Mehdi Hejazi

From 1D Chain to 3D Network: A New Family of Inorganic–Organic Hybrid Semiconductors MO3(L)x (M = Mo, W; L = Organic Linker) Built on Perovskite-like Structure Modules

MO3 (M = Mo, W) or VI-VI binary compounds are important semiconducting oxides that show great promise for a variety of applications. In an effort to tune and enhance their properties in a systematic manner we have applied a designing strategy to deliberately introduce organic linker molecules in these perovskite-like crystal lattices. This approach has led to a wealth of new hybrid structures built on one-dimensional (1D) and two-dimensional (2D) VI-VI modules. The hybrid semiconductors exhibit a number of greatly improved properties and new functionality, including broad band gap tunability, negative thermal expansion, largely reduced thermal conductivity, and significantly enhanced dielectric constant compared to their MO3 parent phases.

Lead-free piezoelectric materials and ultrasonic transducers for medical imaging

Piezoelectric materials have been vastly used in ultrasonic transducers for medical imaging. In this paper, firstly, the most promising lead-free compositions with perovskite structure for medical imaging applications have been reviewed. The electromechanical properties of various lead-free ceramics, composites, and single crystals based on barium titanate, bismuth sodium titanate, potassium sodium niobate, and lithium niobate are presented. Then, fundamental principles and design considerations of ultrasonic transducers are briefly described. Finally, recent developments in lead-free ultrasonic probes are discussed and their acoustic performance is compared to lead-based transducers. Focused transducers with different beam focusing methods such as lens focusing and mechanical shaping are explained. Additionally, acoustic characteristics of lead-free probes including the pulse-echo results as well as their imaging capabilities for various applications such as phantom imaging, in vitro intravascular ultrasound imaging of swine aorta, and in vivo or ex vivo imaging of human eyes and skin are reviewed.

Fabrication and evaluation of a single-element Bi 0.5 Na 0.5 TiO 3 -based ultrasonic transducer

This paper discusses the fabrication and characterization of a single-element ultrasonic transducer with a lead-free piezoelectric active element. A piezoelectric ceramic with composition of 0.88Bi_0.5Na_0.5TiO₃–0.08Bi_0.5K_0.5TiO₃– 0.04Bi_0.5Li_0.5TiO₃ was chosen as the active element of the transducer. This composition exhibited a thickness coupling coefficient (kt) of 0.45, a dielectric constant of 440 (at 1 kHz), and a longitudinal piezoelectric coefficient (d₃₃) of 84 pC?N^–1. To make the transducer, the ceramic was sandwiched between an epoxy–tungsten backing layer and a silver epoxy matching layer. An epoxy lens was also incorporated into the transducer¿s design to focus the ultrasound beam. The focused transducer with a center frequency of about 23 MHz demonstrated a –6-dB bandwidth of 55% and an insertion loss of –32 dB; the –20-dB pulsed length was measured to be 150 ns. A phantom made of copper wires (30 μm in diameter) was utilized to investigate the imaging capability of the transducer. The results indicated that the fabricated transducer, with a lateral resolution of 260 μm and a relatively high depolarization temperature, could be considered as a candidate for replacement of lead-based ultrasonic transducers.

Lead-Free KNN-Based Piezoelectric Materials

Lead-free piezoelectric materials have recently been given vast attention due to environmental issues and the toxicity of their lead-based counterparts. One of the potential lead-free candidates is the xKNbO3–(1−x)NaNbO3 binary system. The K0.5N0.5NbO3 composition in particular exhibits relatively high Curie temperature and the highest electromechaical and ferroelectric response among the possible K x Na1−x NbO3 compositions. The present chapter covers some fundamental issues regarding the crystal chemistry, material processing, and dielectric and ferroelectric behavior of KNN-based piezoelectrics. The topics discussed in this context fall into four categories: KNN-based polycrystalline ceramics, textured ceramics, single crystals, and thin films. The recent research activities concerning the most promising compositions, processing techniques, and some of the challenges encountered in obtaining high performance Pb-free piezoelectric materials have been reviewed for each category.

Lead-free (Bi 0.5 Na 0.5 )TiO 3 -based thin films by the pulsed laser deposition process

We have studied the effect of deposition parameters on the microstructure, crystallinity, and ferroelectric properties of 0.88(Bi_0.5Na_0.5)TiO₃-0.08(Bi_0.5K_0.5)TiO₃-0.04BaTiO₃ thin films grown on SrRuO₃-coated SrTiO₃ substrates by pulsed laser deposition. The parameters studied were the repetition rates, substrate temperatures, oxygen pressures, and laser energies. It was realized that the films prepared at 800°C, 10 Hz, 400 mtorr, and 1.2 J·cm^-2 exhibited the highest ferroelectric properties. The measured remanent polarization, dielectric constant at 1 kHz, and coercive field for this film were about 30 μC·cm^-2, 645, and 85 kV·cm^-1, respectively. Increasing the oxygen pressure during deposition from 200 to 400 mtorr improved the crystallinity, microstructure, dielectric constant, and polarization of the films. The leakage current and dielectric loss were suppressed at 400 mtorr because of the lower concentration of oxygen vacancies and disappearing pinholes and surface undulations in the film deposited at this pressure.

Electromechanical properties of acceptor-doped lead-free piezoelectrics

In this paper, the processing and electrocmechanical of Mn-doped BNT-based ceramics prepared by mixed oxide route have been studied. Different amounts of Mn (0.01, 0.014, 0.015, 0.016, 0.017, 0.02, and 0.022) were doped to the 0.88[Bi_0.5Na_0.5TiO₃]-0.08[Bi_0.5K_0.5TiO₃]-0.04[Bi_0.5Li_0.5TiO₃] ceramics. Samples were sintered at different temperatures (1075-1150 °C) to achieve the highest density and mechanical quality factor. Mn-doping resulted in a considerable enhancement of Q_m in both planar and thickness vibration modes. In 1.5 mol.% Mn-doped ceramics sintered at 1100 °C, a planar Q_m of about 970 and tanδ of 0.88% were obtained. Mn-doping also resulted in decreasing coupling coefficients, piezoelectric charge coefficient, and dielectric constant.

Fabrication and evaluation of a single element lead free-based ultrasonic transducer

This paper discusses the fabrication and characterization of a single element ultrasonic transducer with a lead-free piezoelectric active element. A piezoelectric ceramic with composition of 0.88Bi_0.5Na_0.5TiO₃-0.08Bi_0.5K_0.5TiO₃-0.04Bi_0.5Li_0.5TiO₃ was chosen as the active element of the transducer. This composition exhibited a thickness coupling coefficient (k_t) of 0.45, a dielectric constant of 440 (at 1 kHz), and a longitudinal piezoelectric coefficient (d₃₃) of 84 pC.N^-1. To make the transducer, the ceramic was sandwiched between an epoxy-tungsten backing layer and a silver epoxy matching layer. An epoxy lens was also incorporated into the transducers design to focus the ultrasound beam. The focused transducer with a center frequency of about 23 MHz demonstrated a -6 dB bandwidth of 55% and an insertion loss of -32 dB, while the -20dB pulsed length was measured to be 150 ns. A phantom made of copper wires (30 μm in diameter) was utilized to investigate the imaging capability of the transducer. The results indicated that the fabricated transducer with a lateral resolution of 260 μm and a relatively high depolarization temperature could be considered as a candidate for replacement of lead-based ultrasonic transducers.

Ferroelectric and dielectric properties of bismuth ferrite based thin films by Pulsed Laser Deposition

BiFeO₃ (BFO) has stimulated a great interest in recent years for its room temperature multiferroic behavior with very high remnant polarization. However, the leakage current of BFO films is very high. To lower the leakage current, we have developed thin films with following target compositions on SrRuO₃ buffered on SrTiO₃ substrate by Pulsed Laser Deposition (PLD): (A) BiFeO₃, (B) 0.88Bi_0.5Na_0.5TiO₃-0.08Bi_0.5K_0.5TiO₃-0.04BaTiO₃, (C) 0.6BiFeO₃-0.4(Bi_0.5K_0.5TiO₃) The phase, growth orientation, microstructural characterization and electrical properties were investigated as a function of deposition parameters. The epitaxial bi-layered 300nm BNT-BKT-BT/BFO thin films (150nm each) exhibited ferroelectric behavior as: 2P_r = 44.0 μC.cm^-2, 2E_c = 200 kV.cm^-1 and K = 140. Thin films with a composition of 0.6BFO-0.4BKT have also been deposited with different oxygen pressures of 300-500 mTorr. Preliminary results of 0.6BFO-0.4BKT films show that the leakage current can be suppressed by about 4 orders of magnitude which in turn improves the ferroelectric and dielectric properties of the films. It is observed that the remnant polarization increases from 8 to 19 μC.cm^-2 as the oxygen pressure is changed from 500 to 300mtorr.

Lead-free BNT-BKT-BT thin films by pulsed laser deposition process

We have studied the effect of deposition parameters on the microstructure and ferroelectric properties of pulsed laser deposited 0.88(Bi<inf>0.5</inf>Na<inf>0.5</inf>)TiO<inf>3</inf>-0.08(Bi<inf>0.5</inf>K<inf>0.5</inf>)TiO<inf>3</inf>-0.04BaTiO<inf>3</inf> thin films on SrRuO<inf>3</inf> coated SrTiO<inf>3</inf> substrates. The films were deposited at different repletion rates, substrate temperatures, and laser energies. It has been demonstrated that the film made at 800 °C, 10 Hz, 400 mtorr and 1.2 J.cm⁻² shows the best ferroelectric properties among the investigated films. The remnant polarization for this film was measured to be about 30 µC.cm⁻². We have also discussed the electrical conduction mechanisms in the temperature range of 200–350 K. At low electric fields, the leakage mechanism is governed by the ohmic conduction. With increasing the applied filed, an abrupt increase in the leakage current was observed. This was attributed to a trap-filling process by the injected carries. At sufficiently high electric fields, the leakage current obeyed the Childs trap-free square law suggesting the space charge limited current was the dominant mechanism.

Dielectric and ferroelectric properties of BNT-based thin films by pulsed laser deposition

We have studied the effect of laser repetition rate on the microstructure, crystallographic orientation, dielectric and ferroelectric properties of pulsed laser deposited 0.94(Bi<inf>0.5</inf>Na<inf>0.5</inf>)TiO<inf>3</inf>-0.04(Bi<inf>0.5</inf>K<inf>0.5</inf>)TiO<inf>3</inf>-0.02BaTiO<inf>3</inf> thin films on SrRuO<inf>3</inf> coated SrTiO<inf>3</inf> substrates. The films were deposited at different repletion rates of 2 Hz, 4 Hz and 8 Hz, while the other processing parameters such as the substrate temperature and oxygen partial pressure were kept constant. It has been demonstrated that the film made at 4 Hz has been epitaxially grown on the substrate with a smooth and flawless surface and shows the best ferroelectric properties among the investigated films. The remnant polarization and dielectric constant for this film were measured to be 13.6 µC.cm⁻² and 425 (at 1 MHz). The leakage current density of the optimized film is 1.3>10⁻⁵ A.cm⁻² at 320 kV.cm⁻¹, which is about 3 orders of magnitude lower than those observed in the films prepared at repetition rates of 2 Hz and 8 Hz. The remnant polarizations for the films deposited at 2 Hz and 8 Hz were found to be 8.7 µC.cm⁻² and 7.6 µC.cm⁻², respectively.