Scanning electron microscopy (SEM) has long played a central role in structural characterisation for material scientists. Bombarding the surface of a material with a beam of electrons and detecting those that are emitted or backscattered allows microscopists to see down to resolutions of 10 nanometres or so, giving them intricate details of the material’s structure. However, the requirements of SEM, such as a high vacuum and the need for a thin coating if an insulator is being analysed, mean that certain types of materials have always proved difficult or impossible to image straightforwardly.
Limitations of Conventional SEM

For example, the coating can obscure the fine surface detail on some insulators - although SEMs equipped with field emission guns have made such samples easier to image. Another difficulty arises with wet and damp samples such as paints, inks, emulsions and biological tissue - these materials prove particularly challenging for SEM. The high vacuum requirements in the chamber mean lengthy specimen preparation techniques are required to remove or fix the water before imaging, raising the risk of artefacts being introduced.
Overcoming These Limitations Using ESEM

These problems can now be overcome, thanks to the new environmental scanning electron microscope (ESEM), which permits the imaging of wet systems with no prior specimen preparation. The development of this instrument means that whole new classes of materials, previously undreamed of, can be imaged in their natural state. But the potential of ESEM is even greater than. this. Because the sample environment can be dynamically altered, hydration and dehydration processes can be followed as they happen in the sample chamber.
How Are These Limitations Overcome?

So how is this all achieved? Two basic developments have made it possible for this new generation of SEMs to hit the market. Firstly there was the recognition that by using a system of differential pumping, the electron gun can be maintained at high vacuum while the sample chamber can be kept at a constant pressure of 10-20 torr (1 torr ~ 133 N.m-2).
Pressure Limiting Apertures

In the ESEM instrument, a series of pressure limiting apertures (PLAs) are placed down the column, across each of which a pressure differential is maintained. Figure 1 shows the column and highlights the pressures in the different zones. Consequently, despite the relatively high pressure in the chamber, this design allows ESEMs to operate with LaB6 filaments as well as tungsten, and field emission guns are also becoming available to give superior quality imaging.

Detectors

Having a vapour in the sample chamber presents another problem - the conventional Everhardt-Thornley detector used to pick up the electrons emitted from the sample surface in standard SEM cannot be used for ESEM. So the second crucial development needed was a new type of detector that can operate at a pressure of tens of torrs. The enlarged section in figure 1 shows the environmental secondary detector, which was the first successful detector to operate under these conditions. As its development has been only fairly recent, detector design is still being modified and optimised.
Electron Beam Scattering and Resolution

Scattering of the electron beam between the gun and the sample is an obvious concern, given that there are many atoms/molecules of vapour in the ESEM’s chambers. The question is: if such scattering occurs, can a decent resolution be achieved? The answer is yes, as long as the gap between the final PLA and the sample is kept reasonably small, say 510 mm, and the pressure is kept quite modest, say less than 12 torr.

Under these circumstances, with a mean free path for 20 KeV electrons of a few millimetres, the majority of incident electrons from the gun are not scattered at all. A well defined electron ‘probe’ is still incident on the sample, but with an accompanying ‘skirt’ of scattered electrons which can extend for tens of microns from the central probe. Fortunately, the intensity distribution of the skirt is quite flat and so it can essentially be treated as a background signal, above which the central probe still gives a strong signal. In fact, the probe signal is strong enough to give a resolution of down to 5nm with a LaB6 filament, if the pressure in the sample chamber is kept sufficiently low.
Backscattered Electrons

Of greater importance is what happens to the electrons - both those ‘backscattered’ from the sample surface and ‘secondaries’ produced by the surface bombardment - after they leave the sample and move towards the detector. Owing to the presence of the vapour, these electrons also undergo collisions, many of which lead to ionisation of a gas molecule (particularly in the case of the low energy ‘secondaries’). Each ionisation produces a daughter electron, which can then itself ionise further gas molecules, creating a cascade in the gap between sample and detector, figure 2. The overall effect is considerable signal amplification the extent of this depends on parameters such as gas pressure and type, detector bias and of course the population of electrons leaving the sample.

Wet Imaging

As vapour is tolerated in the sample chamber, ESEM makes it possible to carry out ‘wet imaging’ of samples. In order to view a wet sample such as a colloidal dispersion, the atmosphere in the sample chamber must be carefully controlled at all stages. After the sample is placed in the chamber, air at atmospheric pressure must be replaced by water vapour at a few torr It is important to carry out the pump-down carefully so that premature dehydration of the dispersion does not occur. If done correctly, no accidental aggregation of the particles occurs, and a truly dispersed state will be imaged.
Evaporation and Condensation

During imaging it is also important to ensure that neither water evaporation nor condensation occur. This is achieved by using an atmosphere at the saturated vapour pressure (SVP) of water. However, this raises an additional difficulty in imaging samples at room temperature, because the SVP of water is relatively high at this temperature compared to the modest pressures of a few torr that are acceptable in the sample chamber. The trick is to use a Peltier stage to drop the temperature by a few degrees so that SVP is easily maintained without undue loss of image quality and resolution.
Real Time Hydration/Dehydration Studies

However, for certain experiments it is desirable to move away from these SVP equilibrium conditions to allow deliberate hydration or dehydration of a sample. In principle, changes in structure or state of a wet sample can be dynamically followed as water condenses on to or evaporates from the sample. In practice though, beam damage - always a bugbear for electron microscopists and a particular problem when examining organic materials - may well prevent real time studies being carried out.

It may therefore be necessary to examine the structure by viewing different samples at different times during the hydration or dehydration process. Dropping the temperature to cause water to condense onto a sample surface can be just as useful an experiment. Chemical reactions taking place in water can be followed in this way.
Case Study – Hydration of Cement

The hydration of cement is one important example of a chemical reaction in water that can be followed using the ESEM. Cement is an extremely important and complex commercial material. Controlling the reaction of the grains of material with water is vital in ensuring that the cement sets at the right rate - too fast and it could, say, plug a deep sea oil well, too slow and it holds up construction. Despite its importance, there are still uncertainties about the precise mechanisms involved in the setting process and about how subtle changes in cement composition may affect it. ESEM can look at the developing structure in the cement at different times during the setting reaction.

Cement has a number of different inorganic ingredients, including tricalcium silicate (C3S), tricalcium aluminate, dicalcium silicate, tetracalcium aluminoferrite and gypsum. Within the limited field of view of the microscope it is difficult to examine the individual reactions of each component, and so it is easier to follow the reactions of the components separately.

Initially, water is condensed onto a dry grain by dropping the sample temperature, and the reaction is then allowed to proceed for the required length of time. ESEM cannot produce an image through a significant thickness of water, however, so after this time the sample temperature is raised slightly to evaporate most of the surface water. Only a thin, electron-transparent layer remains, to keep the grain hydrated. By repeating this process, the evolving structure can be followed and related to measurements made by other techniques, such as calorimetry.


Applications of ESEM

ESEM has a number of different applications already, but there is plenty of opportunity to extend this range of uses. ESEMs have been around for a number of years, with more in the US than elsewhere, yet as a technique it appears to be substantially underexploited. This is perhaps especially true of its application to studying wet systems. In part this can be attributed to an early failure to appreciate the careful control of chamber pressure required at all times. With improving awareness of this, ESEM will find a key role in an increasing number of materials characterisation laboratories.

Bodycote Testing Group is the world’s leading supplier of independent sub-contract testing. In the environmental field, we have two established laboratories in the U.K. and are now offering commercial testing from our laboratory in Shotton, North Wales. The Shotton Laboratory currently supports Corus Colors, the UK’s largest coated steel strip processor and has the experienced staff and equipment to undertake a wide range of environmental tests.
Testing Equipment Available at the Shotton Laboratory

The equipment inventory includes: -

· ICP Optical Emission Spectrometers

· Industrial Microwave

· Photometric COD Analyser

· BOD Analyser

· Photometric Ammoniacal Nitrogen Analyser

· pH Meters

· Conductivity Meters

· Gravimetric and Titrimetric Methods

· Infra-Red Oil in Water Analyser
Testing Procedures Available at the Shotton Laboratory

This enables the Bodycote laboratory to carry out a wide range of commonly required tests including environmental effluent monitoring, groundwater and borehole analysis. The range of specific tests available includes: -

· Metals in Solution

· COD

· BOD

· Ammoniacal Nitrogen

· pH

· Conductivity

· Suspended Solids

· Moisture Content

In addition to the above, a wide range of classical wet chemistry techniques are available within the laboratory enabling the testing of, for example, Nitrates and Sulphates.
Ananlytical / Environmental Laboratories at Glasgow

The existing Bodycote Analytical (Environmental) laboratory in Glasgow offers a vast range of testing and consultancy services, and clients who may have a requirement for services not offered by the local laboratory will be contacted by an expert and guided through the testing process.
Bodycote’s Accredited Testing Facilities

Bodycote’s laboratories are accredited by the United Kingdom Accreditation Service (UKAS), which provides assurance of the quality systems in place. The assessment standard is ISO 17025, which is an internationally recognised quality assurance standard, compatible with the ISO 9000 series.
Additional Services to Customers

Bodycote management and technical staff alike pride ourselves on our customer service ethic, and our belief is that understanding and satisfying our customer’s needs is paramount. Routine samples are typically processed and reported within 48 hours of receipt. Also, our laboratory management computer system ensures standardised and efficient reporting, and allows the transmission of certificates by e-mail if required.

Optical Emission analyser designed to identify all the key elements in metals - especially where highest accuracy and/or the analysis of light elements like C, Al, S, P, Mg, Si is needed and when sorting low alloys and aluminum. Ideal, for example, for separation of 316 H (>0.04% C) and 316 L (<0.03% C).


Production Cycle Applications

Use one unit throughout the whole production cycle including:

· Classification of raw material (recycling metal)

· QC of semimanufactured products

· QC of final product
Carbon Analysis Of Steels

Recognising that carbon analysis in steels is of primary concern in many material verification functions, and e.g. in welding work, ARC-MET8000 has the unique capability of measuring carbon in both air and argon modes.
ARC-MET8000 Features

ARC-MET8000 offers:

· Accurate results using Air or Argon measurements

· Fast grade identification and assay

· Only one probe with an integrated display

· Low level carbon analysis using Air burn in a few seconds

· Battery operation, long cable between main unit and probe, and convenient mobility

· Unique to ARC-MET8000: the probe is the heart of the system
Suitable Materials

· Ideal for ferrous and non-ferrous metals.

· Low alloy steels

· Stainless steels

· Tool steels

· Low alloy (white) cast iron

· Aluminum alloys

· Titanium alloys

· Nickel alloys

· Cobalt alloys

· Copper alloys

· Zinc alloys

· Magnesium alloys

Abstract

Alloys of composition Cu100-xHfx with 50 ≥ x ≥ 25 at.% and Cu55Hf45-xTix with 5 £ x £ 45 at.% have been investigated. All alloys investigated in both series were fully vitrified when melt spun to ribbon of thickness ~25 mm, except the binary alloy Cu75Hf25 that showed a crystalline + amorphous structure. The critical size for the binary alloys was limited to ribbons, while the critical diameter is 3 mm for the Cu55Hf25Ti20 and Cu55Hf20Ti25 alloys. On the other hand, 2 mm fully glassy diameter was found for the Cu55Hf30Ti15 and Cu55Hf15Ti30 alloys. The substitution of Hf by Ti causes an increase in the glass-forming ability (GFA). As the Ti content increases, the glass transition temperature (Tg) and crystallization temperature (Tx) decrease for both Cu100-xHfx and Cu55Hf45-xTix alloys. In contrast, the liquidus temperature (Tl) has a minimum value of 1163 K for the Cu55Hf20Ti25 alloy, resulting in a maximum Tg/Tl of 0.61. The alloys with the highest Tg/Tl value showed the best GFA in this Cu-based alloys series.
Keywords

Bulk Metallic Glasses, Suction Casting, XRD, DSC, Critical Glassy Diameter
Introduction

Metallic glasses are non-crystalline alloys formed by continuous cooling from the liquid state. Up until recently [1], such materials could only be formed in thin sections at cooling rates typically in the range 104-106 Ks-1. Such alloys, based on iron, nickel and/or cobalt have established numerous applications as ultra soft magnetic materials in power and high frequency cores for transformers, chokes, inductors and other similar devices [2]. In the past decade, researchers had paid more attention to the metallic glasses not only due to their superior chemical and physical properties, but because they can now be fabricated in bulk form. They are therefore considered as real practical engineering materials and open up new application opportunities. For instance, glassy alloy can be used for making magnetic tapes [3, 4] as it exhibits better wear resistance. Extensive experimental investigations have demonstrated that many compositions in these systems can be formed into the glassy state by using suction casting to form cylindrical or slab-shaped ingots. Maximum section diameters or thicknesses of fully glassy phase range up to 72mm [5], depending on the combination of constituent elements and their precise concentrations. Recently, Inoue et al. [6, 7] reported Cu-based alloys with high glass forming abilities, high tensile strength of over 2000 MPa and lower material cost bulk metallic glasses which can be prepared by copper mould casting. Their excellent mechanical properties and glass forming ability (GFA) make the production of precision mechanical parts possible such as high precision gears [6]. Driven partially by interest in engineering applications, there has been an ongoing effort to identify amorphous alloys with greater strength, elastic modulus, hardness, and ductility. Of particular interest are alloys based on such common metals such as Cu, Al, Co, Fe, Ni, etc. However, the information of a glassy phase in Cu-Hf-Ti alloys is based on 60 at.% Cu. The aims of this investigation are to produce bulk amorphous rods of Cu-Hf and Cu-Hf-Ti and determine the critical glassy diameter of these alloy families.
Experimental

Alloy ingots of nominal compositions Cu100-xHfx (x = 50, 45, 40, 35, 30 and 25 at.%) and Cu55Hf45-xTix (x = 5, 10, 15, 20, 25, 30 , 35 and 40 at.%) were prepared by arc melting mixtures of Hf (crystal bar), Cu (sheet) and Ti (Rod) having purities of 99.5 at.%, 99.99 at.% and 99.8 at.%, respectively. The arc melting was performed in a Ti-gettered high purity Argon atmosphere. Each ingot was re-melted at least four times in the arc melter in order to obtain good chemical homogeneity. Ribbon samples of mean thicknesses ~ 25 mm and width ~ 2 mm were prepared by melt spinning in a controlled Ar atmosphere. Copper die suction casting was employed to produce rods with a stepped profile having diameters decreasing from 4 to 3 to 2 mm, each with a total length of 50 mm. The phase constitutions of the rods were studied by X-ray diffraction (XRD). The thermal stability, defined by the glass transition temperature (Tg) and the crystallization temperature (Tx), was studied by differential scanning calorimetry (DSC) at a heating rate of 20 K/min. The solidus temperature (Tm) and liquidus temperatures (Tl) were determined by differential thermal analysis (DTA) at a heating rate of 20 K/min. Thermal characterization was performed using melt spun ribbons.
Results

The average thickness of the ribbon produced was 25 mm and almost all the ribbons produced were of uniform width. All the alloy ribbons formed, except for one sample, the binary alloy Cu75Hf25, were found to be nominally fully amorphous by XRD analysis. They showed high metallic lustre and could easily be bent through 1800 without fracture, thus showing very good ductility to a high strain and indicating a fully amorphous or almost fully amorphous thickness. However, the Cu75Hf25 alloy ribbon had very poor ductility and it was shown to be crystalline by the XRD analysis. Stepped rods were produced of diameters 2, 3 and 4 mm with a length of 50 mm. Figure 1 shows a photograph of a 2/3/4 mm diameter stepped rod produced by suction casting. The rods produced by suction casting showed good metallic lustre and a low incidence of fabrication defects.

The structure results by X- ray diffraction for all the ribbon and rod samples of the Cu100-xHfx alloy series with x = 25 – 50 are summarised in Table 1. All rods were crystalline or largely crystalline as several distinct peaks can be seen for all these traces with no clear evidence of a diffuse halo corresponding to a glassy phase. The ribbon samples are clearly fully amorphous in each case, with the exception of the composition Cu75Hf25 which shows a small fraction of crystalline structure, with one peak at ~ 43o 2 q corresponding to the phase Cu8Hf3 (111). Figure 2 shows DSC curves for the melt-spun ribbons of this alloy series and values of Tg and Tx are plotted as functions of Hf content in Figure 3. Although no clear glass transition is observed for 30 at.% Hf, the alloys containing 50 at.% to 35 at.% Hf exhibit distinct glass transitions, followed by the glass transitions region before crystallisation, however, the alloy containing 25 at.% Hf was found to be partially crystalline, giving a DSC curve with amorphous + crystalline structure. The Tg and Tx decrease with increasing Hf content from 35 at.% to 45 at.%, despite the fact that the Hf has a much higher cohesive energy than Cu. It is also seen that the supercooled liquid region defined by the temperature interval between Tg and Tx, DTx (=Tx − Tg), shows a relative constant value of ~ 30 K over the range 35 – 50 at.% Hf, but appears to narrow considerably for 50 at.% Hf. The substitution of Hf for Cu in the range 25 – 35 at.% Hf decreases the liquidus temperature, but increasing the Hf content beyond 35 at.% Hf then leads to an increasing Tl. The values of the reduced glass temperature (Trg=Tg/Tl) are plotted as a function of Hf content in Figure 4. Trg increases when decreasing Hf content from 0.55 up to 0.61 for x = 50 and 35 respectively, then decreasing to the value of 0.59 for x = 30. Table 2 shows the thermal analysis results for this alloy series.

Ecoflex is a multi purpose technology that enables a wide range of building solutions to be cost effectively constructed using recycled raw materials. The technology has been proven effective in important applications for four major industries:

· Civil Construction

· Mining

· Commercial Construction

· Agriculture & Aquaculture
Applications
Civil Construction

Retaining walls, sealed and unsealed roads and hard stands, drainage and water flow systems
Mining

Underground roads and retaining walls
Commercial Construction

Concrete slabs and sealed pavement
Agriculture & Aquaculture

Dams, ponds, creek crossings, access roads, wave barriers
Ecoflex Technology

Ecoflex technology has been proven by through the construction of over 450 construction projects, all of which have been independently certified and have demonstrated both the engineering values and the commercial advantages of the technology. Ecoflex enables construction companies and recycling companies to manufacture and supply proven building products and gain competitive advantage in their market place.

Competitive Advantages and Competitive Positioning
Low Cost

On average Ecoflex products enable installed construction cost to be reduced by 25% versus traditional construction solutions. This is a sustainable competitive advantage driven by the use of low cost recycled raw materials and reflecting the low capital and operating costs of the Ecoflex recycling system.
High Performance

Ecoflex products have been proven and certified by independent authorities.
Environmental Solution

Enables recycled waste tyres and recycled aggregate to be used for high value purposes rather than disposed of via land fill. Ecoflex can make a major contribution to construction project waste reduction, can be reused and eliminates waste. Ecoflex meets a pressing need world wide for a high volume, sustainable means of recycling used tyres.
IP Protection

Ecoflex technology is patented in most of the industrialised world. International business development potential is enormous.
Advocates

Ecoflex is an Australian Technology Showcase member and has a very long list of advocates spanning the construction industry, engineering firms, waste management and environmental organisations, government and academia.