Xiogng Guo , Toto Ling , Junfeng Guo , Huisheng Hung , Shuying Kong ,Jinwei Shi , Binfng Yun , Qi Sun

a Chongqing Key Laboratory of Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University,Chongqing, 408100, PR China

b Chongqing Sports Medicine Center, Department of Orthopedic Surgery, Southwest Hospital, The Third Military Medical University, Chongqing, 40038,China

c College of Life Sciences, Chongqing Normal University, Chongqing, 401331, China

Keywords:Electrophoresis assembly Al/WO3 MICs Fluorination treatment Anti-wetting property Exothermic life-span

ABSTRACT For solving the dilemma of the short exothermic life-span of WO3 based metastable interstitial composites (MICs) with extensive application prospect, this paper has firstly designed the promising antiwetting Al/WO3 MICs via electrophoresis assembly of nano-Al and WO3 particles fabricated by hydrothermal synthesis method, followed by the subsequent fluorination treatment.A combination of X ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy(EDX),and Fourier transform infrared spectroscopy(FT-IR) techniques were utilized in order to characterize the crystal structure,microstructure,and elemental composition distribution of target films after different natural exposure tests.The product with uniform distribution and high purity possesses a high contact angle of~170° and a minute sliding angle of~1°, and displays the outstanding anti-wetting property using droplets with different surface tensions.It also shows great moisture stability in high relative-humidity circumstances after one year of the natural exposure experiment.Notably, the heat output of a fresh sample can reach up to 2.3 kJ/g and retain 96% after the whole exposure test, showing outstanding thermo-stability for at least one year.This work further proposed the mechanism of antiwetting Al/WO3 MICs considering the variation tendency of their DSC curve, providing a valuable theoretical reference for designing other self-protected MICs with a long exothermic life-span applied in wide fields of national defense, military industry, etc.

1.Introduction

The film-formation process and performance analysis of transition-metal oxides (TMOs), including MoO3, V2O5, and In2O5,have attracted increasing interest in the field of fuel conversion[1,2].The tungsten trioxide (WO3) is regarded as a typical class of TMOs because of flexible adjustable crystal structures (e.g.tetragonal, orthorhombic, etc.) and a wide band gap.Thus, the WO3exhibits outstanding advantages in gas-sensing due to its high sensitivity and stability toward target gases [3], in electrode designing due to its chemical stability and high electron mobility(10-20 cm2/(V·s)) [4], and other essential technological fields of photochromism, anticorrosive paints,and catalysis[5-9].Notably,combining WO3with highly active metals (e.g., Al) can form metastable interstitial composites (MICs) as a class of promising energetic materials,which can release energy quickly and violently in an instant after the stimulation of tiny energy, showing great application prospects in defense, detonators, triggers, sensors, adhesives,and so on [10-14].

The current research on MICs (e.g.Al/CuO [15], Al/Mn2O3[16],Al/Fe2O3[17]) mainly focuses on the reduction (from micro-to nano-level) of reactant scale,the exploration of novel morphology or microstructure of reactants,and the improvement of exothermic performance, and the main preparation methods include mechanical mixing method [15], magnetron sputtering [16], sol-gel[17], DNA-directed assembly [18], freeze drying [19], etc.For example, M.R.Zachariah reported in-situ microscopy synthesis to analyze the reaction mechanism of nano-Al/WO3MIC and found that heterogeneous condensed-phase reactions and large structural changes had occurred in regions where the Al and WO3were in close proximity at a high heating rate [20].In addition, the electrophoretic assembly is regarded as an useful technology to design various functional coatings [21].Guo et al.also designed Al/WO3MIC coating using this method,and the target film showed the full exothermic process and high heat release (~2.4 kJ/g) [22].P.Gibot et al.[23]used a mechanical mixing technique to fabricate nano-Al/WO3MIC, and mechanical desensitization of the product was realized via carbon addition via the pyrolysis of naturally occurring molecules(carbohydrates),and the carbon-doped Al/WO3MIC still exhibits well reactive properties (ignition and combustion velocity).However, how to improving the lifespan or increasing the exothermic stability of Al/WO3MICs due to the easy decay of exothermic property caused by high activity, hygroscopicity, and hydrophilicity of fuel (Al) [24]has not received much attention at present.

It is worth mentioning that several effective strategies have been developed to improve the stability of MICs.For example,Collins et al.introduced chemical vapor + atomic layer deposition to prepare water-repellent functional Al/CuO MICs [25]with great exothermic property after immersion tests, demonstrating the significance of hydrophobic coating for energy-release protection.In our previous reports,several superhydrophobic MICs(e.g.,Al/Ni/Bi2O3[26], Al/Co3O4[27]) have been fabricated via an electrophoresis assembly technique, and the exothermic stability of some samples can be maintained for more than two years.Moreover,coaxial electrospinning is demonstrated as an effective method to prepare an energetic core/hydrophobic shell nanofiber material of Al/glycidylazide polymer/nitrocellulose/polyvinylidene fluoride whose heat of reaction and the laserinduced plasma characteristics can be controlled, and the diffusion distance between fuel and oxidizer can be decreased, resulting in an enhanced combustion performance [28].However, the design of anti-wetting Al/WO3MICs to improve their structure and exothermic stability has not been reported.In this work,the improved electrophoresis assembly technique combined with surface post-treatment was creatively employed to prepare anti-wetting Al/WO3MICs with a uniform microstructure.The target samples were characterized in terms of structure, hydrophobicity, and exothermic processes.Further investigations were carried out to verify the long life-span or excellent heat-release ability of the product.

2.Materials and methods

2.1.Reagents and materials

Polyethyleneimine, ethanol, isopropyl alcohol, and perfluorodecyltriethoxysilane were purchased from Aladdin Industrial Co., China.The Sinopharm Chemical Reagent Co., Ltd.provided other reagents (e.g., polyethylene glycol-2000, sodium carbonate)that were not treated.The nano-Al particles (~50 nm) were purchased from Dekedaojin Co., China.The commercially available Ti sheets were used as electrode materials, and all other reagents were used without purification.Throughout the experiments,deionized water(R= 18 Ω) was used.

2.2.Design of anti-wetting nano-Al/WO3 MICs

First, after soaking sodium carbonate, ultrasonic cleaning with deionized water and ethanol, and vacuum drying for 15 min, Ti sheets with an effective area of 2.0(length)×1.0(width)cm2were used as working electrode and counter electrode with a perpendicular distance of 1.0 cm.Then, nano-Al and nano-WO3particles with an optimal proportion(mass ratio of 1:12,total mass was 0.1g)were added to the dispersing agent(100 mL),which was made up of isopropyl alcohol and PEG-1000 (volume ratio of 50:1) with a trace amount of polyethyleneimine.The WO3nanoparticles with an average grain diameter of ~50 nm were synthetized by the hydrothermal synthesis method using PVP-40000, PEG-2000, and ammonium metatungstate as precursor reagents [22].Then, a stable suspension with a total loading concentration of 1 g/L was obtained after ultrasound treatment of the dispersing agent at 200 W for 20 min at 293 K.Then,the electrophoresis assembly filmformation process was realized using an electrophoresis apparatus under an applied effective voltage of 200 V for 10 min.Following that, the Al/WO3composite-coated working electrode was taken from the stable suspension and treated by surface functionalization treatment.To be more specific, the target film was immersed in a flow modification fluid including ethanol and perfluorodecyltriethoxysilane (volume ratio of 100:1) at a liquid flow rate of 1 cm/s at 323 K for 1 h,and then heated for 10 min.Finally,the anti-wetting nano-Al/WO3MICs were obtained after natural cooling.The corresponding schematic diagram of the fabrication of promising anti-wetting nano-Al/WO3MICs is displayed in Fig.1.

2.3.Characterization

The microstructure and crystal structures of products were analyzed by a field emission scanning electron microscope(FESEM,Hitachi SU5000+, Oxford Instrument Ultim Max, Japan) equipped with energy dispersive X-ray spectroscopy (EDX) and highresolution transmission scanning electron microscopy (HRTEM,JEOL JEM-2100F, Japan) with a scanning rate of 5°/min.The antiwetting and weather resistance properties of target films were determined by an optical contact angle instrument (HARKE-SPCA,Beijing, China) and a salt spray test chamber (ATEC, Co., Ltd,Chongqing., China),and a high-speed camera (Phantom VEO 410,Vision Research,Inc.,and Wayne,NJ,USA)with a Nikon AF Nikkor lens, respectively.The heat-release and thermos-stability of antiwetting nano-Al/WO3MICs during thermite reaction were analyzed using differential scanning calorimetry (DSC, STA449F3,NETZSCH, Germany) on a separate ceramic crucible.Particularly worth mentioning is that the Ar flow began 40 min before DSC testing to ensure that the detection environment was as pure as possible.

3.Results and discussion

3.1.Microstructure characterization of the anti-wetting nano-Al/WO3 MICs

Fig.1. Schematic diagram of the fabrication of promising anti-wetting nano-Al/WO3 MICs.

Fig.2. The typical XRD spectra of the anti-wetting nano-Al/WO3 MICs.

To analyze the crystal structure of the anti-wetting nano-Al/WO3MICs,the XRD technique is carried out in Fig.2.To begin,the XRD pattern of a fresh sample(SampleF)displays diffraction peaks at 23.143°, 23.643°, 24.366°, 26.831°, 28.870°, 32.976°,34.034°,35.495°,41.905°, 44.715°, 47.253°, 48.375°, 49.872°,53.512°,54.231°,55.114°,and 55.658°,which correspond to crystal planes (002), (020), (200), (120), (112), (022), (202), (122), (222),(132),(004),(040),(-140),(-204),(204),(-142),and(142)of WO3with triclinic structure (δ-phase, and P-1 (2), and cell size of(7.3 × 7.52 × 7.69 ?3with corresponding degree＜88.83 × 90.91 × 90.93>, PDF # 20-1323).Similarly, the crystal planes(111)and(200)of Al(PDF#04-0787,Fm-3m(225))with a cubic crystal structure and a cell side length of 4.094 ? match the diffraction peaks at 38.472°and 44.738°,indicating the existence of the fuel-Al in SampleF.In addition, the sharp crystalline peaks of WO3and Al are identified,showing that SampleFis of a high degree of crystallinity.The absence of other diffraction peaks (e.g.Al2O3,W)indicates that the fresh sample is of high purity and there is no evident redox interaction between Al and WO3during the electrophoresis assembly process.Compared to SampleF, the sample has a similar crystal structure and diffraction peak position after 6 months(Sample6M)or 12 months(Sample12M)of natural exposure experiment,with no additional peaks appearing(Fig.2),indicating that the crystal structure is quite stable.

The microstructures of samples are also investigated by FESEM and EDX techniques in Fig.3.Clearly, in the low-resolution FESEM image of Fig.3(a), the SampleFshows relatively uniform surface roughness with few obvious cracks and humps,demonstrating the successful co-assembly process of anti-wetting nano-Al/WO3MICs.In the high-resolution FESEM image of Fig.3(b), there are lots of clearly visible gaps between Al and WO3nanoparticles on the surface of SampleF, which probably provides an ideal structural foundation for realizing rapid release of heat and designing moisture-resistant surfaces at the same time.The fuel-Al and oxidizer-WO3particles are still nanoscale,probably contributing to increasing the reaction contact area and enhancing the exothermic reaction intensity [26,29].The contact angle of sample can reach~170°with a water-droplet keeping a perfect sphere after contacting the target surface, showing the outstanding water-proof property (Fig.3(c)).The micromorphological stability of samples is also evaluated.After 6 months or one year of exposure, the surface uniformity of Sample6Mor Sample12Mmaintains high stability from the respective low-solution FESEM image (Fig.3(d) and Fig.3(g)).Also,the Al and WO3particles are clearly still in nanoscale with irreplaceable key gaps (Fig.3(e) and Fig.3(h)), and there is little change in the sphericity of the droplets(Fig.3(f)and Fig.3(i))for Sample6Mand Sample12M, indicating the highly stable of microstructures of sample combined with the morphology mechanism analysis diagram of the sample after different exposure tests.

Furthermore, EDX analysis is used to investigate sample homogeneity (Fig.4).All major elemental indications of Al, O and W with homogeneous distribution,as illustrated in Fig.4(b)-Fig.4(d)based on the entire region of top-view image (Fig.4(a)) of sample before exposure test, are clearly visible and correspond with the composition of deposits.Because of the fluorination posttreatment, three extra elements of Si, F, and C with weak signals are also found (Fig.4(e) and Fig.4(g)), and the signal peaks of all elements can be seen in the EDX spectrum(Fig.4(h))for SampleF,as expected.The corresponding mole ratio or percent of all elements is also displayed in Fig.4(i),where the overall mole percent of Al:O:W:C:F:Si is roughly 31%:16.1%:47%:4%:1.9%:1%,and the mole ratio of Al: O: W is close to the theoretical exothermic reaction ratio of 2:1:3.Moreover,after the whole exposure experiment,the surface of Sample12M(Fig.4(j)) still displays the promisingly even distribution of the major elements (Fig.4(k)-Fig.4(p)), and the mole percent variation tendency of all elements calculated by the results from the EDX spectrum(Fig.4(u)and Fig.4(r))is similar compared with that of SampleF,illustrating the chemical composition stability within targeted functional films.

Fig.3. The typical FESEM images ((a), (d) and (g)) low resolution and ((b), (e) and (h)) high resolution of SampleF, Sample6M and Sample12M, and followed by the morphology mechanism analysis diagram of the sample, and the water-droplet contact angle of (c) SampleF, (f) Sample6M and (i) Sample12M.

The stability of the composition and structure of the product can be further confirmed by the FT-IR analysis in Fig.5.For SampleF,the four absorption peaks at around 780 cm-1,1120 cm-1,1207 cm-1,and 1365 cm-1were detected and assigned to the stretching vibration of C-F bonds for perfluorinated decane [30-32].A band due to Si-C deformation vibration in the Si-CH3groups [33]was observed at 1270 cm-1.Two reasonably absorption peaks at 810 cm-1and 1100 cm-1ware attributed to the asymmetric Si-O-Si stretching vibration and Si-O-Si deformation[34]in the model, indicating the fluorosilane molecules with low surface energy self-assembled on the surface of the SampleF,which is also in accord with the EDX results of three additional elements with low content.In addition,all the main characteristic peaks of C-F,Si-C,and Si-O-Si show no significant migration or change for Sample6Mand Sample12M, again proving the excellent stability of the composition of the product.

3.2.Anti-wetting properties

The anti-wetting properties of products are deeply analyzed for the practical application purposes of target functional film-Al/WO3MICs in Fig.6.The whole droplet rolling-off process on the product surface placed on quiet water with an almost negligible angle(Fig.6(a)).Clearly,a water droplet rolls off quickly once it contacts the functional surface,due to the frictional resistance(f)of droplet much smaller than the driving force(F1)forming from combination of gravity(G),support force(Fs),and adhesion force(Fa)(Fig.6(d)),which indicates the outstanding water-proof performance of the product with a sliding angle of ＜1°.The typical immersion experiment of a sample is conducted in Fig.6(b).The immersed part of the target coating under the aqueous solution becomes silverywhite from ash black, demonstrating a silver mirror-like phenomenon[35].This is due to the promising air-cushion layer consisting of a great number of air bubbles collected on a micro/nanostructured Al/WO3MICs rough surface, reflecting obvious natural light, and the detail mechanism is explained as shown in Fig.6(e).The sample surface keeps dry with scarcely any change after the whole immersion cycle.Furthermore,the droplet-bouncing ability is also regarded as a key indicator for evaluating the anti-wetting performance of functional films, and the droplet-bouncing process of a sample is clearly seen in Fig.6(c).Generally, a complete bouncing cycle of a droplet consists of several steps of(i)falling,(ii)contact, (iii) compression, (iv) detachment, and (v) bounce for super moisture resistant samples.Notably, all key steps are clearly observed for SampleFin Fig.6(c), and there is little difference between the rebound height(Hf)and the initial fall height(Hi)of the droplet, mainly due to the rather low dissipation work or energy loss.

In addition, the humid environment plays an essential role in affecting the thermostability of a large proportion of MICs,and the relationship between water-proof ability and relative humidity(RH)is displayed in Fig.7(a).There is a small drop in the value of the contact angle with an increase of RH,and the corresponding sliding angle rises at a slower rate.Even in a high RH (95%) environment,the water contact and sliding angle of SampleFare still >165°and＜2°,further demonstrating great weather-resistance property[36].

The anti-wetting stability of the product in Fig.7(b)-Fig.7(d)is further investigated using exposure experiments and droplets with different surface tensions.The contact angle as a function of exposure time is shown in Fig.7(b), where the contact angle of SampleFis measured as a high value of ~170°with an almost symmetrical sphere placed in the Cassie state on the SampleFsurface [37], even though it is exceedingly simple to roll off from the tested surface.After exposure testing for 6 or 12 months, the Sample6Mor Sample12Mshows a similar value of (>168°), and the corresponding Cassie state is obvious, respectively (Fig.7(b)),showing the promising water-proof stability.Furthermore,different droplets(water,peanut oil,tetradecane,etc.)are collected to explore the anti-wetting behaviors of sample surfaces(Fig.7(c)).To be sure,the contact angle of a functional film typically decreases as droplet surface tensions rise,and all prepared samples exhibit a similar change law.However, it is obviously observed that the contact angle of SampleF,Sample6Mor Sample12Mis still>150°even for tetradecane with an extremely low surface energy of 26.5 mN/m.Furthermore, the energy loss ratio (EL= (Hi-Hf)/Hi) of water droplets for SampleFis less than 6%, and only around 7% after 12 months of exposure test (Sample12M) (Fig.7(d)).It is worth emphasizing that the contact angle of the sample after twenty immersion cycles and a one-year natural exposure experiment shows only a small variation of 1.18%, further confirming the exceptional anti-wetting stability property.

Fig.4. (a)The top-view of sample before exposure test and followed by the EDX mapping images of all elements of(b)Al,(c)O,(d)W,(e)C,(f)F and(g)Si in SampleF,and(h)the EDX spectrums of SampleF followed by (i) the mole percent of all elements, and (j) the top-view of sample after exposure test, and the corresponding EDX mapping images of all elements of(k)Al, (l)O,(m) W,(n)C,(o)F and (p)Si in the sample after the whole exposure test,and(q)the EDX spectrums of Sample12M followed by(r)the mole percent of all elements.

Fig.5. The FTIR spectra of the anti-wetting nano-Al/WO3 MICs after 0, 6 and 12 months of exposure experiments.

3.3.Exothermic life-span studies

In fact, the exothermic life-span is essential for energetic materials, including anti-wetting Al/WO3MICs, for practical application.In this study, the exothermic process and corresponding stability are investigated by DSC technique using Ar (99.999%) atmosphere in Fig.8.The detail DSC curve of target anti-wetting Al/WO3MICs (SampleF) is displayed in Fig.8(a), where there are the three sharp exothermic peaks at 600°C-800°C attributed to the exothermic reaction (2Al+ WO3→Al2O3+ 2W+ ΔQ, ΔQ= 3×106J/kg).The first exothermic peak at around 600°C is caused by the solid state-Al and WO3reaction,and the two other exothermic peaks at around 710°C and 820°C are due to the reaction of liquidstate Al with WO3nanoparticles.Compared with the first exothermic peak, the subsequent exothermic processes are more intense because of the reaction interface changing from solid-solid to solid-liquid,largely promoting the exothermic reaction intensity and speeding up the heat-release reaction rate, and the corresponding schematic illustrations are shown in Fig.9.Clearly,there are two endothermic peaks at ca.660°C and 730°C mainly due to the melting of Al [38]and the crystal transition process of Al2O3[39], respectively.Notably, samples after exposure tests show a similar DSC variation tendency,and the three exothermic peaks are obviously observed for Sample6Mand SampleFMin Fig.8(b).

Fig.6. (a) The typical whole droplet rolling-off process; (b) The immersion process and (c) the fresh sample droplet-bouncing process; (d) The mechanism diagram of droplet rolling down and (e) the silver mirror-like phenomenon of soaked sample.

Moreover,the relationship between the fitted output of heat(Q)and exposure time is shown in Fig.8(c).TheQof SampleFcan reach up to 2.3 kJ/g which shows a very slight decline (0.15 ± 0.02%)compared with sample before surface modification process,indicating a small effect of modification process on the exothermicity of product.In addition,there is a slightly gradual downward trend forQof SampleFwith exposure time,and theQof the sample after the whole exposure test can still be 96%compared with that of the SampleF.For a fixed RH,theQof sample gradually decreases as the exposure time increases (Fig.8(d)), and the higher RH is, the greater the downtrend is.However,the reduction ofQis only ＜4%after one year exposure experiment under super high RH (95%),demonstrating the promising exothermic stability.Furthermore,using the Kissinger Akahira-Sunose method, the activation energy(Ea) of the sample used to investigate exothermic stability is calculated[27]), the activation energy,linear heating rate (K/min), universal constant (8.314 J/(mol K),peak absolute temperature (K), and pre-exponential factor is indicated asEa,β,R,TPand A.The heating rates are 10 K/min,20 K/min,30 K/min,and 40 K/min,respectively.The analyzed data for all samples is listed in Table 1, and the high R2(>0.9) for SampleF,Sample6Mand Sample12Mshow the high degree of fitting.After the calculation analysis, the Eaof SampleF is relatively small value of 313.03 kJ/mol, demonstrating a low activation energy barrier occurring in the process of heat-releasing, further contributing to boosting energy release efficiency.The Easlowly increases with exposure time for the sample after exposure testing, which is probably caused by a minor change in the inner space or radiation from samples with a high surface density,and the small change in value of Eaalso indicate the long life-span of exothermic capacity of or long thermal stability life of anti-wetting Al/WO3MICs

Fig.7. (a)The contact angle and sliding angle as functions of relative humidity,(b)the relationship of contact angle and exposure time,(c)the contact angle as a function of samples,and (d) the energy loss rate and contact angle relative to exposure time.

Fig.8. (a) DSC curves of anti-wetting Al/WO3 MICs fabricated by the electrophoresis assembly and subsequent functionalization; (b) DSC exothermic curves of samples after different exposure time,followed by(c)the change law of fitted output of heat(Q)and exposure time;(d)the heat release of product as functions of exposure time under different humidity environments.

Fig.9. The schematic illustrations of the reaction mechanism of Al/WO3 MICs after heat source stimulation.

Table 1The calculated activation energy (Ea) results of different samples.

4.Conclusions

In brief,the novel anti-wetting Al/WO3MICs with an ultra-long thermal stability life were facilely fabricated by electrophoresis assembly and subsequent surface modification.The target functional coating was a highly crystalline, including the key reactants of fuel-Al and oxidant-WO3, and was of a highly uniform distribution of main elements of Al, W, O, etc., demonstrated by XRD,FESEM, EDX, and FT-IR techniques.The great stability of structural and compositional changes of in product was deeply examined in this work.Moreover, the fresh sample with a high CA of ~170°displayed super-hydrophobicity via the typical tests of droplet impacting,immersion,and natural exposure tests(12 months),and kept anti-wetting stability properties via using different droplets(e.g.tetradecane) and moisture circumstances.The DSC results showed that the anti-wetting Al/WO3MICs can release energy quickly,and the exothermic process and the curve trend are similar for samples after different exposure times,and only about 4%of the energy is lost after a one year exposure test, showing great thermostability with wide applications.Also, the reaction mechanism of Al/WO3was proposed,providing a valuable theoretical basis for improving the energy releasing capacity of other kinds of MICs.Thus,this work provides a highly-effective method for designing or optimizing MICs with excellent thermal stability in complex environments, which is used in lots of engineering fields.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was funded by the financial support from National Natural Science Foundation of China (Grant No 21805014 and No 82102635),Science and Technology Research Project of Chongqing Education Board (Grant No.KJQN201901428).Thanks to eceshi(www.eceshi.com) for the SEM analysis.