Preparation of di-boride coatings by electrophoretic deposition in nanoparticle-containing molten inorganic salts

Molten inorganic salts containing solid nanoparticles with a stable and uniform dispersion have attracted great attention as efficient heat transfer and storage materials and for catalysis for chemical reactions. Electrophoretic deposition in molten inorganic salts containing nanoparticles, have not been reported in the literature, compared with the related wide investigations in aqueous and organic suspensions. Here we report the possibility of electrophoretic deposition of nanoparticles in high-temperature molten salts. In molten fluorides and chlorides, cell voltages of 1.2-1.5 V below the decomposition voltage of the electrolytes, were applied to perform the electrophoretic deposition of nanoparticles (e.g., TiB2 and ZrB2) on different cathode substrates, resulting in compact and adhesive coatings with high hardness. These findings should present opportunities to synthesize additional coatings and films via the proposed process.

a, Schematic of electrophoretic deposition (EPD) in molten salts. b, X-ray diffraction (XRD) pattern of the deposited coating on a molybdenum substrate in TiB2 nanoparticle-containing molten NaF-AlF3 (61:39 mol) system under a potentiostatic mode (1.2 V) at a temperature of 980 ℃. c-e, Scanning electron microscopy (SEM) images (c,d) and photograph (e) (polished) (note the reflection of the pencil) of TiB2 coating. f, Micrographs of the cross-section of TiB2 coating. g, The dependence of indentation hardness and elastic modulus on the indentation depth for the TiB2 coating.
The deposit on the molybdenum cathode was analyzed by X-ray diffraction (XRD) to be determined as a TiB2 coating (Fig. 1b). The surface morphology of TiB2 coating was characterized by scanning electron microscopy (SEM) (Fig.1c), showing a flat appearance. The coating was ground by 400# SiC sandpaper, then exhibited a similarly metallic luster (Fig.1e). The cross-section of TiB2 coating as demonstrated in Fig. 1f, is revealing a flat, compact and adhesive layer, with a thickness of around 12 µm. Fig. 1g shows the dependence of hardness and elastic modulus of the TiB2 coating on the indentation depth, with a maximum indentation depth of 500 nm. After ~150 nm, the hardness and modulus remain relatively unchanged with the increase of indentation depth. Hardness of ~40 GPa and elastic modulus of ~500 GPa are obtained (compared with the reported values of 25-35 GPa for hardness and 500-560 GPa for elastic modulus for TiB2 14 ). From these findings, it is concluded that the EPD of TiB2 nanoparticles has been successfully achieved in molten fluorides, yielding a coherent TiB2 coating with high hardness. In order to figure out the suspension state of added TiB2 nanoparticles in the above melt, we analyzed the local nanoparticles-containing electrolyte. We sampled the upper part of molten NaF-AlF3 salts with the introduction of TiB2 nanoparticles 5 h before, cooled it down to room temperature quickly and characterized it by SEM (Fig. 2). It was found that TiB2 nanoparticles still existed in the sampled solidified NaF-AlF3, which proves the stable suspension of TiB2 in the molten fluorides for at least 5 h. Therefore, the achievement of the stable suspension in molten salts and the EPD on the cathode indicates the formation of stable and positively charged TiB2 colloids in molten NaF-AlF3 indirectly.
It is noted that a dense TiB2 coating, without any inclusion of residual fluoride electrolyte, has been fabricated at only 980 ℃. Actually, powdery TiB2, with a melting point up to 3225 ℃, is difficult to be sintered to full density 16,17 . For example，Rabiezadeh et al. made attempts to sinter TiB2 nanoparticles without sintering additives under hot press at 1700 ℃ for 2 h and obtained 92% of the theoretical density 18 . In the present paper the preparation of dense TiB2 coating is probably due to the occurrence of electroosmotic motion of molten salts during the EPD of TiB2 nanoparticles. As shown in Fig.1a, as TiB2 nanoparticles migrates to the cathode, the electroosmotic flow of molten salts inside the not yet densified TiB2 layer is driven away from it. In fact, it has been reported that during EPD in aqueous suspensions, the nanoparticles rearrangement is accelerated via the electroosmotic flow, leading to higher packing rates of nanoparticles [19][20][21] . It has also been suggested that for liquid assisted flash sintering, the existence of electroosmotic driven flow benefits the densification of sintering of nanoparticles 22 . In this paper, from the SEM image of the surface morphology (with higher magnification) of TiB2 coating (Fig.1d), large nanoplates consisting of original nanoparticles were observed, which represents an intermediate state during the growth of TiB2 nanoparticles. It seems that during the EPD, the EPDed TiB2 nanoparticles on the surface of the cathode gather and grow up to form these large nanoplates with a thickness of 100-200 nm and a length of ~500 nm, with the assistance of electroosmotic flow. Finally, these nanoplates continuously grow up to collect each other to form the dense TiB2 coating. Summarily, the occurrence of EPD of nanoparticles is accompanied by the existence of electroosmotic driven flow of molten salts, which probably promotes the forming of dense coating at a high temperature. These results also provide a novel and substantial addition to electrokinetic phenomena in colloid science.
In the same molten salts at 980 ℃， a graphite cathode was employed to perform the electrophoretic deposition of TiB2 nanoparticles (with a content of 70 g/L), via the similar procedures as the above description. In Fig. 3 a, the photo and micromorphology of the EPDed coating are presented. An apparent coating was observed on the graphite substrate and the surface micromorphology seemed relatively flat. Fig. 3 b,c shows the fractured cross-section and XRD result of the coating, which was proved to be a flat, compact and adhesive TiB2 coating with a thickness of around 50 µm. Its micro-hardness was measured by a Vickers micro-hardness tester, giving an average value of 3500 HV0.1. These results indicate that the successful electrophoretic deposition of TiB2 nanoparticle on a graphite substrate in molten fluorides has been accomplished, which could lead to the possibility of producing wettable TiB2-coated carbon cathode for aluminum electrolysis 23 .  The electrophoretic plated coating, exhibiting a similarly metallic appearance (top-left inset of Fig. 4a), on the graphite substrate was characterized by SEM (Fig. 4a). It was found that the coating was made up of micro-particles, which consisted of nanoparticles as observed under higher magnification. The XRD pattern (Fig. 4b) with broad diffraction peaks proved that it was a ZrB2 coating with possible nanocrystal feature. The fractured cross-section of the ZrB2 layer was observed by SEM (Fig. 4c,d). It is dense and adhesive, with a thickness of approximately 2 µm, over the whole cross-section showing a nanostructure under higher magnification. Fig. 4e shows the transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern (top-right inset of Fig. 4e) of the sample. SAED pattern included continuous but rather broad diffraction rings, which indicated polycrystalline microstructure with possible nanocrystal structure of the ZrB2 coating. High resolution TEM (HRTEM) image is shown in Fig. 4f, which confirmed the presence of nanocrystallites. From the original nanoparticles to nanocrystal and compact ZrB2 coating, the maintaining of the nanocrystal structure reveals the possibility of assembling nanoparticles by EPD in molten chlorides. In addition, the hardness and the elastic modulus of the ZrB2 coating was assessed with a nanoindenter, giving the values of 20 GPa and 146 GPa, respectively. Therefore, the EPD of ZrB2 nanoparticles has been successfully conducted in molten chlorides, resulting in a dense and adhesive ZrB2 coating with a nanocrystal structure.
Compared with that in aqueous or organic solutions, EPD in high-temperature molten salts has the advantages as generally applied in aqueous or organic solutions 7 . Secondly, EPD in high-temperature molten salts results in a dense and adhesive coating in one step. While in aqueous or organic solutions, the resulting deposits with high porosity, low density and poor adhesion generally need further drying and sintering, however, resulting in cracking, which is difficult to be dealt with 7,24 . It is our anticipation that the EPD of nanoparticles in molten inorganic salts should become an important method for synthesis of ceramic coatings and films.
In summary, for the first time the EPD of ceramic di-boride nanoparticles has been demonstrated in molten halides, resulting in dense and adhesive coatings with high hardness. These findings could open opportunities to fabricate coatings and films by the proposed novel process. Molten inorganic salts represent a very attractive class of solvents (including halides, sulfides, nitrates and carbonates) for the stable dispersion and synthesis of nanomaterials (e.g., borides, oxides, carbides and nitrides) [25][26][27] . It may be assumed that the EPD can be applied to a wider range of nanoparticles and in other molten salts systems, possibly resulting in preparing more types of coatings and films with desired properties. Furthermore, if the EPD is employed for in-situ synthetic nanoparticles in molten inorganic salts, it is likely to result in additional new phenomena.

Methods
Nanoparticle-containing solid salts preparation. Two types of solid salts (i.e., fluorides and chlorides) were employed in this paper. The processes for manufacturing mixtures of solid salts and nanoparticles are the same, with the schematic shown in Fig. 5. The mixed solid NaF (>99%, Aladdin) and AlF3 (>98%, Guanfu) or NaCl (>99%, Sinopharm) and KCl (>99%, Sinopharm) were milled with 316 stainless milling balls (φ1mm:φ3mm=3:2 mass) for 3 h in a planetary ball mill (HLXPM-φ10X4, Hengle), with both revolution and rotation speeds of 120 r.p.m. The milled solid salts were blended with nanoparticles (TiB2 (mean size 50nm, 99%, ChaoWei) and ZrB2 (mean size 50nm, 99%, ChaoWei)), in a weight ratio of 1:1, followed by ultrasonic dispersion in acetone at 100 kHz for 2 h. Finally, the mixture in a slurry form was dried in a vacuum drying oven at 120 ℃ for 1 h. Figure 5 Stepwise schematic of preparation of nanoparticles-containing solid salts.
Electrophoretic deposition process in molten salts. For all the experiments of EPD, the procedures of preparation of high-temperature molten salts containing nanoparticles are as follows. The mixture of NaF and AlF3 (61:39 mol) or NaCl, KCl and AlCl3 (>99%, Aladdin) (45:45:10 mol) were introduced to a graphite crucible, which was placed in an electric resistance furnace, heated up to temperatures of 980 ℃ and 710 ℃, respectively. After the mixture was melted, the prepared solid salts containing nanoparticles mentioned in the above section were added to the molten salts, resulting in molten salts including nanoparticles with a required content. The resulting molten mixture stabilized for 10-20 min, followed by the subsequent process. As shown in Fig. 1a, the high-temperature molten salts including nanoparticles were contained in a crucible. A molybdenum or graphite cathode (4*1*25 mm or 8*3*30 mm), with an immersion depth of 10-20 mm, and a graphite anode with an immersion depth of 15-20 mm were placed in the cell. Before use the cathodes were ground by successively finer SiC sandpaper, followed by an ultrasonic cleaning in acetone and dried for use. In molten fluorides and chlorides, the temperature was controlled at 980 ℃ and 710 ℃, respectively. Pre-electrolysis under 1.5 V cell voltage for 1 h was needed to remove some impurities. The EPD was under a constant voltage mode, ranging from 1.2-1.5 V for 1 h. A DC power supply (HLR-3660D, Henghui) was used to perform the potentiostatic control. All the EPD were performed in an argon atmosphere.
After the EPD was terminated, the coated cathode was removed from the cell. After the coated cathode was withdrawn from the molten chlorides, it was immersed in deionized water for 1h to remove the solid chloride salt residues, in acetone for 5min to remove the water, followed by drying. After the coated cathode was withdrawn from the molten fluorides, it was soaked in molten NaCl-KCl at 710 ℃ for 5min to remove the solid fluoride salt residues and the subsequent processing was the same as for coated cathodes prepared in molten chlorides. The coated molybdenum was embedded in epoxy resin at room temperature and ground by successively finer SiC sandpaper, followed by polishing using diamond abrasion paste, resulting in the sample of cross-section of TiB2 coating (Fig. 1f). The coated graphite was broken down to obtain the fractured cross-section of the TiB2 and ZrB2 coatings ( Fig. 3b  and 4c,d).
Characterization. The EPDed coatings were examined using a SEM (Regulus 8220, HITACHI), TEM (JEM-2100, JEOL) and XRD (Smart Lab, Rigaku) to analyze their micromorphology, microstructure and phase composition. TEM samples were prepared using a precision ion polishing system (Model 691，Gatan) for Fig. 4 e,f. The EPDed coatings were ground by successively finer SiC sandpaper to obtain a smooth surface for nanoindentation tests. Their hardness and elastic modulus were assessed by a nanoindenter (Nano Indenter G200, Agilent). Six indentations tests on various parts of each coating were conducted to ensure the reliability.