Synthesis of Novel Electrospun Composite Nano Powders of the Quaternary Ti-Al-O-B System


 In the present work, electrospinning was applied to develop multicomponent oxide, boride, and borate nanostructures in a quaternary Ti-Al-O-B system. Different molar ratios of B/(Ti + Al) (0.8, 1.6, and 2.4) were employed and evaluated. Imaging with the field emission scanning electron microscope (FESEM) and the transmission electron microscope (TEM) revealed that after one hour of thermal treatment at 1100 °C, the hybrid electrospun nanofibers (NFs) in the fibrous platform transformed into nanoparticles (NPs), nano-needles, and nano-whiskers at B/(Ti + Al) molar ratios of 0.8, 1.6, and 2.4, respectively. The binding energies were investigated by X-ray photoelectron spectroscopy (XPS), whereas the phase study was conducted via the X-ray diffraction (XRD) technique. The results confirmed the formation of nanostructured ceramic powder platforms composed of the multiple components, namely oxides (e.g., B doped TiO2; Al2O3), borides (TiB, TiB2, Ti2B5, TiB12, and AlB2), and borates (TiBO3; Al18B4O33). Simultaneous thermal analysis (STA) of the Ti-Al-O-B mats indicated that the borides and borates formed consecutively at temperatures above 800 °C through reactions involving molten B2O3. We found that the obtained NPs were well arranged and sintered together throughout the fibers.


Introduction
Ceramic particles, due to having desirable physical, chemical, and mechanical properties, are widely used as catalysts, semiconductors, electromagnetics, filtration media, thermal insulators, fillers, and reinforcements [1][2][3]. Nanostructured ceramics such as nanoparticles (NPs) and nanofibers (NFs) have drawn much attention in particular applications due to their unique properties such as the high surface area to volume ratio, low density, low heat conductivity, high hardness and excellent toughness [4][5]. The development of multicomponent ceramic products from nanostructured oxides, borides, and borates is an efficient solution for overcoming the hardness, corrosion resistance, high melting point, and abrasive requirements of designed systems [7,8]. Thus, a quaternary Ti-Al-O-B system can logically be used for exhibiting outstanding oxides, borides, and borates of titanium and aluminum components on the nanoscale.
Titanium borides have stoichiometric diversity in the forms of TiB, TiB2, Ti3B4, Ti2B5 as well as TiB12 [8,9]. Several routes, such as pack cementation, magnetron sputtering, and selfpropagating high-temperature synthesis (SHS), have been developed to produce the titanium borides as well as titanium borate (TiBO3) [10][11][12]. However, there is one main problem in the Ti-B system for production of some kinds of titanium borides. Some borides (e.g., Ti2B5 and TiB12) are formed via particular production methods such as SHS [13]. Titanium boride powders have received considerable attention for mechanical applications due to the presence of covalent bonds within the titanium matrix [14,15]. The diffusion coefficient of B into the titanium matrix has a significant role in the formation of the specific kind of titanium boride [16]. According to the literature review, by controlling the production reactions in the solidliquid TiO2-B2O3 binary system, the desired titanium boride and borate species can be generated [17].
Aluminum borates (Al18B4O33 and Al4B2O9) are the main stable phases present in the Al2O3-B2O3 binary system. Al18B4O33 is stable until up to 1900 °C and is generated with lower B2O3 amounts, whereas Al4B2O9 is produced when more B2O3 is present and is stable only below 1100 °C [18]. Aluminum borates have a strong tendency toward taking a needle-like morphology [19]. Production of aluminum borate needles or whiskers occurs between 600 and 800 °C; these products are often utilized in metal alloys for reinforcement, insulation, and improving oxidation resistance [20,21]. It is noteworthy that their application is not limited to metal matrix; Wang et al. [22] utilized the one-dimensional Al18B4O33 as a junction between two alumina pieces at high temperatures. The AlB2 powders can also be prepared via the aluminothermy reaction in the Al2O3-B2O3 binary system. This kind of boride can be used to induce oxidation resistance because it is decomposed into Al and B at high temperatures [23].
Several routes are available for the preparation of nanostructured ceramic NPs and NFs, including sol-gel, crystallization from molten salts, combustion synthesis, and melt spinning [24][25][26]. However, the electrospinning method presents a simple and cost-efficient process that yields continuous fibers with average diameters of nano to micrometers [27,28]. In fact, electrospinning has recently facilitated the synthesis of nanocomposite particles and nanostructured thin films [29,30]. There are two approaches toward developing ceramic NFs via the electrospinning procedure: ceramic reagents are either blended as ceramic NPs or dissolved into a polymer solution as ceramic precursors. It should be added that post-heat treatment may regulate the final product as amorphous to highly crystalized ceramic NFs [28].
Dai et al. [31] were the first to attempt the preparation of borate fibers; they simply synthesized the predominating Al4B2O9 species and traces of Al18B4O33 crystals via the electrospinning method. Later, Ozdemir et al. [32] investigated the role of the viscosity of the electrospinning solution for the production of aluminum borate NFs. In another experimentation with the electrospinning technique, Song et al. [33] studied the formation of mullite-type composite NFs in the Al2O3-SiO2-B2O3 system. Nevertheless, there is a lack of studies regarding the NFs of borides and borates, especially in the ternary TiO2-Al2O3-B2O3 system.
This study aimed to investigate the potential of the electrospinning procedure in the formation of diverse boride and borate nano-arrays of the quaternary Ti-Al-O-B system. The novelty of this study was the aim of achieving synergetic effects between titanium boride NPs and the aluminum borate nano-whiskers. Since the B/(Ti+Al) molar ratio has a significant impact on the physical and mechanical properties of borides [34], an electrospun Ti-Al-O-B powder was first synthesized using a B/(Ti+Al) molar ratio of 0.8. Afterward, Ti-Al-O-B powder samples were developed with two and three times higher boron contents (B/(Ti+Al) molar ratios of 1.6 and 2.4, respectively). Finally, a range of analytical experiments was conducted to facilitate an understanding of the mechanisms involved in the formation of the powders.
The solution was stirred for 2 h at room temperature followed by 45 min of ultra-sonication to obtain a homogenous solution. Secondly, the 0.  Table 1, along with the sample codes.

Characterizations
The microstructures of the as-spun mats were studied by scanning electron microscopy (SEM, model JEOL JSM 840A). The powders were investigated using a field emission scanning electron microscope (FESEM; model T-Scan) equipped with an energy dispersive X-ray spectrometer (EDS). The ImageJ software (1.38x NIH USA) was applied to calculate the size of fibers and particle diameters. The binding energies of Ti, Al, O, and B were evaluated by Xray photoelectron spectroscopy (XPS; model BESTEC) with monochromatic Al Kα radiation (hν=1486.7 eV). High-resolution scanning was performed with a binding energy of 1s for O and B, and 2p for Ti and Al. The XPS analysis was carried out under 10 -10 mbar after sputtering with argon ions. The phase study of powders was conducted by X-ray diffraction technique (XRD; model Philips, PW1730 Xpert; 1º incidence angle; kα Cu; λ =0.154 nm). The X'pert HighScore Plus software (Ver. 2.2b) was used to interpret the phase formation data. The thermal behavior of mats in the range of 25-1000 °C was studied by thermogravimetry (TG) and differential thermal analysis (DTA) with a simultaneous thermal analyzer (STA; model PA Q600). The transmission electron microscope (TEM; model Philips EM208S, 100 kV) facilitated the high-resolution morphological characterization of the prepared ceramic particles.

Results
The synthesis of nanofibrous Ti-Al-O-B powders has not been reported before this study.
Hence, to evaluate the balance of the precursors in the hybrid electrospun solution, the sample with a 0.8 molar ratio of B/(Ti+Al) was synthesized. The micrographs of the electrospun nanofibrous TAOB-0.8 mat before and after calcination are displayed in Fig. 1 (a and b). As seen, straight fibers without beads were formed via the electrospinning procedure. The average diameter of the hybrid fibers was 233 ± 16 nm, providing an appropriate platform for the synthesis of nanostructured ceramic Ti-Al-O-B products. The results showed that the hybrid straight fibers transformed into ceramic particles arranged into a fibrous structure. The average sizes of the particles and fibers were 94 ± 41 and 217 ± 39 nm, respectively. The findings confirmed that a nanostructured ceramic powder was synthesized via the electrospinning method.
To understand the intra-molecular bonds within the developed powder, XPS spectra were  [35]. According to the area under the peaks, it can be said that oxygen had similar direct bonds with the other elements inside the Ti-Al-O-B powder. The B1s spectrum revealed a high peak at 192.8 eV related to B in the B2O3 compound, a medium peak at 190.9 eV related to B in aluminum borate [7], and a small peak at 187.3 eV related to B in titanium borides [14].
Finally, we did not find a peak relevant to the Ti-Al binding energy, meaning that Ti and Al  Fig. 4 (a to c) were examined. As seen, sintered particles with an average size of 94 nm were present besides larger particles (up to 275 nm) in the TAOB-0.8 powder; most particles had polygonal and sphere-like morphology. In contrast, finer particles with needle-like morphology were observed for the powders in the TAOB-1.6 powder. In addition, the larger particles vanished, though some spherical particles remained inside the powde. According to the micrograph illustrated in Fig. 4 (b), needle-like particles with an average width of 44 ± 5 nm and an average length of 113 ± 19 nm were the major component of the TAOB-1.6 powder. By enhancing the B/(Ti+Al) molar ratio up to 2.4, whisker-shaped powders with an average width of 62 ± 18 nm and an average length of 595 ± 186 nm replaced the needle-like powders; few spherical particles were also observed.
The surface EDS spectra of all Ti-Al-O-B powders are provided as graphs in Fig. 5 (a to c).

Discussion
The XPS and XRD analyses indicated that no phase corresponding to the Al-Ti, Al-Ti-O, or Al-Ti-B systems had formed. Therefore, the possible growth mechanism of the Ti-Al-O-B powder from a hybrid electrospun polymer/ceramic mat should be described based on the thermal behaviors and individual chemical reactions between the TiO2-B2O3 and Al2O3-B2O3 oxide systems [36]. Fig. 6 (a) presents a schematic cross-sectional view of a hybrid polymer/ceramic fiber, including an amorphous Al-O-B and TiO2 NPs dissolved in the PVP matrix. The STA curves (including TG and DTA) of all mats are presented in Fig. 7 (a and b).
The results show that weight loss commenced immediately at the beginning of the process. The The results of the STA analysis depicted in Fig. 7 (a and b) confirm the decomposition of Al(NO3)3 at 350-400 °C, which is one of the vital reactions that occurred in the Ti-Al-O-B system. This is because Al2O3 sites are potential nuclei for the formation of aluminum boride and aluminum borate within the fibers. The schematic presentation of this phenomenon is presented in Fig. 6 (c). The decomposition of Al(NO3)3 took place with a weight loss of 20-30 % in all samples due to the remarkable production of the NO2 and O2 off-gases based on Eq. 4 [37].
With further increase in temperature, the homogenously dispersed solid B2O3 became smelted at 450 °C, before the molten B2O3 surrounded the Al2O3 nuclei as well as the TiO2 NPs inside the fibers (Fig. 6 (d)). According to the STA analysis, with a further increase in temperature, the decomposition of PVP was intensified (Fig. 7). The PVP matrix was wholly eliminated at 600-650 °C, and the initial weight of the platforms declined by approximately 78, 82, and 86 % with respect to the TAOB-0.8, TAOB-1.6, and TAOB-2.4 samples, respectively. After the PVP matrix weight residue became zero, a ceramic matrix containing solids and liquids remained throughout the fibers, as schematically presented in Fig. 6 (e). The number and nature of the solids have significant roles in solid-liquid reactions [38]. The EDS spectra confirmed the absence of the C element inside all powders. Above 800 °C, the circumstances for the production of borides and borates are provided via the vapor-liquid-solid reactions as not only solids were covered by the molten B2O3, but also a matrix based on the molten B2O3 was formed [39]. The DTA curves in the range of 800 to 1000 °C shown in Fig. 7 (c) allowed the identification of peaks corresponding with borides and borates. Further studies were performed by the derivative of heat to temperature (dH.dT -1 ) in the range of 800 to 1000 °C to understand the formation of the borides and borates. Figure 7 (d) presents the complex temperatures for assuming the formation of the boride and borate species. According to the studies, the number of exothermic peaks increased, and the heat flow intensified after increasing the B/(Ti+Al) molar ratio. In other words, a higher amount of B2O3 relative to Al and Ti oxides promotes the formation of borides and borates. The unreacted TiO2 NPs reacted specifically with the surrounding molten B2O3, and thus TiB was generated according to Eq. 5.
The O2 gas penetrates the TiO2 NPs and causes particle breakage. The broken TiO2 NPs, having a higher surface area, react more readily with B2O3. The mechanism of TiO2 NP breakage is presented in Fig.6 The vast amount of hot O2 gas caused the oxidation of TiB to TiBO3 shown in Eq. 9 [40]. The presence of titanium borate within all the powders was confirmed by both the XRD and the 2p3/2 and 2p1/2 peaks present in the XPS spectrum of Ti. The re-consumption of TiB in the oxidation reaction was the main reason for its low amount in the final products.
Notably, the reactions between the Al2O3 nuclei and molten B2O3 also took place at above 800 °C [22,32]. The majority of the Al2O3 crystals were replaced with Al18B4O33 according to Eq.
10, and homogenously dispersed Al18B4O33 grains were therefore created alongside the fibers (see Fig. 6 (f)). The AlB2 phase also can be synthesized directly via the reaction between Al2O3 nuclei and molten B2O3 at elevated temperature, as shown in Eq. 11, respectively [23]. The XRD patterns revealed that the amount of AlB2 was low in comparison with Al18B4O33. This is because most of the Al(NO3)3 transformed into the Al2O3 nuclei, and Al18B4O3 subsequently replaced the Al2O3 nuclei.
With increased temperature and the presence of more B2O3 within the powders, all titanium and aluminum borate and boride particles grew in size. As schematically presented in Fig. 6 (g), grain growth continued until the grains could attach well together. Intra-grain diffusion was activated at the flat grain boundaries [41], and the grown grains consequently became sintered together well by a shrinkage throughout the fiber diameter, as shown in Fig.6 (h) [42].
The sintering of borides, borates, and oxides together created the brittle fibers that may break to shorter fragments. The microscopic findings obtained from Fig. 4 (a) obviously confirm the fibrous microstructure constructed by the sintering of NPs. It is worth mentioning that although the borides have difficult sintering behavior due to the presence of strong covalent bonds [43], the nanostructured platform promoted sintering because of the high surface area to volume and the large number of particle junctions. Our findings also demonstrate that with augmentation of the B/(Ti+Al) molar ratio to 1.6, nano needle-like particles became the primary constructive agents of the powder. Grain growth in a preferred crystallography orientation is a significant specification of Al18B4O33 [44]. The reaction between Al2O3 nuclei and molten B2O3 (Eq. 10) intensified when the B/(Ti+Al) molar ratio was increased up to 2.4. In subsequence, the needlelike grain growth of Al18B4O33 also intensified, leading to the formation of Al18B4O33 whiskers, as confirmed by the micrographs shown in Fig. 4