Mechanosynthesis of nanocomposites in TiO2–B2O3–Mg–Al quaternary system

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Mechanosynthesis of nanocomposites in TiO2– B2O3–Mg–Al quaternary system Touraj Adhami, Reza Ebrahimi-Kahrizsangi, Bahman Nasiri-Tabrizi

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S0272-8842(13)01664-7 http://dx.doi.org/10.1016/j.ceramint.2013.12.048 CERI7781

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Cite this article as: Touraj Adhami, Reza Ebrahimi-Kahrizsangi, Bahman Nasiri-Tabrizi, Mechanosynthesis of nanocomposites in TiO2–B2O3–Mg–Al quaternary system, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2013.12.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mechanosynthesis of nanocomposites in TiO2–B2O3–Mg–Al quaternary system Touraj Adhami, Reza Ebrahimi-Kahrizsangi*, Bahman Nasiri-Tabrizi Advanced Materials Research Center, Materials Engineering Department, Najafabad Branch, Islamic Azad University, Isfahan, Iran

Abstract The mechanochemical behavior of TiO2–B2O3–Mg–Al quaternary system to synthesize various composite nanopowders was studied. A mixture of boron oxide and titanium dioxide powders along with different amounts of magnesium and aluminum was milled using a high-energy planetary ball mill to persuade necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). Results showed that the formation of composite nanopowders was influenced strongly by the reducing agents content. In the absence of Al (100 wt% Mg), TiB2 nanopowder was formed after 34 min of milling. In the presence of x wt% Mg–y wt% Al (x = 40 and 70; y = 100-x), mechanical activation was completed after 37–40 min which caused the formation of TiB2–MgFe0.6Al1.4O4 composite nanopowders. In the case of 10 wt% Mg–90 wt% Al, a ternary nanocomposite (TiB2–MgAl2O4–Al2O3) was produced after 43 min of milling. Besides, Al2O3–TiB2 nanocomposite was formed after 90 min of milling in the absence of Mg (100 wt% Al). From the SEM images, mechanochemical process reached a steady state after short milling times where the particles have become homogenized in size and shape. The reaction mechanism steps were proposed to clarify the reactions occurring during mechanochemical process. Keywords: Mechanosynthesis; Nanocomposite; MSR; Structural features; Reaction mechanism.

1. Introduction Borides as ultra high-temperature ceramics are generally desirable materials to be used in a variety of industries due to their improved properties at high temperatures. They have a high melting point and high hardness values at elevated temperatures [1]. Currently, there has been great demand for multiple di-borides such as titanium diboride (TiB2) and zirconium diboride (ZrB2) owing to their exceptional combination of hardness and thermal as well as chemical stability at high temperatures. However, their use in various industries is limited due to their inherent brittleness [2]. Therefore, the improvements of mechanical properties of these materials have been attempted by a number of studies [3–5].

*Corresponding author. Tel.: +98 3312291008; fax: +98 3312291008 E-mail address:[email protected], [email protected] (R. Ebrahimi-Kahrizsangi).

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According to literature, mechanosynthesis of boride-based nanocomposites is a successful strategy for enhancing the properties of monolithic borides [6]. Moreover, considerable research efforts have been directed towards the development of in situ nanocomposites, in which the matrix and reinforcements are formed in situ by exothermal reactions between elements and compounds [7,8]. Using this attitude, nanocomposites with a wide range of second-phase particles (carbides, nitrides, oxides and their mixtures) can be generated [9]. These composite structures are found to exhibit excellent mechanical properties because of the formation of ultrafine and stable ceramic reinforcements [2]. To date, self-propagating high temperature synthesis (SHS) [10], mechanical alloying (MA) [11], carbothermic reduction of titanium and boron oxides [12], boron carbide reduction of titanium oxide [13], combustion synthesis using TiO2, B2O3, Mg and 0–60 % salt mixture [14], and magnesiothermic or aluminothermic reduction of metal oxide–boron oxide mixture [15,16] were investigated to fabricate TiB2 and TiB2-based composites. Among them, the MA process is used to produce any quantity of nanopowder with controlled microstructure. In this approach, melting and precisely controlled conditions are not essential and the products have nanostructural characteristics. The simplicity, reproducibility, and low processing cost are the main advantages of this method [6,17]. Therefore, the MA process as employed in this research can be served, when the mass production of nanopowders is required. Over the past few decades, various mechanochemical processes were used to synthesize nano-metric TiB2 [11,15,18,19]. Welham and Bilgi et al. examined the mechanosynthesis of TiB2 using the activation of TiO2–B2O3–Mg ternary system. They reported that the mechanochemical reaction was completed after 10-15 h of milling [11,15]. In a different approach, nanoparticles of TiB2 were produced after short milling time (lower than 2 h) [18]. However, in many instances formation of borides was also not achieved directly by milling, but only after a suitable heat treatment [19]. For example, Nozari et al. studied mechanochemical behavior of TiO2–B2O3–Si system to produce TiB2 nanoparticles. They found that milling up to 50 h could not lead to the formation of TiB2. After thermal treatment at 1300 °C, the phase compositions were TiB2, Ti2O3, Si and SiO2. According to our recent findings [20], the formation of Al2O3–TiB2 nanocomposite during the mechanical activation of Al–TiO2–H3BO3 ternary system was influenced effectively by the milling time. In the present study, mechanochemical behavior of TiO2–B2O3–Mg–Al quaternary system to synthesize various composite nanopowders was studied for the first time. A mixture of B2O3 and TiO2 powders along with different amounts of Mg and Al (reducing agents) was activated using a high-energy planetary ball mill. The phase compositions, structural features as well as morphological characteristics of the nanocomposites were evaluated to determine the optimum composition. In addition, reaction mechanism steps were proposed to clarify the reactions occurring during the mechanochemical process.

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2. Materials and methods 2.1. Raw materials Boron oxide (B2O3, 99.95 % purity, mean particles size 30±5 m), titanium dioxide (TiO2, 99 % purity, < 2 m), magnesium (Mg, > 97 % purity, < 100 m), and aluminum (Al, 99.7% purity, particles size 40±5 m) were used as the raw materials. All chemicals were purchased from Merck and used without purification.

2.2. Explanation In general, it has been found that the reaction in TiO2–B2O3–Mg ternary system includes two stages: first, reduction of B2O3 and TiO2 with Mg which are highly exothermic reactions (nearly-instantaneous) and second, the reaction of elemental Ti and B which leads to the formation of TiB2 [18]. To examine the influence of the simultaneous presence of both reducing agents (Mg and Al) on mechanochemical behavior of TiO2–B2O3–Mg–Al quaternary system, various samples with different amounts of reductants were prepared. The specifications of the samples and general form of the total reactions are presented in Table 1. Based on this table, it is clear that the overall reaction was different depending on the weight fraction (wt%) of reductants in the powder mixture.

2.3. Mechanosynthesis and characterization Details of mechanosynthesis of nanocomposites in TiO2–B2O3–Mg–Al quaternary system is shown in Fig. 1. Mechanical activation was performed in a high-energy planetary ball mill using hardened chromium steel vials (vol. 125 ml) and balls (20 mm in diameter) without using any process control agent (PCA). The weight ratio of ball-to-powder (BPR), total powder mass, and rotational speed were15:1, 7 g and 600 rpm, respectively. To prevent the oxidation process, the milling vials were filled with high purity argon gas (99.99% purity) before ball milling. All the mechanosynthesized powders were leached in 18% HCl aqueous solution at 60 °C for 30 min to remove unwanted phases. X-ray diffraction (Philips X–ray diffractometer (XRD), Cu–K radiation, 40 kV, 30 mA and 0.02 °S–1 step scan) was employed to evaluate the phase purity and the crystallographic structural properties of the nanocomposites. The XRD patterns were collected over the 2 angular range between 10° and 90° at scan speed of 1°/min. "PANalyticalX'PertHighScore" software was also used to analyze the XRD data. The XRD patterns were compared to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS), which involved card #035-0741 for TiB2, #003-0858 for Mg2TiO4, #0040829 for MgO, #01-071-1237 for MgFe0.6Al1.4O4, #001-1157 for MgAl2O4, #011-0661 for -Al2O3, and #047-1292 for Al2O3. The phase volume fraction of TiB2 (PVFTiB2) was determined using semi-quantitative XRD by comparing the peaks of TiB2 (1 0 1), MgAl2O4 (3 1 1), MgFe0.6Al1.4O4 (3 1 1), -Al2O3 (3 1 1), and -Al2O3 {(1 0 4) and (1 1 3)} from XRD patterns [21].

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ITiB2 (101)

PVFTiB2

ITiB2 (101)  I x

u 100

(I)

Here, ITiB2attributed to XRD normalized integrated intensity values of TiB2 (1 0 1) and Ix represents those belonging to MgAl2O4 (3 1 1), MgFe0.6Al1.4O4 (3 1 1), -Al2O3 (3 1 1), and-Al2O3 {(1 0 4) and (1 1 3)} reflections. The values of adiabatic temperature (Tad), the maximum temperature which could be attained as a result of reaction heat, would be calculated using the following equation [22]: Tm

'Q

q 'H 298 

³

298

¦ C p ( Solid ).dT  'H m 

Tad

³ ¦C

p

( Liquid ).dT

0

Tm

(II) where Cp,

q 'H 298 ,and Q are specific heat capacity, standard enthalpy changes of formation at 298 K, and heat of

reaction, respectively. For microstructural examination, scanning electron microscopy (SEM; Vega©Tescan, Brno, Czech Republic) which operated at 15 kV acceleration voltage was used. X-ray energy dispersion spectroscopy (EDS) and elemental mapping analysis attached to the SEM were utilized for semi-quantitative examination of the samples. In addition, the edge-mode of SEM images was employed to apprise the volume fraction of grain boundary of the nanopowders.

3. Results and discussion 3.1. Phase evolution and structural features (XRD analysis) Fig. 2 shows the XRD profile of the milled sample in the absence of Al (S1) before the leaching process. In this case, the reaction was near-instantaneous and occurred after about 34 min of milling. According to this figure, the un-leached sample was composed of TiB2 and MgO. Besides, several additional peaks corresponding to Mg2TiO4 were detected. It has been reported that the formation of Mg2TiO4 as an intermediate phase may be caused by the reaction between unreacted TiO2 and formed MgO according to the following reaction[14]: TiO2 + 2MgO = Mg2TiO4

(R1)

In a different manner, it has been shown that the formation of Mg2TiO4may be due to the higher temperature of reaction (> 2000 oC) which favors the decomposition of MgO to Mg [23]. Accordingly, the reaction between Mg and un-reacted TiO2 results in the formation of Mg2TiO4 according to the following reaction: 2Mg + 2TiO2 = Ti + Mg2TiO4

(R2)

With the addition of excess Mg, no un-reacted TiO2 remains in the system and consequently the formation of Mg2TiO4 could be prevented [15]. In other un-leached samples, similar findings were obtained.

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The XRD patterns of the leached products are shown in Fig. 3a. It can be seen that the characteristic peaks of MgO and Mg2TiO4 were completely removed in the leaching process. In the absence of Al (S1), single-phase TiB2 was obtained after the leaching process. In the presence of 70 wt% Mg–30 wt% Al (S2), the combustion reaction occurred after 37 min of milling and as a result MgFe0.6Al1.4O4 spinel phase and TiB2 were dominant. In the case of S3 (40 wt% Mg–60 wt% Al), similar to the previous sample, TiB2–MgFe0.6Al1.4O4 composite was formed during the milling process. However, the mechanical activation was completed after 40 min. The manifestation of MgFe0.6Al1.4O4 spinel phase may be because of excessive adhesion of powders to the milling media. In general, during the MA process the powder particles get trapped between the milling media and undergo severe plastic deformation; fresh surfaces are created due to the fracture of the powder particles. In addition, collisions occur between the grinding medium and the vial, and also amongst the grinding balls. All these effects cause wear and tear of the grinding medium which ultimately result in the incorporation of the impurities into the powder. This contamination from milling equipment (vials and balls) is mainly substitutional [6]. Hence, for both samples S2 and S3, the incorporation of the impurity (Fe) into the powder mixture caused the generation of MgFe0.6Al1.4O4 spinel phase. It should be noted that the appearance of the spinel phase confirmed the occurrence of both aluminothermic and magnesiothermic reactions during milling process. When the reducing agents content reached 10 wt% Mg–90 wt% Al (S4), the phase compositions were changed as shown in the XRD patterns. From Fig. 3a, a ternary nanocomposite (TiB2–MgAl2O4–Al2O3) was formed after 43 min of milling as a result of both magnesiothermic and aluminothermic reactions. Magnesium aluminate spinel (MgAl2O4) as a structural ceramic possesses useful physical, chemical and thermal properties, both at normal and elevated temperatures. This spinel shows high resistance to attack by most of the acids and alkalis and has low electrical losses. It has a wide range of application in structural, chemical, optical and electrical industries due to these desirable properties [24]. According to these findings, TiB2–MgAl2O4–Al2O3 composite can be used in numerous industries such as cutting tools, wear-resistant substrates, and light weight armor [20]. It should be mentioned that increasing the Al content to 90 wt% prevented the excessive wearing of the milling media due to increasing the ductility of the system. Hence, no MgFe0.6Al1.4O4 spinel phase was diagnosed. In the presence of 100 wt% Al (S5), the combustion reaction occurred after 90 min milling which led to the formation of Al2O3–TiB2 nanocomposite. This result is in good agreement with the previous studies [20]. In accordance with these observations, the combustion time rose sharply from 34 to 90 min with increasing the Al content from 0 to 100 wt%. This behavior is related to the reactivity of the reducing agents (Mg is more reactive than Al). Isolines plots of the mechanosynthesized powders after leaching process are shown in Fig. 3b. This view displays the overlapping range of all scan data in an isolines (contour lines) plot. Isolines view is typically used to display multiple scans as stemming from a non-ambient experiment or a stress measurement. This method was also used to estimate the fraction of crystalline phase [25]. It can be seen in Fig. 3b that, for all the samples, overlapping range of all scan data was

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very low which confirmed the formation of high crystalline nanostructures after leaching process. This result indicates that the mechanosynthesized composite nanopowders had high thermal stability. Fig. 4 shows the magnified XRD region (3D view) in the range 44–45° and the percentage of TiB2 as a function of reducing agents content. According to this figure, the intensity of TiB2 (1 0 1) reflection decreased gradually with increasing the amount of Al to 100 wt% (Fig. 4a). This suggests that the percentage of TiB2 declined considerably to 26, when the Al content increased to 100 wt%. Depending on the milling conditions, the mechanochemical processes fall into two categories: progressive reaction and mechanically induced self-sustaining reaction (MSR) [2]. In the first approach, the reaction may extend to a very small volume during each collision, resulting in a gradual transformation. In the second attitude, if the reaction enthalpy is sufficiently high, a self-propagating combustion reaction can be initiated after a certain time. In self-propagating reactions, intimate contact between the reactant phases is an essential requirement. In a well-mixed sample, occurrence of the combustion reaction causes a rapid increase in temperature of the milling media, and provides the conditions for quick transformation. This type of reaction mechanism can be predicted by calculating Tad. It has been found that the reaction can be a self-sustained combustion, when the adiabatic temperature of the reaction is higher than 1800 K [22]. The values of Tad as a function of reducing agents content are shown in Fig. 5. As can be seen, Tad is 3282 K in the absence of Al (100 wt% Mg) and decreases to 3097 K, when the Al content rises to 100 wt%. Reduction in Tad by increasing the Al content is related to the heat generated by the reduction reactions. Since the heat generated by the magnesiothermic reaction is higher than that by the aluminothermic reduction, Tad decreased quickly with increasing the Al content and reached a minimum in the presence of 100 wt% Al (S5). Calculations show that the values of adiabatic temperature were higher than 1800 K. Thus, the mode of the reactions for all the samples was MSR.

3.2. Morphological characteristics (SEM observations) In general, particle shape analysis is one of the most difficult problems in powder technology because there is no general shape factor available that clearly differentiates all possible kinds of shapes [26]. However, the morphological features of the mechanically alloyed nanopowders may be determined using SEM images. In the present study, the morphological evaluation was performed for a large number of samples from which typical images (S2 and S3) are presented in Fig. 6. According to this figure, the composite nanopowders had high tendency to agglomerate owing to their relatively large specific surface which were associated with van der Waals interactions (Fig. 6a and d). From mechanochemistry point of view, when two adjacent primary particles collide, the coalescence may occur on the premise that these two particles share a common crystallographic orientation. Accordingly, two primary particles attach to each other and combine into a secondary one. Since the sizes of the secondary particles are still very small, it is reasonable that they will continue to collide and coalesce which may ultimately lead to the agglomeration [27]. At the beginning of MA process, system involved four ductile and soft powders. With continued deformation, the ductile system changed to ductile–brittle system

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by forming the ceramic components. The brittle components (like Mg2TiO4 and so on) were dispersed in the ductile matrix leading to work hardening and transformation of ductile particles into lamellar shapes. The brittle constituents were closely spaced along the interlamellar spacings. With further milling, the lamellae and brittle particles got further refined, the interlamellar spacing decreased, and the brittle components got uniformly dispersed. In fact, the equilibrium between fracturing brittle particles by trapping and ductile particles by work hardening got fine size, and made good dispersion in the matrix. As shown in Fig. 6b and e, the composite nanopowders were composed of several fine spheroidal particles with an average size of about 94 nm for S2 and around 142 nm for S3. This shows that the average particle size of the samples was influenced notably by the weight fraction (wt%) of reductants. In MA process, the rate of structural refinement was found to depend on the rate of mechanical energy input into the process and the work hardening rate of the material being processed [6]. In our samples, the tendency to cold welding has predominated over fracture with increasing the Al content. Accordingly, in the presence of 60 wt% Al (S3) the average particle size increased to 142 nm after 40 min of milling. However, in accordance with the SEM images, after 37–40 min of milling the MA process reached a steady state where the particles have become homogenized in size and shape. This result is in good agreement with the previous studies [28]. Fig. 6c and f display the edge-mode of SEM images of the S2 and S3 samples, respectively. It is clear that the synthesized powders had high volume fraction of grain boundary which confirmed the formation of composite powders with fine microstructures.

3.3.EDS analysis and elemental mapping The EDS analysis and elemental mapping were used to gain a better understanding of the XRD results, as shown in Fig. 7. According to the EDS spectrum (Fig. 7a), the main components of the composite nanopowder in terms of element were oxygen, magnesium, titanium, and aluminum. It should be noted that boron which was also present in the sample, could not be detected due to the strong absorption of the boron K line during the EDS analysis. Besides, Fe contamination caused by the excessive adhesion of powders to the vial and balls was not detected owing to the low relative Fe weight fraction in the milled sample. From elemental mapping analysis (Fig. 7b), the composite nanopowder showed a homogenous distribution of O, Mg, Al, and Ti elements which confirmed the formation of a uniform microstructure after 37 min of milling.

3.4. Reaction mechanism The mechanochemical treatment is a complex process that involves many variables and consequently the mechanism of phase formation during the mechanosynthesis is not well understood. However, using some usual thermodynamic relations could help to know the basic information about reactions and to estimate their behavior during the process [29].

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In the present study, using the obtained results by XRD, SEM, and EDS techniques as well as thermodynamic analysis, the following reaction mechanism steps have been proposed to clarify the reactions occurring during the mechanochemical process. Fig. 8 shows the reaction mechanism of the formation of nanocomposites during the mechanical activation of TiO2–B2O3–Mg–Al quaternary system. Contrary to the solid-state method by which heterogeneous particles with irregular shape are usually produced, powder synthesized using a mechanochemical process usually possesses a well-defined structure due to the perturbation of surface-bonded species as a result of pressure. Besides, the MA process introduces a variety of defects (vacancies, dislocations, grain boundaries, stacking faults, etc.) which raise the free energy of the system making it possible to produce metastable phases. These features enhance the thermodynamic and kinetic reactions between solids [6,30]. In mechanochemical reactions, the raw materials consumption rate may vary depending on the stoichiometric proportionality between the materials and the milling conditions. In the present study, the formation of composite nanopowders in TiO2–B2O3–Mg–Al quaternary system was influenced strongly by the chemical composition of powder mixture (amounts of reducing agents). Here, the thermodynamic calculations were carried out based on three assumptions: (a) milling container vessel was sealed and isolated (b) the reactions occurred in the standard condition (G = G°) (c) the activity coefficient of raw materials was equal to 1. According to these assumptions, the following reaction mechanism is presented. In the absence of Al (S1): In this case, the reaction in TiO2–B2O3–Mg ternary system included two stages: first, reduction of B2O3 and TiO2 with Mg which resulted in the formation of Ti and B (highly exothermic reactions) and second, the reaction of elemental Ti and B which led to the formation of TiB2 [18]. In the presence of 70 wt% Mg–30 wt% Al (S2): According to the presumptions, in the simultaneous presence of both reducing agents (Mg and Al), TiO2 is first reduced to Ti by Al and then the remaining TiO2 (if any) reacts with Mg. This behavior is related to the change in the standard Gibbs free energy, G°298, and the standard enthalpy of formation, H°298, that determines the feasibility and favorability of these reactions. This can be understood by considering the following reactions: (R3)

q CH 298

q 519.4 kJ / mol ,CG298

498.212 kJ / mol

3TiO2 + 4Al = 3Ti + 2Al2O3

(R4)

q 298

q 298

249.813 kJ / mol

TiO2 + 2Mg = Ti + 2MgO

CH

259.2 kJ / mol ,CG °

According to G 298 values, the thermodynamic basis for the occurrence of both reactions at room temperature has been provided owing to the negative standard Gibbs free energy of reactions. Furthermore, negative standard enthalpy of formation at 298 K (H°298) indicates that these reactions are exothermic. Because the standard Gibbs free energy for reaction (R3) is more negative than that for reaction (R4), the reaction (R3) occurs faster than the reaction (R4). On the

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contrary, B2O3 is first reduced by Mg and after that the residual B2O3 (if any) reacts with Al according to the following reactions: (R5)

q CH 298

q 531.3 kJ / mol ,CG298

513.540 kJ / mol

B2O3 + 3Mg = 2B + 3MgO

(R6)

q 298

q 298

387.926 kJ / mol

B2O3 + 2Al = 2B + Al2O3

CH

402.2 kJ / mol ,CG

According to the thermodynamic assumptions, in the presence of 70 wt% Mg–30 wt% Al (S2), the reduction of B 2O3 by Mg occurred first. Then TiO2 was reduced by Al and the residual TiO2 reacted with Mg. In the second stage, the reaction of elemental Ti and B caused the formation of TiB2. In addition, the unwanted phases including MgO and Al2O3 reacted with each other, which resulted in the formation of MgAl2O4. In this case, the incorporation of the impurity into the powder mixture caused by the milling (hardened chromium steel vials and balls) led to the generation of MgFe0.6Al1.4O4 spinel phase. It should be noted that Fe contamination was not diagnosed during the EDS analysis due to the low relative Fe weight fraction in the milled sample. In the presence of 40 wt% Mg–60 wt% Al (S3): In the presence of 40 wt% Mg–60 wt% Al(S2), the reduction of B2O3 by Mg occurred first. After that TiO2 and the remaining B2O3 were reduced by Al, respectively. In the second stage, the reaction of elemental Ti and B caused the formation of TiB2. Also, the reaction of MgO with Al2O3 led to the formation of MgAl2O4. In this case, similar to the previous sample, MgFe0.6Al1.4O4 spinel phase was formed due to the incorporation of the impurity into the powder mixture. The volume fraction of spinel phase went up with increasing the Al content from 30 to 60 wt%, as shown in XRD patterns. In the presence of 10 wt% Mg–90 wt% Al (S4): In this sample, the reduction of B2O3 and TiO2 by Al occurred first and followed by the reduction of residual B2O3 by Mg. In the second stage, the reaction of elemental Ti with B and unwanted phases with each other caused the formation of TiB2 and MgAl2O4, respectively. Due to the excessive increase of Al, a significant amount of alumina remained in the sample. In addition, the volume fraction of MgAl2O4 declined notably as a result of the excessive decrease of Mg in the reaction mixture. Therefore, a ternary nanocomposite (TiB2–MgAl2O4–Al2O3) was produced in this system. In the presence of 100 wt% Al (S5): When the Al content increased to 100 wt%, reduction of B2O3 and TiO2 with Al resulted in formation of Ti, B, and Al2O3. Afterwards, the reaction of elemental Ti and B led to the formation of TiB2 and consequently Al2O3–TiB2 was produced after 90 min of milling. This result is in good agreement with the previous studies [20]. Recalling from the above findings, it can be concluded that the formation of nanocomposites in TiO2–B2O3–Mg–Al quaternary system was influenced noticeably by the type and weight fraction of reductants. From the thermodynamic point of view, all the presumed reactions can be possible to occur due to the negative G°298 values. Meanwhile, the

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feasibility of such reactions has increased during milling through creation of the new surfaces, reduction of the diffusion distances, and a rise in local temperature [28].

4. Conclusions The influence of reducing agents content on mechanochemical behavior of TiO2–B2O3–Mg–Al quaternary system to produce various composite nanopowders was investigated. Examination of the sub-reactions revealed that B2O3 and TiO2 were easily reduced by both Al and Mg. With continued deformation, nanocrystalline TiB2 and several nanocomposites like TiB2–MgFe0.6Al1.4O4, TiB2–MgAl2O4–Al2O3, and Al2O3–TiB2 were formed after short milling times (lower than 2 h). The evaluation of Tad as a function of reducing agents content confirmed that the reaction mode in all the samples was MSR. Thermodynamics analysis showed that overall reaction enthalpy and adiabatic temperature altered by variation of the ratio of reducing agents. According to the SEM images, the average particle size increased from 94 to 142 nm with increasing the Al content from 30 to 60 wt%. However, after 37–40 min of milling the mechanochemical process reached a steady state. Based on the obtained data, it can be concluded that the preparation of various composite nanopowders by aluminothermic and magnesiothermic reductions in TiO2–B2O3–Mg–Al quaternary system was influenced notably by the type and weight fraction of reductants.

Acknowledgment The authors are grateful to research affairs of Islamic Azad University, Najafabad Branch for supporting of this research.

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List of Figures Caption Fig. 1 Details of mechanosynthesis of nanocomposites in TiO2–B2O3–Mg–Al quaternary system. Fig. 2 XRD profile of the milled sample in the absence of Al (S1) before the leaching process. Fig. 3 (a) XRD patterns and (b) isolines plots of the leached products. Fig. 4 (a) Magnified XRD region (3D view) in the range 44–45° and (b) the percentage of TiB2 as a function of reducing agents content. Fig. 5 The values of Tad as a function of reducing agents content. Fig. 6 The morphological features of the composite nanopowders. SEM images of (a,b) S2 and (d,e) S3; Edge-mode of SEM images of (c) S2 and (f) S3. Fig. 7 (a) EDS analysis and (b) elemental mapping images of S2 sample. Fig. 8 Reaction mechanism of the formation of nanocomposites during the mechanical activation of TiO2–B2O3–Mg–Al quaternary system.

List of Table Caption Table 1 Specifications of the samples and general form of the total reactions in TiO2–B2O3–Mg–Al quaternary system.

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Sample S1 S2 S3 S4 S5

Table 1 Specifications of the specimens and general form of the total reactions. Reducing agents (wt%) General form of the total reactions 100 wt% Mg TiO2 + B2O3 + 5Mg = TiB2 + 5MgO 70 wt% Mg–30 wt% Al 2TiO2 + 2B2O3 + 7Mg + 2Al = 2TiB2 + MgAl2O4 + 6MgO 40 wt% Mg–60 wt% Al 3TiO2 + 3B2O3 + 6Mg + 6Al = 3TiB2 + 3MgAl2O4 + 3MgO 10 wt% Mg–90 wt% Al 6TiO2 + 6B2O3 + 3Mg + 18Al = 6TiB2 + 3MgAl2O4 + 6Al2O3 100 wt% Al 3TiO2 + 3B2O3 + 10Al = 3TiB2 + 5Al2O3

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(Reaction 1) (Reaction 2) (Reaction 3) (Reaction 4) (Reaction 5)

Figure1

Figure2

Figure3

Figure4

Figure5

Figure6

Figure7

Figure8