Synthesis and preparation

General wet chemistry

The CMC laboratories at Aarhus University are fully equipped for standard wet chemistry including centrifuges, ultrasonic baths, Slenck line setup and rotational evaporators. The labs also features several glove-boxes for inert-atmosphere chemistry, of which one is used for general wet-chemistry and another is used for dry-matter solid-state chemistry or sample preparation.

High-temperature solid-state synthesis

A wide range of different ovens and furnaces are available to the students within the CMC. Furnaces are used for high-temperature synthesis (e.g. semiconductor-alloys, ceramics, etc.) or for crystallisation, sintering and other post-synthesis treatment. Ovens may also be used for such thermal processing, though at lower temperatures. Their primary function, however, is that of autoclave-syntheses (see below) plus more ordinary tasks of drying, dehydration, compound stability tests, etc.

Induction furnaces

At Aarhus University CMC has the use of two induction furnaces, both with the coil positioned in a pressure chamber (range from 5 bar to vacuum) featuring water-cooled walls. Both instruments are equipped with a lower and an upper pulling shaft allowing the sample to be manipulated at elevated temperatures. This allows a wide range of crystal-growth techniques including Bridgeman growth, floating zone growth and Czochralski pulling. A cold crucible can be mounted inside the pressure chamber.

Conventional furnaces

Conventional furnaces are widely used for solid-state materials synthesis. At CMC, furnaces are available to suit any demand within a wide range of temperatures. Syntheses may also be conducted in temperature gradients, or under various inert or reactive atmospheres. CMC have approximate 11 tube furnaces going op to between 1000-1500 ºC and three box furnaces where the maximum temperature is 1100-1800 ºC.

Ovens

Ovens are employed for synthesis and processing at temperatures which are too low for the operation of furnaces (ambient to 300-400 ºC, typically). They are mandatory for batch autoclave synthesis and extremely useful for testing material stabilty (e.g. prior to a TGA/DSC measurement), for crystallization at benign conditions or studying phase transitions at moderate temperature. Last but not least, low-temperature ovens are invaluable for drying laboratory glass-ware or precursor chemicals prior to synthesis, or synthesis products prior to analysis.

Hydrothermal & Solvothermal synthesis

Hydrothermal and solvothermal synthesis is excellent methods for synthesizing inorganic materials. It is a simple and highly efficient way to produce highly crystalline particles, which with the continous-flow or stop-flow technique may also be produced in large quantities. Hence it is a standard technique in our lab.

Batch autoclave reactors

Batch syntheses are done in stainless steel, Teflon-lined autoclaves. The autoclaves are heated in box ovens, and the syntheses can be done at a large range of temperatures (autogeneous pressures). This technique is very simple and easy to use, which also makes it useful for quick exploration of the basic parameters temperature and residence time, e.g. prior to synthesis with more advanced tools.

Continuous Synthesis in Near- or Supercritical fluids - Single Stage

The Dept. of Chemistry have the use of multiple flow-reactors for synthesis in near- and supercritical media, which is at the disposal of CMC students. The flow reactors can be operated with a broad spectrum of fluids, including water and organic solvents at pressures up to 500 bar and temperatures up to 450 °C.

The single-stage apparatus is presently configured to handle a stream of carrier solvent and a single stream of reagent solution, but due to the very versatile connecting block, it can be readily modified to handle multiple streams of reagents.

It's possible to synthesise a broad spectrum of nanocrystalline materials, which are of scientific and commercial interest. It combines a short process time and continuous flow with pressures and temperatures that are significantly higher than normally obtained with traditional teflon coated autoclaves.

Continuous Synthesis in Near- or Supercritical fluids - Dual Stage

The Dual-Stage Reactor is a flow reactor in which two reactors are connected in series. This allows CMC students to synthesize nanocomposites such as core-shell structures in which a core particle is covered by a shell material. The core particles are synthesized in the primary reactor, while the shell material is deposited on the core particles in the secondary reactor.

A shell material can be applied to enhance properties, protect the core or combine properties into a multifunctional material.

For use in applications (solar cells, batteries, magnets, fuel cells, etc.) a high throughput is a pre

requisite, and the apparatus has a production capacity in excess of 10 g/hour (dry product).

"Extreme conditions" Ultra High Pressure Synthesis

Our huge press weighs 10 tons and can generate a force corresponding to 1000 ton. Using the ingenious geometry of an octahedral Walker module the axial force is transformed into quasi-hydrostatic pressures of up to 25 GPa (250.000 bar) and internal resistive heating allows temperatures of up to 2000°C. High Pressure High Temperature (HPHT) solid state synthesis at these extreme conditions makes it possible to pressure-stabilize new compounds that can be recovered to ambient conditions. A popular example is the production of synthetic diamonds. At CMC the HPHT synthesis methods are exploited as a powerful approach to synthesis of novel materials with interesting structures and properties. The synthesis studies are often combined with high pressure crystallographic characterization performed in diamond anvil cells (DACs) to complement the understanding of pressure effects on materials and their properties.

Ball milling

Ball milling was originally a technique used mainly for mixing, grinding, alloying and mechanical activation. Particles with average size less than 100 nm can be obtained. However, we have in our laboratory developed this technique, so it is applicable for solid state synthesis of new crystalline materials

The CMC laboratories has several types of mills, e.g. a planetary ball mill, a cryo mill and facilities for high pressure milling. The planetary mill is a high energy ball mill, which can operate at 200-400 rpm. A wide range of different vials are available, e.g. a special high pressure vial (p < 150 bar). The temperature and pressure in the vial can be monitored during the ball milling. The cryo mill is a Spex impact mill operated at -196 °C (liquid nitrogen temperature).

Pelleting

Compaction of powders is often useful or even mandatory when studying the chemistry and properties of materials, e.g. as sample preparation for measurements or to cultivate propertes from powders which are normally only present in the bulk. For such purposes the CMC commands a number of pellet-presses suitable for different applications. The inventory also include a double-disk Knuth-rotor for grinding/polishing the as-compacted pellets using SiC paper.

Cold-presses

A cold-press is used for pelleting the least demanding powders, i.e. materials with good binding properties, or for pre-compacting powders for Spark-Plasma Sintering (SPS). Crucibles are available for a range of sample diameters, and the two presses at CMC can exert a compacting force up to 240 kN and 500 kN, repsectively. Cold-pressing is also used for the preparation of pellets for X-ray fluorescence analysis (XRF), sometimes with the aid of an X-ray transparent binder material.  With pelletised samples, XRF-sensitivity to the elements Na, Mg and Al (the lightest elements in the range of the technique) may be improved much.

Spark-Plasma Sintering press

The Spark-Plasma Sintering press (SPS press) is an extremely useful piece of equipment which enable powders to be compacted at elevated temperatures with an extremely rapid heating to set-point temperature (a few minutes) and an entire compaction process to be carried out in 10-30 minutes. This exceptionally allow the preservation of microstructures in the powder since disruptive processes such as phase diffusion or alloying have little time to occur.

As with the cold-presses, a range of crucibles is available for different sample diameters. The SPS press may exert a compacting force of 50 kN while sintering at temperatures up to 2400 °C, enough to pelletise even the most demanding powders (though some materials still require some small addition of a binder).

Due to the combination of compaction and temperature, SPS-pressed powders may attain densities of up to >95% of the bulk density. This is extremely useful as a synthesis-measure since many materials are not easily synthesised in bulk form while they are readily obtainable as powders, and yet the properties of interest may only exist in the bulk.