C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD 607–616 PREPARATION AND PROPERTIES OF SiO 2 /Ag MICROBEADS USING ELECTROLESS PLATING METHOD PRIPRAVA IN LASTNOSTI SiO 2 /Ag MIKRO KROGLIC Z METODO PLATIRANJA BREZ POMO^I ELEKTRI^NEGA TOKA Congcong Zhang, Xiaolei Su * , Jiaqi Yan, Yi Liu School of Materials Science & Engineering, Xi’an Polytechnic University, Xi’an 710048, China Prejem rokopisa – received: 2023-01-31; sprejem za objavo – accepted for publication: 2023-09-18 doi:10.17222/mit.2023.778 SiO2/Ag microbeads were synthesized using the electroless plating method by changing the addition order and the mixed method of electroless plating. Conducting composites were prepared using the prepared SiO2/Ag microbeads and unsaturated polyester resin as the fillers and substrate, respectively. The microstructure and properties of the prepared microbeads and com- posite were characterized with a scanning electron microscope (SEM), X-ray diffractometer (XRD), energy dispersive spec- trometer (EDS), particle size analyzer, DC milliohm meter and vector network analysis tester. Results show that the SiO2/Ag microbeads prepared with the reverse dropping electroless plating method achieved the best uniformity and integrity of the sil- ver coating on the surfaces of glass beads, and its compaction resistance reached 138.80 m ·cm. When the ratio of the prepared SiO2/Ag microbeads and unsaturated polyester resin was 1:2, the obtained coating composite had a resistivity of 2.79 × 10 –5 ·cm, showing good electromagnetic shielding performance. Keywords: SiO2/Ag microbeads, electroless plating, conductive coating, electromagnetic shielding Steklene kroglice mikronske velikosti prevle~ene s srebrom (angl.: SiO2/Ag microbeads) so avtorji sintetizirali s fizikalno-kemijsko metodo oziroma platiranjem brez pomo~i elektri~nega toka (angl.: electroless plating). Pri tem so spremenili vrstni red postopkov in izbiro kemijske raztopine. Za izdelavo prevodnih kompozitov za prevleke so uporabili pripravljene SiO2/Ag steklene mikro kroglice in nenasi~eno poliestersko smolo kot polnilo oziroma podlago (substrat). Mikrostrukturo in lastnosti pripravljenih kroglic in kompozita so okarakterizirali s pomo~jo vrsti~nega elektronskega mikroskopaa (SEM), rentgenskega difraktometra (XRD), energijskega disperzijskega spektrometra (EDS), analizatorja velikostne porazdelitve delcev, merilnika elektri~ne upornosti (angl.: DC Milliohm Meter) in analizatorja za testiranje vektorske mre`e. Rezultati analiz so pokazali, da imajo SiO2/Ag kroglice mikronske velikosti, izdelane z ve~stopenjsko (povratno) mokro metodo platiranja (napr{evanja) brez pomo~i elektri~nega toka odli~no, homogeno in integrirano srebrno prevleko na povr{ini steklenih kroglic. Kompaktna posteljica iz izdelanih SiO2/Ag kroglic je imela upornost 138,8 m ·cm. Pri nasi~enju oziroma razmerju 1:2 med izdelanimi SiO2/Ag mikro kroglicami in nenasi~enim poliestrom je imel izdelani kompozit za prevleke upornost 2,79 × 10 –5 ·cm. Tako izdelane kompozitne prevleke dobro {~itijo pred vplivom elektromagnetnega sevanja in so zato uporabne kot odli~ni oklopi (oklepi). Klju~ne besede: posrebrene steklene kroglice, platiranje brez pomo~i elektri~nega toka, prevodne prevleke, elektromagnetno oklapljanje 1 INTRODUCTION In recent years, potential safety hazards caused by electromagnetic radiation have attracted considerable at- tention due to the growth of the 5G technology and the application of telecommunication and electronic de- vices. 1–5 Among the plentiful kinds of materials for the electromagnetic shielding purpose, core-shell struc- tured 6–8 conductive fillers consisting of SiO 2 cores and silver shells have been of momentous interest because of their light weight and good conductivity. 9 The conven- tional conductive fillers include silver-coated copper, 10 silver-coated aluminum 11 , silver-coated nickel 12 powder, and so on. However, these composite powders are quite heavy, 13 and this limits their applications in lightweight designs. Glass microbeads have the advantages of exhib- iting light weight, low thermal conductivity, high and low temperature resistance, corrosion resistance, good thermal stability, 14 high compressive strength, good dispersibility and fluidity; however, a bead itself has no conductivity or electromagnetic shielding characteris- tic. 15,16 As a result, scholars have made relevant research on silver plating of the surfaces of glass microbeads, dis- covering the excellent performance of core-shell struc- tured fillers. 17–20 So far, there have been many attempts to synthesize SiO 2 /Ag composites, 21 involving the micro-emulsion method, 22 chemical reduction method, 23,24 sol-gel method, 25,26 thermal deposition, 27 electroless plating 28,29 and so on. In particular, the electroless plating method, which overcomes the problems of uneven mixing of the mechanical mixing method and easy grain growth of the sol-gel method in reducing alkali metal oxides, is widely used because it is not restricted to the material shape. 30,31 In addition it is a simple and easy-handling 4,30,31 process for the fabrication of silver-plated glass microbead com- posites with excellent performance. To this end, W.-J. Kim and S.-S. Kim 20 synthesized Ag-coated hollow Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 607 UDK 666.266.4:539.23 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(6)607(2023) *Corresponding author's e-mail: su_x_lei@163.com (Xiaolei Su) microbeads using a two-step procedure of sensitizing and subsequent electroless plating. Wu et al. 32 prepared core-shell SiO 2 /Ag composite microbeads with a dense, completive and scaled silver layer. However, SiO 2 /Ag composites with a complete, uniform and compact silver shell and high-purity are still challenging to be gained. 32 In this study, an optimal process for producing uni- form, continuous, dense, complete silver 33 thin films with excellent electrical conductivity was investigated via altering the addition order and manner of electroless plating. Accordingly, the SiO 2 /Ag core-shell particles 34 were fabricated with a smooth micro-morphology, low resistivity, strong stability, fine silver layer particles and high electromagnetic shielding energy. Meanwhile, prob- able reaction mechanisms for the formation of the SiO 2 /Ag core-shell microbeads prepared with four elec- troplating addition methods were also discussed pro- foundly in this study. Subsequently, conductive coating composites were formed by combining SiO 2 /Ag granules 35 and unsaturated polyester resin with screen printing technology. Finally, the EMI shielding perfor- mance and resistivity of the coating composites were tested, and the prepared conductive powder was tested via a scanning electron microscope, energy spectrometer, X-ray diffractometer, laser particle size analyzer, DC low resistance tester and other instruments. 36–38 2 EXPERIMENTAL PART 2.1 Materials Stannous chloride (SnCl 2 ) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. SiO 2 solid microbeads ( 99 %) with an average size of 48 μm were purchased from Hebei Jinghang Mineral Products Co., Ltd., China. Glucose (C 6 H 12 O 6 ) and tartaric acid (C 4 H 6 O 6 ) were obtained from Tianjin Tianli Chemical Reagents Co., Ltd., China. Silver nitrate (AgNO 3 )a n d sodium hydroxide (NaOH) were procured from Damao Chemical Reagent Factory, China. Hydrofluoric acid (HF), absolute alcohol (C 2 H 5 0H), sodium fluoride (NaF), hydrochloric acid (HCl) and ammonia water (NH 3 ·H 2 O) were supplied by Tianjin Hongyan Chemical Reagent Factory, China. Polyvinylpyrrolidone (PVP) was pro- vided by Shanghai Aladdin Biochemical Technology Co., Ltd., China. The resin and deionized aqueous solutions were prepared in house. All the reagents and chemicals were of analytical grade and were used as re- ceived without further purification. 2.2 Preparation of SiO 2 microbeads The SiO 2 microbeads (48 μm) were degreased with an NaOH solution, washed with deionized water three times and dried in a vacuum oven at 80 °C for 3 h for fu- ture use. In a while, the obtained microbeads were coars- ened with a hydrofluoric acid and sodium fluoride mix- ture that was stirred at room temperature for 10 min at a stirring speed of about 350 min –1 , rinsed with deionized water and dried for future use. The aim of the coarsening reaction was to corrode the surfaces of SiO 2 microbeads, thus increasing the surface roughness and specific area. The coarsened microbeads (40 g) were sensitized in a stannous chloride and hydrochloric acid mixture solution containing 6.4 g SnCl 2 + 24 mL HCl + 320 mL distilled water. Afterward the microbeads were sluiced with dis- tilled water, vacuum filtered and dried in the vacuum ovenat80°Cfor3h. 39 The sensitizing reaction allowed the attachment of a layer of a Sn 2+ film to the SiO 2 sur- faces, providing reducing substances for the next step. 39 The sensitized microbeads (10 g) were activated in a silver ammonia solution (Ag(NH 3 ) 2 OH 2.5 g/L) that was shaken uniformly at 40 °C for 25 min at a stirring speed of about 350 min –1 . Then, the microbeads were washed with distilled water, vacuum filtered and dried at 80 °C. A tier of Sn 2+ ions 40 adsorbed on the surfaces of silica microbeads reacts with the silver ammonia solution, and [Ag(NH 3 ) 2 ] + ions 32 are reduced to Ag particles that are uniformly adhered onto the surfaces of microbeads. The newly reduced Ag particles on the silica surface act as the seeds which provide nucleation sites for a new depo- sition and growth of the silver shell. 20,32 A schematic diagram of the pretreatment process of SiO 2 microbeads is shown in Figure 1. SiO 2 is a micro- bead with a smooth surface. After a variation in the mixed solution with hydrofluoric acid and sodium fluo- ride, the surface of SiO 2 becomes rough and the specific surface area increases. The rough surface is of great ben- efit to the adhesion of the Sn 2+ film 39 and SiO 2 substrate C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD 608 Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 Figure 1: Schematic diagram of the pretreatment process during the sensitization process. Besides, a large specific surface area provides sites for the Ag seed deposition. 41 The Sn 2+ film can supply reducing substances which re- act with the silver ammonia solution used in the activa- tion process to generate silver particles as active sites 41 . After the deposition of Ag seeds on the glass micro- beads, the silver particles are gradually and continuously attached to the surface of SiO 2 to form a SiO 2 /Ag core-shell structure composite via the chemical sil- ver-plating method. 2.3 Synthesis of SiO2 /Ag microbeads The SiO 2 /Ag core-shell particles were synthesized through the electroless plating method. In brief, the sen- sitized microbeads were put in a plating solution contain- ing an oxidizing agent (silver nitrate – AgNO 3 ), a reduc- ing agent (glucose – C 6 H 12 O 6 ), a stabilizing agent (ethanol – C 2 H 5 0H), a dispersing agent (polyvinylpyrro- lidone – PVP), a PH control agent (sodium hydroxide – NaOH) and distilled water. The reducing agent was a mixture of glucose and tartaric acid. The mixing ratio of glucose to tartaric acid was 1:8 by weight. The plating solution was agitated at 40 °C for 20 min, then the NaOH solution was dropwise added into the reaction so- lution and stirred for 20 minutes in order to adjust the pH value of the solution. After reacting, the Ag-coated microbeads were gravity filtered, 41 rinsed with distilled water and dried at 80 °C in the vacuum oven. After thor- ough filtering, rinsing and drying, the SiO 2 /Ag core-shell granules were successfully obtained. In the experiment, four ways of forward dropping, forward mixing, reverse dropping and reverse mixing were used for the electroless silver plating of the pretreated microbeads. A schematic diagram of the add- ing method of electroless plating is shown in Figure 2. The best preparation parameters obtained through exper- iments are as follows: AgNO 3 – 15 g/L, an appropriate amount of ammonia water, C 6 H 12 O 6 – 10 g/L, C 2 H 5 OH – 40 mL/L, PVP – 2 g/L, reaction temperature – 40 °C, PH = 11. 2.4 Fabrication of the SiO 2 /Ag coating composite The SiO 2 /Ag coating composites were prepared using the screen printing technology, and the ratio of SiO 2 /Ag microbeads to unsaturated polyester resin was 1:2 by weight. Firstly, a ceramic rectangular piece was fixed firmly on a screen printer. Then a mixed sizing agent was applied on the ceramic sheet with a scraper. Finally, the ceramic piece was taken out and put into a Petri dish, then dried at 120 °C for2hi nt h ev acuum oven. All samples of the SiO 2 /Ag coating composite manufactured with the screen printing technology were cut into a rect- angular block with a size of 2.0 cm × 2.0 mm × 0.12 mm. 42 2.5 Characterization The morphologies of SiO 2 /Ag microbeads were in- vestigated using a field emission scanning electron mi- croscope (FE-SEM, Quanta-450-FEG+X-MAX50, FEI Company, Netherlands), operating at an accelerating voltage of 30 kV. The compositional information of the silver-plated glass microbeads was acquired with an en- ergy dispersive spectrometer (EDX, Horiba 7021-H2, SUPU, China) installed on FE-SEM. The crystalline structure and the phase composition of the prepared composites were characterized via X-ray diffraction (XRD, DX-2700BH, Dandong Haoyuan Instrument Co., Ltd., China) under a CuK radiation ( = 1.5400 nm) at a scanning range (2 ) of 10–90° and a step size of 0.03°. The particle size and its distribution in the powder were measured with a laser particle size analyzer (JB6100-A, Shanghai Jiubin Instrument Co., Ltd., China) in a size number range of 0.01–1250 μm. The compaction resistance of SiO 2 /Ag core-shell microbeads was tested with a DC milliohm meter (TH2516, Tonghui, China). The resistivity of the conductive coating was gauged as follow: = RS/L (1) C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 609 Figure 2: Schematic diagram of the adding method of electroless plating where R is the resistance, S is the cross-sectional area and L is the length. The electromagnetic interference (EMI) shielding properties of the conductive coating composite were tested with a vector network analyzer (Keysight E5061B ENA, Shenzhen, China), using the waveguide method within 0.5–6 GHz. The S parameter of electromagnetic shielding was obtained with the vector network analyzer, while the transmittance (T), reflectance (R) and absorp- tivity (A) were analyzed based on the S parameter. 43–45 S 11 represents the reflection coefficient on plane T 1 ; S 21 rep- resents the forward transmission coefficient from plane T 1 to plane T 2 . R = IS 11 I 2 , T = IS 21 I 2 (2) A=1–R – T (3) The overall electromagnetic interference shielding ef- fectiveness (SE) of the coating composite was calculated as follows: SE = SE R + SE A + SE M (4) SE R = –10 log (1 – R) (5) SE A = –10 log (T/(1–R)) (6) where SE R is the microwave reflection, SE A is the mi- crowave absorption and SE M is the microwave multiple reflection. When SE = 15 dB, the SE M can be insignifi- cant. 13,43–45 3 RESULTS AND DISCUSSION 3.1 Micromorphology and characterizations of SiO 2 /Ag microbeads 3.1.1 Surface micromorphology The surface micro-morphology of the core-shell SiO 2 /Ag microbeads obtained with different electroless plating methods and under the same magnification of SEM observation is shown in Figure 3. As indicated in Figure 3a, the coating prepared with forward dropping is relatively uniform and complete, while the appearance of SiO 2 /Ag is a little rough because of uneven growth of sil- ver particles. It is found that there is a lot of agglome- ration 33 of white flocs around the silver-plated glass microbeads in Figure 3b where the silver applied with forward mixing on the glass microbead surfaces is not uniform. When comparing Figures 3c and 3b,itisobvi - ous that no white flocs are produced near the surfaces of SiO 2 /Ag microbeads on the former figure where the sil- ver coating is complete and compact. It is also worth not- ing that Figure 3c shows a SEM image of silver-plated silica microbeads with a smooth surface and homoge- neous size. In Figure 3d, the silica microbeads are also completely covered by consecutive silver particles, while the coating effect of the SiO 2 /Ag core-shell particles syn- thesized via reverse mixing is relatively poor. The mechanism diagram of the silver layer growth process is shown in Figure 4. There are two explanations for the reduction mechanism of Ag + . Some scholars think that silver is deposited via a non-autocatalytic pro- cess and can be deposited by itself in a solution. Others believe that silver still has an autocatalytic effect, but the catalytic ability is not strong enough so it is necessary to activate SiO 2 microbeads before silver particles are con- tinuously attached on the substrate surface. The specific synthetic mechanisms of the two for- ward electroless silver plating methods are as follows. Pure silver particles can be relatively uniformly attached and deposited on the surfaces of glass microbeads because the reaction rate via dropping in the forward direction is slow. Furthermore, the oxidation-reduction reaction is generally carried out on the surfaces of microbeads. Meanwhile, the substrate is a glucose solution under this circumstance, which does not form new silver pure ele- ments with the silver nitrate solution as the active site of electroless silver plating so that the preferential growth of Ag occurs in some parts of SiO 2 surfaces and the sil- ver shell coating is not uniform. In contrast, the oxida- tion-reduction reaction rate with forward mixing is too fast, leading to quantities of silver elementary substances in the solution in the form of aggregation. To be specific, before most silver ions reach the surfaces of glass beads, they first react in the electroless plating solution and then form silver elementary substance aggregations in the so- lution. Only a small amount of silver ions 34 react on the surfaces of glass beads, which results in a large number of white flocculent silver elements around SiO 2 /Ag while most of the glass beads are not completely covered with Ag layers. Moreover, the explicit synthetic mechanisms of the two reverse chemical silver-plating methods are also clarified. A comparatively mild reaction rate during re- verse dropping promotes a uniform deposition of Ag par- ticles and makes Ag cover the silica surface completely to form a silver shell. 46 In this case, the silver ammonia solution as the matrix can cause a redox reaction on the surfaces of activated microbeads, forming a thin layer of silver coating that provides stable active sites for an argentic growth in the subsequent Ag plating process. C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD 610 Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 Figure 3: SEM images of SiO 2 /Ag microsbeads: a) forward dropping, b) forward mixing, c) reverse dropping, d) reverse mixing With an increased plating time, Ag particles are depos- ited more uniformly onto the SiO 2 surface, gradually forming a core-shell structure, and eventually the cov- ered silver-coated silica beads become complete, uni- form and compact. When the reverse mixing method is adopted, the preferential growth of silver ions during the crystallization procedure may lead to aggregated lumps and uneven silver layers on the surface. 32 It is also possi- ble that the mixing reaction starts at a very fast rate so that the growth of silver immediately reaches saturation, which makes the silver nucleate rapidly. During the con- tinuation of chemical silver plating, the concentration of the reaction solution decreases and the silver enters the growth stage, resulting in the separation of nucleation and growth of silver, and thus silver lumps appear some- where on the surface as shown in Figure 3d. 3.1.2 EDS characterization The chemical compositions of SiO 2 /Ag microbeads prepared with the reaction methods with different addi- tion orders were analyzed using an energy dispersive X-ray spectrometer (EDS) as shown in Figure 5.Ascan be seen on this figure, only Si, O, Ca, Na and Ag peaks are clearly shown and no other peaks are detected. The content of Ag on the surface of SiO 2 /Ag is about (49.9, 43.7, 54.1 and 51.6) w/% when the preparing methods are forward dropping, forward mixing, reverse dropping and reverse mixing, respectively. This means that an Ag shell with a high purity was acquired on silica microbeads in the current study. 3.1.3 XRD characterization The spectra in Figure 6 show typical X-ray diffrac- tion (XRD) patterns of silver-plated glass beads prepared with the four reaction methods with different addition or- ders to verify the crystalline properties and phase purity of the obtained samples. The four well-resolved diffrac- tion peaks of SiO 2 /Ag core-shell composites are ob- served at 2 angles of 38.12°, 44.30°, 64.44° and 77.40°, in a range of approximately 30–80°, corresponding to the reflections of the (111), (200), (220) and (311) crystal C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 611 Figure 4: Mechanism diagram of the silver layer growth process Figure 5: SEM images with marked EDS measurement areas and EDS spectra of SiO 2 /Ag microbeads: a) forward dropping, b) forward mixing, c) reverse dropping, d) reverse mixing Figure 6: X-ray diffraction patterns of SiO 2 /Ag microbeads planes of face-centered cubic (FCC) metal silver (JCPDS Card No. 87-0720), respectively. 31,32,47 There is no dif- fraction peak of silver compounds, indicating that the pure Ag particles with high crystallinity were deposited to grow on the Ag seeds on the surface of SiO 2 . 48 These XRD results agree well with those of SEM observations. 3.2 Particle size of SiO2/Ag microbeads The particle sizes of SiO 2 /Ag microbeads prepared with different electroless plating methods are shown in Figure 7. It can be seen that the particle magnitude range obtained for the experimental glass microbeads is about 20–67 μm and its D50 is about 34 μm 49 . It is also clear from the figure that there is almost no big difference be- tween the average particle sizes of the samples prepared with the four methods. In addition, the total number of particles below 5 μm in silver plated glass microbeads produced with the reverse method is larger than that ob- tained with the forward method. This is because there are a few dissociative silver particles in the forward reaction process, leading to an increase in the accumulation of ul- tra-fine particles, 50,51 while the cumulant of the sil- ver-plated glass microbeads in this size range is rela- tively limited in the reverse course of reaction. It can be further explained that silver grows better on the surfaces of glass microbeads during reverse electroless silver plat- ing. 3.3 Measurement of resistance of SiO2/Ag microbeads The compaction resistance of SiO 2 /Ag conductive microbeads is shown in Table 1. It can be clearly seen that the electric resistance of the sample obtained with forward mixing is the highest, while that of the sample obtained with reverse mixing is the lowest. 46 3.4 Measurement of resistivity of SiO 2 /Ag coating com- posites The conductive coatings composed of SiO 2 /Ag microbeads were made using the screen printing technol- ogy, and its parameters were 2.0 cm, 2.0 mm, 0.12 mm in length, width and thickness, respectively. The mass ra- tio of SiO 2 /Ag microbeads to unsaturated polyester resin was 1:2. The resistance of these coatings was tested with a DC milliohm meter and then the resistivity of the lay- ers was calculated. The testing information is shown in Table 2. It is clearly seen that the resistivity of the SiO 2 /Ag coating composite obtained with forward mix- ing is the largest, and the one prepared by reverse drop- ping is relatively small, with values of 3.64 × 10 –5 ·cm and 2.79 × 10 –5 ·cm, respectively. It is also worth not- ing that the resistivity of the SiO 2 /Ag coating obtained with reverse mixing is smaller than that obtained with re- verse dropping, which is consistent with the result for the compaction resistance of SiO 2 /Ag microbeads. Table 2: Average resistance and resistivity of SiO 2 /Ag coating com- posites Reaction method Average resistance / Resistivity / ·cm Forward dropping 2.685 3.22×10 –5 Forward mixing 3.036 3.64×10 –5 Reverse dropping 2.324 2.79×10 –5 Reverse mixing 2.129 2.55×10 –5 The resistance of the samples obtained with forward mixing is the highest, while that of the microbeads and its coating obtained with reverse mixing is the lowest. This may be because the matrix is a silver ammonia so- lution which first reacts with the substance on the sur- faces of the glass microbeads, and then the thin layer of Ag is reduced to supply active sites 20 for the subsequent electroless plating during the reverse reaction process. However, the active sites are relatively insufficient in the forward reaction process, leading to a comparatively un- satisfactory coating effort, resulting in an enhancement of the resistance values. The reason why the resistance of the microbeads pre- pared with reverse mixing addition is better than that ob- tained with reverse dropping addition may be as follows. At the beginning of the reaction, the speed of reverse mixing was fast, and in the meantime the silver genera- tion rate had reached saturation, resulting in a rapid nu- cleation. Afterwards, with a decrease in the reaction con- centration, the reaction rate began to slow down, and the nucleation and growth were basically separated, resulting in the phenomenon of explosive nucleation and slow C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD 612 Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 Figure 7: Particle size of SiO 2 /Ag microbeads: a) fForward dropping, b) forward mixing, c) reverse dropping, d) reverse mixing Table 1: Compaction resistance of SiO 2 /Ag microbeads Reaction method Forward dropping Forward mixing Reverse dropping Reverse mixing Sample color Gray white Grey Off white Gray white R/m ·cm 138.50 142.10 138.80 137.75 growth. 52 In addition, too slow a rate at the later stage of reverse mixing reaction may have also brought about the preferential growth of silver, leading to an aggregation of Ag particles on the surfaces of microbeads. Because of the existence of silver nuggets, the resistance of the SiO 2 /Ag microbeads and coating composites gained with reverse mixing is smaller than that obtained with reverse dropping. 3.5 Electromagnetic shielding performance of SiO 2/Ag conductive coatings In this experiment, a shielding test was conducted in an electromagnetic wave frequency range of 0.5–6 GHz. The electromagnetic shielding effectiveness of the SiO 2 /Ag conductive coating is shown in Figure 8. The common trend of the four curves indicates that the SE R value increases as the frequency of electromagnetic waves decreases, significantly increasing from (45.10, 45.6, 46.25 and 46.63) dB to (55.88, 56.42, 57.05 and 57.43) dB due to the electroless Ag-plating methods in- cluding forward mixing, forward dropping, reverse drop- ping and reverse mixing, respectively. When the waves are shorter than 1 GHz, the electromagnetic shielding of SiO 2 /Ag coatings exhibits a comparatively high perfor- mance with a SE R value exceeding 52 dB. Detailed rela- tionship between the electromagnetic shielding effective- ness and efficiency can be found in Table 3. 53 According to the experimental results and measurements, the shield- ing efficiency of all four kinds of SiO 2 /Ag conductive coatings is 99.999 %. It is obvious from Figure 8 that the electromagnetic shielding effectiveness of the conductive coating pre- pared with the reverse mixing method is the best, while the one obtained with the forward mixing method is the poorest. Moreover, the shielding curves of the conductive coatings prepared with reverse dripping and reverse mix- ing are very close. It is well known that the electromag- netic shielding performance increases with a decrease in the resistance and resistivity. This result agrees well with those of resistance and resistivity measurements, and the phenomenon, according to which the shielding curve of the SiO 2 /Ag coating obtained with reverse mixing is slightly higher than that obtained for reverse dropping, is caused by the presence of silver nuggets on the SiO 2 /Ag powder surface. The complex conductive coating is com- posed of many SiO 2 /Ag microbeads, which connect with each other to form a continuous conductive network. The increase in the SE R of these four different SiO 2 /Ag coat- ings is ascribed to the content, uniformity and integrity of the silver adhered over the SiO 2 microbeads. As incident electromagnetic waves run into the coarse Ag layers, they are diminished by scattering, ad- sorption and reflection many times in the formed con- ductive network. The electromagnetic field can induce currents resulting in ohmic losses, effectively shielding the electromagnetic waves. 43 From the above analysis, we can conclude that the electromagnetic shielding ef- fectiveness of the conductive layer changes with the ad- justment of the adding modes when the values for the electromagnetic shielding effectiveness of the coatings consisting of SiO 2 microbeads produced through reverse mixing and dropping are slightly different and the shield- ing efficiency is good. Table 3: Relationship between electromagnetic shielding effectiveness and efficiency 53 Effectiveness (dB) Efficiency (%) 00 10 90 20 99 30 99.9 40 99.99 50 99.999 60 99.9999 70 99.99999 80 99.999999 90 99.9999999 92 99.99999994 4 CONCLUSIONS High-performance SiO 2 /Ag core-shell composites with both superior surface topography and electromag- netic shielding properties were successively synthesized via the reverse dropping electroless plating method and utilized as conductive fillers to manufacture a SiO 2 /Ag conductive coating. 41 Their microstructure, chemical composition, phase composition, particle size, conduc- tivity and electromagnetic interference shielding proper- ties were investigated. The results of SEM, EDS, XRD, particle size analyzer and DC milliohm meter showed that SiO 2 microbeads were covered with a uniform, con- tinuous, complete and compact Ag shell, which allowed silver-plated glass beads to have good conductivity; its compaction resistance was 138.80 m ·cm, the silver layer had an fcc structure, and its D50 was about 34 μm. C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 613 Figure 8: Electromagnetic shielding effectiveness of SiO 2 /Ag coating composite The results also showed that the effect of reverse electro- less Ag-plating was better than that of a forward reac- tion, and the coating effect of dropping was better than that of mixing. Almost no white flocs were produced on the surfaces of silver-plated glass beads prepared with the reverse dropping method, and the coating was dense, uniform and complete. 6 Besides, the Ag content of the coating was relatively high, reaching about 54.1 w/%, and the silver shells did not fall off easily. By analyzing the silver layer growth and SiO 2 /Ag microbead synthesis mechanisms, it can be concluded that both the reaction speed and reactant addition order had a major impact on the deposition and growth of sil- ver particles where neither too fast nor too slow a reac- tion rate was conducive to electroless Ag-plating. In this system of taking Ag-seeds as active sites of Ag-shell growth, surface morphologies and application perfor- mances of the micro-composites can be well controlled and regulated by changing the addition order and using the mixed method of electroless plating. The prepared coating composite fabricated of unsaturated polyester resin and SiO 2 /Ag, obtained through reverse dropping, with a ratio of 1:2, exhibits excellent electromagnetic shielding performance 54–56 including a resistivity of 2.79 × 10 –5 ·cm, SE R value of 57.05 dB and wave fre- quency range of 0.5–6 GHz. This result is mainly attrib- uted to the formation of a conductive path and improve- ment of electrical conductivity through SiO 2 /Ag core-shell 18 particles acting as the conductive fillers. In conclusion, the SiO 2 /Ag microbeads obtained with re- verse dropping have great potentials and promising ap- plications in the field of EMI shielding. Acknowledgments The authors gratefully acknowledge support from the Scientific Research Program Funded by Education De- partment of Shaanxi Provincial Government (Program No.23JC036) and Scientific and Technological Plan Pro- ject of Xi'an Science and Technology Bureau (Program No. 23KGDW0031-2022). 5 REFERENCES 1 S. Wei, C. Zhou, L. Huang, Occupational health and safety: measure- ment and analysis of the electromagnetic radiation produced by radiofrequency devices for rejuvenation, Lasers Med Sci, 38 (2022) 1, 25–25, doi:10.1007/s10103-022-03669-y 2 T. Zhang, D. Wang, R. Liu, Y. Xie, J. Li, L. Wang, A Coral Reef-Like Structure Fabricated on Cellulose Paper for Simultaneous Oil-Water Separation and Electromagnetic Shielding Protection, A C SO m e g a ,5( 2020) 29, 18105–18113, doi:10.1021/acsomega. 0c01666 3 K. Karipidis, R. Mate, D. Urban, R. Tinker, A. Wood, 5G mobile net- works and health – a state-of-the-science review of the research into low-level RF fields above 6 GHz, J. Expo. Sci. Environ. Epidemiol., 31 (2021) 4, 585–605, doi:10.1038/s41370-021-00297-6 4 R. P. Chowdhury, L. A. Stegeman, M. L. Lund, D. Fry, S. Madzun- kov, A. A. Bahadori, Hybrid methods of radiation shielding against deep-space radiation, Life Sciences in Space Research, 38 (2023), 67–78, doi:10.1016/j.lssr.2023.04.004 5 Z. Ding, X. Xiang, J. Li, S. Wu, Molecular Mechanism of Malignant Transformation of Balb/c-3T3 Cells Induced by Long-Term Expo- sure to 1800 MHz Radiofrequency Electromagnetic Radiation (RF-EMR), Bioengineering (Basel), 9 (2022) 2, 43–43, doi:10.3390/ bioengineering9020043 6 Q. Wu, H. Zhang, Y. Zhou, Z. Tang, B. Li, T. Fu, Y. Zhang, H. Zhu, Core-Shell Structured Carbon@Al2O3 Membrane with Enhanced Acid Resistance for Acid Solution Treatment, Membranes (Basel), 12 (2022) 12, 1246–1246, doi:10.3390/membranes12121246 7 Y. Bao, X. Wu, B. Yin, X. Kang, Z. Lin, H. Deng, H. Yu, S. Jin, S. Chen, M. Zhu, Structured copper-hydride nanoclusters provide in- sight into the surface-vacancy-defect to non-defect structural evolu- tion, Chem. Sci., 13 (2022) 48, 14357–14365, doi:10.1039/ d2sc03239b 8 C. Wang, D. Ma, X. Li, D. Luo, L. Wu, An electroless-plating-like solution approach for the preparation of PS@TiO2@Ag core-shell spheres, RSC Adv., 10 (2020) 16, 9341–9346, doi:10.1039/ c9ra10624c 9 B. Guo, J. Liang, J. Chen, Y. Zhao, Highly flexible and ultrathin electromagnetic-interference-shielding film with a sandwich struc- ture based on PTFE@Cu and Ni@PVDF nanocomposite materials, RSC Adv., 12 (2022) 46, 29688–29696, doi:10.1039/d2ra05439f 10 S. Lee, C. Wern, S. Yi, Novel Fabrication of Silver-Coated Copper Nanowires with Organic Compound Solution, Materials (Basel), 15 (2022) 3, 1135–1135, doi:10.3390/ma15031135 11 M. Hao, L. Li, X. Shao, M. Tian, H. Zou, L. Zhang, W. Wang, Fabri- cation of Highly Conductive Silver-Coated Aluminum Microspheres Based on Poly(catechol/polyamine) Surface Modification, Polymers (Basel), 14 (2022) 13, 2727–2727, doi:10.3390/polym14132727 12 Y. Yang, G. Montserrat-Siso, B. Wickman, P. A. Nikolaychuk, I. L. Soroka, Core-shell and heterostructured silver-nickel nanocatalysts fabricated by gamma-radiation induced synthesis for oxygen reduc- tion in alkaline media, Dalton Trans., 51 (2022) 9, 3604–3615, doi:10.1039/d1dt03897d 13 X.-K. Lv, J.-G. Yu, Novel Silver-Plated Nickel-Coated Graphite Powder with Excellent Heat and Humidity Resistance: Facile Prepa- ration and Performance Investigation, Molecules, 27 (2022) 13, 4007–4007, doi:10.3390/molecules27134007 14 C. P. Feng, F. Wei, K. Y. Sun, Y. Wang, H. B. Lan, H. J. Shang, F. Z. Ding, L. Bai, J. Yang, W. Yang, Emerging Flexible Thermally Con- ductive Films: Mechanism, Fabrication, Application, Nano-Micro Letters, 14 (2022) 1, 127–127, doi:10.1007/s40820-022-00868-8 15 Y. Ma, H. Liu, Z. Han, L. Yang, J. Liu, Highly-reproducible Raman scattering of NaYF4:Yb,Er@SiO2@Ag for methylamphetamine de- tection under near-infrared laser excitation, Analyst, 140 (2015) 15, 5268–5275, doi:10.1039/c5an00441a 16 X. G. Cao, H. Y. Zhang, Investigation into conductivity of sil- ver-coated cenosphere composites prepared by a modified electroless process, Applied Surface Science, 264 (2013), 756–760, doi:10.1016/j.apsusc.2012.10.116 17 H. Wang, F. Fotovat, X. T. Bi, J. R. Grace, Tribo-charging of binary mixtures composed of coarse and fine particles in gas-solid pipe flow, Particuology, 43 (2019), 101–109, doi:10.1016/j.partic. 2018.07.001 18 K. Siczek, H. Zatorski, A. Chmielowiec-Korzeniowska, R. Kordek, L. Tymczyna, J. Fichna, Evaluation of anti-inflammatory effect of silver-coated glass beads in mice with experimentally induced colitis as a new type of treatment in inflammatory bowel disease, Pharma- cological Reports, 69 (2017) 3, 386–392, doi:10.1016/j.pharep. 2017.01.003 19 K. Siczek, J. Fichna, H. Zatorski, B. Karolewicz, L. Klimek, A. Owczarek, Development of the rectal dosage form with silver-coated glass beads for local-action applications in lower sections of the gas- trointestinal tract, Pharmaceutical Development and Technology, 23 (2018) 3, 295–300, doi:10.1080/10837450.2017.1359843 C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD 614 Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 20 W.-J. Kim, S.-S. Kim, Preparation of Ag-coated hollow microspheres via electroless plating for application in lightweight microwave ab- sorbers, Applied Surface Science, 329 (2015), 219–222, doi:10.1016/j.apsusc.2014.12.173 21 C. Liu, D. Yang, Y. Jiao, Y. Tian, Y. Wang, Z. Jiang, Biomimetic syn- thesis of TiO2-SiO2-Ag nanocomposites with enhanced visible-light photocatalytic activity, ACS Appl. Mater. Interfaces, 5 (2013)9 , 3824–3832, doi:10.1021/am4004733 22 F. Jiang, X. X. Wei, J. Zheng, Synthesis and electromagnetic charac- teristics of MnFeO/TiO composite material, Materials Research Ex- press, 9 (2022) 10, doi:10.1088/2053-1591/ac97de 23 G. Yang, S. Luo, T. Lai, H. Lai, B. Luo, Z. Li, Y. Zhang, C. Cui, A Green and Facile Microvia Filling Method via Printing and Sintering of Cu-Ag Core-Shell Nano-Microparticles, Nanomaterials, 12 (2022) 7, 1063, doi:10.3390/nano12071063 24 Z. Wang, K. Yliniemi, B. P. Wilson, M. Lundstom, Targeted surface modification of Cu/Zn/Ag coatings and Ag/Cu particles based on sacrificial element selection by electrodeposition and redox replace- ment, Surface & Coatings Technology, 441 (2022), doi:10.1016/ j.surfcoat.2022.128531 25 P. Karo-Karo, S. Sembiring, I. Firdaus, R. Situmeang, S. D. Yuwono, Preparation of silver-doped rice husk silica composites using the sol-gel method, Ceramics – Silikaty, 66 (2022) 3, 365–373, doi:10.13168/cs.2022.0032 26 E. A. Gonzalez, N. Leiva, N. Vejar, M. Sancy, M. Gulppi, M. I. Azocar, G. Gomez, L. Tamayo, X. Zhou, G. E. Thompson, M. A. Paez, Sol-gel coatings doped with encapsulated silver nanoparticles: inhibition of biocorrosion on 2024-T3 aluminum alloy promoted by Pseudomonas aeruginosa, Journal of Materials Research and Tech- nology, 8 (2019) 2, 1809–1818, doi:10.1016/j.jmrt.2018.12.011 27 A. Ablat, L. Hirsch, M. Abbas, Electron beam versus thermal deposi- tion of aluminum top electrode for organic solar cells, Materials Let- ters, 312 (2022), doi:10.1016/j.matlet.2021.131619 28 L. Shen, Y. Zhang, W. Yu, R. Li, M. Wang, Q. Gao, J. Li, H. Lin, Fabrication of hydrophilic and antibacterial poly(vinylidene fluoride) based separation membranes by a novel strategy combining radiation grafting of poly(acrylic acid) (PAA) and electroless nickel plating, J. Colloid Interface Sci., 543 (2019), 64–75, doi:10.1016/j.jcis.2019. 02.013 29 L. Rao, J. Tang, S. Hu, L. Shen, Y. Xu, R. Li, H. Lin, Inkjet printing assisted electroless Ni plating to fabricate nickel coated polypropy- lene membrane with improved performance, J. Colloid Interface Sci., 565 (2020), 546–554, doi:10.1016/j.jcis.2020.01.069 30 S. D. Kim, W. G. Choe, J. Choi, J. R. Jeong, Preparation and charac- terization of silver coated magnetic microspheres prepared by a mod- ified electroless plating process, Powder Technology, 342 (2019), 301–307, doi:10.1016/j.powtec.2018.09.094 31 Y. Zhou, Z. Sun, L. Jiang, S. Chen, J. Ma, F. Zhou, Flexible and con- ductive meta-aramid fiber paper with high thermal and chemical sta- bility for electromagnetic interference shielding, Applied Surface Science, 533 (2020), doi:10.1016/j.apsusc.2020.147431 32 Z. G. Wu, Y. R. Jia, J. Wang, Y. Guo, J. F. Gao, Core-shell SiO2/Ag composite spheres: synthesis, characterization and photocatalytic properties, Materials Science-Poland, 34 (2016) 4, 806–810, doi:10.1515/msp-2016-0121 33 Q. Li, G. Lin, S. Zhang, H. Wang, J. Borah, Y. Jing, F. Liu, Con- ducting and stretchable emulsion styrene butadiene rubber compos- ites using SiO2@Ag core-shell particles and polydopamine coated carbon nanotubes, Polymer Testing, 115 (2022), doi:10.1016/j.poly- mertesting.2022.107722 34 S. Sembiring, A. Riyanto, I. Firdaus, Junaidi, R. Situmeang, Struc- ture and properties of silver-silica composite prepared from rice husk silica and silver nitrate, Ceramics – Silikaty, 66 (2022) 2, 167–177, doi:10.13168/cs.2022.0011 35 K. Pajor, A. Michalicha, A. Belcarz, L. Pajchel, A. Zgadzaj, F. Wojas, J. Kolmas, Antibacterial and Cytotoxicity Evaluation of New Hydroxyapatite-Based Granules Containing Silver or Gallium Ions with Potential Use as Bone Substitutes, International Journal of Mo- lecular Sciences, 23 (2022) 13, 7102–7102, doi:10.3390/ ijms23137102 36 Y. Xiao, L. Lang, W. Xu, D. Zhang, Diffusion bonding of copper al- loy and nickel-based superalloy via hot isostatic pressing, Journal of Materials Research and Technology, 19 (2022), 1789–1797, doi:10.1016/j.jmrt.2022.05.152 37 X. Liu, G. Wang, J. Wang, X. Gong, J. Chang, X. Jin, X. Zhang, J. Wang, J. Hao, B. Liu, Electrochromic and Capacitive Properties of WO3 Nanowires Prepared by One-Step Water Bath Method, Coat- ings, 12 (2022) 5, 595–595, doi:10.3390/coatings12050595 38 C.-H. Kuok, W. Dianbudiyanto, S.-H. Liu, A simple method to valo- rize silica sludges into sustainable coatings for indoor humidity buff- ering, Sustainable Environment Research, 32 (2022) 1, doi:10.1186/ s42834-022-00120-3 39 M. Jose, P. Sienkiewicz, K. Szymanska, D. Darowna, D. Moszynski, Z. Lendzion-Bielun, K. Szymanski, S. Mozia, Influence of Prepara- tion Procedure on Physicochemical and Antibacterial Properties of Titanate Nanotubes Modified with Silver, Nanomaterials, 9 (2019)5, 795–795, doi:10.3390/nano9050795 40 A. Ishikawa, T. Kato, N. Takeyasu, K. Fujimori, K. Tsuruta, Selec- tive electroless plating of 3D-printed plastic structures for three-di- mensional microwave metamaterials, Applied Physics Letters, 111 (2017) 18, 183102–183102, doi:10.1063/1.4986203 41 K. Zhang, C. Wang, Z. Rong, R. Xiao, Z. Zhou, S. Wang, Silver coated magnetic microflowers as efficient and recyclable catalysts for catalytic reduction, New Journal of Chemistry, 41 (2017) 23, 14199–14208, doi:10.1039/c7nj02802d 42 C. Li, H. Zhang, Y. Song, L. Cai, J. Wu, J. Wu, S. Wang, C. Xiong, Robust superhydrophobic and porous melamine-formaldehyde based composites for high-performance electromagnetic interference shielding, Colloids and Surfaces A: Physicochemical and Engi- neering Aspects, 624 (2021), doi:10.1016/j.colsurfa.2021.126742 43 T.-T. Li, Y. Wang, H.-K. Peng, X. Zhang, B.-C. Shiu, J.-H. Lin, C.-W. Lou, Lightweight, flexible and superhydrophobic composite nanofiber films inspired by nacre for highly electromagnetic interfer- ence shielding, Composites Part A: Applied Science and Manufac- turing, 128 (2020) C, 105685–105685, doi:10.1016/j.compositesa. 2019.105685 44 F. Ren, Z. Z. Guo, H. Guo, L. C. Jia, Y. C. Zhao, P. G. Ren, D. X. Yan, Layer-Structured Design and Fabrication of Cyanate Ester Nanocomposites for Excellent Electromagnetic Shielding with Ab- sorption-Dominated Characteristic, Polymers, 10 (2018) 9, 933–933, doi:10.3390/polym10090933 45 F. Ren, H. Guo, Z. Z. Guo, Y. L. Jin, H. J. Duan, P. G. Ren, D. X. Yan, Highly Bendable and Durable Waterproof Paper for Ultra-High Electromagnetic Interference Shielding, Polymers, 11 (2019)9 , 1486–1486, doi:10.3390/polym11091486 46 L.-P. Wu, Y.-Z. Li, B.-J. Wang, Z.-P. Mao, H. Xu, Y. Zhong, L.-P. Zhang, X.-F. Sui, Electroless Ag-plated sponges by tunable deposi- tion onto cellulose-derived templates for ultra-high electromagnetic interference shielding, Materials & Design, 159 (2018), 47–56, doi:10.1016/j.matdes.2018.08.037 47 S. Naseer, M. Aamir, M. A. Mirza, U. Jabeen, R. Tahir, M. N. K. Malghani, Q. Wali, Synthesis of Ni-Ag-ZnO solid solution nanoparticles for photoreduction and antimicrobial applications, RSC Adv., 12 (2022) 13, 7661–7670, doi:10.1039/d2ra00717g 48 T. Liu, D. Li, D. Yang, M. Jiang, An improved seed-mediated growth method to coat complete silver shells onto silica spheres for sur- face-enhanced Raman scattering, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 387 (2011) 1, 17–22, doi:10.1016/j.colsurfa.2011.07.030 49 M. Zhu, G. Xie, L. Liu, P. Yang, H. Qu, C. Zhang, Influence of Me- chanical Grinding on Particle Characteristics of Coal Gasification Slag, Materials, 15 (2022) 17, 6033–6033, doi:10.3390/ma15176033 50 Y. Liu, Y. Zhou, Y. Lin, G. Jia, One-pot microwave-assisted synthesis of Ag2Se and photothermal conversion, Results in Physics, 38 (2022), doi:10.1016/j.rinp.2022.105590 C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616 615 51 Y. Da, J. Liu, Z. Gao, X. Xue, Studying the Influence of Mica Parti- cle Size on the Properties of Epoxy Acrylate/Mica Composite Coat- ings through Reducing Mica Particle Size by the Ball-Milled Method, Coatings, 12 (2022) 1, 98–98, doi:10.3390/coatings 12010098 52 E. R. Wainwright, S. V. Lakshman, A. F. T. Leong, A. H. Kinsey, J. A. Gibbins, S. Q. Arlington, T. Sun, K. Fezzaa, T. C. Hufnagel, T. P. Weihs, Viewing internal bubbling and microexplosions in combusting metal particles via x-ray phase contrast imaging, Com- bustion and Flame, 199 (2019), 194–203, doi:10.1016/j.combust- flame.2018.10.019 53 F. Shahzad, M. Alhabeb, C. B. Hatter, B. Anasori, S. M. Hong, C. M. Koo, Y. Gogotsi, Electromagnetic interference shielding with 2D transition metal carbides (MXenes), Science, 353 (2016) 6304, 1137–1140, doi:10.1126/science.aag2421 54 J. T. Orasugh, S. S. Ray, Graphene-Based Electrospun Fibrous Mate- rials with Enhanced EMI Shielding: Recent Developments and Fu- ture Perspectives, ACS Omega, 7 (2022) 38, 33699–33718, doi:10.1021/acsomega.2c03579 55 C. Ji, Y. Liu, J. Xu, Y. Y. Li, Y. D. Shang, X. L. Su, Enhanced micro- wave absorption properties of biomass-derived carbon decorated with transition metal alloy at improved graphitization degree, Journal of Alloys and Compounds, 890 (2022), doi:10.1016/j.jallcom. 2021.161834 56 Y. Liu, J. N. Qin, L. L. Lu, J. Xu, X. L. Su, Enhanced microwave ab- sorption property of silver decorated biomass ordered porous carbon composite materials with frequency selective surface incorporation, International Journal of Minerals, Metallurgy and Materials, 30 (2023) 3, 525–535, doi:10.1007/s12613-022-2491-7 C. ZHANG et al.: PREPARATION AND PROPERTIES OF SiO2/Ag MICROBEADS USING ELECTROLESS PLATING METHOD 616 Materiali in tehnologije / Materials and technology 57 (2023) 6, 607–616