The use of porous metals with highly porous nano- or submicro-structured walls for energetic materials applications has been considered in recent years. The aim of this work is to construct a stable porous structure for synthetize of copper azide primary explosive. For this purpose, hollow copper and copper oxide microspheres were successfully prepared by electroless plating using polystyrene (PS) templates with diameter of 529 nm. SEM and XRD analysis were conducted to characterize the microstructure and composition of the products, and ICP was used to measure the variation of Cu²⁺ concentration during plating. Results show that the amount of copper deposit increased with increasing pH value. By adding enough NaOH solution during plating, copper ions could be all reduced while the microstructure of copper coatings destroyed. The addition of stabilizers had a negative effect on PS/Cu core-shell structures. Hollow copper microspheres with shell thickness of 100 nm were obtained by sintering PS/Cu core-shell microspheres in N₂ while hollow copper oxide microspheres were obtained in air.

Methods

PS microspheres are synthesized by soap-free emulsion polymerization using a mixed solution of deionized water and methanol as a dispersant, potassium persulfate as an initiator, and styrene as a monomer. The specific steps are as follows: 40 mL deionized water, 7 mL methanol, 0.08 g KPS and 5mL styrene are added in round-bottom flask fitted with two necks, and N₂ is introduced into the bottle to purge O₂ out for about 10 min. The flask is closed with silicone-greased stoppers and clamps, and then reacts in a 75 °C water bath at 500 rpm for 12 h. PS microspheres are obtained by washing the polymerized product and then drying in an vacuum oven at 50 °C for 24 h. The preprocessing of PS microspheres is given in Reference 18.

The electroless plating copper bath is consisted of CuSO₄·5H₂O (0.1 mol×L^-1), Na₂EDTA (0.12 mol×L^-1), pH buffer (NaOH) and 2, 2-bipyridine (0.01g·L^-1), potassium ferrocyanide (0.01 g·L^-1) and methanol (50.00 mL·L^-1)). The preprocessed PS was immersed into a freshly prepared copper bath, and dipped reductant HCHO (15.00 mL·L^-1) under ultrasonic to start reaction. Then, the PS/Cu composite was obtained by centrifugation and rinsed with deionized water for three times and dried in vacuum oven at 50 °C. Finally, nanoporous copper and copper oxide microspheres are obtained by sintering PS/Cu core-shell at 400 °C for 1 h in the N₂ and in the air with the heating rate of 5 °C·min^-1, respectively.

Characterization

Particle size and the particle size distribution of PS microspheres are performed by laser particle size analyzer (Mastersizer 2000, UK). The thermal performances of PS microspheres and PS/Cu composites are measured by thermogravimetric analyzer (TG/DTA6300, Japan) at a heating rate of 10 °C·min^-1 from 25 °C to 600 °C. The IR spectra of PS microspheres are collected by infrared spectrometer (TENSOR 27, Germany). The Cu²⁺ concentration is measured by inductively coupled plasma optical emission spectrometer (ICP-OES, PE OPTIMA 7000DV, USA). The microstructure of samples is observed by scanning electron microscopy (SEM, S4800, Japan). X-ray diffraction (XRD) analysis is performed on a diffractometer (Rigaku, Japan) with Cu Kα radiation in the range from 5° to 80°.

Figure 1 is the particle size distribution and TG-DTG curves of PS. The PS diameter ranges from 100 nm to 1 μm, and the peak particle value is 529 nm which could be considered as the average diameter. The gentle peak distributed from 2 μm to 11 μm is caused by the agglomeration of PS due to its sub-micrometer diameter. The TG-DTG curves in N₂ give the thermal performance of the prepared PS, as can be seen in Figure 1b. PS starts decomposing at about 320 °C and completes until 450 °C. After decomposing, the weight of PS changes to 0 which means PS could be removed completely by sintering.

Figure 2 is the FT-IR spectrums of PS microspheres before and after roughness. The peak at 3440 cm^-1 which corresponded to the stretching vibration of O-H is obviously strengthened and broadened. This result clearly indicates that the hydrophily of PS surface is improved by roughness process. Sensitization is to absorb sensitize ions onto substrate surface. Sn²⁺ is the most used sensitize ions. Activation is a replacement reaction which reduces the activated ion onto substrate surface as catalytic activation center (usually using Pd).

Figure 3 gives the microstructures of PS before and after preprocessing. The PS surface is smooth and clean (Figure 3a). After preprocessing, PS retains good sphere structure while the PS surface becomes rougher. And more importantly, it can be clearly seen that there are many nanoparticles absorbed on PS surface (Figure 3b). Those nanoparticles are considered as the catalytic activity centers (Pd) which are obtained by preprocessing process.

Figure 4 is the SEM images of PS / Cu composite microspheres obtained by reaction for 60 min at the initial pH value of 12.35, 12.85, 13.35 and 13.85, respectively, and the addition amount of PS is 3 g·L^-1. It can be seen that with the increase of pH value, the reduced copper content on the PS surface increases significantly. When the pH value increases to 13.85, the agglomeration of copper particles is obvious. This is due to the high pH value which resulting in a rapid reaction. The deposited copper particles are close to 100 nm when the pH value is small. The content of deposited copper also increases with the increased of pH value, and the copper particles grow up to about 100-200 nm. However, most of PS surface is not coated with copper due to the large specific surface area of PS and the relatively small amount of copper.

Figure 5(a)gives the variations of Cu²⁺ concentration in plating bath and pH value with deposition time, where the initial pH value is 12.85 and the PS loading amount is 3 g·L^-1. It clearly displays that the change laws of Cu²⁺ concentration coincide basically with those of pH value. The concentration of Cu²⁺ and pH value decrease dramatically in first 2 min and after that decrease slowly. HCHO is reductive only in alkaline solution and it reducibility increases with increasing pH value. In the first 2 min, the Cu²⁺ reduction rate is very fast due to the relatively high pH value. The fast copper deposition consumes large amount of OH^- and thus leads to the decreasing of pH value. The Cu²⁺ reduction rate decreases dramatically due to the decrease of HCHO reducibility when pH value is reduced to about 9.5. The Cu²⁺ reduction ratio is 34.94% with deposition time of 20 min. In order to maintain the reducibility of HCHO and thus improve the Cu²⁺ reduction ratio, NaOH solution according to the stoichiometric coefficient (1 mol Cu²⁺ corresponds to 4 mol NaOH) is added steady into the plating bath in first 20 min. And the variations of Cu²⁺ concentration and pH value with deposition time are shown in Figure 5b. In first 2 min, the deposition reaction consumes large amounts of Cu²⁺ and OH^-, leading to rapid decreasing of pH value from 12.5 to 10. After 2 min, pH value increases because the adding amount of NaOH is exceeds the consume amount. While Cu²⁺ concentration is decreases to 0 at 13 min, this result indicates that the Cu²⁺ reduction ratio improves to 100%. However, there are many red copper agglomerates floating at the plating bath surface during the later stage. This implies the adding of NaOH may disturb the growing trend of copper deposits.

Figure 6 gives the microstructures of two PS/Cu composites samples obtained in above experiments. Due to insufficient copper deposits and large PS surface area, the deposited copper particles ranged from 50 nm to 100 nm are scattered sporadically on PS surface (Figure 6a). While adding NaOH solution during electroless plating, the amount of deposited copper increases obviously (Figure 6b). However, the subsequent Cu²⁺ reduces on copper nanoparticles in priority and thus most PS surface is still uncovered. These results demonstrate that adding NaOH solution during electroless plating could improve Cu²⁺ reduction but fail to obtain good PS/Cu core-shell structures.   

Figure 7 shows the morphology of PS / Cu composite microspheres when potassium ferrocyanide (0.01 g·L^-1), 2, 2-bipyridine (0.01 g·L^-1) and 50.00 mL·L⁻¹ methanol is added to the plating solution and stabilizer is not added. It can be seen from Figure 7(a) that when stabilizer is added to the plating solution, a part of PS coating is complete, a small part of PS surface is not completely coated, and many copper particles are scattered. This is because the stabilizers inhibit the process of PS surface chemical plating to a certain extent and disturb the growth of copper, so the morphology of copper coating is damaged and the uniformity is poor. Figure 7(b) shows that the surface of PS microspheres is basically covered by copper coating with good uniformity without stabilizer. However, there are still a small amount of scattered copper particles, which are basically greater than 100 nm, and the copper particles on the copper coating are less than 100 nm. Besides, the scattered copper particles are due to the fact that PS is not fully washed during the pretreatment, and the scattered Pd between PS as the active center will lead to the reduction of copper ions. The surface of PS microspheres is basically coated with copper coating and the uniformity is good, but there are still a small amount of scattered copper particles. The above results show that PS / Cu core-shell structure with complete morphology can be obtained without stabilizer in the plating solution.

Figure 8 shows the TG curves of PS/Cu in N₂ and air atmosphere are presented in. It can be seen in Figure 8(a) that PS started thermal decomposing at about 320 ℃ and completed until 430◦C, and the mass loss ratio of PS/Cu in N₂ is 37.38%. The curve (b) indicates that Cu starts oxidation at 150 ℃ and the weight decreases due to the combusting or decomposing of PS when the temperature is further increased to 260 ℃. The thermal reaction complete at about 370 ℃, and the mass loss ratio of PS/Cu in air is 25.81%, which is consistent with the theoretical calculation.

Figure 9(a) is the XRD pattern of the product obtained in N₂, which closely matches Cu (JCPDS No. 04-0836) and no impurity peak is detected. The peaks at 43.31°, 50.46°and 74.13° are respectively corresponded to crystalline planes of (111), (200) and (220). Meanwhile, the diffraction peaks at 35.68°, 38.86° and 48.79° in Figure 9(b) corresponding to the reflections of (002), (111) and (-202) crystalline planes of copper oxide (JCPDS No. 45-0937), indicating that Cu is oxidized to CuO completely.

Figure 10 gives the microstructures of the sintering products. As can be seen in Figure 10(a, b), Cu deposits maintain good sphere structure and form hollow Cu microspheres with shell thickness of about 100 nm after sintering. Cu nanoparticles are coarsening obviously compared to the microstructure before sintering (Figure 7b). The surface of Cu nanoparticles melts slightly and thus Cu particles connect to each other forming a strong hollow microsphere structure during sintering process. The glass transition temperature of PS is 80-10 °C and thus PS skeleton melts and collapses before Cu oxidation, leading shrink of microspheres. Figure 10(c, d) shows that the copper oxide still maintains a complete hollow spherical shell structure with thickness of about 200 nm, which is larger than copper microspheres (Figure 10a). This is due to the oxidation of copper, which makes the thickness increase and the adjacent microsphere also adheres together. The above results show that hollow copper microspheres can be obtained by sintering PS / Cu in nitrogen and copper oxide microspheres can be obtained by sintering in air.