The Effective Surface Metallization of Hollow Glass Microspheres for Flexible Electromagnetic Shielding Film

BU Fan, SONG Pengcheng,LIUYahui,WANG Jun,WU Xiyuan, LIU Lei,XU Chuanhua,ZHANGJianfeng＊

(1. College of Mechanics and Materials, Hohai University, Nanjing 211100, China 2. Sinosteel Maanshan New Material Technology Co., Ltd, Maanshan 243000, China)

Abstract: The surface of hollow glass microspheres (HGMs) was roughened by a HCl+NH4F strategy,which achieved a broken ratio as 16.10%, and then metallized by electroless plating by Co nanoparticles up to 90 wt% (abbreviated as Co-HGMs). The average grain size of Co was measured to range from 0.4 to 0.5 µm.Then Co-HGMs were mixed with liquid silicone rubber and xylene, and cured on a perspex plate applicable for flexible electromagnetic shielding. By attentive parameter optimization, a film about 0.836 mm in thickness was obtained with a density of 0.729 g/cm3, showing a shielding effectiveness of 15.2 dB in the X-band (8.2-12.4 GHz) at room temperature, which was ascribed to the formation of a conductive network of Co-HGMs inside the film. Simultaneously, the tensile strength of 0.89 MPa at an elongation ratio of 194.5% was also obtained,showing good mechanical properties and tensile strength.

Key words: hollow glass microspheres (HGMs); electroless plating; electromagnetic shielding film;flexible film; lightweight materials

1 Introduction

With the rapid development of modern electronic industries, a rich variety of wireless communication systems and high-frequency electronic devices have increased dramatically in the past two decades[1,2]. Electromagnetic waves emitted by these electronic devices are ubiquitous to the surrounding environment, which not only adversely interferes with the precise operation of electronic equipments, but also causes serious damage to human health[3-6]. Therefore, the development of high performance electromagnetic interference (EMI)shielding materials has attracted wide attention. Although conventional metals, such as Ag, Cu, Ni, Co,and others, are competitive as excellent electromagnetic wave barriers, they are difficult to be used in tiny microelectronic devices for the disadvantages of high density and poor flexibility. Even the deposition of thin metal films onto uneven device surfaces is not an easy task[7]for portable and wearable intelligent electronic products[8-10]. Some electromagnetic shielding materials instead of metals have emerged recently, such as graphene[11-15], carbon nanotubes[16-20], and MXene[21-25].But they are still far from industrial production due to the complex preparation process and high cost.

On the other hand, placing metal fillers in the light flexible matrix is more promising for reduced density and good shielding performance[26-28]. As a new type of light-weight material, hollow glass microspheres (HGMs), mainly composed of silicon dioxide and aluminum oxide, have shown a good potential for functional applications with a high compressive strength, high corrosion resistance, low thermal conductivity, low thermal shrinkage coefficient and other advantages[29-32]. Especially, by coating metallic or magnetic materials, HGMs can be used for electromagnetic shielding with the ultralight characteristic[31,33-35].Wanget alprepared epoxy resin-based composites with Fe3O4-coated HGMs and reduced graphene oxide,whose reflection loss (RL) performance was measured to be -10 dB in a wide frequency range. Poly(hydroxybutyrate) and HGMs composites were also developed by melt intercalation, whose preparation process is a little more complicated[36].

Before metallization treatment of HGMs, roughening is an important preprocess to improve the adhesion and wettability between the substrate and the coating. Traditional HF method is widely used for roughening of HGMs, but the strong corrosion ability of F-damages HGMs easily, resulting in the increase of broken ratio[34]. Some material scientists have also tried to pre-treat the surface of HGMs by NaOH hydroxylating[37], potassiu mdichromate (K2Cr2O7) plus sulfur acid(H2SO4, 98 wt%)[38], or γ-aminopropyltriethoxy silane(APS)[39]. But the weak adhesion between the plating layer and the smooth substrate surface of HGMs still should be conquered urgently[34]. Zhanget al[34]has proposed NaF and HCl instead of HF to alleviate the serious damage of HGMs effectively. An additional alkaline washing process was necessary to remove the impurities of the byproduct Na2SiF6.

In this study, a new kind of HGMs surface metallization pretreatment by a mixed solution of NH4Cl and HCl was adopted in this experiment, by which the alkaline washing was avoided and effectively reduced the broken ratio of HGMs. After that, a new electromagnetic shielding film incorporated with Co-HGMs was obtained, which was measured to have good flexibility and electromagnetic shielding performance, showing a high potential for flexible wearable electronic products.

2 Experimental

2.1 Materials and chemicals

HGMs (43.6 μm in average diameter, and 0.6 g/cm3in density) were supplied by Sinosteel Maanshan Institute of Mining Research Co., Ltd, China, which is mainly composed of silicon oxide and aluminum oxide. The chemical reagents, such as ammonium fluoride (NH4F), hydrochloric acid (HCl), palladium chloride (PdCl2), absolute ethyl alcohol, cobaltous sulfate (CoSO4), sodium hypophosphite monohydrate(NaH2PO2·H2O), boric acid (H3BO3), trisodium citrate dehydrate (C6H5Na3O7·2H2O) were all purchased from Sinopharm Chemical Reagent Co., Ltd, and used as received without any further treatment. The liquid silicone rubber and matched curing agent used at the preparation stage of the film were purchased from Taibeili High Polymer Material Co., Ltd. China.

2.2 Experimental procedure

2.2.1 Preparation of Co coated HGMs

The schematic process for preparation of Co coated HGMs is shown in Fig.1(a), and described in brief as follows.

(a) Roughening: A HCl solution with a concentration of 10 wt% was adopted. 10 g NH4F and 10 g HGMs powder was added successively under mechanical agitation at 300 r/min in a water bath at 25 ℃ for 30 minutes with 100 mL HCl. Afterwards the powders were filtered and cleaned with distilled water for 3 times. Then the powders were dried in oven at 60 ℃for 6 hours.

(b) Activation: Firstly, 0.01 g PdCl2was added into the mixed solution composed of 10 mL ethyl alcohol and 90 mL deionized water in water bath at 60℃ for 30 minutes. In order to prevent alcohol from evaporating, the beaker was sealed with a plastic wrap.Secondly, 1 g roughened HGMs was appended into the solution under mechanical agitation at 300 r/min for 10 minutes. Finally, the powders were filtered and cleaned and dried in oven at 60 ℃ for 6 hours.

(c) Electroless plating: The activated HGMs powder was added directly to a prepared bath with a chemical composition as shown in Table 1 for electroless Co plating. The reaction was also carried out in a water bath of 75 ℃ under mechanical agitation of 300 r/min for 30 minutes. Then the powder was filtered with distilled water for three times, and dried in an oven at 60℃ for 6 hours.

Table 1 The composition of the plating bath＊

2.2.2 Preparation of the electromagnetic shielding film

The main preparation process of the electromagnetic shielding film is shown in Fig.1(b). Co-HGMs were incorporated into an electromagnetic shielding film by tape casting. First, liquid silicone rubber (1 g)and xylene (1-3 g) were fully dissolved. Then, a certain amount of Co-HGMs was added and dispersed by ultrasound for 10 min. After the powder was evenly dispersed, a certain amount of curing agent (1 wt%-5 wt%of the mass of liquid silicone rubber) was added. Afterwards, the solution was poured onto a perspex plate (50 mm×50 mm) and kept for 24 hours. Finally, the flexible electromagnetic shielding film can be obtained by uncovering it.

2.3 Characterization

Fig.1 Schematic procedure for (a) Coating of HGMs by Co; (b) Preparation of electromagnetic shielding films

The phase composition of the powders was determined by X-ray diffraction analyzer atλ= 1.540 6 Å (XRD, Bruker D8 Advance, Germany). The tube voltage was 30 kV, the current was 40 mA, the angle of 2 theta was 5-90°, the step size was 0.02, and the scanning rate was 10°/min, respectively. The surface morphology of the powder was observed by scanning electron microscope (SEM, ZEISS SUPRA 55, Germany). The acceleration voltage was set at 5 kV and the multiplier at 500-10 000 times, respectively. The atomic valence on the surface of the powder was determined by X-ray photoelectron spectrometer (XPS, PHI Quantera, Japan). The coating amount of Cobalt was determined by ultraviolet spectrophotometer (TV-1901,China) through analyzing the change of ion concentration in the bath.

The shielding effectiveness of the film was tested by a vector network analyzer (PNA-N5244A, Agilent Technologies, USA) in the frequency of 8.2-12.4 GHz at room temperature, and the mechanical property of the film was tested by an electronic universal material testing machine (5943, Instron Corporation, USA).

The recovery ratio and broken ratio of the HGMs powders were calculated by weighing the mass change between the powder after being treated and the raw HGMs powder. The recovery ratio and broken ratio can be calculated by equations (1-2).

where,m0is the mass of raw HGMs powder;m1is the mass of roughened HGMs powder floating in the water;m2is the mass of roughened HGMs powder sinking to the bottom of the beaker;w1is the recovery ratio of powder in the roughening process;w2is the broken ratio of powder in the roughening process.

3 Results and discussion

Fig.2 shows the XRD patterns of HGMs after different processes. The presence of SiO2phase in HGMs was validated from the amorphous steamed bread peak at 15°-38° (Fig.2 (a)). The reaction during the roughening process can be deduced just as shown in Eq.(3).As SiF4forms in a gas state, and NH4Cl dissolves into water, no impurity was thus identified from the XRD patterns of roughened HGMs in Fig.2 (b). After the activation process, the XRD patterns also kept almost the same (Fig.2 (c)), the reaction is shown in Eq.(4). But after the plating process, diffraction peaks of Co were identified in Fig.2 (d), just as shown in Eq.(5), suggesting a successful deposition of Co onto the HGMs surface.

Fig.2 XRD patterns of HGMs after different processes: (a) raw HGMs; (b) roughened HGMs by NH4F and HCl; (c) activated HGMs; (d) Co coated HGMs

Fig.3 SEM images of (a) raw HGMs; (b) roughened by NH4F and HCl; (c) liquid separation; (d) activation; (e1-e2) coated by Co

Fig.4 EDS mapping of Co-HGMs

Fig.3 shows the SEM images of HGMs after different processes. The surface of the raw powders was observed very smooth. After being roughened by NH4F and HCl, some of the microspheres were inevitably damaged and broken (Fig.3(b)), so a flotation process was adopted to remove the broken ones ((Fig.3(c))).After the activation and metallization processes in Fig.3(d) and (e1-e2), we can find some fine grains were uniformly deposited on the surface of HGMs, which were hexagonal prismatic and the average grain size was measured to be 0.4-0.5 µm.

To explore the valence state composition of the coating, we performed EDS (Fig.4) and XPS (Fig.5)characterizations on the Co-HGMs. As shown in Fig.4,the element distribution of Co and P fitted well with that of Si. In Fig.5, Co was identified to possess three valence states of Co0, Co2+and Co3+, and the corresponding atomic binding energies were 778.23, 782.06,and 785.95 eV respectively. Moreover, Co2+and Co3+had their respective satellite peaks, but their peaks overlapped each other. Overall, the content of Co0, Co2+and Co3+were calculated to be 5.6%, 52.0% and 42.4%,respectively. P was found only existing in the form of P5+. This indicated that the Co coating was mainly composed of Co-P compounds with only a very small amount of elemental Co.

In order to further declare the effects of NH4F/HCl ratio on the broken phenomenon of HGMs, SEM images were taken and recovery ratio broken ratio of HGMs was calculated, as shown in Fig.6. When the loading of HGMs was set at 200 g/L, the recovery ratio of HGMs was 78.6% and the broken ratio was as low as only 13%. We can see that the surface of HGMs was rough according to Fig.7 (a), which proved that it had a good etching effect after the roughen process. The broken ratio of HGMs began to increase significantly when the loading decreased to 50 g/L, and it can also be seen from Fig.6 (d) that the surface of HGMs begins to break down. When the loading was 40 g/L, the broken ratio was as high as 27.2%. At this time, the recovery ratio was 55.6%, and there was a large amount of damaged HGMs, which greatly reduced the production efficiency. From what has been discussed above, when the loading was 66.7 g/L, the surface roughness of HGMs was good and the broken ratio was low.

Fig.5 XPS spectra of Co-HGMs: (a) coatings; (b) C 1s; (c) P 2p; (d) Co 2p

Fig.6 Recovery ratio and broken ratio of HGMs roughened by NH4F/HCl with different HGMs loadings (the concentrations of H+ are all 2.88 mol/L)

The comparison diagram of HGMs broken ratio after three different roughen methods which all achieve good roughening effect at the same HGMs loading(66.7 g/L) and the same F-concentration (2.88 mol/L) is shown in Fig.8. Due to the violent reaction of HF with SiO2, the broken ratio of HGMs was as high as 27.9%[34]. Although the reaction of NaF and HCl with SiO2runs more smoothly than HF, the alkali washing of byproducts induced the further decrease of the broken ratio of HGMs to 18.4%[34]. In this experiment,NH4F and HCl roughening process induced a broken ratio of 16.10%, suggesting this process not only greatly reduced the damage ratio of HGMs, but also avoided the secondary damage caused by alkaline washing.

Fig.9 shows the tensile strength of composite film with different amount of xylene and curing agent. Curing and forming is a key part in the film forming process, which directly affects whether the composite film can be prepared successfully. Therefore, it is of great significance to study the dosage of solvent and curing agent to optimize the properties of thin films. With the increase of the solvent xylene content in (a), the tensile strength fluctuated, but remained between 0.68 and 0.89 MPa. The elongation at break increased significantly from 85.04% to 194.49%. With increasing the xylene dosage, the elongation at break increased, too. When the amount of xylene was 3 g, the elongation at break and tensile strength reached the highest, and the mechanical properties of the composite film were the best.If the amount of xylene continues to increase, the liquid will spill out from the plexiglass plate.

With the increase of curing agent content in Fig.9 (b), the tensile strength decreased from 0.92 to 0.45 MPa, and the elongation at break decreased from 197.90% to 16.76%. When the amount of curing agent was less than 2 wt%, the composite film had good mechanical properties, but the amount of curing agent was relatively small. As a result, the silicon-oxygen bond crosslinking in liquid silicone rubber was not complete,so it was difficult to remove the complete composite film on the perspex plate. When the amount of curing agent was more than 2 wt%, the edge of the cured composite film started to tilt up and cannot recover.Through the analysis of the above data, the amount of xylene and curing agent should be controlled at 3 g, 2 wt%, respectively, which can not only obtain a better flexible electromagnetic shielding film, but also ensure the smooth removal of the complete composite film.

Fig.7 SEM images of HGMs roughened by NH4F/HCl with different HGMs loadings: (a) 200 g/L; (b) 100 g/L; (c) 66.7 g/L; (d) 50 g/L; (e)40 g/L, (the concentrations of H+ are all 2.88 mol/L)

Fig.8 The comparison diagram of broken ratio achieved by three roughen methods (the HGMs loading was 66.7 g/L and the concentrations of H+ are 2.88 mol/L)

Fig.10 shows the electromagnetic shielding performance of the films with different coating amount in the X-band at room temperature. The electromagnetic shielding effectiveness of the composite film increased with the increase of the cobalt coating amount in (a),and the maximum electromagnetic shielding effectiveness of the composite films with the cobalt coating amount of 30 wt%, 50 wt%, 70 wt% and 90 wt% were respectively 3.8, 6.3, 9.8 and 15.2 dB. When the cobalt coating amount was less than 90 wt%, the electromagnetic shielding effectiveness of the composite films were all below 10 dB. When the Co coating amount was 90 wt%, the electromagnetic shielding effectiveness of the composite films were the largest, up to 15.2 dB, which meant that the coating was highly dense and had a strong shielding ability against electromagnetic waves, and the microstructure of the HGMs can be seen in Fig.11.

Fig.9 The tensile stress of composite film with different amount of (a) xylene (the amount of curing agent was 2 wt%); (b) curing agent (the amount of xylene was 3 g)

In order to explore the shielding mechanism of the composite film, the electromagnetic shielding effectiveness (SET), reflection loss (SER) and absorption loss(SEA) of Co-HGMs flexible electromagnetic shielding films with different coating amount were studied and the amount of coated HGMs was 2 g, just as shown in Fig.10 (b). When the coating amount was 30 wt%, the reflection loss was greater than the absorption loss. The shielding mechanism of composite film was mainly reflected loss. When the coating amount was further increased to 50 wt%, the absorption loss increased and the shielding mechanism turned to be dominated by absorption loss. This was because when the amount of cobalt was low, some HGMs was not totally covered by cobalt or the thickness of cobalt layer was small, and the absorption capacity of electromagnetic wave was relatively weak. When the amount of cobalt increased,the cobalt layer was densified and thickened, and the absorption capacity of electromagnetic wave was enhanced.

Fig.10 (a) SE of the composite films with different coating amount; (b) SET, SER, SEA of the composite film; (c) electromagnetic shielding mechanism

Fig.11 Metallographic microscope images of HGMs with different cobalt plating: (a) 30 wt%；(b) 50 wt%；(c) 70 wt%；(d)90 wt%

Above all, when the content of HGMs was 2 g and the coating amount was 90 wt%, the film had good electromagnetic shielding performance at about 15.2 dB in the X -band. The size (50 mm×50 mm), thickness (0.836 mm), mass (1.529 3 g) of the film can be obtained and we can get that the film is 0.729 g/cm3.We can also deduce the electromagnetic shielding mechanism of the film, just as shown in Fig.10 (c).Electromagnetic waves incident on the surface of the film reflected part of them first at the interface due to impedance mismatch. The remaining electromagnetic waves were reflected many times inside the film and absorbed gradually. The HGMs coated with metal form a conductive network in the film, which further promotes the loss of electromagnetic waves.

4 Conclusions

A preparation scheme of flexible electromagnetic shielding film with low cost and simple process was proposed to realize electroless plating on HGMs.

a) A new HCl+NH4F strategy was performed to roughen the HGMs, and the HGMs were then metallized by electroless plating of Co. Controlling the HGMs loading as 66.7 g/L, the broken ratio can be reduced as 16.10% and at the same time the roughening effect can be ensured. The Co coating of hollow glass microspheres is complete and uniform. The fine grains on the surface of HGMs were mostly hexagonal prismatic and the average grain size was measured to be 0.4-0.5 µm, which was mainly composed of Co-P compounds with only a very small amount of elemental Co.

b) The film was made from coated HGMs and liquid silicone rubber. When the amount of xylene was 3 g, and the amount of curing agent was 2 wt%, the fracture toughness was 0.89 Mpa, and the elongation at break was 194.49%. The shielding effectiveness increased with the addition of metal coating amount and the amount of HGMs. When the cobalt coating amount was 90 wt%, the amount of coated HGMs was 2 g, the thickness of the film was 0.836 mm, and the shielding effectiveness of the composite film was 15.2 dB in the X-band at room temperature, which has good electromagnetic shielding performance. At this point, the shielding loss of the film was mainly absorption and mainly came from the formation of the conductive network inside the film.