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Scalable Preparation of Fully Coated Ag@BaTiO3 Core@Shell Particles via PVP Assistance for High-k Applications Gang Jian, Cheng Zhang, Chao Yan, Kyoung-Sik Moon, and C.P. Wong ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00220 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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ACS Applied Nano Materials

Scalable Preparation of Fully Coated Ag@BaTiO3 Core@Shell Particles via PVP Assistance for High-k Applications Gang Jian*,†,‡, Cheng Zhang‡, Chao Yan†, Kyoung-Sik Moon*,‡, and C.P. Wong*,‡ †

School of Materials Science and Engineering, Jiangsu University of Science and Technology,

Zhenjiang 212003, China ‡

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta 30332,

United States ABSTRACT: This paper reports a poly (vinyl pyrrolidone) (PVP) assisted synthesis of Ag@BaTiO3 (BT) core@shell particles, consisting of smooth and fully coated BT shells on Ag cores. The PVP adheres to Ag by coordination attraction, and it is present as a framework on Ag surface. Driven by the adhesion forces from PVP, fully coated BT shells forms on the template of PVP framework. The shell phase is formed through a modified low temperature direct synthesis (LTDS) method. SEM examination of morphological evolution reveals smooth surfaces of the oxide coatings on Ag cores. XRD pattern, EDX spectra, line scan and mapping confirm the phase, chemical composition, and the configuration of the shells. The synthesis method in this paper can achieve controllability of growth rate and shell thickness. Due to the fully coated shells of BT, the polydimethylsiloxane (PDMS) composites filled with the synthesized core@shell fillers have very small leakage currents of ~10–8 – ~10–7 A/cm2. A high permittivity of 202 (εr/εm = 84) and a low dielectric loss of 0.003 are achieved @1 MHz in the composites filled with 40 vol.% Ag@BT fillers. Lich.’s theory containing equivalent permittivity of core@shell Ag@BT originating from the interfacial polarization is used to account for the dielectric constant of the composite. The smooth and fully coated Ag@BT core@shell particles can be used in composite materials for high dielectric constant and low loss applications.

KEYWORDS: core@shell particles, Ag@BaTiO3 (BT), driving forces, composites; high k and low loss 1 Environment ACS Paragon Plus

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1. INTRODUCTION Core@shell particles are a kind of multi-phasic particles with an inner core and an outer shell made of different materials. These particles have been of interest as they can exhibit unique properties due to the combination of the core and the shell materials. Core@shell particles have been designed so that the shell material can improve the reactivity,1 thermal stability,2 dielectric,3 resistivity,4 and oxidation stability

5

of the core material. Thus, they have applications in

biomedicine,6 catalysts,7 plasmonic,8 electric,9 and energy storage fields.10 Particularly, particles containing a metal core such as Al, Ag, Ni, etc and a dielectric shell (e.g., Al2O3, εr ~ 11; TiO2, εr ~ 48, etc.) were used as fillers to make high-k polymer-based composites which can be applied to embedded capacitors.10–13 In this type of material, a high k value can be obtained from the enhanced equivalent permittivity of core@shell particle. Also, a suppressed dielectric loss and an improved breakdown capability can be achieved from the blocking effect of electron transfer by continuous insulative shells. Researchers have reported that Al@Al2O3

10

and Ag@TiO2

11

core@shell nano-particles and

composites containing the core@shell fillers showed permittivity over 70 at a frequency of 10 kHz. However, effects of the shell properties (permittivity, volume fraction, etc) on the effective permittivity of composites and the mechanisms inside were not deeply studied, especially a highk shell had not been employed in the enhancement of permittivity in composites. BaTiO3 (BT) shows a large intrinsic permittivity (εr ~1500) and thus it proves a good candidate as the shell. As a multi-metal-elements compound, it is difficult to prepare BT shells through a one-step hydrolysis from relative alkoxides, compared to other single-metal-element oxides (e.g., TiO2,11 SiO2,14 ZrO2,15 etc.). In addition, to produce fully coated BT shells on large metal cores, the decreased surface energy of the cores with large radius presents issues: the BT shell tends to form sparse dots rather than a conformally coated structure. The surface energy γ (J/m2) of a particle is given by:16

 ∂G 

γ =   ∂A  P ,T

(1) 2 Environment ACS Paragon Plus

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where G is Gibbs free energy of particles, A is surface area, and P and T are pressure and temperature, respectively. Surface energy of the core γ c must be larger than that of the shell γ s , i.e., surface energy difference ∆γ = γ c − γ s > 0 , for the formation of core@shell structures.17 As

γ c tends to decrease with increasing the particle size since less external work is applied for changing particle size, it is difficult to coat on large cores purely driven by ∆γ . It is believed that to make coating onto large metal cores, additives could be used to provide adhesion forces between the core and the shell and to improve the wetting and spreading of the shell on the cores. Among efforts to make BT onto metal cores, Lee et al 18 and Zhang et al 19 reported pioneer work of fully coated core@shell Ni@BT using a two-step approach: in the first step, TiO2 is coated onto Ni, and then TiO2 is transformed to BT via ion diffusion. However, the volume expansion caused by the transformation of TiO2 to BT and the residual untransformed TiO2 phase may result in poor reproducibility of the BT shell on Ni particles.20,21 In order to avoid a stoichiometric imbalance and to guarantee kinetic controllability, a new strategy of one-step coating by employing a polymer is studied in this paper, where the key is a one-step formation of BT in moderate conditions. Among methods to synthesize fine BT particles such as hydrothermal,22 sol-gel,23 low temperature direct synthesis (LTDS),24–26 etc., we use LTDS with moderate synthesis conditions to prepare BT. In LTDS, BT is formed under the driving forces of neutralization heat,24 the simultaneous hydrolysis of Ba and Ti alkoxides,25 or usage of a supersaturation solution.26 In this study, we use poly (vinyl pyrrolidone) (PVP) as an additive for surface modification to prepare micron scale core@shell Ag@BT particles. The PVP framework on Ag surfaces provides a template for the growth of smooth and fully coated shells. The BT shells were produced by the modified LTDS method. The shell layers grew slowly at relatively low temperatures, and in solution-based conditions, which are beneficial to obtaining a high-quality shell layer. The morphology, phase, composition, shell configuration of the synthesized core@shell particles were examined. I-V and dielectric properties of composites filled with core@shell fillers were 3 Environment ACS Paragon Plus


investigated. The performance of such new Ag@BT particles makes them potentially applicable in high-k components such as the PCB-embedded dielectric components. 2. RESULTS AND DISCUSSION Figure 1 presents the schematic of preparation method of fully coated core@shell Ag@BT assisted by PVP. The process includes Ag functionalization, shell growth, and residual PVP removal. It is a solution-based process, where the solvents are used to disperse particles, to dissolve reagents, and to dilute reactants. Ethylene glycol (EG) was used because its boiling point is higher than the reaction temperature. Figure 2A shows the SEM morphology overview of Ag particles used for the core. The Ag particles are of spherical shape with size ranging from 5 to 15 µm. Figure 2B shows the detailed SEM morphology of a typical individual Ag particle. Particle surface before coating appears to be rough. Figure 2C shows SEM images of PVP-functionalized Ag particles. PVP molecules are adsorbed onto Ag surface and thus change the original particle surface. Figure 2D shows the detailed morphology of a PVP modified Ag particle. The adsorbed PVP forms a framework on Ag surface and the patterns differ from domain to domain, as seen in EDX mapping of nitrogen which represents the PVP in Figure 2E. Functionalization of Ag surface with PVP is the key to obtaining fully coated shell. PVP can be easily adsorbed to Ag particles with a tight contact, and the attractive force results from the coordination bonding between Ag and oxygen and nitrogen of the pyrrolidone ring of PVP.27 And PVP forms regular patterns with the rotation movements of Ag particles during stirring. The significance of adsorbed PVP in the coating process lies in following aspects: Firstly, the PVP framework acts as a template for shell growth, the morphology of capped PVP determines that of ceramic shells. The PVP framework also acts as a stabilizer. The long-chain PVP on Ag surfaces provides inter-particle steric repulsion

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to disperse particles and to avoid coagulation which

greatly increases the possibility of monodisperse core coating. Finally, PVP is a binder to adsorb reactants onto the Ag surface for shell growth. The adsorption force, also a driving force for 4 Environment ACS Paragon Plus

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coating originates from the attraction between polar amide groups (–NH–CO–) in PVP and fine BT particles suspended in the solution.29,30 Figures 3A–B present the SEM morphology of as-synthesized core@shell Ag-BT (Ag@PVP/BT, reaction time for 3 h). Different from Ag and Ag@PVP particles, the Ag@PVP/BT particles have regular round shapes and smooth surfaces, indicating a high quality coating of the BT shell. At this step, smooth surfaces of particles can be confirmed by the shiny particle surface, which could be observed using an optic microscope (not shown here). We adopted a modified LTDS process for BT coating, in which the reaction mixtures were diluted with EG. The purpose of the modification in LTDS is to facilitate particle dispersion and grow rate control. BT is formed from the reaction of titanium n-butoxide (TNBT) and the supersaturation solution of Ba(OH)2 at ~ 96 °C. Ti(OC4H9)4 + Ba(OH)2 + H2O → BaTiO3 + 4 C4H9OH

(R1)

It is a one-step formation of BT, which is driven by external heating and dissolution heat.26 A reaction similar to R1 can be found from simultaneous-hydrolysis synthesis of BT,25 in which barium alkoxide’s hydrolysis product, Ba(OH)2 reacts with titanium alkoxide to form BT at a temperature around 80 °C. Reactions where Ba:Ti ratio >1 gives out a phase-pure BT,18,31 which is by 1.2 : 1.0 with a slightly excessive amount of Ba in this synthesis. It is believed that the shell coating is mainly driven by PVP adhesion between the core and the shell in this synthesis, as the smooth and complete shell coating is successfully produced through the PVP pretreatment. And a small amount of coating may be driven by surface energy differences, because of a sparse coating of BT on Ag from the same approach without PVP. The total driving force Ft for shell-coating contains surface energy differences between the core and the shell ( ∆γ = γ c − γ s ) and work of adhesion: Ft = ∆γ + Wa

(2)

The expression of surface energy γ of a particle with radius of r is:32

γ=

Ecluster − Ecohesive 4π r 2

(3) 5 Environment ACS Paragon Plus


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where Ecluster is the total binding energy, Ecohesive is the cohesive energy. For a bulk material, the value of surface energy is smaller compared to that of nanoparticles (NPs) (Eq. (3), Table S1). The surface energy of Ag γ Ag is ~ 7.2 J/m2 for NPs, whereas that of the bulk Ag is ~ 1.065 J/m2.33 As the mean surface energy of BT NPs γ BT is ~ 1.0 J/m2,34 the surface energy differences ( ∆γ = γ Ag −γ BT ) between Ag and BT for Ag NPs and macron sized Ag are ~ 6.2 J/m2 and ~ 0.065 J/m2, respectively. According to the adsorption theory, adhesion process includes two steps: in the first step, objects to be adhered are close to each other by diffusion movements; in the second step, adhesion forces are generated when inter-molecules distances reach to 10 Å. The adhesion energy Ea (kJ/mol) of PVP originated from the interactions of polar molecules can be expressed by the

van der Waals' attractions:35 Ea =

 2  µ4 3 + αµ 2 + α 2 I E  6  8 δ  3k BT 

(4)

where δ is the adsorption distance, normally less than 10 Å, k B is Boltzmann constant, T is temperature, α is molecule polarizability, µ is molecule dipole, ~ 0.7 for PVP, I E is molecule ionization energy. From Eq. (4), Ea increases with increasing µ , which indicates a large Ea value for PVP can be obtained due to the strong polarity of amide groups. Work of adhesion Wa (J/m2) can be obtained from Ea by:

∑M ∑A

PVP

Wa = Ea RPVP = Ea

i

(5)

i

where RPVP is the adsorption rate for PVP (mol/m2), M PVP is the molar amount of PVP in core@shell particle i , A is surface area of core@shell particle i . The adhesion energy Ea for PVP with strong polarity is about 40 kJ/mol,36 using 10 µm size of Ag and 5 wt.% of PVP to calculate the Wa gives about 1.27 J/m2 (Table S2). The driving force for BT coating in this

6 Environment ACS Paragon Plus

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synthesis is mainly attributed to work of adhesion of PVP. Furthermore, as adsorption rate RPVP is a constant with Ag radius, Wa is steady with Ag sizes. It should be noted that this synthesis method can be applied to prepare nano-scale Ag@BT, as a higher driving force Ft for make fully coated structures. Figures 3C–D show SEM images of core@shell Ag@BT particles heat-treated at 550 °C in air for 2 h, where very smooth surfaces and uniform spherical shapes of particles are observed. The removal of PVP from Ag@BT/PVP caused a slight change in particle shape. No considerable pore or crack in the surface are observed, while a continuous and homogenous shell is presented (Figure 3E). During the heat treatment process, crystalline grains grow (to ~ 100 nm, Figure 3F) and migrate filling pores possibly produced by vapor of PVP molecules at the heat treatment. And the yield strength or the load of the shell could be greater than the stress generated by expansion of the core during heat treatment, which is also confirmed by the drastic cooling and heating experiment of the core@shell particles without cracks. Figure 4A shows the TGA curves of pure PVP, where the PVP is completely decomposed in O2 at 550 °C and in N2 at 460 °C, respectively. The different TGA behaviors in O2 and N2 can be accounted for the formation of peroxide residues in the interval of temperatures 200–400 °C in the presence of oxygen.37 The results indicates that PVP framework is removed by heat treatment at 550 °C in air (~80 vol.% N2 & ~20 vol.% O2). Figure 4B shows TGA curves of Ag@BT/PVP in N2 and O2 atmospheres. A weight loss of ~ 0.5 wt.% and Ag, BT and PVP densities of 10.49, 6.01, and 1.14 g/cm3 are used to estimate the PVP layer thickness, which turned out to be ~ 100 nm. Figure 5 shows the compositional characterization of core@shell Ag@BT particles after heat treated for 2 h at 550 °C. From SEM and the corresponding energy dispersive X-ray (EDX) mapping images of Ag@BT (Figures 5A–E), both Ba and Ti are homogeneously distributed throughout the particles which indicates the successful coating of continuous BT (rather than tiny particles decorated) for the whole population. The mapping images of Ba and Ti (Figures 5B–C) are consistent with the shapes and sizes of particles in SEM images (Figure 5A), while the Ag 7 Environment ACS Paragon Plus


mapping (Figure 5D) indicates that Ag particles have smaller areas and appear in the interior positions since they are used as the cores. The overlap of the mapping of Ag, Ti, and Ba in Figure 5E indicates circular rings of Ba and Ti in the outer region of Ag cores, which suggests the core@shell structures. The mapping and the cross-sectional compositional line-scanning profiles of Ag, Ba and Ti for an individual Ag@BT particles in Figures 5F–G further confirm the core@shell structures. Size distribution of Ag and core@shell Ag@BT particles were obtained from SEM and corresponding EDX mapping images, and were summarized in Figure 6. The Ag particles (the cores) have diameters ranging from 5 to 15 µm and a mean diameter of 10 µm; the coated BT shells have thickness ranging from 50 to 400 nm and a mean thickness of 150 nm. Thus, the ratio of core diameter / shell thickness is calculated to be 68 for Ag@BT. Figure 7A shows the powder X-ray diffraction analysis results of Ag and Ag@BT (550 °C, 2 h) particles. The diffraction pattern of Ag can be classified into JPCDS no. 87-0717 with a face centered cubic crystal structure (a = 0.409 nm).38 The core@shell Ag@BT has two sets of peaks, which are consistent with Ag, and BT with JPCDS no. 31-0174 (cubic, a = 0.403 nm),39 respectively. Figure 7B shows the EDX spectra reflecting element distributions on the surface of core@shell Ag@BT particles. Besides peaks belonging to Ag, an overlap peak of Ti and O in the energy range of 0.4–0.5 keV, and the peaks of Ba, Ti, and O in the energy range of 4.5–5.0 keV are observed. The presence of Ba and Ti with a molar ratio of 1:1 confirms the chemical composition of BT. The effects of precursor concentrations and reaction time on the thickness of shells are investigated. For the BT coating, the shell thickness is increasing linearly with the precursor concentration and reaction time (Figure S1). Shell thickness (coating amount I ) follows Fick’s equation, i.e., I = ∫ Jdτ = − ∫ D∇Cdτ , where J the is coating rate, ∇C is the gradient of concentration of reaction products, D is a diffusion coefficient, and τ is the reaction time. It should be noted that the linear kinetics is attributed to the solution-based condition and steady


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and homogenous growth process in this synthesis process. Shell coating rate J is mainly determined by the precursor concentration. Low growth rate facilitates the formation of the smooth shell, while high growth rates tend to result in an uneven coating structure. The synthesized Ag@BT particles were used to make polydimethyl siloxane (PDMS) based composites, for investigating the effect of Ag@BT on dielectric properties of the composite. For comparison, PDMS based composites filled with ceramic (BT) and metal (Ag) particles are also prepared. Figures 8A–C show the SEM images of PDMS composites filled with the same amount (40 vol.%) of BT, Ag and Ag@BT fillers. Figure 9 shows the I-V characteristics of PDMS composites filled with 40 vol.% of Ag and 10–40 vol.% of Ag@BT fillers. The composites filled with Ag fillers show electrically conductive behavior at a low electric field due to the conduction pass formed by connecting Ag particles (Figures 9A,C). However, the composites filled with the core@shell particles show decent insulative properties with very small leakage currents (~10–8 – ~10–7 A/cm2, Figures 9B–C, and left inset of Figure 9C), which is far lower than that of percolation system without barrier layers.40 This is attributed to continuously coated insulative shells which prevent the short circuit between Ag particles (Figure 9B). The I-V curves of composites filled with core@shell fillers have a jump of current which results from the breakdown of composite at the applied voltage (Figure 9C). The breakdown strengths for composites with Ag@BT filler loading of 10, 20, 30, and 40 vol.% are 11.8, 10.2, 9.1, and 7.2 kV/cm, respectively (summarized in right inset of Figure 9C). The differences in breakdown strength among composites with different filler loadings are attributed to the different inter-particle distance of Ag. Figure 10A shows the dielectric properties of composites filled with 40 vol.% of Ag, BT, and core@shell Ag@BT fillers. Relative permittivity and dielectric loss show a decrease at low frequencies (10 kHz–40 kHz) and stabilize at high frequencies (100 kHz–10 MHz). This frequency dependent behavior is a universal phenomenon in polymer based composite, which is attributed to the interfacial polarizations exhibiting a long relaxation time.41 The permittivity of



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composites with 40 vol.% Ag@BT is 202 (εr/εm = 84; for PDMS, εm = 2.4) at 1 MHz, which is much larger than 98 of Ag composites and 60 of BT composites. Figure 10B shows the dielectric properties versus volume fractions of fillers in composite filled with core@shell Ag@BT. The permittivity increases with increasing filler loading amount, the dielectric loss keeps at a low level (~0.003) with different filler contents. The composite shows a different behavior from pure percolation system with metal particles loaded, as dielectric constant increases with increasing filler loading and no critical percolation fraction (between 20– 30 vol.% ) is observed.42 This means composites can be highly loaded with core@shell metalinsulatvie fillers; the more metal phases, the greater chance of increased permittivity as minor electrodes. The permittivity of composites filled with Ag@BT fillers is larger than the reported results, of composites filled with 40 vol.% Al@Al2O3 (εr/εm ~ 30), 10 Ag@TiO2 (εr/εm ~ 34) 11 or Ag@SiO2 (εr/εm ~ 5)

13

fillers at 1 MHz. This indicates the shell properties in this study are

effective in achieving high permittivity of composites. Various models have been proposed to predict the effect permittivity of composites and to elucidate their dielectric performance mechanisms. These include the Lich.’s mixing rule,43 serial mixing rule,44,45 parallel mixing rule,44,45 logarithmic mixing rule,46 and effective medium theory (EMT).47 The different models are expressed as Lich.’s mixing rule: ε rα = (1 − f ) ε mα + f ε αf Serial mixing rule:

1

εr

=

1− f

εm

+

f

εf

(6) (7)

Parallel mixing rule: ε r = (1 − f ) ε m + f ε f

(8)

Logarithmic mixing rule: log ε r = (1 − f ) log ε m + f log ε f

(9)

  f (ε f − ε m )  EMT: ε r = ε m  1 +  ε m + n (1 − f ) ( ε f − ε m )   

(10)


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where α is a parameter that determines the type of mixing (it varies from –1 to 1), f is the volume fraction of filler, n is filler morphology fitting factor in EMT. ε r , ε m and ε f are permittivity of the composite, the matrix and the filler, respectively. From the experimental and theoretical comparison in Figure 10B, the logarithmic model is consistent with the experimental results for 10, 20, and 30 vol.% Ag@BT loadings. When the content increases above 30 vol.%, the logarithmic model deviates from the experimental results. This is because as the volume fraction increases, agglomeration of Ag@BT particles increases and thus destructs the polymer continuity.48 It should be noted that logarithmic mixing rule is the case when α → 0 in Lich.’s rule. The Lich.’s rule with a slight decrease of α at α = −0.05 fits well with the results of 40 vol.% loading of Ag@BT. The serial and parallel models fits with special mixing situations which is not suitable for the spherical fillers; EMT does not reflect the changing of effective permittivity of composites with high-k core@shell fillers. In all the theoretical predictions in this study, the core@shell Ag@BT particle is treated as a hybrid filler. The equivalent permittivity of the filler is obtained from the interfacial polarization relations (Figure 10C inset). The hybrid core@shell Ag@BT particle can be treated as a twophasic composite, which contains an insulative matrix (BT) and a conductive filler (Ag), belonging to the well-known water-in-oil (W-O) system. The Ag@BT hybrid is also an an extreme example of W-O system, as the matrix has a large permittivity and a low electrical conducvitivty while the filler has a large electrical conductivity and a low permittivity. Due to

ε BT σ Ag

( ≠ ) ε Agσ BT

, ( ε BT , ε Ag are permittivity of BT and Ag; σ BT , σ Ag are electrical

conductivity of BT and Ag, respectively), there are strong interfacial polarizations inside the hybrid particle. According to Hanai’s theory, the equivalent permittivity of the hybrid particle originates from interfacial polarizations can be given as:49

ε Ag @ BT =

ε BT

(1 − f Ag )

(11)

3



where ε Ag @ BT is the equivalent permittivity of core@shell Ag@BT particles, ε BT is the permittivity of BT shell, and f Ag is the volume fraction of Ag core in the Ag@BT particle. In the case of 10 µm size of Ag and 150 nm of BT shell, the resultant ε Ag @ BT is calculated to be 2.4 ×106 , which is about 1600 times of permittivity of BT (~1500). Composites with the same amount of Ag@BT fillers (40 vol.%) show much lower dielectric loss (tanδ ~ 0.003, Figures 10A–B) than that of Ag composite (tanδ = 0.7, inset of Figure 10A) which makes them better applied to practical capacitors. It is believed that the shells restrict electron transfer among Ag particles and thus render the Ag-BT composites a low dielectric loss. Figure 10C shows the breakdown strengths of Ag@BT composites are enhanced compared with Ag composites and are in the same level of BT composite and pure matrix. The increased breakdown strength is also attributed to the blocking effect of insulative layer with large dielectric strengths. By employing metal-dielectric core@shell fillers, these composites exhibit enhanced dielectric constant, suppressed dielectric loss and enhanced breakdown strength simultaneously (as shown in Figure 10D), and thus are a promising material for capacitors.

3. CONCLUSION In summary, this study demonstrates the preparation of Ag@BT core@shell particles with well-defined structures. The process includes following steps: Ag functionalization, shell growth, and residual PVP removal. The PVP adheres to Ag by coordination attraction, and it is present as a framework on Ag surface. With the effect of PVP, fully-coated oxide shell can form on the template of the framework. SEM examination of morphology evolutions from Ag, Ag@PVP, Ag@PVP/BT to Ag@BT indicates smooth surfaces of the oxide coatings. XRD pattern, EDX spectra, line-scanning and mapping confirm the phase, chemical composition, and configuration of the shells. Shell thickness and growth rate are tunable by adjusting precursor concentrations and reaction time. The entire synthesis process is carried out with low cost equipment, which makes it possible to scale up the fabrication process. Due to continuous shells of insulative BT, PDMS based composites filled with core@shell fillers exhibit a very small leakage current of


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~10–8 – ~10–7 A/cm2. A high permittivity of 202 (εr/εm = 84) and a low dielectric loss of 0.003 are achieved @1 MHz in the composites filled with 40 vol.% Ag@BT fillers, respectively, which makes the core@shell particles potentially applied in high-k and low loss dielectric composites. Lich.’s theory containing equivalent permittivityof core@shell Ag@BT from interfacial polarizations is used to account for the dielectric constant of the composite.

4. EXPERIMENTAL SECTION Silver particles (Ag, 99%) with spherical shape in a size range of 5–15 µm were supplied by Sigma Aldrich. The particles were heat-treated for 30 min at 320 °C to decompose possible Ag2O on Ag particles.

4.1. Functionalization with PVP. Using the method of Graf et al,50 3 g Ag powder was added to 160 mL deionized water with 8 g poly (vinyl pyrrolidone) (PVP; Mw = 55000, Sigma Aldrich) dissolved. The mixture was stirred for 24 h to allow PVP to be fully adsorbed. After that, the particles were settled, the aqueous solution was filtrated out. Then the particles can be directly used in shell coating step.

4.2. BT shell coating. Functionalized Ag particles were transferred into 100 mL ethylene glycol (EG, C2H6O2, 99%, Sigma Aldrich) followed by mixing with a supersaturated solution formed by dissolving 1.28 g barium hydroxide octahydrate (Ba(OH)2·8H2O, 99%, Sigma Aldrich) into 8.72 g deionized water and stirring at 90 °C for 1 h then cooling down. Under stirring, 8 g ethanol (C2H5OH, 99%, Sigma Aldrich) containing 1.2 g titanium n-butoxide (TNBT, Ti(OC4H9)4, 97%, Sigma Aldrich) was added into the mixture for 5 min. Then the reaction mixture was refluxed at 96 °C and stirred for 3 h. After shell coating, particles were centrifugally separated for 5 min at 7000 rpm, sequentially washed with ethanol and ethanol/water (1:1 in volume), and dried at 75 °C for 40 min. By this step, shiny particle surface can be observed using an optic microscope (Leica DC300). Then the particles were heat-treated for 2 h at 550 °C to remove the residual PVP and to densify the BT coating.



4.3. Preparation of composites. 0.15 g polydimethylsiloxane (PDMS) base containing 0.015 g curing agent were compounded with a certain amount of BT (~1 µm, 99%, Nippon Chemical), Ag or Ag@BT particles with a volume fraction in a range of 10–40 vol.%. The mixtures were cast onto glass to form a film with thickness of ~120 µm. The silicone compound was polymerized for 30 min at 150 °C. Ag paste was printed on the surfaces of the composites as electrodes for electrical tests.

4.4. Characterization. X-ray diffraction (XRD) patterns were recorded on the powder sample using a PANalytical B.V. diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 0.02 degree/s. Scanning electron microscopy (SEM) measurements were carried out using Hitachi SU8010 at an acceleration voltage of 5 kV. Energy dispersive X-ray (EDX) analyses were performed using Noran spectrometer at Hitachi S4800 at 20 kV. Thermogravimetric analyses (TGA) were conducted using Netzsch 409 in the following conditions: sample weight 6–8 mg, heating rate 10 °C/min, two atmospheres of N2 and O2. The current-voltage (I-V) curve was measured using a Keithley 2612A source meter. The dielectric properties were measured with a HP 4263A LCR meter, at frequencies from 10 kHz to 10 MHz.

ASSOCIATED CONTENT Supporting Information is included about: size dependent surface energy of particles; parameter values for the calculation of work of adhesion; plots of kinetics of shell growth of BT shells (PDF).

ACKNOWLEDGEMENTS This work is supported by Natural Science Foundation of Jiangsu Province in China (grant number BK20140517) and University Natural Science Project of Jiangsu Province in China (grant number 14KJB430011).

REFERENCES


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Figure 1. Schematic illustration of PVP assisted preparation for Ag@BT core@shell particles. The process includes following steps: Ag functionalization, shell growth of BT by LTDS, and removal of residual PVP.


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Figure 2. (A) Low- and (B) high-magnification SEM images of Ag particles. (C) Low- and (D) high-magnification SEM images of Ag particles functionalized with PVP. (E) EDX mapping of PVP (nitrogen detected) for an individual particle shown in (D).



Figure 3. SEM image of core@shell Ag@BT. (A) Low- and (B) high-magnification SEM images of morphology of as-synthesized Ag-BT (Ag@PVP/BT) particles. (C) SEM images of overview morphology of Ag-BT particles heat-treated for 2 h at 550 °C. (D–F) SEM images of surface morphology of an individual Ag@BT particle (550 °C, 2 h) with high-magnifications. i–ii in (D) denote the domains on the surface. 22 Environment ACS Paragon Plus

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Figure 4. (A) TGA curves of pure PVP in atmospheres of O2 and N2. (B) TGA curves of Ag@PVP/BT in atmospheres of O2 and N2.



Figure 5. (A) SEM and (B–E) the corresponding EDX mapping images of Ag@BT core@shell particles (550 °C, 2 h). (F) SEM, the corresponding EDX mapping images, and (G) EDX line scan of an individual Ag@BT core@shell particle (550 °C, 2 h).


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Figure 6. (A) Diameter distribution of Ag particles (the cores), and (B) thickness distribution of BT shells in core@shell Ag@BT particles (550 °C, 2 h).



Figure 7. (A) XRD patterns of Ag and core@shell Ag@BT (550 °C, 2 h) particles. (B) EDX spectra of Ag@BT core@shell particles (550 °C, 2 h).


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Figure 8. SEM images of PDMS composites filled with different types of fillers. (A) 40 vol.% of ceramic (BT), (B) 40 vol.% of metal (Ag), and (C) 40 vol.% of core@shell Ag@BT.



Figure 9. (A) Schematic of the possible conduction passes in metal fillers filled composites. (B) Schematic of the blocking effect of core@shell fillers composites. (C) I-V characteristics of composites using 10 – 40 vol.% of core@shell Ag@BT particles and 40 vol.% of Ag as fillers. The insets are detailed I-V plot of composites filled with Ag@BT fillers (left) and breakdown strength (Eb) of composites filled with Ag@BT fillers (right).


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Figure 10. (A) Relative permittivity and dielectric loss vs frequency in composites filled with 40 vol.% of Ag, BT, and core@shell Ag@BT fillers. The inset plot is frequency dependent dielectric loss of composites filled with 40 vol.% of Ag. (B) Experimental and theoretical relative permittivity and dielectric loss vs volume fraction of fillers in composites filled with core@shell Ag@BT fillers @ 1 MHz. (C) Breakdown strength of PDMS matrix and composites filled with 40 vol.% of metal (Ag), 40 vol.% of ceramic (BT), 10 & 40 vol.% of core@shell Ag@BT fillers. Inset is schematic illustration of calculation of permittivity of Ag@BT from interfacial polarization. (D) Comparison of properties of εr/εm, tanδ and Eb/Em of composites filled with 40 vol.% of Ag, BT, and core@shell Ag@BT fillers (parameters of PDMS matrix: εm = 2.4, Em = 14.5 kV/cm).



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