Cellular Ceramics / p2

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2.8

Hollow Spheres

Srinivasa Rao Boddapati and Rajendra K. Bordia

2.8.1

Introduction

Hollow spheres have found applications in such diverse areas as refractory thermal insulation, lightweight composites, fiber-optic sensors, laser-fusion targets, encapsulation, and gas and chemical storage. Here, attention is restricted to ceramic hollow spheres. Advancements in processing, properties, and applications of solid and hollow spheres up to 1994 were discussed as a focused symposium at the 1994 fall meeting of the Materials Research Society held in Boston [1]. The focus of this chapter is on processing of ceramic hollow spheres, although a general overview of properties and applications of hollow spheres is also given. A variety of processing methods have been developed to produce hollow spheres over the years: spray techniques such as spray drying [2–4], spray pyrolysis [5–11], and the coaxial-nozzle method [12–15], the sacrificial-core method [16–26], the sol–gel/emulsion method [27–30], and layer-by-layer deposition on colloidal templates [31–36]. Each method has its own advantages and disadvantages. Spray techniques are most suitable for bulk production and larger diameter spheres. On the other hand, solutionor colloid-based techniques yield hollow spheres of controlled morphology and size, but they are not suitable for bulk production. All techniques without exception use some solvent, either for holding the ceramic particles (slurry or colloid) or for dissolving the salts or chemical compounds to be coated onto the core particles. After drying the droplets, the dried hollow spheres are sintered at high temperatures (> 400 C, depending on the particle size) to improve their mechanical properties. Before the 1990s, most studies on inorganic hollow spheres were confined to hollow glass spheres, which were made by blowing molten glass, driven by research into targets for iner- tial-confinement fusion [13]. The two most important parameters of hollow spheres are sphere size and aspect ratio (the ratio of sphere diameter to wall thickness), which govern their properties. Various processing methods employed to fabricate ceramic hollow spheres are described first, followed by their properties and applications.

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

178Part 2 Manufacturing

2.8.2

Processing Methods

The processing of ceramic hollow spheres (CHS) can be broadly classified into five different categories: 1) sacrificial-core method, 2) layer-by-layer deposition, 3) emul- sion/sol–gel method, 4) spray and coaxial-nozzle methods, and 5) reaction methods. Other methods such as spray drying and aerosol-based techniques can also be grouped with the nozzle techniques, as these methods employ a droplet-generation technique as a primary means for generating hollow spheres. In the coaxial-nozzle technique, a slurry containing ceramic particles or a precursor solution flows through a narrow orifice while an inert gas flows through an inner nozzle concentric with the outer nozzle. The sacrificial-core method uses a sacrificial core such as polystyrene spheres onto which the shell material of the required hollow sphere is coated and later heated to high temperatures to remove the core and sinter the shell material. A variation of the sacrificial-core method is the layer-by-layer (LbL) deposition technique. Although it can be viewed as a variation of the sacrificial-core method, it gives greater flexibility in controlling the shell wall thickness of hollow spheres by deposition of individual layers of polyelectrolyte and hollow-sphere material. The emulsion/sol–gel method utilizes the principle of limited or no miscibility between two liquids in forming an emulsion. The emulsified globules of one of the liquids acts as a template for coating with the hollow-sphere material, and the liquid in the core is either extracted or evaporated during subsequent heating to higher temperatures. Occasionally, polymer particles have also been used in the emulsion/ sol–gel method as a template for making hollow ceramic spheres. Each of the techniques mentioned above is described and discussed in detail in the following subsections.

2.8.2.1

Sacrificial-Core Method

The sacrificial-core method is one of the most widely used for making ceramic hollow spheres (CHS). In this method a spherical polymer particle (core) is coated with a slurry or solution of ceramic material, the core–shell composite is dried, and the core is removed by chemical dissolution or by heating to a temperature at which the polymer decomposes. Sometimes the polymer sphere is negatively or positively charged, and this enhances adsorption between the sphere and the material to be coated (ceramic particle in colloidal form or ceramic-producing salt in solution). The thus-formed hollow spheres are heated to higher temperatures to sinter the particles in the wall and impart adequate mechanical strength and rigidity to the hollow sphere. A number of ceramic materials have been processed into hollow-sphere form by using this technique: Y2O3 [16, 37], Pb(Zr0.52Ti0.48)O3 [28, 38], SiO2 and Fe3O4 [17], hematite [20, 24], SiO2 [18], TiO2 [23, 25, 39, 41, 42], TiO2 and SnO2 [40], silver/TiO2 [43], and BN [44]. CHS have also been made by using combustible cores [26], mechanofusion [45], the reactant itself as a core [19, 44], vesicles as cores [21], and micellized diblock copolymer as template [46]. Yang et al. [23] modified the

2.8 Hollow Spheres 179

sacrificial-core technique to produce tunable-cavity hollow spheres from core–shell gel template particles which were prepared by inward sulfonation of polystyrene particles with concentrated sulfuric acid. Yin et al. [22] and Aoi et al. [47], functionalized interior surfaces of TiO2 hollow spheres with silver and platinum nanoparticles, respectively. This kind of functionalization of the interior surface of the CHS eliminates the step of diffusing the substance to be immobilized or functionalized into the interior of the hollow sphere. Along similar lines, inorganic cores such as SiO2 have been used to fabricate polymer hollow spheres by coating polystyrene nanospheres onto the SiO2 cores and later removing the SiO2 by etching [48]. Among the different template methods used to make CHS, the catanionic (a mixture of cationic and anionic surfactants) vesicle template method gives very small diameters (60–120 nm) and wall thicknesses (1–2 nm). Diameter and wall thickness of hollow spheres are the two most important parameters which control their properties. In some applications (inertial-confinement fusion targets), the surface roughness also plays an important role. The wall or shell of a CHS is formed by coating the ceramic material from a solution onto a core by different means: sol–gel, precipitation, hydrolysis of ceramic-producing salts, mechanofusion, and so on. The timescale of this process is on the order of hours or, in some cases, days [19]. The method is amenable to producing CHS having a diameter ranging from a few tens of nanometers to millimeters. The diameter of the hollow spheres is controlled by the diameter of the core or template particle, and the wall thickness, in most cases, is controlled by the concentration of the solution used for coating the cores and the exposure time.

2.8.2.2

Layer-by-Layer Deposition

Layer-by-layer deposition has been used to make hollow spheres of different ceramic materials [31–36, 49, 50]. In this technique, colloidal core particles are coated with alternating layers of polyelectrolytes and charged ceramic particles. The charge on the polyelectrolytes and the ceramic particles are opposite in nature. As a result, the ceramic particles in a solution are adsorbed onto the polyelectrolyte layer by electrostatic interaction. A schematic depicting various stages of the process is shown in Fig. 1. A polymer sphere such as negatively charged polystyrene is used as core. Adsorption of positively charged polyelectrolytes such as poly(diallyldimethylammonium chloride), PDADMAC, onto the polystyrene in solution lead to a polyelectrolyte layer with excess positive charge, onto which negatively charged ceramic particles (mostly nanosize) are adsorbed by electrostatic interaction. By repeating the process of alternating adsorption of polyelectrolyte and ceramic particle layers, the required wall thickness of the hollow sphere is obtained. Sometimes, three or five alternating layers of PDADMAC and polystyrenesulfonate (PSS) are coated onto the polystyrene or polymer core particles to provide a uniformly charged and smooth polyelectrolyte surface [32, 34, 35, 50]. The cavity or void size of the hollow sphere is governed by the diameter of the polystyrene or polymer core particle and shrinkage during sintering. The core is removed by thermal or chemical means, and the

180 Part 2 Manufacturing

Fig. 1 Schematic of layer-by-layer deposition method [Reprinted with permission from F. Caruso, R.A. Caruso, and H. M.hwald, Science, 1998, 282, 1111–1114. Copyright (1998) AAAS].

resulting green hollow spheres are generally sintered at temperatures above 400 C. If thermal heating is employed to remove the core material, the sintering step is combined with the core-removal step. In the case of SiO2/PDADMAC layer pairs, after sintering, the diameter of hollow spheres shrank by about 5–10 % [33]. The number of polyelectrolyte and ceramic particle layers deposited on the core controls the wall thickness of the hollow spheres. This method has been used to obtained hollow spheres of silica [31, 35, 36], titania [34], silica/Fe3O4 composite [32], laponite [36], and zeolite [49, 50] (see Tab. 1). Figures 2–4 show the magnetic (Fe3O4) and Fe3O4/SiO2 composite hollow spheres prepared by depositing ceramic nanoparticle layers in alternation with polyelectrolyte layers [each layer contains PDADMAC/PSS/PDADMAC (PE3)] on initially PE3-coated polystyrene particles. This technique has been used extensively to coat planar substrates, colloidal particles, and self-assembled colloidal particles. More detailes can be found in Ref. [51]. The parameters governing successful generation of ceramic hollow spheres depend on the selection of core materials, polyelectrolytes, particle size, shape and concentration of ceramic nanoparticles in the solution or inorganic molecular precursors, core removal, and sintering of the hollow spheres. Although it is possible to control the wall thickness of the hollow sphere by the number of cycles used to deposit polyelectrolyte and ceramic layers, the excess polyelectrolyte must be removed each time the ceramic layer is adsorbed or coated onto the polyelectrolyte, which is time-consum- ing. In addition, repeated use of centrifugal forces for this purpose can disrupt the integrity of the ceramic layer. When ceramic precursor is used instead of a ceramic colloidal particle for coating or infiltrating the polyelectrolyte-coated polymer core, at least certain minimum number of layers of ceramic precursor is required to bring about a mechanically stable shell [34]. Separate preparation of ceramic colloidal particles is required for coating a ceramic layer onto the polyelectrolyte, unless inorganic molecular precursors are used, as the process is mainly driven by electrostatic

182 Part 2 Manufacturing

interaction. Another major shortcoming is maintaining individual coated particles as well as separation of coated particles without disturbing their geometry. Centrifuging is used to separate the coated particles and is ineffective if the particle size is smaller than 100 nm. Filtration is used to separate the coated particles when the particles are smaller than 100 nm, although they lead to frequent clogging of pores of the filter and hence frequent replacement of filters and attendant cost.

Table 1 Ceramic hollow spheres produced by layer-by-layer deposition.

2.8.2.3

Emulsion/Sol–Gel Method

A water-extraction sol–gel process, patented by Sowman [52], has been used to prepare hollow ceramic spheres. Because water extraction and the dispersion were carried out simultaneously, control of the particle size was a challenge, however, and the product had a broad particle size distribution. The emulsion/sol–gel method developed by Liu and Wilcox, Sr., [27, 53], is an improvement on the water-extraction sol–gel technique. In this method (Fig. 5), hollow ceramic spheres are prepared by forming an emulsion of aqueous oxide/hydroxide sols by dispersing them in an organic liquid using a suitable surfactant followed by water extraction from the emulsified sol globules. The process parameters controlling both aspect ratio and size of hollow microspheres are the colloid concentration, water-extraction rate, and droplet size. Colloid concentration affects the initial droplet size as it influences the viscosity of the colloid and thereby partly controls the droplet size during the emulsion step. The effect of colloid concentration on the aspect ratio of hollow spheres depends on the critical concentration for gelation of a particular sol. At a fixed droplet size and water-extraction rate, dilute colloids lead to the formation of hollow spheres with lower aspect ratios than those from concentrated colloids, as the shrinkage of the droplet is inversely proportional to colloid concentration. Wall

2.8 Hollow Spheres 183

thickness is controlled by the water-extraction rate, and at a fixed colloid concentration and droplet size, higher water-extraction rate leads to quick formation of shell membranes. As a result, higher aspect ratio hollow spheres were obtained due to reduced shrinkage and thinner shell wall. The droplet size affects whether a droplet will form a hollow or a solid sphere. Liu and Wilcox [27] reported that the microspheres smaller than 1 mm were found to be solid in most cases. They attributed this to the small diffusion distance, which led to a homogenous colloid concentration, rather than a gradient in colloid concentration from the surface to the interior of the droplet, and hence homogenous gelation rather than surface gelation. The advantage of this method, compared to that of Sowman [52], is that the emulsion and water-extraction steps are separate, and hollow-sphere size can be controlled during the water-extraction step independent of emulsion step. By using this technique hollow spheres of SiO2 for removal of heavy metal ions from wastewater [54], Si/Al composite oxide [55], SiO2 for preparation of hollow gold spheres [56], and SiO2 from tetraethoxysilane (TEOS) [57] have been maufactured. Yang and Chaki [28] prepared lead zirconate titanate (PZT) hollow spheres by coating polyacrylamide latex microspheres (20–100 mm) in an emulsion with PZT sol and subsequently burning out the polymer core. In this case, polymer spheres are used as templates, unlike in usual emulsion-based techniques in which oil droplets are used as templates. The PZT sol was prepared from lead nitrate Pb(NO3)2, zirconium n-butoxide Zr(C4H9O)4, and titanium isopropoxide Ti(OC3H7)4 and the molar ratio of the chemical compounds used for processing was such that the cation ratio in the final compound was maintained as Pb(Zr0.52Ti0.48)O3. The zirconium and titanium alkoxides were poured into Pb(NO3)2 solution and stirred to obtain PZT sol. The polyacrylamide spheres were dispersed in water-free toluene by stirring followed by addition of NH3. The resultant mixture was stirred so that an emulsion of NH3 droplets was formed. The droplets of NH3 which came in contact with polymer spheres were adsorbed, and this was responsible for gelation of PZT sol when it came in contact with NH3. The diameter of the PZT hollow spheres ranged from 20 to 95 mm.

Collins et al. [58] reported the formation of TiO2 hollow spheres in nonaqueous emulsions such as oil in formamide from titanium alkoxide precursors. Among the tested precursors, titanium(iv) ethoxide alone gave intact hollow spheres, whereas titanium(iv) propoxide, although successful to some extent, often gave very thin shells which were susceptible to damage. In contrast, titanium(iv) butoxide failed to produce intact hollow spheres due to the reduction in the rates of hydrolysis and condensation because of the large alkyl chains. Wu et al. prepared mesostructured lead titanate by an oil-in-water emulsion-mediated route using titanium butoxide, lead acetate, and dodecylamine in 1-butanol as the source of TiO2, PbO, and template for mesostructure in the shell wall, respectively [30]. It was shown that the presence of KOH as a mineralizer is essential in the appropriate concentration (i.e., at molar ratios of K2O/H2O of 4.0/1780 to 10.0/1780) for producing mesostructured hollow spheres. Sun et al. prepared hollow silica spheres with multilamellar structure in the shell using poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer based emulsion in combination with sodium silicate [29]. Recently, Liu et al. produced Cu2O hollow spheres using a multiple-emulsion sys-

2.8 Hollow Spheres 185

2.8.2.4

Spray and Coaxial-Nozzle Techniques

Hollow spheres have been prepared by a host of droplet generation techniques using solutions of the salts of the target material or slurries of the target material as the starting materials. The solution or slurry is either sprayed through a hot chamber [13, 60] or through a coaxial nozzle [12, 13, 61–63] set up to generate spherical droplets. Torobin used the coaxial nozzle technique to make hollow porous microspheres as substrates and containers for catalysts [61, 64, 65].

In a spray pyrolysis process, hollow spheres were generated by atomizing the inorganic precursor in solution form, evaporating the solvent during flight, and precipitating the inorganic salt on the surface. The remaining solvent was removed by drying, and the precipitated salt was pyrolyzed at higher temperatures (> 250 C) to form the desired ceramic phase in the shell of a hollow sphere. These pyrolyzed spheres or particles were sintered at higher temperatures to achieve sufficient mechanical strength. Messing et al. described in detail various steps involved in spray pyrolysis and also the conditions required for producing solid and hollow particles, fibers, thin films, and composite particles [66, 67]. Pratsinis and Vemury reviewed the formation of particles in gases and described the advantage of processing particles in gases [68]. Most of the spray pyrolysis methods differ in the way the droplets are produced. Hollow spheres of zircon (ZrSiO4) were prepared by mechanically mixing a zirconium salt and a silica sol in proportions corresponding to the zircon composition and spraying the aqueous solution as a mist into a preheated (150 C) chamber [5]. The effect of calcination temperature on the phase evolution and the size and morphology of the hollow spheres was investigated. Tartaj et al. synthesized silica-coated c-Fe2O3 hollow spheres by aerosol pyrolysis of a methanol solution containing iron ammonium citrate and silicon ethoxide [7]. Aerosol techniques have been used to produce fine metal and metal oxide particles [8], as well as for encapsulation of oil droplets with metal oxide [9] and mesostructured SiO2 [69]. Pyrolysis of aerosol droplets was employed to generate hollow spheres of CuO from copper sulfate pentahydrate [10], and of NiFe2O4 from nickel and iron nitrates [6]. Jokanovic et al. used ultrasonic spray pyrolysis for producing TiO2 hollow spheres [11]. Tani et al used emulsion combustion to prepare hollow spherical particles of Al2O3, TiO2, ZrO2, and Y2O3 from metal precursors [70]. Spray drying is one of the most versatile methods of powder processing [71]. In spray drying (see Fig. 7), wateror organicbased slurries of ceramic particles are sprayed in the form of droplets into a chamber containing hot air or other, inert gases. Spray drying (atomizing and drying) leads to a large variety of particle shapes depending on the processing parameters and slurry characteristics: solid, elongated, pancake, donut-shaped, needlelike, or hollow particles. The effect of spray-drying conditions on the morphology of a mesoporous hollow silica particle is shown in Fig. 8. It has been shown in the case of Al2O3 slurry that a flocculated slurry leads to solid particles, whereas a dispersed suspension leads to hollow particles [72]. Spray drying has been applied successfully to produce hollow spheres of TiO2 [4], hydroxyapatite [3], and SiO2 [2]. The success of most of the spray pyrolysis and spray drying techniques in generating hollow