and FineFine Mechanics, Shanghai, China)China) withwith aa certain flowflow raterate of nitrogen (99.999%).(99.999%). The sample was calcined toto deaminizedeaminize atat 550550 ◦°CC for 20 min and pre-reduced at 650 ◦°CC for 2.5 h, and then heated to the desired temperaturetemperature forfor reductionreduction andand nitridationnitridation forfor aa certaincertain of time. Subsequently Subsequently,, thethe product waswas cooled toto roomroom temperaturetemperature inin nitrogennitrogen atmosphere.atmosphere.

Figure 1. Process ﬂowflow sheet for preparation of vanadiumvanadium nitridenitride (VN).(VN).

2.3. Material Characterization The nitrogennitrogen contentcontent (N(N content)content) waswas determineddetermined employingemploying distillation-aciddistillation-acid basebase titrationtitration analysis. The X-ray diffractiondiffraction (XRD)(XRD) patternspatterns werewere obtainedobtained byby usingusing aa Rigaku Rigaku D/MAX-RB D/MAX-RB X-rayx-ray diffractometer (Rigaku, Akishima, Akishima, Japan) Japan) with with Cu Cu K Kαα radiationradiation to to analyze analyze the the phase phase compositions compositions in inthe the products. products. Microscopic Microscopic observation observation and and elementa elementall analysis analysis (SEM–EDS; (SEM–EDS; SEM: SEM: Scanning Scanning 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.

Metals 2017, 7, 360 4 of 12 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 ◦ −1 −1 linear heating rate of 10 C·min under N2, and a gas ﬂow rate of 50 mL·min . 3. Results and Discussion 3. Results and Discussion

3.1. Effect of the Main Reaction ConditionsConditions on the NN ContentContent inin VNVN Product

3.1.1. Effect of C/V22OO5 5MassMass Ratio Ratio

The effect of of C/V C/V2O2O5 mass5 mass ratio ratio (m(C:V (m(C:V2O52))O on5)) the on N the content N content in VN in product VN product is shown is shownin Figure in ◦ Figure2, obtained2, obtained at a reaction at a reaction temperature temperature of 1250 of 1250°C, a reactionC, a reaction time of time 3 h, of and 3 h, anda N2 aflow N2 ﬂow rate rateof 300 of 300mL·min mL·−min1. −1.

Figure 2. (a) EffectEffect ofof C/VC/V22OO55 massmass ratio ratio on on the the N N content content in in VN VN product product with a reaction temperature ◦ −−11 of 1250 °C,C, reaction timetime ofof 33 hh andand NN22 ﬂowflow raterate ofof 300300 mLmL·min·min ;;( (bb)) XRD XRD (X-ray (X-ray diffraction) diffraction) patterns ◦ of thethe productsproducts obtainedobtained atat differentdifferent C/VC/V2OO55 massmass ratios ratios with with a a reaction temperature of 1250 °C,C, −11 reactionreaction timetime ofof 33 hh andand NN22 ﬂowflow rate of 300 mLmL·min·min . .

As evident from Figure 2a, the m(C:V2O5) has a pronounced effect on the N content in VN, as the As evident from Figure2a, the m(C:V2O5) has a pronounced effect on the N content in VN, as the N N content increased from 15.37% to 17.03% when m(C:V2O5) increased from 0.27 to 0.30. Beyond 0.30, content increased from 15.37% to 17.03% when m(C:V2O5) increased from 0.27 to 0.30. Beyond 0.30, thethe NN contentcontent decreaseddecreased fromfrom 17.03%17.03% toto 10.17%.10.17%. The low N content was a possible result of thethe incomplete reduction, due to the shortage of carbon. This may be confirmed by the appearance of incomplete reduction, due to the shortage of carbon. This may be conﬁrmed by the appearance of V2O3 V2O3 (Figure 2b) at an m(C:V2O5) value of 0.27. Conversely, when the m(C:V2O5) exceeded 0.30, the (Figure2b) at an m(C:V2O5) value of 0.27. Conversely, when the m(C:V2O5) exceeded 0.30, the excess carbonexcess carbon and the and formation the formation of vanadium of vanadium carbonitride carbonit solidride solution solid solution led to the led decrease to the decrease of the N of content. the N However,content. However, there were there no were peaks no that peaks could that be could indexed be indexed to C or VCto C in or Figure VC in2 Figureb, which 2b, waswhich due was to thedue amorphousto the amorphous state of state carbon of blackcarbon and black very and similar very diffraction similar diffraction patterns of patterns VC and VNof VC [31 ],and respectively. VN [31], respectively. Therefore, 0.30 was considered as the most suitable C/V2O5 mass ratio for the reaction. Therefore, 0.30 was considered as the most suitable C/V2O5 mass ratio for the reaction.

3.1.2. Effect of Reaction Temperature To investigateinvestigate thethe effecteffect ofof thethe reactionreaction temperaturetemperature onon thethe NN contentcontent inin VNVN product,product, severalseveral experiments werewere performedperformed atat aa C/VC/V2O55 massmass ratio ratio of of 0.30, 0.30, a reaction time of 3 h and aa NN22 ﬂowflow rate of 300 mLmL·min·min−11. .The The experimental experimental results results are are depicted depicted in in Figure Figure 3.3.

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Figure 3. ((a)) EffectEffect ofof reactionreaction temperaturetemperature onon ththe N content in VN product with a C/V22O55 massmass ratioratio Figure 3. (a) Effect of reaction temperature on the N content in VN product with a C/V2O5 mass of 0.30, reaction time of 3 h and N22 flowflow raterate ofof 300300 mL·minmL·min−−11−;; 1((b)) XRDXRD patternspatterns ofof thethe productsproducts ratio of 0.30, reaction time of 3 h and N2 ﬂow rate of 300 mL·min ;(b) XRD patterns of the products obtained at different temperatures with a C/V22O55 mass ratio of 0.30, reaction time of 3 h and N22 flow obtained at different temperaturestemperatures withwith aa C/VC/V2OO5 massmass ratio ratio of of 0.30, 0.30, reaction reaction time of 3 h and N2 flowﬂow raterate of of 300 300 mL·min mLmL·min·min−−11..1 .

As shown in Figure 3a, the N content in VN product increased rapidly from 4.69% to 16.70% as As shown in Figure3a, the N content in VN product increased rapidly from 4.69% to 16.70% as the thethe reactionreaction temperaturetemperature increasedincreased fromfrom 950950 °C°C toto 11501150 °C;°C; thethe NN contentcontent thenthen stayedstayed atat aa constantconstant reaction temperature increased from 950 ◦C to 1150 ◦C; the N content then stayed at a constant level over levellevel overover thethe elevatedelevated reactionreaction temperature.temperature. FiFigure 3b exhibits XRD patterns of the products the elevated reaction temperature. Figure3b exhibits XRD patterns of the products obtained at different obtained at different temperatures. At 950 °C, the peaks were identified as V22O33 andand VN,VN, indicatingindicating temperatures. At 950 ◦C, the peaks were identiﬁed as V O and VN, indicating that the reduction and thatthat thethe reductionreduction andand nitridationnitridation ofof thethe precursorprecursor were2 not3 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 temperaturetemperature ofof 11501150 °C°C waswas chosenchosen asas ththe optimal condition in this experiment. chosen as the optimal condition in this experiment.

3.1.3. Effect Effect of Reaction Time The effect of reaction time on the N N content content in in VN VN product product was was investigated investigated under under the the conditions conditions of a C/V22O55 massmass ratioratio ofof 0.30,0.30, aa reactionreaction temperaturetemperature ofof 11501150 ◦°C°C andand aa NN22 flowflow raterate ofof 300300 mL·minmL·min−−−11.. of a C/V2O5 mass ratio of 0.30, a reaction temperature of 1150 C and a N2 ﬂow rate of 300 mL·min . The results are shownshown inin FigureFigure4 4..

Figure 4. Effect of reaction timetime onon thethe NN contentcontent inin VNVN productproduct withwith aa C/VC/V22OO55 massmassmass ratioratio ratio ofof 0.30,0.30, ◦ −1 reactionreaction temperature temperature ofof 11501150 °C°CC and and N N22 flowflowﬂow raterate ofof 300300 mL·minmL·min mL·min−−11.. .

ItIt cancan bebe observedobserved thatthat thethe NN contentcontent inin VNVN productproduct increasedincreased stronglystrongly fromfrom 13.80%13.80% toto 16.38%16.38% as the reaction time increased from 0.5 h to 1 h and then remained almost constant beyond 1 h, which

Metals 2017, 7, 360 6 of 12

MetalsIt 2017 can, 7, be 360 observed 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, whichindicated indicated that the that reduction the reduction and nitridation and nitridation of the precursor of the precursor was almost was complete almost completeat 1 h. Thus, at 1the h. optimumThus, the optimumreaction time reaction was determined time was determined to be 1 h. to be 1 h.

3.1.4. Effect of N 2 FlowFlow Rate Rate

The effect of N 2 flowﬂow rate rate on on the the N N content content in in VN VN prod productuct was was investigated investigated under the conditions ◦ of a C/V 22OO5 5massmass ratio ratio of of 0.30, 0.30, aa reaction reaction temperature temperature of of 1150 1150 °CC and and a a reaction reaction time time of 1 h. The The results results are shown in Figure5 5..

Figure 5. Effect of N2 flow rate on the N content in VN product with a C/V2O5 mass ratio of 0.30, Figure 5. Effect of N2 ﬂow rate on the N content in VN product with a C/V2O5 mass ratio of 0.30, reaction temperature of 1150 ◦°CC and reaction time of 1 h.

Figure 5 indicates that the N2 flow rate substantially influenced the N content in VN product. Figure5 indicates that the N 2 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−1 to 15.43% with an increasing flow rate. There was an optimum N2 flow rate of 300 mL·min , at which−1 a maximum N content was N2 flow rate. There was an optimum N2 flow rate of 300 mL·min , 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 N partial pressure and decrease the CO which might accelerate the reduction and nitridation reactions.2 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. −1 2 mL·min was the optimal−1 N flow rate. Therefore, 300 mL·min 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 V2O5 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 Figure6 revealed that the precursor consists of (NH 4)2V6O16·1.5H2O 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 NH4Al(SO4)2·12H2O. 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. The XRD pattern of the precursor. To have a visualized comprehension of the distribution of (NH4)2V6O16·1.5H2O and carbon black in theTo precursor, have a visualized SEM–EDS comprehens analysis wasion ofconducted the distribution and the of results(NH4)2 Vare6O 16presented·1.5H2O and in Figurecarbon 7.black As To have a visualized comprehension of the distribution of (NH4)2V6O16·1.5H2O and carbon shownin the precursor,in Figure 7a, SEM–EDS there was analysis C in the wascenter conducted and V was and plentiful the results around are C presented in a single in particle, Figure which 7. As black in the precursor, SEM–EDS analysis was conducted and the results are presented in Figure7. indicatedshown in Figurethat the 7a, carbon there was black C in was the centersurrounded and V wasby vanadium plentiful around during C the in aprocess single particle, of vanadium which As shown in Figure7a, there was C in the center and V was plentiful around C in a single particle, precipitation.indicated that Figure the carbon 7b is a black partially was enlarged surrounded view byof thevanadium edge of theduring particle the shownprocess in of Figure vanadium 7a. It which indicated that the carbon black was surrounded by vanadium during the process of vanadium canprecipitation. be clearly seenFigure that 7b V is is a located partially at enlargedthe edge ofview the ofparticle, the edge with of Cthe being particle in the shown interior. in Figure There were7a. It precipitation. Figure7b is a partially enlarged view of the edge of the particle shown in Figure7a. It can manycan be vanadium-coatedclearly seen that V carbonis located structures at the edge (V-coated of the particle, C structures), with C beingas shown in the in interior. the red There circles were in be clearly seen that V is located at the edge of the particle, with C being in the interior. There were many Figuremany vanadium-coated7c. carbon structures (V-coated C structures), as shown in the red circles in vanadium-coated carbon structures (V-coated C structures), as shown in the red circles in Figure7c. 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)Figure 7. SEM element (scanning mapping electron of: (a) Themicroscope) precursor; images(b) the partialwith enlargedEDS (energy-dispersive view of the edge ofX-ray the image of the overall distribution of the precursor. particle;spectroscope) (c) SEM element image mapping of the overall of: (a dist) Theribution precursor; of the (b precursor.) the partial enlarged view of the edge of the particle; (c) SEM image of the overall distribution of the precursor. The formationformation processprocess of of the the V-coated V-coated C C structure structure is schematicallyis schematically illustrated illustrated in Figurein Figure8, where 8, where the blackthe blackThe parts formationparts and and orange processorange parts partsof representthe represent V-coated carbon carbonC structure black black and is and schematically APV, APV, respectively. respectively. illustrated Due Due toin theFigureto the presence presence8, where of theofthe the disperedblack dispered parts carbon carbonand orange black, black, theparts the nuclei nuclei represent of of the the carbon vanadium vanadium black precipitates precipitates and APV, inrespectively. in the the slurry slurry growDuegrow to onon the thethe presence surface ofof the carboncarbon dispered black,black, carbon which black, served the asnuclei thethe of heterogeneousheterogene the vanadiumous nucleationnucleation precipitates site, in the instead slurry ofgrow formingforming on the discretediscrete surface precipitates.of carbon black, As the which precipitation served as proceeded, the heterogene APV particlesous nucleation grew gradually site, instead and coatedof forming carbon discrete black completely,precipitates. thethe As typical typicalthe precipitation coatedcoated structurestructure proceeded, formedformed APV withwith particles thethe corecore grew andand gradually thethe coatingcoating and layer coated was carbon black andcompletely, APV,APV, respectively;respectively; the typical the coatedthe uncoated uncoated structure carbon carbon formed black black with located located the core around around and APV. the APV. coating The The overall layeroverall distribution was distribution carbon ofblack the of precursortheand precursor APV, wasrespectively; was the V-coatedthe V-coated the Cuncoated structure C structure carbon which which black was was surrounded located surrounded around by the byAPV.uncoated the The uncoated overall carbon carbon distribution black. black. of the precursor was the V-coated C structure which was surrounded by the uncoated carbon black.

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Figure 8. FormationFormation process process sketch sketch of of the the precursor. precursor. (APV:(APV: AmmoniumAmmonium polyvanadate.)polyvanadate.)

TheThe V-coatedV-coated CC structurestructure evidentlyevidently promotedpromoted thethe reactionreaction betweenbetween vanadiumvanadium oxideoxide andand carboncarbon blackblack due to to the the following following reasons. reasons. First, First, on on the the internalternal internal interphaseinterphase interphase ofof thethe of theV-coatedV-coated V-coated CC structure,structure, C structure, COCO COproduced produced by direct by directreduction reduction of vanadium of vanadium oxides by oxides carboncarbon by blackblack carbon couldcould black notnot escapeescape could duedue not toto escape thethe coatedcoated due tostructure;structure; the coated hence,hence, structure; itit continuedcontinued hence, toto reactreact it continued withwith vanadiumvanadium to react oxideoxide with byby vanadium indirectindirect reduction.reduction. oxide by indirectSubsequently,Subsequently, reduction. COCO22 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 temperature.isis toto say,say, insideinside That isthethe to say,vanadiumvanadium inside the oxides,oxides, vanadium thethe oxides,direct thereduction direct reductionand indirect and indirectreduction reduction occur occursimultaneouslysimultaneously simultaneously atat thethe at internalinternal the internal interphase.interphase. interphase. SecondSecond Secondly,ly,ly, outsideoutside outside thethe thevanadiumvanadium vanadium oxides,oxides, oxides, thethe the uncoateduncoated uncoated CC Creactedreacted reacted withwith with vanadiumvanadium vanadium oxidesoxides oxides simultaneouslysimultaneously simultaneously ononon thethe the externalexternal external interphase.interphase. interphase. Lastly,Lastly, Lastly, thethe the V-coatedV-coated V-coated CC Cstructurestructure structure increasedincreased increased thethethe contactcontact contact areaarea area betweenbetween between carbcarb carbonon black black and andvanadium vanadium oxides oxides remarkably remarkably and andprovided provided a more a more homogeneous homogeneous chemical chemical composition composition than thantraditional traditional mechanical mechanical blending. blending. APV decomposed into V22O55 andand VV22O55 waswas reducedreduced andand nitridednitrided graduallygradually duringduring thethe synthesissynthesis ofof VN.VN. TheThe APV decomposed into V2O5 and V2O5 was reduced and nitrided gradually during the synthesis of VN.coatingcoating The structurestructure coating structure alwaysalways existedexisted always withwith existed thethe coatingcoating with the layerlayer coating changingchanging layer fromfrom changing APVAPV from toto lower-valentlower-valent APV to lower-valent vanadiumvanadium vanadiumoxides, vanadium oxides, carbide vanadium and carbide vanadium and nitride, vanadium until nitride, the precursor until the converted precursor into converted VN completely. into VN completely.Hence, the V-coated Hence, the C structure V-coated Cobtained structure by obtained adding carboncarbon by adding blackblack carbon toto thethe black strippingstripping to the solutionsolution stripping beforebefore solution thethe beforeprecipitation the precipitation of vanadium of vanadium significantly signiﬁcantly reduced th reducede reaction the temperature reaction temperature and shortened and shortened the reaction the reactiontime.time. time.

3.3.3.3. Phase Evolution of Preparing VNVN from the PrecursorPrecursor TheThe progressprogress of thethe phase formation during the preparation preparation of of VN VN was was monitored monitored by by interrupting interrupting thethe reactionreaction intermittentlyintermittently at at differentdifferent temperaturestemperatures and and thenthen thethe samplessamples werewere analyzedanalyzed byby XRDXRD analysis.analysis. Figure 9 9 shows shows the the XRD XRD patterns patterns of of the the precursor precursor and and products products at at different different temperatures. temperatures.

FigureFigure 9.9. XRDXRD patternspatterns ofofof thethethe precursor precursorprecursor and andandproducts productsproducts at atatdifferent differentdifferent temperatures temperaturestemperatures with withwith a aa C/V C/VC/V222OO555 massmass ◦ ◦ ◦ ◦ ratioratio ofof 0.30:0.30: ( (aa)) The The precursor;precursor; ((bb)) 550550 °C;°C;C; ( (cc)) 650 650 °C;°C;C; ( (d)) 950 950 °C;°C;C; (( ee)) 11501150 °C.°C.C.

4 2 6 16 2 4 4 2 2 TheThe precursorprecursor was was composed composed of of (NH (NH4)42))V2V66OO1616··1.5H·1.5H1.5H22OO andand aa smallsmall amountamount of of NH NH44Al(SOAl(SO44)))22··12H·12H12H22OO 6 13 2 asas shown in curve curve (a). (a). TheThe diffraction diffraction peaks peaks were were identified identiﬁed as as V6 VO613O and13andand VOVO VO2 [32–35][32–35]2 [32– 35inin ] curvecurve in curve (b),(b), 44 22 66 1616 22 22 55 33 22 55 (b),attributed attributed to the to decomposition the decomposition of (NH of)) (NHV O4)·1.5H·1.5H2V6O16O· into1.5H V2OO into andandV NHNH2O5;; then,andthen,NH thethe 3 VV; then,O waswas the initiallyinitially V2O5 reducedreduced toto VV66O1313 andand VOVO22 byby NHNH33 underunder 550550 °C.°C. AsAs shownshown inin curvcurve (c), all peaks corresponded to

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◦ was initially reduced to V6O13 and VO2 by NH3 under 550 C. As shown in curve (c), all peaks corresponded to V4O7, which indicated that the further reduction of V6O13 and VO2 to V4O7 may be ◦ by the progressive reduction of V6O13 to VO2 and VO2 to V4O7 below 650 C. The appearance of V2O3 and VN in curve (d) illustrates that V4O7 was reduced to V2O3 continuously and the VN phase was ◦ ◦ formed from V2O3 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)2V6O16 · 1.5H2O = 3V2O5 + 2NH3 ↑ +2.5H2O (1)

3V2O5 + 2NH3 = V6O13 + N2 ↑ +2H2O + H2 ↑ (2)

3V2O5 + 2NH3 = 6VO2 + N2 ↑ +3H2O (3)

2V6O13 + 5C = 3V4O7 + 5CO ↑ (4)

V6O13 + C = 6VO2 + CO ↑ (5)

4VO2 + C = V4O7 + CO ↑ (6)

V4O7 + C = 2V2O3 + CO ↑ (7) θ −1 2V2O3 + 2N2 = 4VN + 3O2 ↑, ∆G = 1371222 − 82.73T,J·mol (8) θ −1 V2O3 + 5C = 2VC + 3CO ↑, ∆G = 535438.31 − 490.18T,J·mol (9) θ −1 2VC + N2 = 2VN + 2C,∆G = −185495.60 + 147.56T,J·mol (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 V2O3 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)2V6O16·1.5H2O to VN took place in the following sequential order:

(NH4)2V6O16·1.5H2O → V2O5 → V6O13 → VO2 → V4O7 → V2O3 → 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 V6O13 and VO2 without the appearance of residual unreacted V2O5, ◦ which has a low melting point (670 C) and high saturation vapor pressure. V6O13 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 V2O5 to V6O13 and VO2, which corresponded to the Reactions (1)–(3). The third weight loss corresponded to the further reduction of V6O13 and VO2 to V4O7 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 Metals 2017, 7, 360 10 of 12

Metalsescalated 2017, 7, 360 with increasing temperature because the reduction reaction was endothermic and increasing10 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 V O to 6 13 6 13 corresponded to the decomposition temperature of V O . 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 O . The last weight loss was assigned to the phase transformation from V O to Reactions (7), (9) and6 (10).13 4 7 V2O3 and V2O3 to the desired phase VN, which corresponded to Reactions (7), (9) and (10).

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

4. Conclusions 4. Conclusions A novel synthesis method of a (NH4)2V6O16·1.5H2O 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 characterizedblack shale. by VN adding with an carbon N content black of to 16.38% the strippi was successfullyng solution prepared during by the thermal vanadium processing recovery of the from ◦ −1 blackprecursor shale. VN at with 1150 anC forN content 1 h with of a C/V16.38%2O5 wasmass su ratioccessfully of 0.30 andprepared a N2 flow by thermal rate of 300 processing mL·min of. the precursorLow synthesis at 1150 °C temperature for 1 h with and a C/V short2O period5 mass forratio the of preparation 0.30 and a ofN2 VNflow was rate due of to300 the mL·min V-coated−1. Low synthesisC structure temperature of the precursor, and short whichperiod enlarged for the prepar the contactation area of VN between was due reactants to the V-coated significantly C structure and of theprovided precursor, a more which homogeneous enlarged chemical the contact composition. area between In addition, reactants the simultaneous significantly direct and reduction provided a moreand homogeneous indirect reduction chemical at the composition. interphase caused In addition, by the coating the structuresimultaneous evidently direct accelerated reduction the and indirectreaction. reduction XRD at patterns the interphase and thermodynamic caused by the analysis coating indicated structure that evidently the phase accelerated evolution fromthe reaction. the · → → → XRDprecursor patterns toand VN thermodynamic took place in the analysis following indicated order (NH that4)2 Vthe6O 16phase1.5H 2evolutionO V2O 5fromV 6theO13 precursorVO2 to → V4O7 → V2O3 → VC → VN. The precursor converted to V6O13 and VO2 completely after being VN took place in the◦ following order (NH4)2V6O16·1.5H2O → V2O5 → V6O13 → VO2 → V4O7 → V2O3 → calcined at 550 C, indicating that the pre-reduction of V2O5 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. Therefore,Acknowledgments: this methodThis is an research effective, was simple funded and by low-cost the National route Natural to prepare Science VN. Foundation of China (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 51404174performed and No. the 51474162) experiments and and the analyzed Project the in data;the National Yimin Zhang, Science Tao & Liu Technology and Jing Huang Pillar contributed Program of reagents, China (No. 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.

References

1. Huang, J.; Peng, H.; Xia, G. Microwave synthesis of vanadium nitride for industrial applications. Ironmak. Steelmak. 2009, 36, 110–114. 2. Yu, S.; Li, W.; Ji, Z.; Fu, N.; Sui, Z. Effect of technical parameters on preparing for vanadium nitride. Adv. Mater. Res. 2010, 150–151, 480–483.

Metals 2017, 7, 360 11 of 12

References

1. Huang, J.; Peng, H.; Xia, G. Microwave synthesis of vanadium nitride for industrial applications. Ironmak. Steelmak. 2009, 36, 110–114. [CrossRef] 2. Yu, S.; Li, W.; Ji, Z.; Fu, N.; Sui, Z. Effect of technical parameters on preparing for vanadium nitride. Adv. Mater. Res. 2010, 150–151, 480–483. [CrossRef] 3. Morel, A.; Piron, Y.B.; Porto, R.L.; Brousse, T.; Belanger, D. Suitable conditions for the use of vanadium nitride as an electrode for electrochemical capacitor. J. Electrochem. Soc. 2016, 163, A1077–A1082. [CrossRef] 4. Zhou, X.; Chen, H.; Shu, D.; He, C.; Nan, J. Study on the electrochemical behavior of vanadium nitride as a promising supercapacitor material. J. Phys. Chem. Solids 2009, 70, 495–500. [CrossRef] 5. Wu, M.; Guo, H.; Lin, Y.; Wu, K.; Ma, T.; Hagfeldt, A. Synthesis of highly effective vanadium nitride (VN) peas as a counter electrode catalyst in dye-sensitized solar cells. J. Phys. Chem. C 2014, 118, 12625–12631. [CrossRef] 6. Krawiec, P.; Cola, P.L.D.; Gläser, R.; Weitkamp, J.; Weidenthaler, C.; Kaskel, S. Oxide foams for the synthesis of high-surface-area vanadium nitride catalysts. Adv. Mater. 2006, 18, 506–508. [CrossRef] 7. Gajbhiye, N.S.; Ningthoujam, R.S. Low temperature synthesis, crystal structure and thermal stability studies of nanocrystalline VN particles. Mater. Res. Bull. 2006, 41, 1612–1621. [CrossRef] 8. Smolik, J.; Mazurkiewicz, A.; Słomka, Z.; Bujak, J.; Gołacka, J.K.; Garbacz, H.; Wieciński, P. Nanomultilayer coatings based on vanadium nitride. Solid State Phenom. 2015, 237, 15–20. [CrossRef] 9. Caicedo, J.C.; Zambrano, G.; Aperador, W.; Escobar-Alarcon, L.; Camps, E. Mechanical and electrochemical characterization of vanadium nitride (VN) thin films. Appl. Surf. Sci. 2011, 258, 312–320. [CrossRef] 10. Duan, X.; Srinivasakannan, C.; Zhang, H.; Zhang, Y. Process optimization of the preparation of vanadium nitride from vanadium pentoxide. Arab. J. Sci. Eng. 2015, 40, 2133–2139. 11. Tripathy, P.K.; Sehra, J.C.; Kulkarni, A.V. On the carbonitrothermic reduction of vanadium pentoxide. J. Mater. Chem. 2001, 11, 691–695. [CrossRef] 12. Pan, H.; Zhang, Z.; Peng, J.; Zhang, L.; Li, W. Densification of vanadium nitride by microwave-assisted carbothermal nitridation. Adv. Mater. Res. 2011, 201–203, 1787–1792. [CrossRef] 13. Chen, Z.; Xue, Z.; Wang, W.; Yu, Y.; Liu, Q.; Li, P. One-step method of carbon thermal reduction and nitride to produce vanadium nitrogen alloy. Adv. Mater. Res. 2012, 476–478, 194–198. [CrossRef] 14. Cai, P.; Yang, Z.; Wang, C.; Xia, P.; Qian, Y. Synthesis of nanocrystalline VN via thermal liquid-solid reaction. Mater. Lett. 2006, 60, 410–413. [CrossRef] 15. Yeh, C.L.; Chen, Y.D. Combustion synthesis of vanadium carbonitride from V-C powder compacts under nitrogen pressure. Ceram. Int. 2007, 33, 365–371. [CrossRef] 16. Cheng, F.; He, C.; Shu, D.; Chen, H.; Zhang, J.; Tang, S.; Finlow, D.E. Preparation of nanocrystalline VN by

the melamine reduction of V2O5 xerogel and its supercapacitive behavior. Mater. Chem. Phys. 2011, 131, 268–273. [CrossRef] 17. Azargohar, R.; Dalai, A.K. Production of activated carbon from luscar char: Experimental and modeling studies. Microporous Mesoporous Mater. 2005, 85, 219–225. [CrossRef] 18. Zhang, B.; Li, Z. Synthesis of vanadium carbide by mechanical alloying. J. Alloys Compd. 2005, 392, 183–186. [CrossRef] 19. Yao, W.; Makowski, P.; Giordano, C.; Goettmann, F. Synthesis of early-transition-metal carbide and nitride nanoparticles through the urea route and their use as alkylation catalysts. Chem. Eur. J. 2009, 15, 11999–12004. [CrossRef][PubMed] 20. Chen, L.; Gu, Y.; Shi, L.; Yang, Z.; Ma, J.; Qian, Y. A room-temperature synthesis of nanocrystalline vanadium nitride. J. Eur. Ceram. Soc. 2010, 30, 2099–2107. [CrossRef] 21. Zhao, Z.; Liu, Y.; Cao, H.; Ye, J.; Gao, S.; Tu, M. Synthesis of VN nanopowders by thermal nitridation of the precursor and their characterization. J. Alloys Compd. 2008, 464, 75–80. [CrossRef]

22. Liu, A.; Liu, Y.; Ma, S.; Qiu, Y.; Rong, P.; Ye, J. Synthesis of (Cr,V)2(C,N) solid solution powders by thermal processing precursors. Mater. Chem. Phys. 2017, 193, 196–202. [CrossRef]

23. Cai, Z.; Zhang, Y.; Liu, T.; Huang, J. Mechanisms of vanadium recovery from stone coal by novel BaCO3/BaO composite additive roasting and acid leaching technology. Minerals 2016, 6, 26. [CrossRef] 24. Zhang, X.; Yang, K.; Tian, X.; Qin, W. Vanadium leaching from carbonaceous shale using ﬂuosilicic acid. Int. J. Miner. Process. 2011, 100, 184–187. [CrossRef] Metals 2017, 7, 360 12 of 12

25. Zhang, Y.; Bao, S.; Liu, T.; Huang, J. The technology of extracting vanadium from stone coal in China: History, current status and future prospects. Hydrometallurgy 2011, 109, 116–124. [CrossRef] 26. Liu, Y.; Yang, C.; Li, P.; Li, S. A new process of extracting vanadium from stone coal. Int. J. Miner. Metall. Mater. 2010, 17, 381–388. [CrossRef] 27. Nguyen, T.; Lee, M. Solvent extraction of vanadium (V) from sulfate solutions using LIX 63 and PC 88A. J. Ind. Eng. Chem. 2015, 31, 118–123. [CrossRef] 28. Li, X.; Wei, C.; Deng, Z.; Li, M.; Li, C.; Fan, G. Selective solvent extraction of vanadium over iron from a stone coal/black shale acid leach solution by D2EHPA/TBP. Hydrometallurgy 2011, 105, 359–363. [CrossRef]

29. Li, X.; Wei, C.; Deng, Z.; Li, C.; Fan, G.; Li, M.; Huang, H. Recovery of vanadium from H2SO4-HF acidic leaching solution of black shale by solvent extraction and precipitation. Metals 2016, 6, 63. [CrossRef]

30. Gu, H.M.; Shih, W.Y.; Shih, W.H. Single-calcination synthesis of pyrochlore-free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method. J. Am. Ceram. Soc. 2003, 86, 217–221. [CrossRef] 31. Ivashchenko, V.I.; Turhci, P.E.A. Phonon softening and the phase transition in VN. Phys. Rev. B 2008, 78, 224113. [CrossRef] 32. Vernardou, D.; Apostolopoulou, M.; Louloudakis, D.; Spanakis, E.; Katsarakis, N.; Koudoumas, E.;

McGrath, J.; Pemble, M.E. Electrochemical properties of opal-V6O13 composites. J. Alloys Compd. 2014, 586, 621–626. [CrossRef] 33. Vernardou, D.; Pemble, M.E.; Sheel, D.W. In-situ FTIR studies of the growth of vanadium dioxide coatings

on glass by atmospheric pressure chemical vapour deposition for VCl4 and H2O system. Thin Solid Films 2007, 515, 8768–8770. [CrossRef] 34. Vernardou, D.; Louloudakis, D.; Spanakis, E.; Katsarakis, N.; Koudoumas, E. Functional properties of

APCVD VO2 layers. Int. J. Thin Films Sci. Technol. 2015, 4, 187–191. 35. Vernardou, D.; Bei, A.; Louloudakis, D.; Katsarakis, N.; Koudoumas, E. Oxygen source-oriented control of

atmospheric pressure chemical vapor deposition of VO2 for capacitive applications. J. Electrochem. Sci. Eng. 2016, 6, 165–173. 36. Bennett, L.H.; Massalski, T.B.; Giessen, B.C. Alloy Phase Diagrams. Mater. Res. Soc. Symp. Proc. 1983, 19.

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