Microemulsion mediated synthesis of BaTiO 3 – Ag nanocomposites

BaTiO3 – Ag composite nanopowders were synthesized via microemulsion mediated synthesis through the hydrolytic decomposition of mixed metal alkoxide solutions as precursor for the BaTiO3 and the reduction of silver nitrate in the presence of polyvinylpyrrolidone (PVP) as source for the Ag nanoparticles. The X-ray diffraction (XRD) patterns indicate that BaTiO3 and Ag phases were successfully synthesized in the composite powders. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the synthesized BaTiO3 nanoparticles were aggregates of nanosized primary particles as small as 10 nm in diameter and the average particle size of nanocrystalline Ag was about 100 nm. Calcination and sintering studies reveal that there exists a difference in the sintering behaviour of BaTiO3 and Ag in the composite nanopowders. Thermogravimetric analysis (TGA) shows weight losses due to the burnout of organic residues arising from the synthesis, the release of water from the surface and separation of hydroxyl ions from the lattice of BaTiO3 nanoparticles. A dilatometric study of BaTiO3-Ag composite confi rmed a strong difference in the shrinkage behaviour compared to that of the pure BaTiO3 obtained by microemulsion mediated synthesis.


I. Introduction
Ceramic-metal nanocomposite materials are extensively used in many technological applications due to enhanced mechanical, optical, electrical or magnetic properties, which are not achievable for monolithic materials [1][2][3].It makes them excellent candidates for the fabrication of multifunctional devi ces with unique features and much effort has been un dertaken to design and control their microstructure for new applications.Several processes have been develo ped for the preparation of nanoscale ceramic-metal composite powders to be sintered into consolidated nano-cermets.These processes include mechanical milling [4], reactive metal penetration [5], electro chemical infi ltration [6], the combustion method [7], self-propagating high-temperature synthesis (SHS) [8], the sol-gel method [9], and the heterogeneous precipitation method [10].Although each technique has its own advantages, most of them have been found to be inadequate to produce composite nanopowders with an average particle size smaller than 100 nm.
With respect to this, colloidal approaches such as microemulsion mediated synthesis are believed to have a high potential for the realization of tailor-made nanocomposites because the particle size and size distribution can be controlled easily on the nanoscale and mixing of all constituents, ceramic and metallic phases, can be carried out very homogeneously.In recent years, several technologically important ceramic systems have been synthesized from water-in-oil microemulsions [11][12][13].Microemulsions are liquid, transparent, optically isotropic and thermo dynamically stable dispersions of a polar and a non-polar solvent, typically water or an aqueous solution and a liquid hydrocarbon.Droplets of only 1-100 nm in size are stabilized by the addition of surfactants and thus particle sizes down to a few tens of nanometers or even only several nanometers can be achieved by restricting crystal growth during particle formation.Herrig and Hempelmann [14] reported that micro emulsion-derived ceramic powders of BaTiO 3 are much fi ner in particle size, narrower in particle size distribution and higher in both composition homo geneity and sinterability than those prepared via many other chemical synthesis routes.However, there are only very few reports on the preparation of BaTiO 3metal composite nanoparticles by microemulsion until now.
BaTiO 3 and its related compounds are widely used as thermistors, capacitors, gate dielectrics, memories, and power-storage devices in recent years [15][16][17].Materials with high dielectric constant are crucial to satisfy the current trend of miniaturization and improved performance in electronic devices and BaTiO 3 -Ag composites has attracted much attention due to the strongly enhanced dielectric constant [18].
In this study, BaTiO 3 -Ag composite nanopowders were synthesized via microemulsion mediated synthesis.This modifi ed synthetic route has been developed in order to prepare homogeneously dispersed composite nanoparticles of metal and ceramic.The microstructure evolution and sintering behaviour are investigated.

II. Experimental
BaTiO 3 -Ag composite nanopowders with 20 wt.% of silver were synthesized by mixing a silver nanoparticle dispersion with a barium titanate nanoparticle dispersion.The synthetic procedures for the preparation of the colloidal BaTiO 3 suspension were performed using the Schlenk technique under argon.CO 2 contam-ination of the argon gas was minimized using a CO 2adsorber column for gas purifi cation.2.355 g of pure Ba metal (> 99% metal basis, Alfa Aesar GmbH & Co., KG, Karlsruhe, Germany) was dissolved in 150 ml of methanol which was dried over a molecular sieve (0.3 nm, beads, Merck Eurolab GmbH).4.875 g of titanium isopropoxide (synthetic grade, Merck Eurolab GmbH, Germany) was added to the previously prepared barium methanolate solution.All starting chemicals used for precursor preparation were stored in gastight glass vessels in an Ar-glove box.After the preparation of the precursor solutions, 8.253 g of a microemulsion was added dropwise through a dropping funnel into the reaction vessel in order to form the nano-sized BaTiO 3 particles.The microemulsion contained 3.74 wt.% of CTAB (cetyl trimethyl ammonium bromide, Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany), 72.82 wt.% of cyclohexane (analytical grade, Riedel de Haën, Seelze, Germany), 12.21 wt.% of 1-pentanol (analytical grade, Merck, Eurolab GmbH, Germany), and 11.23 wt.% of degassed water.For the preparation of the Agdispersion, 1.574 g of silver nitrate (Merck, Darmstadt, Germany) was dissolved in 150 ml methanol (Merck, Darmstadt, Germany) in the presence of 2.4 g polyvinylpyrrolidone (PVP, average molar weight M = 10000 g/mol, Fluka, Steinheim, Germany) and heated under refl ux for 1 hour.Subsequently, the obtained dark red solution which contained the silver nanoparticles was mixed with the methanolic solution of the BaTiO 3 na- With the synthesized powders, phase identifi cation was performed by powder X-ray diffraction (XRD, Philips X'PERT, Koninklijke Philips Electronics N.V., Eindhoven, the Netherlands).The incident X-rays had a wavelength of 1.5418 Å due to combined application of Cu K α1 and Cu K α2 radiation.The diffraction pattern was scanned from 20° to 80° (2θ) with a step size of 0.01°.The particle size and homogeneity of BaTiO 3 and Ag distribution of the synthesized composite nanopowders were characterized by fi eld emission scanning electron microscopy (FE-SEM, LEO1530, Carl Zeiss AG, Jena, Germany) operating at 20 kV in combination with energy dispersive X-ray analysis (EDX) and transmission electron microscopy (TEM, CM-200, Koninklijke Philips Electronics N.V., Amsterdam, the Netherlands) operating at 200 kV.The samples for TEM were pre-pared by dropping the solution mixture of BaTiO 3 and Ag on a carbon coated Cu-grid and drying in air.Thermogravimetric analysis of the initial powders (TGA, STA 429, Netzsch Gerätebau GmbH, Selb, Germany) was carried out under Ar-atmosphere in the temperature range from 25°C to 900°C with heating and cooling rates of 20 K/min, respectively.The sintering behaviour of the raw powder was investigated using dilatometry (DIL 402 C, Netzsch Gerätebau GmbH, Germany) under Ar atmosphere in the temperature range from 25°C to 1000°C also with heating and cooling rates of 20 K/ min, respectively.For this purpose a cylindrical sample with a diameter 10 mm was compacted by uniaxial pressing applying a force of 63.7 MPa followed by cold isostatic press at 815 MPa.The green density of the powder pellet was 3.844 g/cm 3 (55.4% of the theoretic density).The surface morphology and microstructure of the sintered samples after the dilatometric measurement were studied using optical microscopy (OM, Axioplan 2, Carl Zeiss MicroImaging GmbH, Jena, Germany) and scanning electron microscopy (SEM, model S-4100, Hitachi, Tokyo, Japan).

III. Results and discussion
The X-ray diffraction (XRD) patterns of the as-synthesized BaTiO 3 -Ag composite powders with 20 wt.% of silver and that of calcined powders at 600°C for 1hr are shown in Fig. 2. Observed Bragg diffraction peaks were identifi ed to belong either to BaTiO 3 (JCPDS 31-0174) or Ag (JCPDS 03-0921) and BaCO 3 appears as indicated in the spectra.However, we found no evidence of carbonate formation as the reaction was performed [19].This indicates that the carbonate formation is due to aging at air during sample storage.After annealing at 600°C for 1hr, the diffraction peak of BaTiO 3 and Ag increased in intensity and sharpened relatively to those measured for the raw product due to crystal growth.The XRD diffraction patterns indicate that no other major phases than BaTiO 3 and Ag were present in the composite powders.Fig. 3 shows SEM and TEM pictures of the as-synthesized BaTiO 3 and silver nanoparticles.The average silver particle size was estimated to be about 100 nm and that of BaTiO 3 was less than 10 nm [19].Fig. 3b clearly shows that BaTiO 3 particles are formed by the aggregation of nanometer-sized fi ne particles, smaller than 10 nm in diameter.
The phase morphology of BaTiO 3 -Ag composite powders calcined at 600°C for 1hr is shown in Fig. 4a.The results of EDX analysis shown in Fig. 4b indicate that the micron-size bright particles which are seen in Fig. 4a consist of pure silver, possibly slightly oxidized, with no trace of Ba, Ti, O.The EDX analysis of the matrix area marked in Fig. 4a gave evidence for the presence of Ba, Ti, O and traces of Ag, which implies that ultrafi ne Ag nanoparticles are also distributed in the BaTiO 3 matrix.Fig. 4c, an enlarged SEM photograph, clearly shows that ultrafi ne Ag nanoparticles as well as larger Ag particles are distributed in the BaTiO 3 matrix.
Fig. 5 shows the TGA curve of the as-prepared Ba-TiO 3 -Ag composite nanopowders.The results of these measurements reveal several steps of mass losses.The fi rst weight loss amounts to 21.91 % occurs up to a temperature of approximately 450°C and is attributed to the burnout of residual organics arising from the synthesis (e.g.surfactant) from the composite powders and the release of physisorbed water on the surface of BaTiO 3 nanopowders [20,21].The second weight loss of 9.81 % occurs in a wide range of temperatures up to 900°C and mainly results from the release of chemisorbed water and/or incorporated hydroxyl ions in the lattice of Ba-TiO 3 .Thus, it is apparent that the as-prepared BaTiO 3 -Ag nanopowders possess high concentration of organics, water, and hydroxyl ions.
In order to study the sinterability of the synthesized composite nanoparticles a dilatometric analysis was  carried out.The linear shrinkage curve of the sample is presented in Fig. 6.With the introduction of Ag nanoparticles into the BaTiO 3 , the dilatometric shrinkage observed up to approximately 700°C is about 17.0 % which is much higher compared to that of pure BaTiO 3 obtained by microemulsion mediated synthesis (linear shrinkage about 5.0 %) [19].The coalescence of Ag is probably related to the enhanced sinterability of composite compared to the pure BaTiO 3 .
Fig. 7a shows the polished surface of a BaTiO 3 -Ag composite sintered at 700°C for 1h.As observed by optical microscopy, it shows that silver particles (bright) agglomerate during sintering and form fi laments.In an enlarged SEM photograph (Fig. 7b), no evidence of substantial grain growth for the ultrafi ne BaTiO 3 nanoparticles can be observed.Silver nanoparticles, however, form micron-size particles upon sintering and grain growth.These results refl ect a different sintering behaviour for BaTiO 3 and Ag nanoparticles.
With the addition of silver, previous reports showed that ceramic-Ag composite can be sintered at much lower temperatures with enhanced sinterability compared to the pure ceramic phase [22][23][24].Hwang et al. for example showed that the densifi cation of PZT was accelerated by the addition of silver particles without the formation of fi laments [24].Panteny et al., on the other hand, founded that silver addition reduced the final sintered density in BaTiO 3 -Ag composites [25].In our study, the synthesized silver nanoparticles are 100 nm or less in size and consequently they start to sinter at much lower temperatures around 125°C.Moon et al., also presented that coalescence of the Ag nanoparticles (about 20 nm) starts at signifi cantly lower temperature (~ 150°C) compared to that of the bulk material [26].Additionally, fi lament formation was observed, for the reasons not yet understood.Further investigations are needed in order to obtain a clear picture of the sintering behaviour of BaTiO 3 -Ag composites synthesized by microemulsion mediated synthesis.

IV. Conclusions
BaTiO 3 -Ag nanocomposite powders were successfully prepared via microemulsion-mediated synthesis.The synthesized BaTiO 3 nanoparticles were aggregates of nanosized primary particles as small as 10 nm and the size of Ag nanoparticles was about 100 nm.Calcined and sintered BaTiO 3 -Ag composite powders reveal a different sintering behaviour of BaTiO 3 compared to Ag nanoparticles in composite nanopowders.More specifi cally the ultrafi ne Ag nanoparticles start to form elongated fi laments at 700°C due to the coarsening process.It is anticipated that modifi ed microemul- 2.5 μm sion synthetic route can be adapted for the preparation of the novel BaTiO 3 -metal nanocomposite powders for new concepts in ceramic materials and their potential applications.

Figure 1 .
Figure 1.Flow chart for the synthesis of BaTiO 3 -Ag composite nanopowders using microemulsion-mediated synthesis

Figure 2 .Figure 3 .
Figure 2. XRD patterns of the as-synthesized BaTiO 3 -Ag composite powder and that of calcined one (600°C for 1h)

Figure 5 .
Figure 5. TGA traces measured with a heating rate of 20 K/min in Ar atmosphere from 25°C up to 900°C for the as-prepared BaTiO 3 -Ag composite nanopowders

Figure 7 .Figure
Figure 7. Optical micrograph (a) and scanning electron micrograph (b) of the polished surface of BaTiO 3 -Ag composite sintered at 700°C for 1h