Lead-free Bi0 Research Articles

Temperature Dependence on Ferroelectric, Energy Storage Density, and Electric Field-Induced Strain Response of Lead-Free Bi0.485(Na0.388K0.097)Ba0.021Sr0.009TiO3 Ceramics

In this research, Bi0.485(Na0.388K0.097)Ba0.021Sr0.009TiO3 or BNKBSrT ceramic sintered at various temperatures from 1100 °C to 1150 °C were investigated. The optimum density (5.80 g/cm3), mechanical (HK = 5.3 GPa, HV = 4.1 GPa, E = 62 GPa, and KIC = 1.35 MPa m1/2), dielectric (εr = 1525, tanδ = 0.0566), piezoelectric (d33 = 172 pC/N, g33 = 12 × 10−3 Vm/N), electric field-induced strain (Smax = 0.32%, d*33 = Smax/Emax = 640 pm/V, Q33 = 0.0340 m4/C2), and energy storage (W = 0.55 J/cm3, η = 67.2% @ 150 °C) were obtained for the ceramic sintered at 1125 °C.

Excellent electric field-induced strain with high electrostrictive and energy storage performance properties observed in lead-free Bi0.5(Na0.84K0.16)0.5TiO3-Ba(Nb0.01Ti0.99)O3-BiFeO3 ceramics

In this research, ceramics with a high electromechanical strain based on the composition (1-x)[0.97Bi0.5(Na0.84K0.16)0.5TiO3-0.03Ba(Ti0.99Nb0.01)O3]-xBiFeO3 or (1-x)[BNKT-0.03BNbT]-xBFO (with x = 0, 0.03, 0.05, 0.07, and 0.09 mol fraction) were investigated. All compositions were synthesized via a conventional solid-state mixed oxide method. The effects of the addition of BFO on the phase structure, dielectric, ferroelectric, and piezoelectric properties and electric field-induced strains behavior of BNKT-0.03BNbT ceramics were systematically studied in this work. X-ray diffraction (XRD) analysis revealed that all ceramics exhibited a pure perovskite structure. The coexistence of rhombohedral (R) and tetragonal (T) phases was observed for all compositions. The electric field-induced strain increased with increasing BFO content and attained a maximum value of Smax = 0.65% and d*33 = 1161 pm/V (@40 kV) for the composition x = 0.05 at room temperature (RT). The bipolar strain behavior exhibited minimal asymmetry and a low driving electric field. The composition also exhibited a high electrostrictive response (Q33 ∼ 0.039 m4/C2). In addition, the x = 0.09 composition showed excellent energy storage properties, with an energy storage density of 0.91 J/cm3 at 125 °C, high normalized energy storage density (∼0.14 μC/mm2), thermal stability (7% for 75-150 °C) and energy storage efficiency (η = 87% @125 °C). These results suggest that the (1-x)[BNKT-0.03BNbT]-xBFO ceramics are promising as lead-free piezoelectric material candidates, and are also suitable for both actuator and high temperature energy storage applications.

Synthesis and structural characterization of BNT–BT based ferroelectric thin films

Lead-free Bi0.506Na0.46Ba0.08–3x/2LaxTiO3 thin-films, with x = 0, 0.01, 0.02 and 0.03, have been synthesized via Sol-Gel process and the spin-coating technique. High quality thin-films have been obtained, revealing the formation of the perovskite structure with the minor Bi2Ti2O7 pyrochlore phase. A shift of the reflection in the 2θ range of 31–34° has been observed with the increase of the lanthanum dopant, suggesting the selective substitution of La3+ ions at the A-site of the BNT-BT perovskite structure.

Raman and 23Na solid-state NMR studies on the lead-free ferroelectrics Bi0.5(Na1-xKx)0.5TiO3 in the morphotropic phase boundary region

The local structural changes, due to the substitution of the smaller Na+ by the larger K+ ion, in the lead-free piezoelectric ceramic compositions Bi0.5(Na1-xKx)0.5TiO3 have been studied using Raman and 23Na solid-state NMR spectroscopy. Different close compositions in the solid solution series Bi0.5(Na1-xKx)0.5TiO3 (0 ≤ x ≤ 0.36, Δx = 0.02) are studied in the morphotropic phase boundary (MPB) region arising from the different crystal structures of the end members Bi0.5Na0.5TiO3 and Bi0.5K0.5TiO3. Close correlations between the Raman and NMR parameters with the performance parameters of the system have been observed, suggesting the role of the local structural changes in determining these parameters. Raman and 23Na NMR studies showed that the onset of the MPB region is at x = 0.16 and the MPB region corresponds to 0.16 ≤ x ≤ 0.24 where better performance parameters are observed.

Thickness-dependent microwave dielectric and nonlinear optical properties of Bi0.5Na0.5TiO3 thin films

Abstract Lead-free Bi0.5Na0.5TiO3 (BNT) films were grown on Pt(111)/Ti/SiO2/Si and quartz substrates for various thickness by using pulsed laser deposition technique. The impact of thickness on structural, microstructural, Raman-spectroscopy, microwave dielectric and nonlinear optical properties have been investigated in detail. X-ray diffraction pattern of BNT films deposited at 700 °C showed rhombohedral crystal symmetry with a small secondary phase of Bi4Ti3O12 and also revealed that the lattice parameters, lattice volume, strain and crystallite size were enhanced with film thickness. The film thickness, refractive index and optical bandgap have been determined by Swanepoel's envelope method and Tauc relation and are in the range of 208 nm–1300 nm, 3.79 eV–3.28 eV and 2.41–1.80, respectively. The third order nonlinear optical properties of films were estimated by Z-scan technique and the 208 nm film exhibited strong nonlinear refractive index (n2 = 4.62 × 10−6 cm2/W) and nonlinear absorption coefficient (β = 0.796 cm/W). The enhancement in microwave dielectric properties was observed with film thicknesses which were measured by using SPDR technique. The leakage current density exhibited the space charge limited current conduction behavior. The enhanced microwave dielectric and nonlinear optical properties of BNT films are promising for microwave tunable and nonlinear photonic device applications.

Effect of crystallization temperature on energy-storage density and efficiency of lead-free Bi0.5(Na0.8K0.2)0.5TiO3 thin films prepared by sol–gel method

Abstract Lead-free Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT) ferroelectric films were synthesized on Pt/Ti/SiO2/Si substrates via the chemical solution deposition. The influence of the crystallization temperature on the microstructures, the ferroelectric and energy-storage properties of the films was investigated in detail. The results showed that the BNKT films have reached the well crystallized state in the single-phase perovskite structure at 700 °C. Ferroelectric and energy-storage properties of the films were significantly enhanced by increasing the crystallization temperature. The remnant polarization (2Pr) and maximum polarization (2Pm) reached the highest values of 18.4 μC/cm2 and 61.2 μC/cm2, respectively, under an applied electric field of 300 kV/cm. Thanks to the strong enhancement in 2Pm and the large Pmax - Pr value, the highest energy-storage density (Jreco) and efficiency of 2.3 J/cm3 and 58.2%, respectively, were obtained. These results indicate that the BNKT films have application potentials in advanced capacitors.

Influence of Lead Free Bi0.5Na0.5TiO3 Substitution on the Structural and Electrical Properties of Ferromagnetic Metallic Phase La0.8Sr0.2MnO3

The preparation and properties of ferromagnetic La0.8Sr0.2MnO3, Bi0.5Na0.5TiO3 and their composites with formula (1 − x) La0.8Sr0.2MnO3 (LSMO) + x Bi0.5Na0.5TiO3 (BNT) (x = 0.125 and 0.25) are hereby reported. The X-ray diffraction data analysis and thereafter the Rietveld refinement of the XRD data revealed rhombohedral structure (R3c) for both the La0.8Sr0.2MnO3 and Bi0.5Na0.5TiO3. Raman scattering spectroscopy technique further verified the lattice structure. Microstructural studies using a sensitive field emission scanning electron microscopy (FESEM) and the verification elemental composition employing energy dispersive analysis of X-ray (EDAX) technique was carried out. The BNT compound and the composites were studied for dielectric behavior where the dielectric constant was found high and the loss value displayed decreasing trend with increasing concentration of the BNT phase in the composites. The pristine LSMO and the composites were probed for dc resistivity using four-probe technique. The LSMO displayed metallic character initially and later showed transition to insulating phase while as the composites exhibited semiconducting behavior.

Enhancement of the electrical-field-induced strain in sodium bismuth titanate-based lead-free ceramics by co-doping with Mn and Nb

Lead-free (Bi0.5Na0.5)0.935Ba0.065Ti1−x(Mn0.5Nb0.5)xO3 piezoelectric ceramics with excellent strain properties were prepared by conventional solid state reaction route. The effects of Mn/Nb on the structure and electrical properties of the ceramics have been investigated. The replacement of Ti by Mn/Nb induced a ferroelectric–ergodic relaxor phase transition with the degradations of the long-term ferroelectric properties. Near the ferroelectric–ergodic phase boundary, the electric-field-induced strain was significantly enhanced and reached the highest strain value of 0.37% (80 kV/cm, corresponds to a Smax/Emax = 463 pm/V) at 0.005 Mn/Nb addition. Temperature dependent measurements of polarization, permittivity, large signal piezoelectric constant and strain indicate that a reversible field-induced phase transformation from ergodic relaxor phase to ferroelectric is the main reason for the large strain response. Moreover, due to the reversible field-induced phase transition, the material with high-strain showed good fatigue resistance (up to 105 cycles), which can be considered as one of promising candidate used in actuator applications.

Effect of Sintering Temperature on Mechanical and Electrical Properties of Lead-Free Bi<sub>0.5</sub>(Na<sub>0.4</sub>K<sub>0.1</sub>)Ti<sub>0.98</sub>Zr<sub>0.02</sub>O<sub>3</sub> Piezoelectric Ceramics

Effect of sintering temperatures on phase formation, mechanical and electrical properties of lead-free Bi0.5(Na0.8K0.2)0.5Ti0.98Zr0.02O3 or BNKTZ piezoelectric ceramics were investigated. The BNKTZ ceramics were prepared via a conventional solid-state sintering technique under various sintering temperature range 1100-1150°C for 2 h. The phase formation and microstructure of the ceramics were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM) method, respectively. XRD analysis indicated that all samples exhibited a single perovskite structure and no secondary phase. SEM microscopy study revealed an increase in grain size with increasing sintering temperature. The maximum values of density and maximum dielectric constant of the ceramics sintered at 1125 °C were 5.79 g/cm3 and 3446, respectively. In addition, the ceramics sintered at 1125 °C showed highest mechanical properties (HV = 4.32 GPa, HK = 5.87 GPa, E = 143 GPa and KIC = 1.30 MPa.m1/2). The highest values of ferroelectric and piezoelectric were found at this sintering temperature.

Ultra-slim pinched polarization-electric field hysteresis loops and thermally stable electrostrains in lead-free sodium bismuth titanate-based solid solutions

Electric field-induced strains (electrostrains) with high amplitude, high thermal stability and ultra-low hysteresis are of great interest both in fundamental researches and industrial applications, such as high-precision displacement actuators. In this work, we present a strategy to fulfil these requirements via a relaxor antiferroelectric-like material that possesses intriguing antiferroelectric and relaxor characteristics simultaneously. Its antiferroelectric state would result in high electrostrains because of the electric-field induced antiferroelectric-to-ferroelectric phase transition, while the relaxor state would boost the temperature stability and ultra-slim hysteresis of electrostrains. According to this strategy, we obtained high electrostrains (>0.1% at 60 kV/cm) accompanying with good temperature stability (variation of strain < 20% from 30 °C to 120 °C) and ultra-low hysteresis (<7%) in lead-free (Bi0.5Na0.5)TiO3-x(Sr0.7Bi0.2)TiO3 (BNT-xSBT) solid solutions. The thermal stability of the electrostrains is directly correlated to the thermal stability of polarization based on electrostrictive effect. The electrostrictive coefficient in this system is discussed in detail. This work not only proves the effectiveness of this strategy, but also paves a way to search high electrostrains with good thermal stability and ultra-low hysteresis in unique BNT-based solid solutions.

Enhanced energy storage properties in Nb-modified Bi0.5Na0.5TiO3–SrTiO3 lead-free electroceramics

Lead-free (1 − x)Bi0.5Na0.5TiO3–xSrTiO3 + ywt%Nb2O5 electroceramics were synthesized using the conventional method to investigate the influences of SrTiO3 content and Nb2O5 doping on the material’s phase structure, microstructure, and energy storage properties. All the investigated compositions possessed a single perovskite phase structure of pseudocubic symmetry. The introduction of SrTiO3 and Nb2O5 led to a weakened relaxor characteristic and a decrease of Tm. The microstructure was refined by Nb2O5 doping, and the average grain size of the 0.6Bi0.5Na0.5TiO3–0.4SrTiO3 sample decreased drastically from ~ 10 to ~ 1 µm when 2.5 wt% Nb2O5 was added. Enhanced electrical breakdown and polarization were achieved for the samples with the refined microstructure. Among the various Bi0.5Na0.5TiO3-based ceramics, 0.6Bi0.5Na0.5TiO3–0.4SrTiO3 + 2.5 wt%Nb2O5 ceramic exhibited excellent energy-storage properties with thermal stability, in which large recoverable energy-storage density Wrec ~ 1.82 J/cc and efficiency η ~ 81% were both achieved. Our results indicate that this ceramic is a promising material for lead-free energy storage applications.

The improved pyroelectric properties, high depolarization temperature and excellent thermal stability in lead-free (1−x)(Bi0.5Na0.5)TiO3-xBa(Ni0.5Nb0.5)O3 ceramics

(1−x)(Bi0.5Na0.5)TiO3-xBa(Ni0.5Nb0.5)O3 (x = 0–0.04) lead-free ceramics (abbreviated as (1−x)BNT-xBNN) were synthesized by a conventional solid-state reaction method. The effect of BNN contents on phase structure, microstructure, ferroelectric, dielectric, pyroelectric properties and thermal stability of (1−x)BNT-xBNN ceramics were systematically investigated. The pyroelectric coefficient (p) at room temperature increased gradually from 3.01 × 10−8 C cm−2K−1 at x = 0 to 5.94 × 10−8 C cm−2K−1 at x = 0.04. The dielectric loss (tan δ) and dielectric constant (εr) decreased first with the small amount of BNN content, then increased slightly with further BNN addition, acquiring the minimum tan δ and εr at x = 0.02. When x = 0.02, the optimal over-all properties were observed with pyroelectric figure of merit (FOMs) of Fv = 3.82 × 10−2 m2 C−1 and figure of merit of Fd = 2.74 × 10−5 P a −1/2. More importantly, the depolarization temperature (Td) of 0.98BNT-0.02BNN ceramics is up to ∼195 °C which is higher than most other reported BNT-based ferroelectric ceramics. In addition, it can be exposed to temperature up to ∼145 °C with negligible deterioration of pyroelectric properties, showing excellent thermal stability. The overall properties reveal that 0.98BNT-0.02BNN ceramics is a promising pyroelectric material for infrared detectors.

Influence of Crystallization Temperature on Structural, Ferroelectric, and Ferromagnetic Properties of Lead-Free Bi0.5(Na0.8K0.2)0.5TiO3 Multiferroic Films

Lead-free Bi0.5(Na0.8K0.2)0.5TiO3 (abbreviated as BNKT) films have been synthesized via a sol-gel technique on Pt/Ti/SiO2/Si substrates, and the dependence of the physical properties in BNKT films were investigated as a function of the crystallization temperature. The BNKT films were annealed at different temperatures (600, 650, 700, and 750°C) for 60 min in the air. The results of this study showed that the optimal crystallization temperature is 700°C. At this, the BNKT films exhibited a single perovskite phase structure and high-dense surface. Besides, the remanent (Pr) and maximum (Pm) polarization reached their highest values of 9.2 µC/cm2 and 30.6 µC/cm2, respectively. All the films showed a weak ferromagnetic behavior with the maximum saturated magnetization (Ms) of 2.1 emu/cm3. These values are equivalent to the highest Pr and Pm values in previous reports on lead-free films.

Influence of Crystallization Time on Energy-Storage Density and Efficiency of Lead-Free Bi0.5(Na0.8K0.2)0.5TiO3 Thin Films

Lead-free Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT) ferroelectric films were synthesized on Pt/Ti/SiO2/Si substrates via chemical solution deposition. The influence of crystallization time on the microstructures and ferroelectric and energy-storage properties of the films was investigated in detail. The XRD analysis showed that the BNKT films have well crystallized corresponding to the single-phase perovskite structure at 700°C. Ferroelectric and energy-storage properties of the films were significantly enhanced by increasing crystallization time. With crystallization time of 60 min, the remnant polarization (Pr), maximum polarization (Pm), and Pm-Pr values reached the highest values of 7.9 µC/cm2, 28.9 µC/cm2, and 21.0 µC/cm2, respectively, under the electric field of 400 kV/cm. Thanks to the strong enhancement in Pm and the large Pma-Pr value, the highest recoverable energy-storage density (Jreco) gets the value of 2.9 J/cm3. The obtained results indicate that the BNKT films have application potentials in advanced capacitors.

The evolution of phase structure, dielectric, strain, and energy storage density of complex-ions (Sr1/3Nb2/3)4+ doped 0.82Bi0.5Na0.5TiO3-0.18Bi0.5K0.5TiO3 ceramics

Lead-free Bi0.5(Na0.82K0.18)0.5Ti1-x(Sr1/3Nb2/3)xO3 (abbreviated as BNKT-xSN, x = 0.02–0.045) ceramics were fabricated via the conventional ceramic process, and the (Sr1/3Nb2/3)4+ complex-ions were used to modify the phase transition and multifarious electrical responses of BNKT-xSN ceramics. The SN complex-ions absolutely incorporate into the lattice of BNKT matrix to form perovskite structure. The grain morphologies and size are almost unaffected by SN complex-ions. The phase transition concerning ferroelectric type transforming into ergodic relaxor happens with increasing SN content, and the corresponding critical point is x = 0.035. The phase transformation process results in the improvement of energy storage density (W = 0.754 J/cm3) at 80 kV/cm and a high bipolar strain (S = 0.25%) with small hysteresis. The dielectric constant at Tm peaks gradually decreases with increasing SN content, and the ferroelectric-relaxor transition temperature (TF-R) is depressed to room temperature. The evolution behaviors might facilitate our cognition about the mechanism between phase structure and multiple electrical properties of BNT-based ceramics.

Enhanced piezoelectric, electrocaloric and energy storage properties at high temperature in lead-free Bi0.5(Na1-xKx)0.5TiO3 ceramics

The piezoelectric, electrocaloric and energy storage properties were systemically investigated in lead-free Bi0.5(Na1-xKx)0.5TiO3 ceramics from room temperature to high temperature region. These ceramics can be poled completely to obtain large piezoelectric coefficient (104–153 pC/N) at low electric field of ~30 kV/cm. The piezoelectric property shows good thermal stability due to high depolarization temperature (Td). For BNKT20, a large low electric field-induced strain of 0.36% is obtained at 120 °C under 50 kV/cm, the corresponding normalized strain coefficient is up to 720 pm/V, which is larger than other BNT-based ceramics at high temperature region. The electrocaloric properties of these ceramics are studied via indirect and direct methods. Large EC value (~1.08 K) in BNKT20 ceramic is obtained at 50 kV/cm using indirect calculation. Above 100 °C, the dielectric energy storage density and efficiency of BNKT20 is still up to ~0.85 J/cm3 and 0.75, respectively. The BNKTx ceramics may become promising candidates in the fields of actuators, electrocaloric cooling and energy storage at high temperature region.

Effect of BaTiO3 particles with different shape on electrical properties of (Bi0.5Na0.5)TiO3 piezoceramics

Lead-free (Bi0.5Na0.5)TiO3-based materials are becoming a viable alternative to lead-based material in developing piezoelectricity strategies, due to its high piezoelectric performance as well as large strain response. In the present work, three kind of micro-sized, nano-sized, and plate-like BaTiO3 particles were employed to tailor the electrical properties of (Bi0.5Na0.5)TiO3 piezoceramics. The piezoelectric, strain properties, and phase transition temperature could be improved when employing BT nano-sized particles instead of BT micro-sized and plate-like particles as the raw material. Besides, addition of BT nano-sized particles could also yield a decreased critical poling filed of 22 kV/cm, compared with BT micro-sized (24 kV/cm) and plate-like (28 kV/cm) particles. Most intriguingly, an enhanced frequency-dependent unipolar strain in relative to other lead-free ceramics was also realized in the ceramics with BT nano-sized particles. In addition, the fatigue response of plate-like BT particles-modified ceramics was superior to that of nano- and micro-sized ones, which was mainly attributed to the lower defect density. This study demonstrates the superiority of uniform and nano-scale modifier for BNT-based ceramics, and also provides new opportunities to tailor and control piezoelectric activity for actuator applications.

Impact of multiple phases on ferroelectric and piezoelectric performances of BNKT–BZT ceramic

Lead-free (1−x)Bi0.5(Na0.8K0.2)0.5TiO3–xBaZr0.1Ti0.9O3 (with x = 0.0, 0.02, 0.025, 0.030, 0.035, 0.040) were prepared by conventional solid state reaction method. The effect of different amount of barium zirconate titanate (BZT) on structural, microstructural, ferroelectric and piezoelectric properties was examined experimentally through X-ray diffraction, scanning electron microscope and field-induced polarization and strain measurement. The coexistence of rhombohedral/tetragonal phase has been observed in pure BNKT. Now, with the addition of BZT contents in pure BNKT, the tetragonal phase has been observed to be dominant over rhombohedral, but still, mixed phase coexists in entire composition range. The temperature dependent dielectric constant and room temperature ferroelectric hysteresis show a strong dependence on their crystallographic phases. Piezoelectric properties reveal that the BNKT–0.025BZT ceramic has a large unipolar strain of 0.21% (Smax/Emax = 430 pm/V) at room temperature under an external field of 5 kV/mm. Addition of BZT reduces the remnant polarization and hysteresis loss, suggesting ferroelectric ceramics for future energy storage applications.

Raman Spectroscopy and Low Temperature Dielectric Properties of (Bi0.5Na0.5)TiO3 Ceramics

In the present study, lead-free Bi0.5Na0.5TiO3 (BNT) ceramics were prepared by solid state reaction route and the structural, microstructural and dielectric properties have been reported. The Rietveld refinement method and Raman spectroscopy were used to analyze the structural properties of BNT ceramics. The Rietveld refinement analysis of all the sintered samples revealed the rhombohedral crystal symmetry with R3c space group. The sintered (1100 ºC) BNT ceramics exhibited homogeneous grain distribution with an average grain size of 1.40 µm. The dielectric properties of BNT ceramics were investigated over the frequency range of 1 MHz-100 MHz in the temperature range of 133K – 653K. The obtained values of dielectric constant (ϵr), dielectric loss (tanδ) and Curie temperature (TC) of the ceramics are 525, 0.067 and 603K respectively, as measured at 1 MHz. The temperature dependent AC-conductivity (σac) of the BNT ceramics was analyzed using Arrhenius law and the activation energies (EA) were estimated in different regimes. The obtained EA values are in the range of 12.37 – 23.99 meV in the temperature regime 1 ( R1: 133K – 278K), whereas in regime 2 (R2: 320K – 603K) in the range 51.5 – 126.2 meV. The frequency (1 MHz – 100 MHz) dependent σac measured at different temperatures was analyzed using Jonscher’s power law and super linear power law (SLPL). The obtained results indicate the BNT ceramics have potential applications in high frequency tunable devices.

Dielectric and piezoelectric properties of 2-2 connectivity lead-free piezoelectric ceramic Bi0.5Na0.5TiO3/Portland cement composites

In this paper, 2-2 connectivity pattern lead-free piezoelectric ceramic Bi0.5Na0.5TiO3/Portland cement composites with ≈ 30–70 vol% of Bi0.5Na0.5TiO3 ceramic has been successfully fabricated. Dielectric and piezoelectric properties of the composites were investigated. The results showed that Bi0.5Na0.5TiO3 ceramic content has an effect on dielectric property of the composite, that is the dielectric constant value increased as Bi0.5Na0.5TiO3 ceramic content increased while the dielectric loss value reduced at higher Bi0.5Na0.5TiO3 ceramic content. The piezoelectric coefficient (d33) values of composites obtained were in the range of 48 pC/N to 73 pC/N. Moreover, composite with Bi0.5Na0.5TiO3 ceramic content of ≈ 30 vol% exhibited highest piezoelectric voltage coefficient (g33) value of 26.06 mV m/N. Therefore, these promising results showed that the 2–2 connectivity pattern lead-free piezoelectric ceramic Bi0.5Na0.5TiO3/Portland cement composite has the potential to be used as a sensor for structural health monitoring application.