The Magnetic And Microwave Absorbing Properties Of Novel Nanosized Titanium Nitride

Among transition metal nitrides, titanium nitride (TiN), as a kind of high-performance ceramics, hasbeen highly focused on. TiN possesses high melting point (2950℃) and high hardness, as well as goodelectrical and thermal conductivities and excellent environmental stability, showing promising applicationas an important conductive ceramic in electronic industry. Besides, TiN coating can be used to increase thehardness and improve the anticorrosion ability and chemical stability of cutting tools. However, fewreports are currently available about TiN as a novel microwave absorbing material and dilute magneticmaterial. Therefore, a simple route is established to synthesize nanosized TiN powders by calcinatingnanotubular titanic acid (NTA) precursor in flowing ammonia. The microstructure and composition ofas-synthesized TiN nanoparticles are analyzed by means of high-resolution transmmison electronmicroscopy, X-ray diffraction (XRD), scanning electron microscopy, and energy dispersive spectrometry.The effects of crystal structure, composition and especially structural defects on the microwave absorbingperformance and magnetic properties of as-synthesized TiN are systematically investigated. The maincontents and results are as follows:1. Titanium nitride nanoparticles have been synthesized by calcination of NTA precursor in flowingammonia. The crystal structure and chemical composition of as-prepared TiN nanoparticles have beenanalyzed, and their electromagnetic loss and microwave absorption performance have been examined. Ithas been found that after4h heat-treatment in NH3flow at900℃, the tubular structure of NTA iscompletely broken down, and NTA is converted into nanoparticles with an average size of20–30nm. Thecontent of N doped into the lattice of TiN and the lattice constants rise with extending heat-treatment timein flowing ammonia, while the XRD pattern of TiN nanoparticles tends to shift towards small2θ valuetherewith. Different from commercial TiN, as-prepared novel TiN nanoparticles can nearly completelyabsorb light with a wavelength of400~2500nm and show excellent electromagnetic loss behavior in afrequency rangy of2~18GHz. In particular, when the thickness of the wave-absorbing layer is3.0mm,corresponding TiN-paraffine composites show a minimum reflection loss (RL) value of-27dB at6.6GHz,suggesting that as-synthesized TiN nanoparticles have good microwave-absorbing characteristics. Incombination with the crystal structure and chemical feature of as-prepared novel TiN nanoparticles, we speculate that their unique electromagnetic properties may be closely related to their structural defects.Considering the good high temperature stability of TiN, we suppose that as-prepared TiN nanoparticlesmight be potential candidate of high temperature microwave-absorbing material.2. A superconducting quantum interference device has been performed to measure the magneticproperties of as-prepared novel TiN nanoparticles. It has been found that as-prepared TiN nanoparticlesexperience a gradual diamagnetic to ferromagnetic transition at room temperature, and those TiNnanoparticles obtained after long enough heat-treatment in flowing NH3possess robust room temperatureferromagnetism even without introducing any magnetic elements. As-prepared novel TiN nanoparticle, as atypical superconductor with low superconducting transition temperature, provides a rare example of such asuperconductor with room-temperature magnetic transition characteristic. We speculate that the uniquemagnetic properties of as-prepared TiN nanoparticles may be highly dependent on their structural defects,and there may have some essential relationship between their composition and magnetic properties.3. Novel TiN nanoparticles have been doped with various elements so as to acquire different structuraldefects with different content. The effects of the type and content of doped elements on the magnetic andelectromagnetic properties of novel TiN nanoparticles have been investigated. It has been found thatnitridation process allows dopant elements to be evenly doped into the lattice of TiN, resulting in latticedistortion. In the meantime, whether doped by magnetic element like Fe or by non-magnetic element likeLi or B, as-doped TiN nanoparticles possess room-temperature ferromagnetism, while Fe dopingcontributes to improve the microwave absorbing performance of as-prepared TiN nanoparticles. Thisindicates that the ordered ferromagnetism of doped TiN nanoparticles is highly dependent on theirstructural defects.