Jamil Ur Rahman

Structure–property relationship in lead-free A- and B-site co-doped Bi0.5(Na0.84K0.16)0.5TiO3–SrTiO3 incipient piezoceramics

In this work, a phase diagram of A- and B-site co-substituted 0.96[{Bi0.5 (Na0.84K0.16)}1−x−yLixMgy(Ti1−zNbz)O3]–0.04SrTiO3 (abbreviated as LMN-doped BNKT–ST), where x, y and z = 0.00–0.030, was schematically constructed on the basis of crystal structure and electromechanical, dielectric and piezoelectric properties. The underlying mechanism of the compositionally-induced non-ergodic (NR) to ergodic relaxor (ER) phase transition was explored, and emphasis was given on relating the chemically-induced polymorphic structural phase transition to the dynamics of polar nano-regions (PNRs) and their random fields, which strongly affect the dielectric, ferroelectric, piezoelectric and field-induced strain properties of the investigated system. X-ray diffraction patterns revealed that LMN doping resulted in a transition from coexistence of rhombohedral and tetragonal phases to a pseudocubic phase. Both the dielectric constant and the ferroelectric–relaxor transition (TF–R ∼ 100 °C) temperature decreased with an increase in LMN content. The piezoelectric and ferroelectric responses of the BNKT–ST ceramics were significantly decreased by the addition of LMN. However, the destabilization of the piezoelectric and ferroelectric properties was accompanied by significant enhancements in the bipolar and unipolar strains. A large electric-field-induced strain (S = 0.28%) and a corresponding dynamic piezoelectric constant (Smax/Emax) of 560 pm V−1 were observed under the driving field of 5 kV mm−1 when 1.5 mol% LMN was substituted on respective sites. This significant strain enhancement at this composition, with LMN = 0.015, may be attributed to both the field-induced reversible structural transition and the compositionally-induced NR to ER phase transition. The composition- and temperature-dependence of the energy storage density (W) were studied to demonstrate the practicability of the LMN-doped BNKT–ST. It was found that the addition of LMN enhanced the difference between maximum polarization and remnant polarization, resulting in an improvement of the energy storage properties. For the composition with LMN = 0.020, a nearly temperature-invariant large recoverable energy density (W = 0.70 J cm−3) was achieved under 5.5 kV mm−1 in the wide temperature range of 100–150 °C. These properties indicate that the synthesized system might be a promising lead-free candidate for actuator and energy storage capacitor applications.

Ferroelectric and piezoelectric properties of SrZrO3-modified Bi0.5Na0.5TiO3 lead-free ceramics

Abstract The lead-free SrZrO 3 -modified Bi 0.5 Na 0.5 TiO 3 (BNT-SZ100 x , with x= 0–0.15) ceramics were fabricated by a conventional solid-state reaction method. The effects of SZ addition on BNT ceramics were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), ferroelectric and electric field-induced strain characterizations. XRD analysis revealed a pure perovskite phase without any traces of secondary phases. Ferroelectric and bipolar field induced-strain curves indicated a disruption of ferroelectric order upon SZ addition into BNT ceramics. A maximum value of remnant polarization (32 µC/cm 2 ) and piezoelectric constant (102 pC/N) was observed at 5% (mole fraction) of SZ. Maximum value of the electric field-induced strain ( S max =0.24%) corresponding to normalized strain ( S max / E max = d * 33 = 340 pm/V) was obtained at BNT-SZ9.

Mechanical properties of CNT reinforced hybrid functionally graded materials for bioimplants

Abstract The hybrid functionally graded materials (FGM) of hydroxyapatite (HA), stainless steel 316L (SS316L) and carbon nanotubes (CNT) were synthesized for biomedical implants. Three different types of FGM were produced by the combination of SS316L and CNT to reinforce HA in discrete layers of FGM. In the first type of FGM, concentration of SS316L was varied from 10% to 40% (mass fraction) with an increment of 10% to reinforce micro HA. In the second type of FGM, 0.5% (mass fraction) functionalized CNT was added by maintaining the rest of composition as that of the first type of FGM. In the third type of FGM, mixture of micro and nano HA (mass ratio1:1) was used, keeping rest of composition similar to the second type of FGM. All types of FGM were subjected to uniaxial compaction and sintered by pressureless sintering technique at similar compaction and sintering parameters. The results show that the densification is enhanced with the addition of CNT and nanocrystalline HA in the FGM. Hardness and fracture toughness increase in both FGM reinforced with CNT, but the increase of the hardness and fracture toughness are more pronounced in FGM with micro and nanocrystalline HA.

Structure and temperature dependent electrical properties of lead-free Bi0.5Na0.5TiO3- SrZrO3 ceramics

Lead-free SrZrO3-modified Bi0.5Na0.5TiO3 (BNT-SZ) ceramics were fabricated by a conventional solid state reaction method. X-ray diffraction analysis reveals a pure perovskite phase without any traces of secondary phases. Scanning electron microscopy images depicts dense grain morphology. The temperature dependences of the dielectric behavior was measured in the temperature range of 50-500 °C at 100 kHz. With the increase in SZ content, the dielectric constant (er) constantly decreased and the maximum dielectric constant temperature (Tm) shifted towards lower temperatures. In addition to this, ferroelectric hysteresis loops indicated a disruption of ferroelectric order and increase in the relaxor character of BNT ceramics with increase in SZ concentration. A maximum values of remnant polarization (32 μC/cm2) and piezoelectric constant (100 pC/N) were observed at SZ5 and SZ4, respectively.

Impedance Spectroscopy of Sodium Excess Ta-Modified (K0.5Na0.5)NbO3 Ceramics Prepared by Reactive Templated Grain Growth

Polycrystalline, sodium excess Ta-modified (K0.470Na0.545)(Nb0.55Ta0.45)O3 ceramics were synthesized by reactive templated grain growth method. The crystal structure and microstructure of the synthesized (K0.470Na0.545)(Nb0.55Ta0.45)O3 ceramics were examined with X-ray diffraction and scanning electron microscopy. Complex impedance spectroscopy measurements have been carried out to investigate electrical properties in wide frequency range (0.1 Hz – 1 MHz) at different temperatures. Contributions from grain boundaries and from the grain interiors can be distinguished in the complex impedance plots. Moreover, (K0.470Na0.545)(Nb0.55Ta0.45)O3 ceramics were found to exhibit negative temperature coefficient of resistance.

Thermoelectric Properties of n-Type Half-Heusler Compounds Synthesized by the Induction Melting Method

The n -type Hf0.25Zr0.25Ti0.5NiSn0.998Sb0.002 Half-Heusler (HH) alloy composition was prepared by using the induction melting method in addition to the mechanical grinding, annealing, and spark plasma sintering processes. Analysis of X-ray diffraction (XRD) results indicated the formation of a pure phase HH structured compound. The electrical and thermal properties at temperatures ranging from room temperature to 718 K were investigated. The electrical conductivity increased with increasing temperatures and demonstrated nondegenerate semiconducting behavior, and a large reduction in the thermal conductivity to the value of 2.5 W/mK at room temperature was observed. With the power factor and thermal conductivity, the dimensionless figure of merit was increased with temperature and measured at 0.94 at 718 K for the compound synthesized by the induction melting process.

Structural, Ferroelectric and Field-Induced Strain Response of Nb-Modified (Bi0.5Na0.5)TiO3-SrZrO3 Lead-Free Ceramics

Lead-free ceramics system of 0.96[Bi0.5Na0.5Ti(1−x)NbxO3]-0.04SrZrO3 BNT-SZ-Nb100x (with Nb100x = 0–4) were prepared by a solid-state reaction method. X-ray diffraction analysis revealed the formation of pure perovskite phase with x ≤ 0.03. The temperature dependence of dielectric curves showed that maximum dielectric constant temperature (Tm) shifted towards lower temperatures and the curves become more diffuse with increasing Nb doping. Ferroelectric curves indicated a disruption of ferroelectric order upon Nb addition. From Nb0 to Nb2 ceramics, maximum values for the field-induced strain (Smax) was enhanced from 0.09 to 0.22%. The corresponding normalized strain (Smax/Emax) was increased from 125 to 315 pm/V.

Ferroelectric and impedance response of lead-free (B0.5N0.5) TiO3-BaZrO3 piezoelectric ceramics

Lead-free piezoelectric (0.96B0.5N0.5TiO3)-0.04BaZrO3 (BNT-BZ4) was synthesized by using a solid-state reaction method. SEM micrograph shows dense microstructure. X-ray diffraction (XRD) indicated the formation of a BNB-BZ4 single phase having pseudocubic symmetry. A maximum value of remnant polarization (30μC/cm2) and piezoelectric constant (112 pC/N) was observed for BNT-BZ4 ceramic. The temperature dependences of the dielectric properties of BNT-BZ4 were investigated in the temperature range of 25-600°C at various frequencies (0.1 Hz-1 MHz). The maximum dielectric constant value (r) reaches a highest value of 4046 (at 10 kHz). The electrical properties were investigated by using complex impedance spectroscopy and provided better understanding of relaxation process.

Localized double phonon scattering and DOS induced thermoelectric enhancement of degenerate nonstoichiometric Li1−xNbO2 compounds

We report the synthesis and thermoelectric properties of a new p-type oxide thermoelectric material (Li1−xNbO2, with x = 0–0.6), in which Li-vacancies play a significant role in the enhancement of the thermoelectric performance. The electrical conductivity drastically increases due to Li-vacancies, resulting in an increase in the hole carrier concentration. A remarkable enhancement in the power factor is observed, which is comparable to well-known oxide thermoelectrics. The carrier concentration was not significantly dependent on the temperature, while the Hall mobility shows negative temperature-dependence. The Seebeck coefficient is linearly proportional to temperature, and the density of the state effective mass was estimated by using the Pisarenko relation. The thermal conductivity was substantially reduced by Li-vacancies and Li-vacancy induced localized double phonon scattering. Density functional theory (DFT) calculations reveal that the enhancement of the thermoelectric properties is mainly due to the increase in the electronic density of states (DOS) at the Fermi energy, which increases with hole concentration. All of the samples after high-temperature measurements are highly stable, which suggests that the well-synthesized nonstoichiometric Li1−xNbO2 could be a new promising candidate material for high temperature thermoelectric applications.

Structural and dielectric properties of La and Nb co-substituted Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO 3 -SrTiO 3 ceramics

In this article La and Nb co-substitution effect on the crystal structure, microstructure, ferroelectric and dielectric properties of 0.96[{Bi_0.5 (Na_0.84 K_0.16)_0.5}_1-xLa_x(Ti_1-y Nb_y)O₃]-0.04SrTiO₃ (BNKLTN-ST, where (x=y) = 0.00 - 0.020) ceramics has been investigated. The ceramics with x,y ≤ 0.010 have pure perovskite structure, while a secondary phase appears in the sample with x=y = 0.020. X-ray diffraction patterns revealed that La and Nb co-doping resulted in a transition from coexistent rhombohedral and tetragonal phases to pseudocubic symmetry. The co-doping of La and Nb slightly increased the average grain size. The maximum permittivity temperature (T_m) increased with increasing La and Nb contents, but the values of depolarization temperature (T_d) as well as maximum dielectric constant (ε_m) decrease gradually. La and Nb addition interrupted the polarization and reveals that the ferroelectric (FE) order of unmodified BNKT-ST ceramics lead to a declination of remnant polarization along with coercive field. The results shows that La and Nb co-doping into BNKT-ST ceramics induces a phase transition from relaxor FE to ergodic relaxor phase with a substantial deterioration of the long-range FE order.