In this Division we investigate the process of ceramic production from the synthesis of novel starting powders through to densificaiton and characterisation of material properties.
Our current research interests are in the following materials:
- nano-powders formed by wet chemical methods
- hot isostatic pressing (HIP) for transparent component formation lasers and sensors
- doping for functional properties
MAX-phases (Ti3SiC2, Ti2AlN)
- synthesis using both traditional methods and wet chemical methods
- vaccum densification and hgh temperature stability
Novel MAX phases (CrAlN)
Yttrium oxide (Y2O3)-based ceramic material is used for various applications such as lasers and cutting tools due to properties such as high thermal stability and transparency to infrared radiation. As an optical ceramic it transmits well in the infrared range from 1 to 8 micron wavelength. The high infrared transmission together with good resistance to erosion and thermal shock makes it an ideal material for protection domes for infrared sensors. It is an excellent refractory material, because of its high melting point and low thermal expansion coefficient. Applications include: chemically stable substrates, crucibles for melting reactive metals, nozzles for jet casting molten rare earth-iron magnetic alloys and cutting tools.
Transparent yttria ceramics are used as IR windows in heat-seeking rockets, luminous pipes and solid state laser host material. Yb doped Y2O3 ceramic appears to be a potential candidate material for laser applications.
MAX-phase materials are a relatively new class of ceramic materials which has received quite a lot of attention, especially from the aerospace industry. The name, MAX, is derived from the general formula, which is as follows: MnAXn-1, where M is an early transition metal, A is an element belonging to groups 12-16 in the periodic system of the elements (see figure below) and X is either carbon or nitrogen; n is an integer 2, 3 or 4.
Figure 1 shows the periodic table of the elements and the groups of elements that may be combined to create a MAX phase. It also shows the crystal structure of the most common MAX-phase; titanium silicon carbide or Ti3SiC2.
The crystal structure is essential for the properties of the MAX phases. Although there are many ternary carbides and nitrides (i.e. with the same type of chemical formula as MAX), only a few of them can be classified as MAX. The MAX phases are in fact natural nano-composites constituted by layers of a metal carbide or -nitride interleaved with monatomic A-element layers (see figure above). This laminar structure provides the MAX phases with an extraordinary combination of mechanical and electric properties.
Indeed, the MAX phases seem to combine some of the most attractive properties of ceramics and metals. They possess thermal and electrical conductivity, machinability and ductility at elevated temperatures – properties otherwise associated with metals, while at the same time they are resistant to wear, chemical attacks and extreme temperatures, just like most other ceramics. This makes MAX-phase materials very interesting for applications in jet engines and for any other load bearing detail operating at extremely high temperatures.
The most well-known MAX-phase of them all is the titanium silicon carbide, the Ti3SiC2, and in this division work on this MAX-phase is performed in two areas: the development of a wet chemical synthesis method for Ti3SiC2- and Ti2AlN powder and a metallurgical powder synthesis route.
The wet chemical synthesis aims to produce Ti3SiC2 and Ti2AlN in the form of powders through a controlled chemical reaction between chlorides and sodium metal (Na). The powders are then annealed in a controlled atmosphere.
The metallurgical synthesis uses a starting powder mixture that does not include the strongly oxidizing titanium metal powder, which is commonly used in Ti3SiC2-production today. The aim is to produce dense, monolithic bulk samples using an easily up scaled process.
Novel MAX phases
Another area of interest is the synthesis of new MAX-phases. As can be seen from the figure 1 above, many elements can combine to form MAX phases, though not all combinations seem to consist of the right, laminar structure. There are also three different classes of MAX phases, depending on the crystal structure of the metal carbide or –nitride included (see figure below).
Figure 2 shows the different crystal structures of the three classes of MAX-phases.
For example: in a 211-phase, such as Ti2AlN, the aluminium atoms (the A-element) are separated by a layer of titanium nitride (Ti2N) of formula M2X whereas in a 312-phase, such as the Ti3SiC2, the silicon atoms are separated by titanium carbide of formula M3X2. Most of the MAX-phases known today belong to the 211-class. Our research on novel MAX phases focus on the Cr-Al-N – system. No MAX phase from this system has been reported to date.