Professor Milo Shaffer’s research team at Imperial College London is making great progress in the search for new techniques to enable the clean, scalable, production of Multi Walled Carbon Nanotubes (or MWCNTs to use the appropriate acronym).

Since the discovery of Buckminster Fullerine or ‘Buckyballs’ by the 1996 Nobel Prize laureates Robert F. Curl, Harold W. Kroto, and Richard E. Smalley, the capability of carbon atoms to form well-defined, covalently-linked structures such as balls, sheets and tubes at first fascinated and subsequently excited scientists with their range of prospective technological applications. Some of these opportunities are already being put into practical use. Bulk nanotubes are being used as composite fibres in polymers, improving the mechanical strength, as well as the thermal and electrical properties of finished aerospace and automotive composite structures. Used in inks, MWCNTs enable direct printing of electrical circuits and show great promise as the basis for new generations of biosensors and fuel cell components. The list of applications seems to grow on a daily basis.

The holy grail for many researchers is to discover the means to precisely control the submicron quality, morphology and dimensions of these structures and to optimise yields from catalysts and precursors, enabling the industrial scale precision manufacture of these materials.

Using a multiple zone Lenton PTF ‘Precision Tube Furnace’ and precisely controlling the precursor injection into the apparatus, Professor Shaffer and his team of researchers at Imperial College are able to control the length diameter and alignment of the nanotubes films as they initiate and grow onto quartz substrates. Many different growth conditions have been explored, but a typical synthesis involves injecting a solution ranging from 0.2% to 9.6% ferrocene by weight, dissolved in toluene, into the Lenton PTF ‘Precision Tube Furnace. The vapours are carried in a flow of argon-hydrogen carrier gas preheated to 200°C and reacted at temperatures ranging from 550°C to 940°C within the main furnace tube. The overall nanotube diameters can be varied appreciably in the range from 10nm to 100nm as the ferrocene concentration is adjusted and also as the furnace temperature is adjusted. Extending the injection time over which the precursors are injected up to 7 hours causes the tube lengths to increase, with scanning electron micrographs showing lengths up to several millimetres. This phenomenal length to diameter ratio, which can reach over a million to one, is particularly intriguing and can provide important benefits in many applications. The ability to precisely optimise production of MWCNT for specific reaction times and temperatures using the Lenton furnace, as well as optimising catalyst and precursor ratios are key first steps in attaining large scale industrial production.

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Two specially designed furnaces from Lenton are enabling Stryker Biotech to sinter orthobiologic materials in clean room conditions at closely controlled temperatures up to 1200°C at its factory in the Irish Republic.

The furnaces are used in the manufacturing process for tricalcium phosphate (TCP) granules, the main ingredient in an absorbable filler developed by Stryker to repair bone defects.

The Lenton furnaces have chambers measuring 460mm wide x 460mm high x 1060mm deep, which allow 28 trays of material to be processed in each one at a time. A special loading trolley allows the trays to be positioned easily in the chambers, and specially designed doors with gas-tight seals have been fitted to prevent ingress of air. Each furnace also has an exhaust chimney with a damper valve which is opened to allow fumes to escape and closed at higher temperatures to improve temperature uniformity.
Heating is provided by six resistance wire coils on each side of the chambers, with separate temperature control provided for the lower and upper elements in order to achieve the required temperature uniformity when the equipment is fully loaded. A
16-segment programmer is supplemented by over-temperature protection, and a sealable port is included in the doors to provide access for survey thermocouples.

Lenton has completed an order for three custom-built furnaces with fast cooling performance for heat-treating powder injection moulded components
in controlled atmospheres at temperatures up to 1000°C.

The furnaces, which have been built for Singapore-based Solid Micron Technologies, are designed for burning off binders in injection-moulded components at between 400°C and 500°C and then sintering them at approximately 1000°C. The accelerated cooling system has been included in order to reduce the complete processing cycle to eight hours and maximise throughput.

The furnaces have cylindrical gas-tight stainless steel retorts with capacities
of 170, 340 and 510 litres which are fed with nitrogen gas throughout the heating process to prevent oxidisation of the components. Three-zone heating control ensures uniform and consistent temperatures throughout the retorts.

After-burners are fitted to minimise exhaust emissions to atmosphere, and all electrical equipment has been designed for operation in ambient temperatures from 34°C to 40°C and relative humidity of 98 per cent.

In order to accelerate the cooling phase of each cycle, Lenton has supplied a heat exchanger that can be moved between the furnaces and used to force-cool the hot nitrogen gas, which is then circulated within the retorts. This system has reduced cooling times from about six hours (in the case of a full load in the largest furnace) to about three hours, allowing a full cycle to be completed in a single shift.