Douglas E. Burkes

The Application of Self-Propagating High-Temperature Synthesis of Engineered Porous Composite Biomedical Materials

The term “self-propagating high-temperature synthesis” (SHS), or “combustion synthesis”, refers to an exothermic chemical reaction process that utilizes the heat generated by the exothermic reaction to ignite and sustain a propagating combustion wave through the reactants to produce the desired product(s). The products of combustion synthesis normally are extremely porous: Typically 50 percent of theoretical density. Advantages of combustion synthesis over traditional processing routes, e.g., sintering, in the production of advanced materials such as ceramics, intermetallic compounds and composites include process economics, simplicity of operation, and low energy requirements. However, the high exothermicity and rapid combustion propagation rates necessitate a high degree of control of these reactions. One research area being conducted in the Center for Space Resources (ISR) at the Colorado School of Mines (CSM) is the application of SHS to synthesize advanced, engineered porous TiB–Ti, NiTi, NiTi–TiC, TiC–Ti, and multiphase/heterogeneous calcium phosphate (HCaP) for bone tissue engineering and drug delivery systems. Processing conditions, such as reaction stoichiometry, green density, and gasifying agent, greatly affect the structure and properties of the SHS porous products. In general, all porous composite materials produced in this work for tissue engineering, including NiTi, NiTi + TiC, TiC + Ti, TiB + Ti, and multiphase/heterogeneous calcium phosphate (HCaP), appear to be biocompatible with no overt toxicity observed either in vitro or in vivo. An example of an SHS reaction is Ni + Ti = NiTi. The extent of the porosity and the size of the pores can be controlled using certain ‘gasifying agent such as B2O3, CaCO3. The article discusses the synthesis of porous TiB–Ti, NiTi, Ni3Ti–TiC, TiC–Ti, and multiphase/heterogeneous calcium phosphate (HCaP) for bone tissue engineering and drug delivery systems.

Production of Ni3Ti-TiCx intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi-TiC (stoichiometric) or Ni3Ti-TiCx (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni3Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiCx, for which 0.3 ≤ x ≤ 0.5, has an important controlling effect on the formation of either Ni3Ti or NiTi; i.e., formation of TiC0.7 results in a depletion of titanium and formation of Ni3Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni3Ti-TiCx composites are produced. Ignition, combustion, and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiCx particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented.

Combustion Synthesis of a Functionally Graded NiTi-TiCx Composite

Combustion synthesis (CS) is an alternative technique for producing advanced materials and is dependent upon a highly exothermic chemical reaction to become self-sustaining after only a short energy pulse is applied to initiate the reaction. A NiTi-TiC x functionally graded material (FGM) was investigated that combines superelastic and shape memory capabilities of NiTi with the high hardness, wear, and corrosion resistance of TiC x . CS was employed to produce a FGM from 100% TiC x ceramic to 100% NiTi intermetallic. Temperature and burning velocity data of the CS reaction were recorded. XRD of the final product layers was conducted to determine phase composition. The combustion temperature, burning velocity, and cooling rate in each layer decreased with increasing NiTi content. Large blowholes were present in the high NiTi content layers as a result of outgassing of volatile species from the reactant powders. XRD analysis revealed the presence of Ni-Ti intermetallics along with a substoichiometric TiC (TiC 0.7 ). Production of a NiTi-TiC x FGM is possible through use of a CS reaction employing the propagating mode (SHS). The material layers were observed as functionally graded in both composition and porosity.