Patents

  1. 1
    Fabrication of piezoelectric composites using high temperature dielectrophoresis technique
    Filed on 8th August 2016 in United States
  2. 2
    Method for making high piezoelectric voltage coefficient Lead free piezoelectric composites
    Filed on 1st April 2015 in United States

Publications

Abstract

December 10, 2016

Flexible and Low Cost Lead Free Composites with High Dielectric Constant
Highly flexible lead free composite film having random distribution of ceramic filler was synthesized using Barium Titanate (BT) as a filler and inexpensive Thermoplastic Polyurethane (TPU) as a matrix. The results show that the 30 vol. % BT-TPU composite has a dielectric constant of ~31 which is comparable to the expensive and difficult to produce PVDF based composites. With a breakdown field of 150 KV/mm, an energy density value of ~3 J/cm3 was estimated. These lead-free TPU based composites provide an alternative to PVDF based composites for energy storage applications.

Abstract

November 16, 2016

Effect of the piezoelectric ceramic filler dielectric constant on the piezoelectric properties of PZT-epoxy composites
Piezoelectric composites made from soft and hard lead zirconium titanate (PZT) particles as filler and an epoxy as the matrix were prepared by dielectrophoresis and studied for their piezoelectric properties. It was found that the dielectric constant of the piezoelectric filler plays a significant role in determining the final piezoelectric properties of the composites. Composites with lower dielectric constant for the PZT filler material showed better piezoelectric properties compared to the composites with high dielectric constant filler. This can be ascribed to a more efficient poling of the piezoelectric filler particles. The aging behaviour of these composites was compared to that reported for monolithic ceramics.

Abstract

June 14, 2016

A low-firing Ca5Ni4(VO4)6 ceramic with tunable microwave dielectric properties and chemical compatibility with Ag
A Ca5Ni4(VO4)6 low loss microwave dielectric ceramic with A site deficient garnet structure was prepared via the conventional solid state reaction method. Ca5Ni4(VO4)6 sintered at 980 °C for 4 h to a relative density of 96.2% exhibits favorable microwave dielectric properties such as a permittivity of 10.9, a Q×f value of 96,500 GHz, and a τf value of −63.6 ppm/°C. Its large negative τf could be compensated by forming a solid solution with LiCa2Mg2V3O12, that led to improved properties with a near-zero τf=−3.7 ppm/°C, εr=10.2, Q×f=59,300 GHz for 0.8Ca5Ni4(VO4)6−0.2LiCa2Mg2V3O12 after sintering at 955 °C for 4 h. Further on, 0.8Ca5Ni4(VO4)6−0.2LiCa2Mg2V3O12 proved to be chemically compatible with Ag electrodes, so it might be a possible candidate for LTCC applications.

Abstract

Li4WO5: A temperature stable low-firing microwave dielectric ceramic with rock salt structure

October 09, 2015

A Li4WO5 ceramic with rock salt structure was prepared by the solid-state reaction method and its microwave dielectric properties was demonstrated for the first time. It could be well densified at relatively low sintering temperature (∼890 °C). XRD and DTA analysis revealed a phase transformation from cubic to orthorhombic occured at 700 °C. Excellent microwave dielectric properties with a near-zero temperature coefficient of resonant frequency ∼−2.6 ppm/°C, a relative permittivity ∼8.6 and a quality factor ∼23,100 GHz (at 11.0 GHz) was obtained. Li4WO5 was found to be chemically compatible with silver powders when sintered at 890 °C. All the results indicate that the Li4WO5 ceramic is a promising candidate as a base material in low temperature cofired ceramic technology.

Abstract

Reduced thermal conductivity by nanoscale intergrowths in perovskite like layered structure La2Ti2O7

March 31, 2015

A series of ceramics with a general formula, La2Ti2-xTaxO7, in which x = 0.05, 0.1, 0.2, and 0.3, were prepared by Spark Plasma Sintering (SPS). Effects of tantalum substitution for titanium on the structure, dielectric, and piezoelectric properties were studied. Results revealed that the structure changed gradually from 4-layer to 3-layer due to the higher valence of Ta. The solid solution limit of tantalum in La2Ti2O7 lattice was in the proximity of x = 0.2. The ferroelectric Curie temperature (Tc) decreased with increasing tantalum doping content. dc resistivity reached a maximum value at x = 0.2 with a value of ∼ 1.0 × 108Ω ⋅ cm at 600°C. The influence of texture on the piezoelectric properties of La2Ti2-xTaxO7 ceramics was also investigated. A maximum d33 value ∼2.1 pC/N was obtained at x = 0.2.

Abstract

Study on properties of tantalum-doped La2Ti2O7 ferroelectric ceramics

February 18, 2015

The effect of substitution and oxidation-reduction on the thermal conductivity of perovskite-like layered structure (PLS) ceramics was investigated in relation to mass contrast and non-stoichiometry. Sr (acceptor) was substituted on the A site, while Ta (donor) was substituted on the B site of La2Ti2O7. Substitution in PLS materials creates atomic scale disorders to accommodate the non-stoichiometry. High resolution transmission electron microscopy and X ray diffraction revealed that acceptor substitution in La2Ti2O7 produced nanoscale intergrowths of n = 5 layered phase, while donor substitution produced nanoscale intergrowths of n = 3 layered phase. As a result of these nanoscale intergrowths, the thermal conductivity value reduced by as much as ∼20%. Pure La2Ti2O7 has a thermal conductivity value of ∼1.3 W/m K which dropped to a value of ∼1.12 W/m K for Sr doped La2Ti2O7 and ∼0.93 W/m K for Ta doped La2Ti2O7 at 573 K.

Abstract

Enhancement of thermoelectric properties by atomic-scale percolation in digenite CuxS

April 17, 2014

Atomic-scale percolation phenomena of electric and thermal conductivity were found in digenite CuxS. Copper ions are randomly located between the closely packed sulphur ions and jump to another position via unoccupied interstices. Near the percolation threshold value (fc ∼ 0.3) in the conductive region, a 60% enhancement of ZT value can be obtained. This indicates that the percolation phenomena can provide a new strategy to optimize the properties of thermoelectric materials, especially for quasi disordered materials.

Abstract

Large ZT enhancement in hot forged nanostructured p-type Bi0.5Sb1.5Te3 bulk alloys

March 04, 2014

Bi2Te3 based alloys play a dominant role in commercial applications in the fields of thermoelectric energy generation and solid state cooling. By combining the densification of nanostructured powders followed by a two-step hot forging process, hierarchical nanostructured p-type Bi0.5Sb1.5Te3 alloys with good preferred orientation were successfully fabricated. The Seebeck coefficient in the direction perpendicular to the pressing force, which is highly anisotropic, is much greater than that of the material sintered via one-step sintering. Meanwhile, the nanostructure and crystal defects produced during hot forging also contribute to both higher Seebeck coefficient, and lower thermal conductivity due to more effective and preferential scattering of phonons than electrons. As a result, a 50% enhancement of ZT value (from 1 to above 1.5) in the orientated, hierarchical, nanostructured alloys was obtained.

Abstract

Utilizing the phonon glass electron crystal concept to improve the thermoelectric properties of combined Yb-stuffed and Te-substituted CoSb3

November 1, 2013

A cost-effective and rapid mechanical alloying method was used to prepare Yb-stuffed and Te-substituted CoSb3 to study the thermoelectric properties of this material. Yb rattles inside the cage-like structure of CoSb3 which effectively reduces the phonon mean free path and results in a lattice thermal conductivity comparable to those of costly nanostructured CoSb3 materials. A very low lattice thermal conductivity of 1.17 W m−1 K−1 at ∼550 K was obtained. A zT value of ∼0.7 was reported at 600 K due to the very low lattice thermal conductivity.

Abstract

Graphene reinforced alumina nano-composites

August 01, 2013

Graphene was prepared using liquid phase exfoliation and dispersed in an alumina matrix using an ultrasonication and powder processing route. Al2O3–graphene composites with up to 5 vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8 vol% graphene. However for higher graphene contents the improvement in fracture toughness was limited. Graphene changed the mechanism of crack propagation for the alumina matrix from inter-granular to trans-granular. The formation of an inter-connecting graphene network promoted easy fracture for concentration ⩾2 vol%. Elastic modulus remained nearly constant for up to 2 vol% and decreased significantly for 5 vol% due to the formation of the inter-connecting graphene network. Fracture toughness measured with the indentation and chevron notch methods were consistent up to 2 vol% and at 5 vol% the percolating network of graphene resulted in easy crack propagation with significant discrepancy between the results for the two methods.

Abstract

Dielectric relaxation, lattice dynamics and polarization mechanisms in Bi0.5Na0.5TiO3-based lead-free ceramics

July 2, 2013

In 0.95[0.94Bi0.5 Na 0.5TiO3-0.06BaTiO3]-0.05CaTiO3 ceramics, the temperature TS (dielectric permittivity shoulder at about 125 °C) represents a transition between two different thermally activated dielectric relaxation processes. Below TS, the approximately linear decrease of the permittivity with the logarithm of frequency was attributed to the presence of a dominant ferroelectric phase. Above TS, the permittivity shows a more complicated dependence of the frequency and Raman modes indicate a sudden increase in the spatial disorder of the material, which is ascribed to the presence of a nonpolar phase and to a loss of interaction between polar regions. From 30 to 150 °C, an increase in the maximum polarization with increasing temperature was related to three possible mechanisms: polarization extension favoured by the simultaneous presence of polar and non-polar phases; the occurrence of electric field-induced transitions from weakly polar relaxor to ferroelectric polar phase; and the enhanced polarizability of the crystal structure induced by the weakening of the Bi-O bond with increasing temperature. The occurrence of different electric field induced polarization processes with increasing temperature is supported by the presence of additional current peaks in the current-electric field loops.

Abstract

Ultra low thermal conductivity of disordered layered p-type bismuth telluride

January 24, 2013

Disordered layered p-type bismuth telluride was obtained by high pressure (1 GPa) and high strain deformation along the c-axis direction of commercially available single crystals. After initial deformation the p-type bismuth telluride flakes were subsequently fully densified by cold pressing (800 MPa at room temperature). As a result of the severe plastic deformation, the samples showed highly anisotropic electrical and thermal conductivities. In particular, the thermal conductivity measured along the pressing direction was as low as 0.34 W m−1 K−1, which is one of the lowest values reported for fully dense p-type bismuth telluride. The full set of thermoelectric properties of the disordered bismuth antimony telluride is critically discussed.

Abstract

Synthesis of multiwalled carbon nanotube-based infrared radiation detector​

August 27, 2012

Different morphologies (worm-like, straight and twisted) of multiwalled carbon nanotubes (MWCNT) are synthesized by carefully controlling the C2H2 flow rate. Unique mixed ferrite nanoparticles were synthesized and characterized as the appropriate catalyst for the growth of MWCNT. Scanning electron microscope and transmission electron microscope was extensively used to explore the morphological and structural properties. Electrical properties of MWCNT suggest semiconducting behavior. Infrared (IR) radiation detection is demonstrated using MWCNT and prototypes of room temperature and low temperature. IR radiation detectors have been made using different morphologies of MWCNT as the active sensor material.