Haikuan Kong

Characterization of high temperature piezoelectric crystals with an ordered langasite structure

Piezoelectric single crystals with the ordered langasite structure A3BC3D2O14, including Sr3TaGa3Si2O14, Sr3NbGa3Si2O14, Ca3TaGa3Si2O14, and Ca3TaAl3Si2O14 (CTAS), were studied as a function of temperature, up to 900 °C. The dielectric permittivity e11 and piezoelectric coefficient d11 of the ordered crystals were found to be on the orders of 12–16 and 4–5 pC/N, respectively, slightly lower than langasite (La3Ga5SiO14-LGS) or langanite (La3Ga5.5Nb0.5O14) crystals which possess a disordered structure. The mechanical quality factor Q and electrical resistivity ρ, however, were found to be greatly improved at elevated temperatures ≥500 °C, being one to two orders of magnitude higher, due to cation ordering. Of particular interest is the CTAS crystal, in which, the Ga cations are totally replaced by low cost Al cations. Together with its thermally stable piezoelectric properties and high electrical resistivity, CTAS crystals offer a competitive material for high temperature sensing applications.

From ab initio forecast of piezoelectric properties to growth of piezoelectric single crystals

The piezoelectric properties of two langasite type compounds, Ca3TaAl3Si2O14 and Ca3NbAl3Si2O14, were predicted by first-principle calculation and single crystal of Ca3TaAl3Si2O14 was successfully grown by conventional Czochralski method. The complete elastic and piezoelectric properties of Ca3TaAl3Si2O14, measured by resonant-antiresonant method, indicated that Ca3TaAl3Si2O14 was a promising substitute for quartz in the surface acoustic wave application. The high veracity of calculated results compared with measured ones suggested that first-principle predictions could be considered as a pertinent tool for the development of new materials.

Piezoelectricity of A3BC3D2O14 structure crystals

First-principle calculations are performed to reveal the microscopic origin of piezoelectricity of ordered crystals with A3BC3D2O14 structure. The calculated piezoelectric constant e11 and atomistic piezoelectric contribution from every single atom type in a primitive cell of Sr3NbGa3Si2O14 (SNGS), Sr3TaGa3Si2O14 (STGS), Ca3NbGa3Si2O14 (CNGS), and Ca3TaGa3Si2O14 (CTGS) structures showed that the piezoelectricity comes mostly from one of three atoms in A positions. The percentages of the contributions from the A positions to the total internal piezoelectric constant exceed 69% in CNGS and CTGS and 81% in SNGS and STGS, respectively.

Investigation of room temperature ferromagnetism of 3C-SiC by vanadium carbide doping

Undoped and vanadium carbide (VC) doped 3C-SiC powders have been prepared, and an in-depth study is performed on the VC-doping dependence of room temperature ferromagnetism (FM). It is demonstrated that the FM originates in vacancy defects. The saturation magnetization (Ms) of VC is about 800 times than that of undoped 3C-SiC, while the Ms of VC-doped 3C-SiC is even smaller than that of the undoped one. The increase of doping concentration would result in the decrease of vacancy concentration and the increase of carrier concentration, suggesting that the FM of 3C-SiC is related to both vacancy and carrier concentrations.

Advances in design, growth and application of piezoelectric crystals with langasite structure

Crystals with langasite structure consist of more than one hundred compounds. Only about 20 compounds are grown and characterized. In this work, we first use first-principles calculation to simulate the crystal structure of 29 ordered langasite compounds and then forecast their piezoelectric related properties including dielectric constants, elastic constants, piezoelectric coefficients and electromechanical coefficients. Four known ordered crystals including SNGS, STGS, CTGS, CNGS, and several novel crystals including BTGS, CTAS, CNAS were grown by Czochralski method and characterized at room temperature and at high temperature from 400 to 900°C. The experimental results of piezoelectric properties verified the validity of theoretical forecast and that errors between experimental and theoretical results were in reasonable range. Gallium-free crystals shown good prospect in application of piezoelectric sensors, combining the advantages of low cost of raw materials and high performance. More efforts should be paid to the growth of these novel crystals.

Effect of annealing temperature on the contact properties of Ni/V/4H-SiC structure

A sandwich structure of Ni/V/4H-SiC was prepared and annealed at different temperatures from 650 °C to 1050 °C. The electrical properties and microstructures were characterized by transmission line method, X-ray diffraction, Raman spectroscopy and transmission electron microscopy. A low specific contact resistance of 3.3 × 10-5 Ω·cm2 was obtained when the Ni/V contact was annealed at 1050 °C for 2 min. It was found that the silicide changed from Ni3Si to Ni2Si with increasing annealing temperature, while the vanadium compounds appeared at 950 °C and their concentration increased at higher annealing temperature. A schematic diagram was proposed to explain the ohmic contact mechanism of Ni/V/4H-SiC structure.

Study of Triangle-Shaped Defects on Nearly On-Axis 4H-SiC Substrates

Triangle-shaped defects are one of the most common surface defects on epitaxial growth of 4H-SiC epilayer on nearly on-axis SiC substrate. In this paper, we investigate the feature and structure of such defects using Nomarski optical microscopy (NOM), micro-Raman spectroscopy and high resolution transmission electron microscopy (HR-TEM). It is found that triangle-shaped defects were composed of a thick 3C-SiC polytype, as well as 4H-SiC epilayer.

Study of Nitrogen Concentration in Silicon Carbide

This work focused on studying the nitrogen concentration (CN) in SiC. The variations of CN in the synthesis of SiC powder as well as the transport during SiC crystal growth have been investigated for broad ranges of temperature and Ar pressure. Before SiC crystal growth, SiC powders were synthesized from high-purity silicon and carbon powders. The concentrations of nitrogen, free C, and free Si in the as-prepared powders were all measured. CN in the SiC source powder decreased with increasing temperature and decreasing Ar pressure, whereas it did not show a remarkable trend with the molar ratio of free Si to free C. SiC crystal was then grown by the physical vapor transport (PVT) technique using the as-prepared powder. The distribution of CN in the remaining material indirectly indicated the temperature field of crystal growth. In addition, compared with introducing N2 during SiC crystal growth, doping with nitrogen during synthesis of the SiC source powder might be a better method to control CN in SiC crystals.

Effect of vanadium on the room temperature ferromagnetism of V-doped 6H—SiC powder

This study focuses on the effect of V-doping on the ferromagnetism (FM) of 6H—SiC powder. The X-ray diffraction results indicate that V is inserted into the 6H—SiC lattice. The Raman spectra reveal that with a V concentration of 25 ppm, the crystalline quality and carrier concentration of 6H—SiC are hardly varied. It is found that after the V-doping process, the saturation magnetization (Ms) and the vacancy concentration of 6H—SiC are both increased. From these results, it is deduced that the effect of V might contribute mainly to the increase of vacancy concentration, thus resulting in the increase of Ms of V-doped 6H—SiC.

Synthesis of Source Powder for SiC Crystal Growth Using High Purity Silicon and Carbon Powder

This work focused on the synthesis of source powder for SiC crystal growth. SiC powder was prepared using high purity silicon and carbon powder. Broad ranges of temperature and Ar pressure were studied on the property of the as-prepared powder. X-ray diffraction (XRD) results show that SiC polytypes were determined by synthesis temperature, while not related to the variation of Ar pressure. The lattice constant of SiC would expand when Ar pressure decreased. Raman results revealed that the variation of Ar pressure would influence SiC crystallization. It was found that the concentrations of free C, free Si and nitrogen all varied with the variation of temperature or Ar pressure.