Laser-induced combustion synthesis of 3D functional materials: computer-aided design

Self-propagating High-temperature Synthesis (SHS) can be used to synthesize inorganic materials that in some cases cannot be prepared by conventional methods. SHS is an exothermic process whereby a green mixture is ignited by point source initiation—usually by a flame or heated wire. A hot synthesis wave moves through the material converting reactants to products. The most promising means for controlling the parameters of an SHS reaction include the use of ultrasound, laser radiation, electron beam and ac/dc electromagnetic fields. On appropriate choice of the frequency/amplitude of an applied field the combustion parameters such as velocity and temperature can be influenced. For a reaction to be selfpropagating the reaction must be sufficiently exothermic such that the combustion process provides enough energy to overcome the heat losses to the surroundings. In conventional SHS work this is quantified by the term Tad, the adiabatic combustion temperature. Reactions that have a calculated Tad greater than 1450 uC generally propagate from a point source of ignition sending a combustion wave through the whole solid mass. Those reactions with a Tad below 1450 uC in general do not propagate through the solid but do undergo reaction at the point of initiation with some incipient reaction in the surrounding area. It is conventional in traditional SHS only to concentrate on reactions that have Tad w 1450 uC. 3 Important limitations of SHS in forming ‘net-shape’ products are (1) its multi-stage character and (2) its limited controllability—the reaction is in effect a thermal explosion. As a result, SHS-produced ingots or monoliths often strongly deviate from the desired shape, which in turn requires additional expensive processing. Another important objective in SHS is the formation of functionally graded materials (FGMs) and related items. These materials can be widely used in the aerospace, chemical, and nuclear power industries. The design of FGMs can be carried out by using either Rapid Prototyping & Manufacturing (RP&M) or Solid Free Form Fabrication (SFFF). At the Computer-Aided Design/Computer-Aided Engineering (CAD/ CAE) level, it is possible to make functionally graded parts by means of Laser Stereolithography, Selective Laser Sintering (SLS), Fused Deposited Modeling (FDM) and Laser Engineered Net Shaping (LENS). Recently it has been shown that laser-induced SHS reactions and SLS can be combined in the same process. In this case, an SHS reaction is only initiated within the spot of a focused laser beam. The laser beam is then scanned over the powder mixture surface to create a desired computer-designed configuration. After the first layer of material has reacted a second fresh layer of starting material is added and the process repeated. In this step-wise fashion a monolith can be built up—the depth of each added layer can be calculated such that the SHS reaction proceeds through the layer and bonds to the underlying layer. Using the stoichiometric mixtures Ni–Al, Ti–Al and Ni–Ti and a continuously working (cw) YAG : Nd laser, this technique was used to prepare sintered intermetallic items of desired configuration. Later, this process was termed Reactive Rapid Prototyping (RRP). This technique also has the potential to be applied to the synthesis of smart micro devices (Micro Electro Mechanical Systems, or MEMS devices) such as: sensors; filters; piezoelectric detectors and pumps. The purpose of this paper is to present the first SHS–SLS of oxide materials and show that this technique is particularly applicable to the formation of shaped porous complex oxides. We also show that the SHS–SLS technique can be used for making shaped intermetallic monoliths with embedded oxides, complex monoliths and shaped PZT ceramics.