(70ab) Probing of the Microstructural Evolution of Nano-Powder Compacts Using High Resolution X-Ray Microtomography

ABSTRACT

The use of nano-engineered materials is becoming incredibly diverse; such applications include, bio-implants, pharmaceutical and cosmetic formulations, aircraft and engine parts, and electronic devices and sensors. In all cases, the tailor-made microstructure is essential to achieving highly specific functionality. Whether materials are designed empirically, ?top-down?, or by molecular design, ?bottom-up?, the dynamic evolution of the material microstructure requires careful monitoring both during the formulation of the materials and during the intermediate processing stages of the parts and components. There are a number of comprehensive review articles on the subject of the development of various formulations and processing routes developed during the past decade [1-2]. The close monitoring of the dynamic evolution of the microstructure during processing is required to ensure that the nano-intensive properties of the materials are preserved and the specific functionality required of the materials are retained in the manufactured parts and components.  

In nano-material formulation, the structural properties are conventionally monitored and characterised using imaging techniques such as SEM, TEM, surface spectroscopy and AFM.  These highly expensive pieces of analytical equipment require carefully prepared static samples that are taken off-line from the formulation and the processing stages. As such, whilst helping to probe and characterise structures down to molecular and atomic scales, they do not offer dynamic monitoring of the manufacturing process.  What is needed is a non-invasive imaging technique which can examine the three-dimensional structure of the evolving microstructure of the parts and components as a function of the processing conditions.

The work undertaken by this project was the construction, installation, testing and delivery of novel equipment that interfaces three complementary high resolution, non-invasive, imaging techniques: Three-dimensional X-ray microtomography; Transmission X-ray diffraction; Optical microscopy. These are used to facilitate the simultaneous probing of both the volume-based and the surface-based morphology of the nano-engineered particulates in sol gel solutions during simultaneous drying and uni-axial compaction; see [3] for further details.

Figure 1: a) X-ray microtomography system constructed from the highest specification components available. 

b) Specially-constructed environmental vacuum chamber for material processing incorporating Optical, X-ray tomography and X-ray diffraction capabilities.

High Precision Instrument Development: Figure 1(a) shows the X-ray microtomography system built from parts and components.  This system is capable of achieving reconstructions, without significant artefacts, at a spatial resolution of 5mm Figure 1(b) shows the environmental vacuum chamber with beryllium windows for horizontal axis X-ray imaging and optical sapphire windows for viewing of the sample from above. The processing conditions within the chamber can also be controlled. The atmospheric conditions can be varied from vacuum (below 10^-6 mbar) through a range of partial pressures, up to 1 atmosphere, using a range of back-filled gases (including air). The temperature heating cycle can be controlled with an accuracy of 1K up to a maximum final temperature of 900K.  The samples could also be mechanically loaded within the chamber up to a uni-axial load of 2.5MPa.  A sample stage which allowed the sample to be rotationally manipulated through 360° allowed X-ray microtomography to be conducted in-situ, preventing the sample being changed or disturbed in any way.

The bulk-density, moisture content, inter-particle spacing, and particle shape are all important parameters which affect the macro-scale bulk properties of the particle systems post-sintering.  It is this micro-scale particle system which is modelled in computational fluid dynamics simulations.  There is currently a missing link between the micro-scale structure of particles as they dynamically evolve during sintering and the way in which this affects the bulk properties.

 Nano-Engineered Powders: Novel silica-polyviologen hybrids were used in bulk compaction experiments, where the cationic polymer appears to act as a template in the sol-gel synthesis.  The hybrids are precursors to largely mesoporous, high surface area silicas, with potential uses in catalysis and separation processes.  The resultant hybrid materials are amorphous aggregates of roughly spherical particles with mean diameter of 60- 80nm.  Particle sizing by scanning electron microscopy (SEM) shows that there are also occasional monolith structures which are much larger and do not appear to consist of an aggregate of spherical particles.  This polymer-nano silica hybrid system is suitable to show the capability of our system in the temperature range available with our equipment. The sintering temperature of many of the simple ceramic and metallic oxide powders is outside our current temperature limit (i.e. > 1200K in most cases).  However, we did also use fumed Silica nano-powder of (15-20 nm mean diameter) in a few benchmarking experiments of the micro-structural behaviour of simple oxides.

Process System Parameters: A variety of compaction processing conditions were applied to the polyviologens-nano silica hybrid samples of several mm in size (somewhat larger than  a pharmaceutical capsule or an electrical or medical device component) under a variety of processing parameter combinations applied within the environmental chamber seen in Figure 1 (b) above:

•  Uni-axially loaded (up to 2.5MPa) or load-free
•  Vacuum or atmospheric pressure
•  Unheated, hot-plate heating to 900K, or microwave heating (2 minutes at 2kW)

The apparatus was then used with the following operating parameters for the analysis of the samples: X-ray tube voltage 60kVp and current 0.1mA from a molybdenum transmission target with a 100mm zirconium filter.  The magnification used resulted in the reconstruction of a (1024)³ cube of data with a voxel linear dimension of 5mm. The three-dimensional reconstructed data, was than processed, and the average agglomeration size distribution determined. Figure 2 shows three-dimensional renders of the sample (a) and (b), and the processing and labelling of a the data (c) (thresholding (d) → region labelling (f) → volumetric analysis (incorporation of (e) and (f)) shown in two dimensions for clarity but actually conducted in three dimensions.

 SHAPE  \* MERGEFORMAT

            (a)                       (b)                          (c)                   (d)                                    (e)                       (f)

Figure 2: (a) 3-D reconstruction. (b) Section of 3-D. (c) Original reconstructed slice. (d) Gray scale thresholded. (e) Coloured to density scale.  (f) Labelled agglomerate images after connectivity and cluster analysis.

Analysis of Micro-structural Compaction Data:  Bulk density and agglomerate size and shape distributions can then be compared (over the entire three dimensional data set or in specific regions) for the processing conditions tested.  Figure 3 shows examples of central two-dimensional slices for each of the different processing regimes; the volumetric analysis show in table 1 is extracted from the entire three-dimensional data set. Analysing the region labelled three-dimensional data is then histogrammed to reveal agglomerate size distributions.  The mean agglomerate volume for each processing condition is shown in Table 1.

Changes in the agglomerate structure and density can clearly be seen with different processing routes. Comparing the data sets given in table 1 below, it appears that vacuum and uniaxial loading help to preserve much smaller agglomerate sizes during sintering of the polyviologen powder whilst micro-wave heating appears to reduce the most probable agglomerate size by half compared to conventional conduction heating. 

REFERENCES

[1] G.M.Whitesides and J.C.Love "The Art of Building Small", (2001), Scientific American, September, pp 33-41.

[2] S.V. Patwardhan, N.Mukherjee, M. Steinitz-Kannan and S.J. Clarson, "Bio-Inspired Synthesis of New Silica Structures". (2003), Chem. Commun., RSC, pp 1122-1123.

[3] Jenneson, P. M, Luggar, R. D., Morton E. J., Gundogdu, O., Tuzun, U., 2004, ?Examining nano-particle assemblies using high spatial resolution X-ray micro-tomography?, J. Appl. Physics, 96, 2889-2894.