Interaction forces between α-alumina fibres in aqueous electrolyte measured with an atomic force microscope

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Abstract

The surface charging properties of polycrystalline α-alumina fibres in aqueous electrolyte solutions have been investigated by direct force and streaming potential measurements. The presence of both Al and Si on the surface of the fibres resulted in a chemically heterogeneous surface. The heterogeneous distribution of Si resulted in large attractive forces between the fibres at moderate to low pH values and a pzc/iep at a pH value of approximately 5.5. The origin of this force was electrostatic in nature as the force profiles were well described by the DLVO theory of colloid stability. The agreement between the direct force and streaming potential measurements was good both in terms of the magnitude of the potentials and the position of the pzc/iep. By acid washing the fibres the chemical heterogeneity of the surface was reduced and the attractive force profiles at lower pH values were not observed. Instead repulsive forces were observed which were well described by DLVO theory at all separation distances greater than 8 nm. At smaller separation distances an additional repulsive force was measured which was attributed to the presence of a Al(OH)3 like layer on the surface of the alumina. The acid washing treatment also resulted in a shift in the pH at which the pzc/iep occurred to a value of 6.5, presumably due to a lower surface silica concentration.

Introduction

Alumina fibres are important reinforcements in ceramic matrix composites (CMCs) [1] as well as metal matrix composites (MMCs) [2]. One method of processing CMCs involves infiltrating suspensions of ceramic powders into ceramic fibre preforms [3]. In this methodology the fibres should be dispersed from each other to allow the powder to easily fill the void spaces between the fibres. When powder cannot be uniformly packed between the fibres, voids and inhomogeneous areas lead to flaws in the composite, reducing its strength and reliability. To obtain dispersed and therefore uniformly packed particles, a large repulsive interaction energy between the particles is necessary. An obvious way of achieving this goal is to adjust the pH of the suspension so that the surface charge, and therefore surface potential, of the particles is large. It is also important to control the interactions between powder and fibre as well as between the powder particles. It is important to know the surface charging behaviour of the fibres. With this knowledge, the solution conditions may be adjusted to allow the powder to best infiltrate the fibres. A common assumption for alumina fibres is that they have the same isoelectric point (iep) as α-alumina powders dispersed in aqueous solutions (ca. pH 9) [4], [5], [6]. This work and a number of recent results suggest that this is not always the case. A better knowledge of the iep and surface charge of the fibres is thus critical to more reliable composite materials.

For the last two decades it has been possible to measure the interaction forces between smooth, macroscopic surfaces using the Surface Forces Apparatus (SFA) [7]. This technique has demonstrated the validity of the Derjaguin and Landau [8], Verwey and Overbeek [9], or DLVO theory. The results suggest the theory describes the interaction forces between most surfaces immersed in simple symmetric electrolytes (<0.1 M concentration) at separation distances less than 10 nm. Indeed there are several studies between very smooth, single crystal α-alumina surfaces using this technique in the literature [10], [11]. We have chosen to use the Atomic Force Microscope (AFM) which has recently become popular for measuring the interaction forces between two spheres or a sphere and a flat surface. The applicability of the technique is evidenced by the wide variety of surfaces that have been investigated [12], [13] since the first studies in 1991 [14], [15]. One of the advantages of this technique is that interaction forces between colloidal sized spherical particles can be measured. In this study, the aim was to measure the interaction forces between two chemically identical surfaces, not between, for example, a silica sphere and an alumina flat surface, as in previous AFM studies [16], [17]. The system of interest was α-alumina fibres in the crossed cylinders geometry. To the authors knowledge, this is the first time this geometry has been used in interaction force experiments using the AFM.

The results presented in this study have wider implications than those stated above. Direct force measurements involving alumina surfaces [10], [11], [16], [17] have typically used single crystal samples. The surface chosen here were polycrystalline alumina surfaces. Furthermore, there is a great deal of controversy surrounding the pH at which α-alumina surfaces are nett neutral (i.e. the point of zero charge or pzc). For example, the results obtained by Horn et al. using either two single crystals of alumina [10] or one alumina and one silica surface [18] suggested a pzc somewhere between pH values of 6.5 and 10. Veeramasuneni et al. [16] using silica and alumina surfaces in the AFM found a pzc at about pH 9.3 whilst Larson et al. [17], using the same system and technique, found a pzc for alumina at a pH around 4.5. It should be pointed out that Larson et al. [17] found no evidence of silica contamination of the surfaces using X-ray Photoelectron Spectroscopy (XPS). The presence of silica (which has a pzc at around pH 2–3) on the surface will lower the observed pzc of the alumina [19]. Karaman et al. [20] observed a pzc at about pH 7 between a single crystal alumina surface and an amorphous alumina spherical particle. This lack of consensus is reflected in electrokinetic measurements on powdered aluminas. For example, many electrokinetic studies find the isoelectric point (iep) of alumina to be around pH 9.0 [4], [5], [6], [21], however, there are also studies on powders and single crystal samples which find an iep at a much lower pH value [22], [23], [24].

The surfaces used in this study are also rough (on the nm scale) and so provide an opportunity to investigate how the presence of roughness modifies the interaction forces measured from that predicted via the DLVO theory. Also, under some conditions the alumina fibres used here are expected to be chemically heterogeneous on the sub-micrometer scale. To the authors knowledge this phenomenon has not been previously investigated experimentally, although there are some theoretical studies in the literature for neutral surfaces that have an uneven surface charge density [25].

Section snippets

Materials

Nextel 610 polycrystalline α-alumina fibres (3M Corporation, St. Paul MN) were chosen as the fibres for this investigation because of their high purity (>99% α-alumina), smooth surface (grain size approximately 60 nm) and diameter appropriate for the AFM experiments (10–12 μm) [26]. The Nextel-610 α-alumina fibres, as received, are sized with a PVA solution (1%). For the AFM experiments, this was removed by ultrasonication in water over several days with frequent changes of water. The fibres

Characterization of fibres

The alumina fibres used in this study were characterized using AFM and XPS. Images of the fibre surface were obtained with the AFM in contact (constant force) mode. Fig. 2 presents a typical image of a 1×1 μm2 region of a fibre (acid washed once) and clearly shows the granular nature of the surface with a grain size on the order of 60 nm. The morphology and grain size are in agreement with manufacturers specifications and other workers findings [34]. Analysis of five images gave average peak to

Discussion

In general the forces measured for once acid washed fibres presented in Fig. 4, Fig. 5, Fig. 6 are quite well described by DLVO theory at most separation distances. In particular the distance from which the surfaces jumped to contact are well described. However, one would not predict attractive forces at lower pH values between surfaces of identical surface composition. Indeed attractive electrical double layer forces, such as those presented in Fig. 4, Fig. 6, are normally only observed

Conclusions

The surface charging properties of α-alumina fibres in aqueous KBr solutions have been investigated by AFM direct interaction force and streaming potential measurements. The main findings are:

(i) The interaction forces between Nextel 610 α-alumina fibres are well described, for most separation distances, by DLVO theory under the conditions investigated in this study.

(ii) Deviations from DLVO theory occur mostly at separation distances smaller than 8 nm. The first of these consisted of jumps to

Acknowledgements

The authors would like to acknowledge Jeff Haslem and Professor Fred Lange for providing the fibres and background information and Dr Patrick Hartley for assistance with the streaming potential apparatus. We would also like to thank Dr Steven Carnie, and Professors Derek Chan and Thomas Healy for helpful discussions, Dr Leong Mar for the XPS analysis and interpretation of spectra and Professor David Boger for his support. We also acknowledge discussions with Dr Mark Fagan and David Wilson of

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