The influence of process parameters on the chemical and structural properties of solution combustion prepared vanadium pentoxide

Materials Letters 261 (2020) 127095

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The influence of process parameters on the chemical and structural properties of solution combustion prepared vanadium pentoxide Esma Yilmaz 1, M. Seref Sonmez ⇑ Istanbul Technical University, Faculty of Chemical & Metallurgical Engineering, Department of Metallurgical, & Materials Engineering, 34469 Istanbul, Turkey

a r t i c l e

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Article history: Received 22 October 2019 Received in revised form 26 November 2019 Accepted 29 November 2019 Available online 30 November 2019 Keywords: V2O5 Mix-fuel Combustion synthesis Powder technology, ceramic

a b s t r a c t The present study shows the effects of the process parameters on the combustion synthesised’ powder properties using a systematic design of experiments. It describes the fabrication of high purity V2O5 powders with the manipulation of fuel types and fuel-to-oxidizer ratios. Additionally, for the first time the mixture of fuels was used to demonstrate clearly the role of the fuel on the powder properties. Flame temperature of the reaction was calculated for each parameter to understand how the reaction kinetics affect the powder properties especially primary particle size and morphology. Correlation between the physical and chemical properties of the powders and the precursor’s condition was successfully built. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Transition metal oxide (TMO) has been widely used in the field of electrochemistry due to the better physical properties and high chemical stabilities [1–3]. One of the most preferred TMOs as a functional material isV2O5 which is the most stable form among different oxidation states, because of their layered structure and relatively low band-gap (Eg = 2.3 eV) [1,2,4]. Most of the studies used conventional methods such as hydrothermal, spray pyrolysis, solid-state, and sol–gel to fabricate V2O5 [5,6]. Cathodic deposition of a binder- and conductive-agent-free V2O5 thin film electrode was also coated for sodium storage [7]. Due to the high cost requirement, time-consuming process parameters or highpressure application of these methods, as an alternative fabrication procedure, solution combustion synthesis (SCS) has attracted great attention as it allows the production of high purity oxides and requires low energy during processing. Besides, SCS also allows for the production of powder with desired properties by the manipulation of process parameters [1,8]. A few reports have been found about combustion synthesis of V2O5 [1,4]. However, contrary to the literature this paper aims to highlight the influence of the process parameters on the powder features with a detailed explanation of the science behind. The fab-

⇑ Corresponding author. E-mail addresses: [email protected] (E. Yilmaz), [email protected]. tr (M.S. Sonmez). 1 Present address: University of Manchester, School of Materials, United Kingdom. https://doi.org/10.1016/j.matlet.2019.127095 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

rication of V2O5 using unique fuel sources including starch and mix fuel was studied, the structural and morphological properties were analysed. The relationship between fuel type and fuel-to-oxidizer ratio was proved to affect crystallinity and particle agglomeration. Additionally, thermochemical calculations were investigated in depth and adiabatic flame temperatures (Tad) were calculated for different process conditions. 2. Experimental procedure Ammonium meta-vanadate (NH4VO3, Sigma-Aldrich) was dissolved in pure water with the addition of nitric acid (Eq. (1)) for the preparation of required oxidizer form of vanadium.

NH4 VO3ðsÞ þ 3HNO3ðlÞ ! VOðNO3 Þð3ÞðaqÞ þ 2H2 OðlÞ þ NH3ðgÞ

ð1Þ

Subsequently, the fuel (citric acid, glycine, starch, and mix fuel) was presented in the mixture (Eq. 2–5) and all precursors were mixed until a homogenous solution was obtained.

5 15 ð/  1ÞO2 2VOðNO3 Þð3Þ ðaqÞ þ /C 6 H8 O7 :H2 O þ 3 2 ðsÞ ðgÞ 25 /H O þ 3N2ðgÞ ! V 2 O5ðsÞ þ 10/CO2ðgÞ þ 3 2 ðgÞ 10 15 /C 2 H5 NO2 þ ð/  1ÞO2 3 2 ðsÞ ðgÞ
 20 25 5 ! V 2 O5ðsÞ þ /CO2ðgÞ þ /H O þ 3 þ / N2ðgÞ 3 3 3 2 ðgÞ

ð2Þ

2VOðNO3 Þð3Þ ðaqÞ þ

ð3Þ

2

E. Yilmaz, M.S. Sonmez / Materials Letters 261 (2020) 127095

Table 1 Relevant thermodynamic values [9]. Compound

DHf (kJ/mol)

Compound

DHf (kJ/mol)

Cp (J/(mol.K))

NH4VO3(s) C6H8O7H2O(s) (C6H10O5) n(s) C2H5NO2(s) HNO3(l)

1053.10 1543.80 1680.17 528.61 174.1

V2O5(s) H2O(g) N2 (g) CO2(g) NH3 (g)

1550.6 241.82 0 393.51 46.11

194.8–0.0163T 30 + 0.0107T 27.87 + 0.0043T 43 + 0.0110T 37.32 + 0.0187T

Fig. 1. Effect of the process parameters on the synthesized powder.

5 15 ð/  1ÞO2 2VOðNO3 Þð3Þ ðaqÞ þ /C H10 O5 þ 4 6 2 ðgÞ ðsÞ 15 25 /CO2ðgÞ þ /H2 O þ 3N2ðgÞ ! V 2 O5ðsÞ þ 2 4 ðgÞ

ð4Þ

the impurities and dried at 80 °C for 24 h. The black powder was calcined in a muffle furnace for 4 h at 500 °C, which lead to the unique yellow-orange colour change for V2O5. Tad was calculated by the Eq. (6) as a function of fuel compositions with using the thermodynamic data listed in Table 1;

Z

DH0 ¼ DH0f ¼  5 15 2VOðNO3 Þð3Þ ðaqÞ þ /ðC 6 H8 O7 þ 2C 2 H5 NO2 Þ þ ð/  1ÞO2 6 2 ðsÞ ðgÞ
 25 25 5 /CO2ðgÞ þ /H O þ 3 þ / N2ðgÞ ! V 2 O5ðsÞ þ ð5Þ 3 3 6 2 ðgÞ Moreover2, fuel-to-oxidizer ratio was adjusted at three different conditions; fuel rich (U < 1), stoichiometric (U = 1), and fuel lean (U > 1). After the homogenization, the mixture was heated up to 200 °C by using a heating mantle to promote the reaction. After a self-propagating spontaneous reaction, a fragile black foam was formed. The obtained powders were rinsed with ethanol to remove

2

Reaction was obtained to add Eq. (2) and (3), equilibrated in stoichiometric ratio and adjusted to give 1 mol of V2O5.

T ad

T0

Dcp products dT

ð6Þ

where DHf is the enthalpy value, To is the room temperature, and Cp is the molar heat capacity of the products at constant pressure. Thermal decomposition of the dried powder was analysed by DTA/TGA in air with the heating rate of 5 °C/min by a Perkin Elmer Diamond TGA instrument. The phase determination was carried out by a PANalytical X’pert PRO X-ray diffractometer using monochromatic Cu Ka radiation. The average crystallite size of the samples was calculated by Scherrer’s equation (Eq. 7);

D¼

K:k b:cosh

ð7Þ

where b is the full width of half maxima (FWHM), k is the wavelength of the X-ray, h is the angel of the reflection plane and the K value is a constant. The morphology of the particles was analysed

E. Yilmaz, M.S. Sonmez / Materials Letters 261 (2020) 127095

by a Quanta Feg 250 scanning electron microscopy. Grain size of the powders were measured by Nano-FlexTM Particle Size Meter.

3. Results and discussion Fuel can change the reduction rate, combustion temperature, and amount of gases generated during the redox reactions. This resulted in the synthesized powders having different structural properties. Fig. 1 demonstrates the effect of fuel sources with various fuel-to-oxidizer ratios on the chemical and physical properties of synthesized powder. As each fuel has different decomposition temperature and burning energy, Tad values depend on it and combustion types varies (Tad < 1000 °C; smouldering, Tad > 1000 °C; flaming). Fig. 1 shows mixed fuel has the lowest Tad while citric acid has the highest. Hereby, mix fuel provides lower particle size because violent reaction results in particle agglomeration. Additionally, fuel-tooxidizer ratio affect the Tad due to changing the total enthalpy value of the reaction that heats the products up to flame temperature. Theoretically Tad reaches the maximum in the stoichiometric condition (if participation of the oxygen from the air is neglected) so in the fuel rich state flame temperature is found in the lowest range which results in the finest powder [10]. However, starch shows anomalous results, although its Tad is lower than that of citric acid and glycine, it produces a larger particle size. To explain this, crystalline size and agglomeration degree (particle size/crystal

3

size) were calculated and a higher agglomeration degree was found for starch compared to other fuels. Firstly, the longest chain structure of starch creates binding between powder particles and secondly the amount of the released gas products. The evolution of a larger volume of gas (CO2 and H2O) directly affects the porosity, crystallinity, and particle size [1,11] because the movement of gas products results in the breaking of particle agglomeration. Mix fuel has the lowest flame temperature compared to the other fuels and besides this at end of the reaction between oxidizer and mix fuel, larger amount of gas product were released. Consequently, a recipe for the fabrication of finest particles can be summarised by lower Tad (mix fuel, fuel rich; 978 K) with the higher amount of gas products (20.5 mol for fuel rich condition for mix fuel). Fig. 2.A indicates the results of the TG-DTA analysis for V2O5 powder in the temperature range 25 °C to 750 °C with 25 °C/min scan rate. The TGA curve reflected nearly 13% of weight loss for each fuel source except mix fuel which lost 40%. Weight loss can be correlated with volatilization of excess water and ethanol up to 100 °C and the decomposition of functional groups up to 400 °C [12]. One sharp endothermic peak at about 680 °C equals to the melting point of V2O5 [2] with a small error band due to particle size difference. However, mix fuel contributed a different thermograph than the others with two exothermic peaks (at 230 °C and 340 °C) and one endothermic peak (at 400 °C). The first exothermic peak corresponded with the formation of vanadium metaphase, V6O13 while it transformed into V2O5 after 340 °C [13]. As a result, calcination

Fig. 2. A) Thermal behaviour of synthesized V2O5, B) XRD pattern of the synthesized V2O5 (a) different fuels (b) various fuel-to-oxidizer ratios for citric acid.

4

E. Yilmaz, M.S. Sonmez / Materials Letters 261 (2020) 127095

Fig. 3. SEM images of synthesized V2O5.

temperature was determined as 500 °C in order to overtake all phase transformation and weight loss. X-ray diffraction analysis was carried out after the calcination as given in Fig. 2.B. All patterns belong to the standard orthorhombic V2O5 (JCPDS No: 01-077-2418). SEM images of V2O5 were depicted in Fig. 3. Morphology is related with Tad implicitly with the fuel source. Citric acid and glycine (Fig. 3a-b) had nearly the same asymmetrical morphology and flake like shape due to the similar range for Tad and total amount of gas products while spherical morphology was observed for the mix fuel (Fig. 3c). Sponge like morphology was obtained with having bigger particle size for the starch (Fig. 3d).

defected on the chemical structure. Flake like morphology was formed for V2O5 powders synthesised by both citric acid and glycine, while lower particle size were obtained for glycine due to higher amount of gases evolved during reaction. According to the SEM images and particle size analysis, mix fuel led to lowest particle size due to the high amount of gas products and lower Tad. In overall, this study is stand out previous works in literature not only selecting difference fuel sources but also examining detailed thermochemical properties.

4. Conclusion

Esma Yilmaz: Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. M. Seref Sonmez: Conceptualization, Methodology, Software, Resources, Writing - original draft, Writing - review & editing, Supervision.

V2O5 powders were fabricated via combustion method with the manipulation of process parameters. The morphology of V2O5 powders varied for different fuels. However, any difference could not be

CRediT authorship contribution statement

E. Yilmaz, M.S. Sonmez / Materials Letters 261 (2020) 127095

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