Effect of Lead Content on the Structure and Electrical Properties of Pb((Zn1/3Nb2/3)0.5(Zr0.47Ti0.53)0.5)O3 Ceramics

The effects of lead content on the structure and electrical properties of Pb((Zn_1/3Nb_2/3)_0.5(Zr_0.47Ti_0.53)_0.5)O₃ ceramics were investigated. Specimens with various lead concentrations were prepared by the conventional oxide-mixing method. When the lead concentration was slightly less than the stoichiometric amount, a large amount of pyrochlore phase was formed along with the perovskite phase. On the other hand, excessive amounts of lead led to the formation of PbO on the surface of the specimen. These second phases were seriously detrimental to electromechanical properties. The highest piezoelectric properties were observed when an excess of 1 mol% lead was added. By optimizing the specimen composition, excellent piezoelectric and dielectric properties ( k _p= 0.7, d ₃₃= 490 pC/N, and ɛ_m= 15000) were obtained.     

Lanthanum-Modified (1 – x)(Bi0.8La0.2)(Ga0.05Fe0.95)O3·xPbTiO3 Crystalline Solutions: Novel Morphotropic Phase-Boundary Lead-Reduced Piezoelectrics

(1 – x )(Bi_0.8La_0.2)(Ga_0.05Fe_0.95)O₃· x PbTiO₃ (BLGF-PT) crystalline solutions have been fabricated by solid-state reactions. BLGF-PT has single perovskite phase structure with a rhombohedral–tetragonal (FE_r-FE_t) morphotropic phase boundary (MPB) at a PT content of x = 0.43. Lanthanum substitution has been found to increase the insulation resistance and decrease the coercive field down to 20 kV/cm, which results in significant improvements in dielectric and piezoelectric properties of BLGF-PT. The dielectric constant, loss tangent, Curie temperature, remnant polarization, piezoelectric d ₃₃ constant, and planar coupling factor of 1760, 0.05, 264°C, 33 μC/cm², 295 pC/N, and 0.36, respectively, have been achieved for BLFG-PT in the vicinity of the MPB. Compared with conventional Pb(Zr,Ti)O₃ (PZT) piezoelectric ceramics, the BLGF-PT is a competitive alternative piezoelectric material with decreased lead content.     

Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xAgSbO3 Lead-Free Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x AgSbO₃ lead-free piezoelectric ceramics were prepared by normal sintering. The effects of the AgSbO₃ on the phase structure and piezoelectric properties of the ceramics were systematically studied. These results show that the AgSbO₃-modified (K_0.50Na_0.50)NbO₃ lead-free piezoelectric ceramics form stable solution with orthorhombic structure, and the Curie temperature and the polymorphic phase transition of the ceramics decreased with increasing AgSbO₃. The result shows that the piezoelectric properties of the ceramics strongly depend on the AgSbO₃. The ceramics with x =0.05 possess optimum properties ( d ₃₃=192 pC/N, k _p=43%, T _c=348°C, T _o−t =145°C, ɛ_r∼632, and tan δ∼3.5%). These results indicate that the ceramic is a promising candidate material for lead-free piezoelectric ceramics.     

Sodium bismuth titanate-based lead-free RAINBOW piezoelectric devices

The piezoelectric properties of lead-free piezoelectric ceramics are normally lower than those of lead oxide-based ceramics. In order to enhance the electromechanical performance of lead-free piezoelectric ceramics, an asymmetric chemical reduction was applied to sodium bismuth titanate (NBT)-based piezoelectric ceramics. Similar to the lead-containing ceramics, a curvature structure can be induced by the reduction in the NBT-based materials and lead-free RAINBOW (reduced and internally biased oxide wafer) devices can be fabricated. A large displacement (approximately 17 μm) under an electric field of 900 V/mm and high piezoelectric sensitivity (>4000 pC/N) under a stress, which are related to the reduction induced curvature, can be measured in the NBT-based devices. The apparent piezoelectric response of the lead-free RAINBOW devices is comparable to that of Pb(Zr,Ti)O₃-based devices. We proposed that apart from the piezoelectric properties, flexoelectric effect could also be a contributing mechanism for the observed apparent piezoelectric response in RAINBOW devices.     

Cermets From Thermite Reactions

The process whereby the components of a thermite are pressed into a desired shape and ignited to form a cermet is described. In this process, the oxide of the cermet (Al₂O₃) is produced from the oxidation of powdered aluminum and the metallic phase is produced from the resulting reduction of its oxide. A third component (clay, Al₂O₃, or MgO) is added to act as a control agent. Methods of compacting and firing are described. A table of oxides reduced by aluminum using this process is presented. The formation of metallic silicides and borides as the metallic phase of a cermet from the appropriate silicates and borates, or metallic oxides with silica or boric acid, is discussed. As an example, ZrSi₂ is produced by the reduction of either ZrSiO₄ or ZrO₂ and SiO₂, and TiB₂ by the reduction of TiO₂ and B₂O₃. The following advantages may be obtained by this method: (1) inexpensive compositions, (2) low ignition temperatures (1800°F.), (3) high reaction temperatures (5000°F.), (4) short firing time (1 hour), and (5) controlled atmosphere unnecessary. A technique was developed whereby cermet test specimens could be prepared from the thermite reaction between aluminum and zirconium silicate. The tensile strength of the finished cermet is given at room temperature and at 2200°F. The apparatus used for determining these tensile strengths is described. Modulus of rupture data also are given. Other oxide thermites were added to the basic zirconium silicate thermite mixture. The effect of these thermite additions on the strength of the basic thermite is described.     

Characterization of the Piezoelectric Properties of Pb0.98Ba0.02(Mg1/3Nb2/3)O3–PbTiO3 Epitaxial Thin Films

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) (70/30) thin films were deposited by pulsed laser deposition using two growth strategies: adsorption controlled deposition from lead-rich targets (∼25–30 mass%) and lower-temperature deposition ( T _d≤600°C) from targets containing a small amount of excess lead oxide (≤3 mass %). The substrates used were (001) SrRuO₃/LaAlO₃. Typical remanent polarization values ranged between 12 and 14 μC/cm² for these films. The longitudinal piezoelectric coefficient ( d _33,f) was measured using in situ four-circle X-ray diffraction, and the transverse coefficient ( d _31,f or e _31,f) was measured using the wafer flexure method. d _33,f and e _31,f coefficients of ∼300–350 pm/V and ∼−11 C/m² were calculated, respectively. In general, the piezoelectric coefficients and aging rates were strongly asymmetric, suggesting the presence of a polarization bias. The large, extremely stable piezoelectric response that results from poling parallel to the preferred polarization direction is attractive for miniaturized sensors and actuators.     

Phase Transitions and Electrical Properties of (Na1−xKx)(Nb1−ySby)O3 Lead-Free Piezoelectric Ceramics With a MnO2 Sintering Aid

Lead-free piezoelectric ceramics (Na_{1− x} K _x )(Nb_{1− y} Sb _y )O₃+ z mol% MnO₂ have been prepared by a conventional solid-state sintering technique. Our results reveal that Sb⁵⁺ diffuses into the K_0.5Na_0.5NbO₃ lattices to form a solid solution with a single-phase orthorhombic perovskite structure. The partial substitution of Sb⁵⁺ for B-site ion Nb⁵⁺ decreases the paraelectric cubic-ferroelectric tetragonal phase transition ( T _c) and the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ( T _O–F), and retains strong ferroelectricity. A small amount of MnO₂ is enough to improve the densification of the ceramics. The co-effects of MnO₂ doping and Sb substitution lead to significant improvements in ferroelectric and piezoelectric properties. The ceramics with x =0.45–0.525, y =0.06–0.08, and z =0.5–1 exhibit excellent ferroelectric and piezoelectric properties: d ₃₃=163–204 pC/N, k _P=0.47–0.51, k _t=0.46–0.52, ɛ=640–1053, tan δ=1.3–3.0%, P _r=18.1–22.6 μC/cm², E _c=0.72–0.98 kV/mm, and T _C=269°–314°C.     

Fabrication of Porous PZT–PZN Piezoelectric Ceramics With High Hydrostatic Figure of Merits Using Camphene-Based Freeze Casting

Porous lead zirconate titanate–lead zinc niobate (PZT–PZN) piezoelectric ceramics with interconnected pore channels were fabricated using the camphene-based freeze-casting method. In this method, warm PZT–PZN/camphene slurries with various solid loadings (10, 15, 20, and 25 vol%) were prepared by ball milling at 60°C and then cast into molds at 20°C, resulting in the formation of solidified green bodies comprised of three-dimensionally interconnected camphene dendrite networks and concentrated ceramic particle walls. After the removal of the frozen camphene via sublimation, the samples were sintered at 1200°C for 2 h. All of the fabricated samples showed highly porous structures, consisting of fully dense PZT–PZN walls without defects, such as cracks or pores. As the initial solid loading was decreased from 25 to 10 vol%, the porosity was linearly increased from 50% to 82%. This increase in the porosity led to a reduction in the permittivity, a moderate decline in the d ₃₃ value, and a rapid decline in the d ₃₁ value, which endowed the porous samples with a high hydrostatic figure of merit (HFOM). The highest HFOM value of 35650 × 10⁻¹⁵ Pa⁻¹ was achieved for the sample with a porosity of 82%, as well as ɛ₃₃=284, d _h =298 pC/N, and g _h =118 × 10⁻³ V·(m·Pa)⁻¹.     

Effect of BiScO3 and LiNbO3 on the Piezoelectric Properties of (Na0.5K0.5)NbO3 Ceramics

Lead-free potassium sodium niobate-based piezoelectric ceramics (1− y )(Na_{0.5−0.5 x} K_{0.5−0.5 x} Li _x )NbO₃− y BiScO₃ (  y =0.01, x= 0–0.06) have been prepared by an ordinary sintering process. The XRD analysis showed that the structure changes from orthorhombic to tetragonal with the increase of x (at y =0.01, abbreviated as KNNBSL100 x ). At room temperature, the polymorphic phase transition from the orthorhombic to the tetragonal phase was identified at approximately 0.02≤ x ≤0.04. The piezoelectric and ferroelectric properties were significantly enhanced. The temperature dependences of the relative permittivity revealed that the Curie temperature was increased with the addition of LiNbO₃. These solid solution ceramics are promising as potential lead-free candidate materials.     

Ferroelectric and Piezoelectric Properties of Fine-Grained Na0.5K0.5NbO3 Lead-Free Piezoelectric Ceramics Prepared by Spark Plasma Sintering

Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO₃ and KNbO₃ have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na_0.5K_0.5NbO₃ ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920°C, the density of the Na_0.5K_0.5NbO₃ solid solution ceramics was raised to 4.47 g/cm³ (>99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na_0.5K_0.5NbO₃ ceramics with submicron grains. The crystal phase of the Na_0.5K_0.5NbO₃ has an orthorhombic structure. The Curie temperature is 395°C and the piezoelectric parameter ( d ₃₃) of the Na_0.5K_0.5NbO₃ ceramics reached 148 pC/N.     

Compositional Dependence of Piezoelectric Properties in NaxK1−xNbO3 Lead-Free Ceramics Prepared by Spark Plasma Sintering

Lead-free piezoelectric Na _x K_{1− x} NbO₃ ( x =20–80 mol%) ceramics were fabricated using spark plasma sintering at a low temperature (920°C). All the Na _x K_{1− x} NbO₃ ceramics showed a similar orthorhombic phase structure, while the corresponding lattice parameters decreased from the KNbO₃ side to the NaNbO₃ side with increasing Na content. A discontinuous change in lattice parameter close to composition of 60 mol% Na indicated the presence of a transitional area that is similar to the morphotropic phase boundary (MPB) in Na _x K_{1− x} NbO₃ ceramics. The sintered density of the Na _x K_{1− x} NbO₃ ceramics decreased with increasing Na content, from a relative density of 99% for the K-rich side to 92% for the Na-rich side. The piezoelectric constant d ₃₃ and planar mode electromechanical coupling coefficient k _p showed a maximum value of 148 pC/N and 38.9%, respectively, due to the similar MPB effects in the PZT system.     

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃–(Bi_0.5K_0.5)TiO₃ solid solution ceramics were successfully fabricated, exhibiting a continuous phase transition with changing x at room temperature from orthorhombic, to tetragonal, to cubic, and finally to tetragonal symmetries. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was found at 2–3 mol% (Bi_0.5K_0.5)TiO₃ (BKT), which brings about enhanced piezoelectric and electromechanical properties of piezoelectric constant d ₃₃=192 pC/N and planar electromechanical coupling coefficient k _p=45%. The MPB composition has a Curie temperature of 370°–380°C, comparable with that of the widely used PZT materials. These results demonstrate that this system is a promising lead-free piezoelectric candidate material.     

Structure of Aluminum Nitride/Aluminum and Aluminum Oxide/Aluminum Composites Produced by the Directed Oxidation of Aluminum

Unreinforced AlN/Al ceramic/metal composites were produced by the directed oxidation of molten Al alloys in nitrogen. The microstructures of these composites were compared with those of previously studied Al₂O₃/Al composites. In both composite systems the ceramic phase was interconnected and oriented in a columnar structure. The metallic phase showed no significant crystallographic orientation and appeared as both interconnected channels and isolated inclusions. The columns in the nitride system were found to be of micrometer size and to contain subgrains weakly defined by lattice defects, unlike the oxide system, where the columnar structure was shown to be of millimeter size and to contain well-defined micrometer-sized subgrains. Finer structures were obtained in both systems via the addition of Ni to the parent alloy.     

Dielectric and Ferroelectric Properties of Ceramics in the Pb(Zn 1/3Nb2/3)O3-BaTiO3-PbTiO3 System

The use of Pb(Zn_1/3Nb_2/3)O₃ ceramics is restricted by the formation of a pyrochlore phase detrimental to both dielectric and piezoelectric properties. Recently it has been shown that a 6 mol% addition of BaTiO₃ to PZN suppresses the formation of pyrochlore phase. Phase relations and dielectric properties of ceramics in the PZN-BT-PT system are reported here. Compositions with the perovskite structure, having high dielectric constant and low temperature coefficient of capacitance, have been identified.     

High Piezoelectric and Dielectric Properties of La-Doped 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb(ZrxTi1−x)O3 Ceramics Near Morphotropic Phase Boundary

La-doped 0.3Pb(Zn_1/3Nb_2/3)O₃–0.7Pb(Zr _x Ti_{1− x} )O₃ ( x =0.5–0.53) piezoelectric ceramics with pure perovskite phase were synthesized by a two-step hot-pressing route. The piezoelectric properties of various compositions near the morphotropic phase boundary (MPB) were systematically investigated. Not only was the exact MPB of this system determined via X-ray diffractometry analysis, but also the peak of piezoelectric properties was found near the MPB. The optimum piezoelectric properties of this series were observed in the specimen with Zr/Ti=51/49. The piezoelectric coefficient ( d ₃₃) and electromechanical coupling factor ( k _p) were 845 pC/N and 0.70, respectively, which have not been reported in this system so far. Large permittivity (ɛ_r=4088) and permittivity maximum (ɛ_m=29 500) were also obtained for the poled specimens. The temperatures ( T _max) of the permittivity maxima ranged from 206° to 213°C with various Zr/Ti ratios.     

Piezoelectricity and Phase Transitions of the Mixed-Layer Bismuth Titanate Niobate Bi7Ti4NbO21

The piezoelectiric properties of the bismuth titanate niobate Bi₇Ti₄NbO₂₁ have been studied. The two phase transitions at 677° and 856°C are anlayzed based on the measured piezoelectric properties. It is shown that the Bi₇Ti₇NbO₂₁ compound is a ferroelectric until 856°C. Bi₃TiNbO₉ clusters are shown to exist within Bi₇Ti₇NbO₂₁ granis. These clusters may be responsible for the small additional piezoelectric activity of the studied material at temperatures higher than 856°C.     

Microstructure, Piezoelectric, and Ferroelectric Properties of Bi2O3-Added (K0.5Na0.5)NbO3 Lead-Free Ceramics

Lead-free piezoelectric (K_0.5Na_0.5)NbO₃– x wt% Bi₂O₃ ceramics have been synthesized by an ordinary sintering technique. The addition of Bi₂O₃ increases the melting point of the system and improves the sintering temperature of (K_0.5Na_0.5)NbO₃ ceramics. All samples show a pure perovskite phase with a typical orthorhombic symmetry when the Bi₂O₃ content <0.7 wt%. The phase transition temperature of orthorhombic–tetragonal ( T _{O − T} ) and tetragonal–cubic ( T _C) slightly decreased when a small amount of Bi₂O₃ was added. The remnant polarization P _r increased and the coercive field E _c decreased with increasing addition of Bi₂O₃. The piezoelectric properties of (K_0.5Na_0.5)NbO₃ ceramics increased when a small amount of Bi₂O₃ was added. The optimum piezoelectric properties are d ₃₃=140 pC/N, k _p=0.46, Q _m=167, and T _C=410°C for (K_0.5Na_0.5)NbO₃–0.5 wt% Bi₂O₃ ceramics.     

Enhanced Piezoelectric Properties in Mn-Doped 0.98K0.5Na0.5NbO3–0.02BiScO3 Lead-Free Ceramics

Mn-doped 0.98K_0.5Na_0.5NbO₃–0.02BiScO₃ (0.98KNN–0.02BS) lead-free piezoelectric ceramics have been prepared by a conventional sintering technique and the effects of Mn doping on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of Mn can improve the densification of the ceramics effectively. Because of the high densification, fine grain, and Mn doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. Very good piezoelectric and dielectric properties of d ₃₃=288 pC/N, k _p=0.46, ɛ_r=1591, and T _C=328°C were obtained for the 0.98KNN–0.02BS ceramics doped with 0.8 mol% Mn. Therefore, the 0.98KNN–0.02BS ceramics containing a small amount of Mn are a good candidate material for lead-free piezoelectric ceramics.     

Electron Irradiation Induced Transformation of (Pb5Ca5)(VO4)6F2 Apatite to CaVO3 Perovskite

Crystallochemical changes of (Pb₅Ca₅)(VO₄)₆F₂ apatite under electron irradiation were examined by transmission electron microscopy. The apatite, a synthetic analog of vanadinite, was moderately stable towards a less intense 300-keV LaB₆ source, while it changed rapidly in structure when exposed to the higher flux of a 200-keV field emission gun. The electron beam induced transformation of vanadinite proceeds sequentially by (i) migration and loss of fluorine, (ii) lead volatilization and conversion to 2–5-nm platelets of a glaserite-type structure, and (iii) the reduction of V⁵⁺ to V⁴⁺ with the removal of lead and calcium oxide that leads to single-crystal CaVO₃ perovskite as the ultimate product. The phase transformations are interpreted based on the crystallographic relations among the CaVO₃ perovskite, the (Pb₅Ca₅)(VO₄)₆F₂ apatite and the glaserite-type structures, and compositional changes under electron irradiation.     

Fabrication and Electrical Properties of Lead Zirconate Titanate Thick Films

Thick films of lead zirconate titanate of the morphotropic phase boundary composition, Pb(Zr_0.52Ti_0.48)O₃, have been fabricated on platinum-buffered silicon using a modified sol–gel spin-coating technique. Crack-free films of 12-μm thickness can be uniformly deposited on 3-in.-diameter wafers with high yield and properties comparable to those of bulk ceramics. The thickness dependence of film structure and the dielectric, ferroelectric, and piezoelectric properties have been characterized over the thickness range of 1–12 μm. A strong (100) texture develops as film thickness increases above 5 μm; the films were marked by saturation values of longitudinal piezoelectric coefficient d ₃₃, 340 pC/N; remanent polarization, 27 μC/cm²; and dielectric permittivity, 1450. PZT films in this thickness range are extremely well-suited to application as electromechanical transduction media in silicon-based microelectromechanical systems (MEMS).