The influence of processing parameters on morphology and granulometry of a wet-milled sol-gel glass powder

Abstract

A quaternary bioactive sol-gel glass of high silica content was heat treated at different temperatures, and then wet ball milled under different balls-to-powder ratios. A total of sixteen experiments were performed to study in detail the effects of both experimental variables on the structure, morphology, particle size distributions and nitrogen adsorption isotherms. The balls–to–powder ratio exerts a tremendous influence on the final particle size distribution of the powders, while its effects on the pore volume and morphology are minimal. These structural features are mostly governed by the changes in calcination temperature. Therefore, understanding the specific roles of each experimental parameter is of paramount importance towards achieving optimum powders with the desired properties. This work sheds light on the importance of using a suitable combination of these two parameters for tuning the morphology and the granulometry of the sol-gel derived bioactive glass powders.

Introduction

Powdered raw materials are widely used to produce ceramics in many industrial sectors [1]. The ceramic parts can be shaped either by dry or wet powder processing routes. The dry processing route includes pressing the powder to form a green body, and often requires the addition of processing additives such as binders to assist in the consolidation process, and to confer enough strength to the green bodies to enable safe handling and densification behaviour along the subsequent processing steps [2]. The wet routes involve mixing the powders with a compatible liquid in the required proportions, and adding suitable amounts of processing additives with different roles, which might include dispersants, binders, plasticisers, thickening agents, etc. The aim is to obtain homogeneous mixtures, and to confer on them suitable rheological properties for the selected shaping technique. The traditional wet consolidation techniques might start from: (i) relatively concentrated pastes of clayey based materials or of advanced ceramics plasticised with suitable processing additives, and be accomplished by plastic forming (extrusion, roll forming, or jiggering) [3]; (ii) fluid suspensions that consolidate upon partially removing the liquid by slip casting [4] or pressure casting [5], or by evaporation such as in tape casting [6].

However, it was shown that liquid removal driven by the capillary suction exerted by plaster moulds (slip casting), eventually assisted by an external applied pressure (pressure casting), is prone to lead to particle segregation by a process known as the clogging effect [7]. This segregation mechanism is particularly favoured when fine particles are free enough to move quickly along with the liquid, under the effects of driving forces, leaving behind the coarser and heavier ones. In other words, all the factors that contribute to lowering the viscosity of the medium (high degree of dispersion, decrease in solids loading) enhance this phenomenon. On the other hand, the deposition kinetics also determine the extent of particle segregation. It was shown that the driving force exerted by plaster moulds (about 0.15 MPa) maximised the clogging effect. Lower driving forces favoured segregation by sedimentation, while higher ones did not allow the particles to order in the highest possible packing arrangements [7]. Aiming at overcoming the drawbacks of wet consolidation techniques involving liquid removal, and further exploring the potential advantages of colloidal processing, a number of direct consolidation techniques have been developed. Homogeneous suspensions are cast in non-porous moulds, and transformed into rigid bodies without liquid removal, thus preserving the homogeneity achieved in the starting suspensions. The consolidation can be achieved through different setting mechanisms, including, Starch Consolidation [8], Gel Casting [9], Direct Consolidation Casting [10], Hydrolysis-Assisted Solidification [11], Temperature Induced Gelation [12], Epoxy Gel Casting [13], etc. Direct consolidation techniques enable the reduction of the number and the size of structural defects, thus enhancing the reliability of the advanced ceramics, and expanding the applications of colloidal processing. To achieve high green density and the desirable final properties [14], particle size (PS), particle size distribution (PSD) and particle morphology of the powders, and all the relevant processing parameters need to be optimised. This also applies to the modern additive manufacturing techniques when starting from powders, as in 3-D printing [15], or from extrudable pastes for robocasting [16]. All of these motives justify a renewed emphasis on controlling the properties of the powders involved in the processing of ceramic and glass materials.

The reduction in particle size occurring during milling results from the accumulated stresses inside the particles due to the applied mechanical energy, which induces cracks that propagate through them, leading to particle breakage [17].

Particle breakage, comminution, pulverisation and milling are all interchangeable terms, commonly used with apparently identical meanings. The powder milling process may be conducted under dry conditions (dry milling) or in wet environments (wet milling). The later one is more efficient, commonly being recommended when high surface energy induces agglomeration between particles [18].

Many important parameters should be taken into consideration while performing wet ball milling. For example, the liquid-to-powder ratio (LPR), i.e., the solids loading, the type of milling machine, and the speed at it works, need to be selected according to the rheological behaviour of the suspension [19]. Increasing heat treatment temperatures gradually lead to the formation of hard agglomerates that are difficult to destroy on milling, which strongly affect the microstructure and the properties of the final products. A few examples include power transmission [20], optical properties [21], photocatalytic activity [22], [23], drug loading and release [24], phase transformations [25], and sintering ability [26]. The effects of milling ball–to–powder ratio (BPR) on phase formation, and the resulting crystallite size, have been particularly investigated in works related to mechanochemical synthesis [27], [28], [29]. However, most of these studies aimed at disclosing some interdependencies between the experimental variables and the measured properties for certain particular applications.

To the best of the authors’ knowledge, the effects of heat treatment temperature and the balls–to–powder ratio (BPR) on the wet milling performance, granulometry and morphology of the powders have still to be better documented. The aim of this work is to perform a systematic investigation into the effects of these two experimental variables on particle size distributions of the powders, crystalline phase assemblage, pore volume, and morphology of sol-gel derived bioactive glass particles.