Papers by Keyword: Tunability

Abstract: Heusler alloys are intermetallic compounds formed in two combinations: Full-Heusler (X₂YZ) and Half-Heusler (XYZ). X and Y can be any transition element, and Z belongs to the main group. This shows that there can be a huge variation in the combinations, leading to various properties and applications. We aimed at predicting the combination leading to shape memory properties using machine learning tools and then synthesizing the same. The predictions are done by training the tool with input data. We employed the lattice strain, valence electron concentration ratio, mechanical stress, difference in entropy, and saturation magnetization as input features. The correlation between the martensitic and austenitic temperature was evaluated in terms of regression metrics. The random forest and decision tree modeling were executed. Test scores were obtained using frequency ordering, PCA, linear regression, and correlation matrix to forecast magnetically controlled shape memory effect. The silhouette score matched the transition temperature at which the material showed shape memory behavior. Additionally, from 70% of the training data, a combination of Iron (Fe), Nickel (Ni), and Aluminum (Al) as Full Heusler alloys stimulated the algorithms in gaining the accuracy of predictive modeling by minimizing the error. Through DFT-based bandgap and density of states calculations, the Fe₂NiAl Heusler compound is hypothesized to behave as a half-metallic ferromagnet by considering the atomic number, the number of valence electrons, and the local magnetic moment. The experimental validation will be done along with magnetization studies, magneto-transport, and magneto-caloric measurements.

Abstract: Using 2×2 transfer matrix method, we numerically investigate a kind of structure in which two symmetric layers with defects are sandwiched in one-dimensional photonic crystals (PCs). The PCs are made of ordinary dielectrics and placed in the air. When a light beam is incident into PCs, the two resonant peaks can be achieved which constitute a couple of photonic channels. The transmittance of the resonant peaks can nearly reach up to 1. Furthermore, the tunability of the resonant peaks is discussed in detail, the results shows that the position of the resonant peak depending on the value of the incident angle. These properties can provide a theoretical basis for design of a new type of tunable double-channel photonic crystal filter.

Abstract: BaZr_xTi_1-xO₃-based (BZT) ceramics with Al₂O₃ and MgO addtives were prepared by the conventional solid state method with BaCO₃, ZrO₂, TiO₂, Al₂O₃ and MgO as raw materials and B₂O₃ and Li₂CO₃ as sintering additive. The morphologies were analysized by scanning electron microscopy (SEM). The dielectric constant and dielectric loss of ceramics were measured by LCR meter. The temperature dependences of dielectric constant were measured by high-low temperature incubator tank and LCR meter at 1 MHz and a temperature range-55 to 125 °C. The tunabilities were tested by C-T-V converter and LCR meter at 1 MHz at room temperature. The results show that with the increase of Zr/Ti, BZT ceramic dielectric constant increases, the loss increases, the Curie temperature moves to a lower temperature, and dielectric bias field coordination is relatively lower. The SEM images show that the grain size reaches about 1-2 μm when the sintering temperature is 1100 °C, and the addition of Al₂O₃ and MgO promote the grain growth and densification of the composite ceramics. The Curie peaks are broadened and depressed with the addition of Al₂O₃ and MgO. The tunability is improved to 9.59% under a DC electric field of 7.0 kV/cm after the addition of Al₂O₃. The dielectric constant and dielectric loss of BaZr_0.25Ti_0.75O₃-30wt%Al₂O₃ and BaZr_0.25Ti_0.75O₃-30wt%MgO are 586, 0.011 and 486, 0.003, respectively. The optimistic dielectric properties make it a promising candidate for the application of tunable capacitors and phase shifters.

Abstract: A structural simulation strategy was developed for characterizing the thin films of ferromagnetic - ferroelectric nanocomposites consisting of a spinel and a perovskite. The (1-y) Ni_0.5Zn_0.5Fe₂O₄ - y BaTiO₃ magnetoelectric ceramic, with (1-y) = 0.5 - 0.9 obtained by ferrite grains embedding into the BaTiO₃ matrix has found multiple applications for magnetoelectric sensors, four-state memories, anti-electro-magnetic interference (EMI) devices, etc. The tunability of the electromagnetic properties, considered until now as a matter of chemistry, strictly depending on the synthesizing process of the nanocomposite, can be reached by the 3D simulation methods, which reproduce the structure and simulate the interactions between constituents and with external fields. The thin film samples were simulated in the field of a horn antenna (4 ÷ 18 GHz), above the ferromagnetic resonance of the pure ferrite. The effective permittivity, respectively permeability were determined and their evolutions with different internal and external parameters (relative volume fractions, substitution ion radii in the ferrite, polarizing fields) were linked on the intrinsic characteristics of the constituent phases. The obtained surface plots indicates us the sets of optimal control parameters which have to be correlated in practice in order to obtain the desired value of an effective parameter, in a considered frequency subdomain. It appears that the system behaves optimal at a frequency around 8.9 GHz, where the values of the correlated control parameters is convenient for applications at microwave devices. A ferromagnetic - ferroelectric system tunability ranging from 13 to 37% was achieved, depending upon each structure characteristics.

Abstract: The operation of the radar technology is based on the mechanical movement of the antenna. To overcome the flaw of the mechanical movement an electronically tunable reflectarray antenna based on non-linear properties of Liquid Crystal materials has been introduced. This paper presents a detailed analysis of the tunability performance of different X-band reflectarray resonant elements printed on 1 mm thick grounded Liquid Crystal materials. Dynamic phase range and frequency tunabilty of rectangular, dipole and ring elements have been investigated by using CST computer model. Non-linear material properties have been used to develop an algorithm based on Method of Moment, for dynamic phase distribution of three resonant elements. It has been shown that the ring element offers a maximum dynamic phase range of 248° as compared to dipole and rectangular elements which offer 238° and 160° respectively. Moreover a maximum frequency tunabilty of 796 MHz, 784 MHz and 716 MHz can be achieved for rectangular, dipole and ring elements respectively with a dielectric anisotropy of 0.45. Waveguide simulator measurements of passive reflectarray unit cells demonstrate that rectangular element is observed to offer a minimum reflection loss of 1.6 dB as compared to dipole and ring elements which offer 3.3 dB and 3.6 dB respectively.

Abstract: BaxSr1-xTiO3 (x=0.4, 0.5, 0.6) ceramics were fabricated by the conventional solid-state reactions method. The temperature dependences of the dielectric constant and tunability were investigated under high DC electric field. It was found that the change of dielectric constant and tunability under the applied electric field were closely related to ferroelectric phase, phase transition region and paraelectric phase states. The Curie temperature (Tc) was gradually shifted to higher temperature and were broadened and depressed with increasing of DC electric field. The tunability dependence of temperature exhibits different trends in a wide temperature range and reaches a maximum value near the ferroelectric-paraelectric phase transition. These results may be helpful in understanding the mechanism of dielectric response under higher electric field.

Abstract: The Pb0.25Ba0.15Sr0.6TiO3 (PBST) thin films have been deposited on Pt/Ti/SiO2/Si substrates at different temperatures by radio frequency (rf) magnetron sputtering method. The microstructure, surface morphology, dielectric and tunable properties of PBST thin films were investigated as a function of deposition temperature. It’s found that the orientation of PBST thin films was adjusted by deposition temperature. The PBST thin film deposited at room temperature shows (100) preferred orientation and its dielectric constant and tunability are higher than that of PBST thin film deposited at 450 °C. Furthermore, the PBST thin film deposited at room temperature shows lower dielectric loss and leakage current, which makes it exhibit higher FOM of 49.47 for its appropriate tunability of 44.38% and low dielectric loss of 0.00897.

Abstract: Pb0.3Sr0.7TiO3 (PST) thin films have been prepared from precursor solution sonicated for different time on Pt/Ti/SiO2/Si substrates by sol-gel method. The effect of ultrasonic processing on the chemical bonding of PST precursor solution and the thermal evolution of PST dry gel are characterized with FT-IR and TG-DT technology. The structure, dielectric properties and leakage current density of PST thin films are investigated as functions of sol ultrasonic processing time. It’s found that ultrasonic processing brings about the partial sol structure rearrangement, the delay of PST formative temperature, the reduction of crystallization and dielectric constant, and the significant improvement of leakage current, dielectric loss and the figure of merit (FOM). The results suggest that the enhancement in the quality of thin films prepared with sol-gel method can be obtained by the suitable ultrasonic processing on precursor solutions. The PST thin film prepared from precursor solution sonicated for 8 h shows the highest FOM for its appropriate tunability of 57.36% and low dielectric loss of 0.0144.

Abstract: Bismuth zinc niobate titanium (Bi1.5Zn0.5 Nb0.5Ti1.5O7) (BZNT) thin films were deposited on PtTiSiO2Si substrates by radio frequency (rf) magnetron sputtering. The microstructure, surface morphology, stress, dielectric and tunable properties of thin films were investigated as a function of initial annealing temperature. It’s found that high initial annealing temperature increases the grain size, dielectric constant and tunability of BZNT films simultaneously and decreases the tensile stress in films. The BZNT thin film annealed from 500 °C to 700 °C shows the highest FOM value of 45.67 with the smallest dielectric loss and upper tunability.

Abstract: The effect of high energy electron irradiation on poly(vinlidene fluoride-trifluoroethylene) copolymer film with thickness about 180 nm has been studied. Both dielectric spectroscopy investigation and X-Ray Diffraction show that all-trans conformation of pre-irradiated films is transformed to trans-gauche conformation after irradiation. The relaxor behavior of irradiated sample obeys the modified Curie-Weiss and Vogel-Fulcher law. And the tunability is increased from 42% to 63% after irradiation.