Demonstration of Copper Co‐Fired (Na,K)NbO3 Multilayer Structures for Piezoelectric Applications

A Li and Ta modified (Na, K)NbO₃ piezoelectric ceramic has been successfully co‐fired with inner copper electrodes in a reduced atmosphere. Highly dense NKN ceramics (95% relative density, 4.64 g/cm³) were obtained by sintering the samples in a low oxygen partial pressure (low pO₂) atmosphere at 1050°C. The poly(propylene carbonate) binder system was used to permit a clean burnout at low temperature in N₂ atmosphere, and also prevent the electrode copper particles from undergoing any oxidation. No interdiffusion of copper, chemical reactions, and/or carbon residues were observed in the grains, grain boundaries, or at the electrode–ceramic interface of the co‐fired samples from a detailed transmission electron microscopy (TEM) analysis. Dielectric and piezoelectric properties were characterized from those co‐fired prototyped samples. The samples displayed high relative dielectric permittivity above 800, with low dielectric loss about 3.6%. A normalized strain coefficient (max. strain/max. electric field) of = 220 pm/V was obtained under unipolar converse electromechanical measurement at 20 kV/cm. This paper presents the feasibility of co‐firing a Cu inner electrode with NKN ceramics toward multilayer lead‐free piezoelectric applications, providing an engineering route to narrow the performance differences between soft lead‐based piezoelectrics and lead‐free materials.     

Structural and piezoelectric properties of <001> textured PZT‐PZNN piezoelectric ceramics

Rhombohedral 0.69Pb(Zr_0.47Ti_0.53)‐0.31Pb(Zn_0.6Ni_0.4)NbO₃ (PZT‐PZNN) ceramics were textured using 10.0 vol. % BaTiO₃ (BT) platelets along the <001> direction at 950°C with a high Lotgering factor of 95.3%. BT platelets did not react with the PZT‐PZNN ceramics, and the textured PZT‐PZNN ceramic had a tetragonal structure. The PZT‐PZNN ceramics exhibited a strain of 0.174% with a piezoelectric strain constant (d*₃₃) of 580 pC/N at 3.0 kV/mm. The textured PZT‐PZNN ceramic showed an increased strain of 0.276% and d*₃₃ of 920 pC/N at 3.0 kV/mm, which can be explained by the domain rotation. However, the d₃₃ values of the textured specimens are smaller than those of the untextured specimens because of the small remanent polarization and relative dielectric constant of BT platelets. The textured PZT‐PZNN ceramic synthesized in this work can be used for piezoelectric multilayer actuators because of its large strain and low sintering temperature.     

Improved thermal stability of the piezoelectric properties of (Li,Ag)‐co‐modified (K,Na) NbO3‐based ceramics prepared by spark plasma sintering

High‐performance lead‐free piezoelectric ceramics 0.94(K_0.45Na_0.55)_1?xLi_x(Nb_0.85Ta_0.15)O₃–0.06AgNbO₃ (KNNLTAg‐x) were successfully prepared by spark plasma sintering technique. The doping effect of Li on the structural and electrical properties of KNNLTAg‐x (x=0, 0.02, 0.04, 0.06, 0.08 and 0.10) ceramics was studied. The lattice structure, ferroelectric and piezoelectric properties of the KNLNTAg‐x ceramics are highly dependent on the Li doping level. In particular, the Li dopant has a great impact on both Curie temperature T_c and orthorhombic‐tetragonal transition temperature T_O‐T. The 4% Li‐doped sample exhibited relatively high T_O‐T of 95°C, leading to a stable dynamic piezoelectric coefficient (d₃₃*) of 220‐240 pm/V in a broad temperature range from 25°C to 105°C. Additionally, the 2% Li‐doped sample shows a high d₃₃* of 320 pm/V at 135°C, suggesting its great potential for high‐temperature applications.     

Low‐Temperature Co‐Fired Ceramic Substrates for High‐Performance Strain Gauges

Recent advances in the development of high gauge factor thin films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion (9.4 ppm/K) and a low modulus of elasticity (82 GPa) for the application as support material for thin‐film sensors. In the first part, constantan foil strain gauges were fabricated from this material by tape casting, pressure‐assisted sintering, and subsequent lamination of the metal foil on the planar ceramic substrates. The accuracy of the assembled load cells corresponds to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the low‐temperature co‐fired ceramic (LTCC) substrates for fabrication of accurate strain gauges. In the second part, to facilitate the deposition of thin‐film sensor structures to the LTCC substrates, pressure‐assisted sintering step is modified using smooth setters instead of release tapes, which resulted in fabrication of substrates with low average surface roughness of 50 nm. Titanium thin films deposited on these substrates as test coatings exhibited low surface resistances of 850 Ω comparable to thin films on commercial alumina thin‐film substrates with 920 Ω. The presented material design and advances in manufacturing technology are important to promote the development of high‐performance thin‐film strain gauges.     

Enhanced Ferroelectric Phase Stability and High Temperature Piezoelectricity in PN Ceramics via Multisite Co‐Doping

The tungsten bronze‐type lead metaniobate (PbNb₂O₆, PN) is a promising material for high‐temperature piezoelectric devices, while its application is limited by the difficulty in fabrication. In this study, the microstructure and electrical properties of Ca‐doped PN and Ca, Mn‐co‐doped PN ceramics sintered at different temperatures were investigated. Doping promoted the formation of the originally metastable ferroelectric orthorhombic phase. This might be partly attributed to the increased lattice distortion of the orthorhombic phase in the doped samples compared to that reported for pure PN. However, in single A‐site Ca‐doped PN the ferroelectric orthorhombic structure showed a low stability and started to transform to paraelectric phase far below 300°C, resulting in unstable high temperature piezoelectric properties. Interestingly, such a phase transition was completely depressed by A‐site Ca and multisite Mn‐co‐doping, which also improved the piezoelectric performance (d₃₃ = 71 pC/N) and thermal stability in both structure and piezoelectricity. The better stability and performance of the co‐doped samples were explained by the improved sintering behavior and poling efficiency as well as its ability to occupy different sites in the TTB lattice.     

Low‐Temperature Co‐Fired Ceramics System for Light Absorbance Measurement

The article describes technology of the low‐temperature co‐fired ceramics (LTCC) structure that enables light absorbance measurements of liquid sample. The manufactured ceramic structure contains buried microfluidic channels. The structure consists of two co‐fired glass windows that separate the light source and detector from the test solution. A construction of an electronic measurement system is described as well. The signal from three light‐emitting diodes (LED)s — red, green, and blue — can be used in the absorbance measurements. The light intensity is measured by the TCS 3414CS (TAOS, Plano, TX) color detector. Optical properties of the fabricated microfluidic LTCC system is investigated with several concentrations of potassium permanganate (KMnO₄) in water solution. The system can be applied in microbiology for constant monitoring of bacterial growth.     

Fine‐Scaled Piezoelectric Ceramic/Polymer 2‐2 Composites for High‐Frequency Transducer

A novel technique was utilized to fabricate fine‐scaled piezoelectric ceramic/polymer 2‐2 composites for high‐frequency ultrasonic transducers. Lead zirconate titanate (PZT) was used as raw material. Tape‐casted acetylene black tapes were used to define kerfs after sintering. A one‐directional supporter was utilized to avoid distortion of PZT elements. PZT elements with 20 ± 2 μm width exhibited good consistency in longitudinal direction. A resonant method was utilized to evaluate the piezoelectric and dielectric properties of the composites. A 72‐μm‐thick composite with an aspect ratio of ~3.6 exhibited a k_t of 0.61 with satisfied piezoelectric and dielectric properties. A prototype high‐frequency ultrasonic transducer was fabricated and evaluated by an underwater pulse‐echo test. The center frequency was found to be 23.75 MHz, with ?6 dB bandwidth of 5.5 MHz.     

Low‐Temperature Sintering of Bi0.5(Na,K)0.5TiO3 for Multilayer Ceramic Actuators

We investigated the influence of CuO amount (0.5–3.0 mol%), sintering temperature (900°C–1000°C), and sintering time (2–6 h) on the low‐temperature sintering behavior of CuO‐added Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (BNKT22) ceramics. Normalized strain (S_max/E_max), piezoelectric coefficient (d₃₃), and remanent polarization (P_r) of 1.0 mol% CuO‐added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 μC/cm², respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO‐added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O₃‐based device in the frequency range 0.66–20 kHz. This result highlights the feasibility of using low‐cost multilayer ceramic devices made of lead‐free BNKT‐based piezoelectric materials in sound speaker devices.     

Further Enhancing Piezoelectric Properties by Adding MnO2 in AgSbO3‐Modified (Li,K,Na)(Nb,Ta)O3 Lead‐Free Piezoceramics

This work investigated the effect of MnO₂ addition on the phase structure, microstructure, and electrical properties of AgSbO₃‐modified (Li,K,Na)(Nb,Ta)O₃ (abbreviated as LKNNT‐AS) lead‐free piezoelectric ceramics with an optimized composition endowed with a state of two‐phase coexistence. A small amount (0.1 wt%) of MnO₂ can significantly further enhance the piezoelectric property of LKNNT‐AS ceramics, whose piezoelectric constant d₃₃ and converse piezoelectric coefficient d₃₃* as well as planar electromechanical coupling factor k_p reach 363 pC/N, 543 pm/V, and 0.49, respectively. The temperature stability of piezoelectricity in MnO₂‐modified LKNNT‐AS samples also improved, which is associated with the more uniform and better thermally stable ferroelectric domains that were revealed by piezoresponse force microscopy.     

Simultaneously enhanced piezoelectric response and piezoelectric voltage coefficient in textured KNN‐based ceramics

<001> ‐textured 0.99(K_0.49Na_0.49Li_0.02)(Nb_0.97‐xSb_0.03Ta_x)O₃‐0.01CaZrO₃ [abbreviated as 0.99KNLN_0.97‐xST_x‐0.01CZ, x = 0.03, 0.07, 0.10, 0.15, 0.20, 0.25] ceramics were prepared by templated grain growth (TGG) method and a two‐step sintering process. Giant longitudinal piezoelectric coefficient d₃₃ (391 pC/N) and piezoelectric strain coefficient d₃₃* (630 pm/V under an AC E‐field of 20 kV/cm) can be obtained in the textured ceramics with x = 0.25. All textured ceramics display superior k_p (>54%) and g₃₃ (>23 × 10^?3 Vm/N) which are in an order of magnitude with PZT ceramics. The maximum value of k_p (~63.3%) obtained in textured ceramics with x = 0.15 is higher than that of famous textured LF4 ceramics. Excellent comprehensive properties suggest that <001> ‐textured 0.99KNLN_0.97‐xST_x‐0.01CZ ceramics are promising candidates in the field of lead‐free piezoelectric materials.     

Piezoelectric Properties of a Pioneering 3‐1 Type PZT/Epoxy Composites Based on Freeze‐Casting Processing

Lead zirconate titanate (PbZr_1 ? xTi_xO₃, PZT)/epoxy composites with one‐ dimensional epoxy in PZT matrix (called 3‐1 type piezocomposites) have been fabricated by tert‐butyl alcohol (TBA)‐based directional freeze casting of PZT matrix and afterward infiltration of epoxy. The composites with PZT volume fraction ranging from 0.36 to 0.69 were obtained by adjusting initial solid loading in freeze‐casting slurry. The effect of poling voltage on piezoelectric properties of the composites was studied for various volume fraction of PZT phase. With the increasing of PZT volume fraction, relative permittivity (ε_r) increased linearly and piezoelectric coefficient (d₃₃ and d₃₁) increased step by step. The resultant composites with 0.57 PZT volume fraction possessed the highest hydrostatic piezoelectric strain coefficient (d_h) value (184 pC/N), voltage coefficient (g_h) value (13.6 × 10^?3 V/m Pa), and hydrostatic figure of merit (HFOM) value (2168 × 10^?15 Pa^?1).     

Large electric‐field‐induced strain and enhanced piezoelectric constant in CuO‐modified BiFeO3‐BaTiO3 ceramics

In this work, we fabricated the (1‐x)BiFeO₃‐xBaTiO₃+y‰ mol CuO ceramics by the modified thermal quenching technique. The pure perovskite phase was formed and a morphotropic phase boundary (MPB) was observed in the ceramics with x = 0.30‐0.33. The addition of CuO can significantly enhance the density of the BiFeO₃‐BaTiO₃ material. Importantly, an enhanced piezoelectric constant (d₃₃=165 pC/N), a large electric‐field‐induced strain (?S = 0.54%: peak to peak strain) and a large piezoelectric actuator constant (d₃₃*=449 pm/V) together with a high Curie temperature (T_C) of 503°C were observed in the ceramics with x = 0.30 and y = 5. As a result, the enhanced piezoelectricity and large electric‐field‐induced strain could significantly stimulate further researches in BFO‐based ceramics.     

Phase Transition and Large Electrostrain in Lead‐Free Li‐Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

Large piezoelectric effect is achieved in Li‐doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃(BCTZ) ceramics by use of tuning the phase boundaries. Rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) multiphase coexistence is constructed in the ceramics by changing Li contents. The high piezoelectric constant d₃₃ (493 pC/N) and large electrostrain (dS_max/dE_max = 931 pm/V) have been observed in the Li‐doped (Ba, Ca)(Ti, Zr)O₃ ceramics at low sintering temperature (1350°C/2 h). The significant enhancement in materials properties is ascribed to the multiphase region around room temperature induced by Li‐doped effect.     

Small Reduction of the Piezoelectric d33 Response in Potassium Sodium Niobate Thick Films

We have investigated the electromechanical response of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. The high‐field strain hysteresis loops and weak‐field converse piezoelectric d₃₃ coefficient of the films were measured and compared with those of KNN bulk ceramics under the same electric field conditions. The converse d₃₃ values of the thick films and bulk ceramics were equal to 82.5 and 138 pm/V, respectively, at 0.4 kV/mm. The fundamental difference between the piezoelectric response of the KNN films and the ceramics was studied in terms of the effective (“clamped”) piezoelectric d₃₃ coefficient. The reduction in the piezoelectric d₃₃ coefficient of the KNN films, resulting from the clamping by the substrate, was compared to lead‐based ferroelectric thick films, including Pb(Zr,Ti)O₃ (PZT) and (1 ? x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ (PMN‐PT). We propose a possible explanation, based on the particular elastic properties of KNN, for the small relative difference observed between the “clamped” and “unclamped” (“bulk”) d₃₃ of KNN, in comparison with lead‐based systems.     

Properties and structures of nonstoichiometric (K,Na)NbO3‐based lead‐free ceramics

The 0.968[(K_0.48Na_0.52)]Nb_0.95+xSb_0.05O₃–0.032(Bi_0.5Na_0.5)ZrO₃ [KNN_xS–BNZ] lead‐free ceramics with nonstoichiometric niobium ion were fabricated via conventional solid‐state sintering technique and their piezoelectric, dielectric and ferroelectric properties were investigated. When x = 0.010, enhanced piezoelectric properties (d₃₃ ≈ 421 pC/N and k_p ≈ 0.47) were obtained due to the construction of rhombohendral—tetragonal phase boundary near room temperature. The KNN_xS–BNZ ceramics possesses enhanced Curie temperature (T_c) with improved piezoelectric constant. A large d₃₃ of ~421 pC/N and a high T_c ~256°C can be simultaneously induced in the ceramics with x = 0.010. Especially, good thermal stability was observed in a broad temperature range. The results indicated that our work could benefit development of KNN‐based ceramics and widen their application range.     

Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature

Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi_1/2Na_1/2TiO₃–0.08Bi_1/2K_1/2TiO₃–0.04BaTiO₃ (BNKBT88) with copper. A combination of Bi₂O₃, CuO, ZnO, Li₂CO₃, and B₂O₃ are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10^?8 atm, which is necessary for the coexistence of Cu metal and Bi₂O₃. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.     

PNN‐PZN‐PMN‐PZ‐PT Multilayer Piezoelectric Ceramic with Low Sintering Temperature

Multilayer piezoelectric ceramic material with a composition of 0.1Pb(Ni_1/3Nb_2/3)O₃‐0.35Pb(Zn_1/3Nb_2/3)O₃‐0.15Pb(Mg_1/3Nb_2/3)O₃‐0.1PbZrO₃‐0.3PbTiO₃‐4 mol% excess NiO (0.1PNN‐0.35PZN‐0.15PMN‐0.10PZ‐0.3PT‐0.04NiO) was fabricated by a roll‐to‐roll tape casting process and co‐fired with Ag/Pd electrode at low temperature of 950°C. Their dielectric, piezoelectric, and ferroelectric properties were evaluated. The effective piezoelectric coefficient d₃₃ of the obtained multilayer piezoelectric material was 412 pm/V, while d₃₃ for the ceramic pellet was 503 pm/V. Piezoelectric displacement measurements revealed small displacement hysteresis for the multilayer material. The combined characteristics of the multilayer piezoelectric material using the selected composition showed the potential for high power, high strain, and high force actuation applications. In addition, as the composition had a tetragonal phase, which substantially deviated from morphotropic phase boundary (MPB), the excellent properties may be more tolerant to stoichiometric fluctuation, which can allow larger processing and composition window as desired for scalable production.     

Phase,domain, and microstructures in Sr2+ substituted low-temperature sintering PZT-based relaxor ferroelectrics

The Ag-Pd internal electrode of multilayer piezoelectric ceramics needs to be sintered below 1000°C, and lead wires and components need to be welded with lead-free solder at 260°C. PNN–PMW–PZT–xSr piezoelectric ceramics with high Curie temperature (T_c > 260°C) were synthesized at a low sintering temperature (960°C) to meet the requirements of multilayer piezoelectric devices. The relationship between structures (phase, domain, and microstructures) and electrical properties (piezo/ferroelectric properties, and dielectric relaxation) in the Sr²⁺ substituted ceramics was investigated. Rietveld refinement and Raman spectra show that Sr²⁺ substitution can cause the phase change and increase the force constant of [BO₆] octahedron. The piezoelectric response increases with increasing the content of the tetragonal phase (CTP) in the rhombohedral-tetragonal (R-T) coexisted ceramics. The ceramics with 0.6 mol% Sr²⁺ substitution have minimum activation energy for domain wall movement (E_a) of 0.0362 eV which favors the formation of nanometer-sized domains, and possess excellent electrical properties (d₃₃ = 623 pC/N, d₃₃^* =783 pm/V, T_c =295°C). The higher the CTP, the lower the E_a. The lower E_a favors the rotation of polarization direction and extension, and is beneficial to the generation of the nanometer-size domains, resulting in high piezoelectric properties.     

Low‐temperature sintering and enhanced piezoelectric properties of random and textured PIN–PMN–PT ceramics with Li2CO3

Low‐temperature sintered random and textured 36PIN–30PMN–34PT piezoelectric ceramics were successfully synthesized at a temperature as low as 950°C using Li₂CO₃ as sintering aids. The effects of Li₂CO₃ addition on microstructure, dielectric, ferroelectric, and piezoelectric properties in 36PIN–30PMN–34PT ternary system were systematically investigated. The results showed that the grain size of the specimens increased with the addition of sintering aids. The optimum properties for the random samples were obtained at 0.5 wt% Li₂CO₃ addition, with piezoelectric constant d₃₃ of 450 pC/N, planar electromechanical coupling coefficient k_p of 49%, peak permittivity ε_max of 25 612, remanent polarization P_r of 36.3 μC/cm². Moreover, the low‐temperature‐sintered textured samples at 0.5 wt% Li₂CO₃ addition exhibited a higher piezoelectric constant d₃₃ of 560 pC/N. These results indicated that the low‐temperature‐sintered 36PIN–30PMN–34PT piezoelectric ceramics were very promising candidates for the multilayer piezoelectric applications.     

Low Temperature Sintering and Enhanced Piezoelectricity of Lead‐Free (Na0.52K0.4425Li0.0375)(Nb0.86Ta0.06Sb0.08)O3 Ceramics Prepared from Nano‐Powders

(Na_0.52K_0.4425Li_0.0375)(Nb_0.86Ta_0.06Sb_0.08)O₃ powders were synthesized via sol–gel and solid‐state reaction methods as a raw material for the preparation of the ceramics. Dependence of piezoelectric properties and microstructure on sintering temperatures was investigated in this study. Sol–gel‐derived nano‐powders could be densified at a lower temperature of 940°C and exhibited excellent electrical properties after sintering at 1020°C (d₃₃ = 424 pC/N, d₃₃^* = 780 pm/V, k_p = 52.1%, and T_c = 265°C). The enhanced electric properties were most likely due to the coexistence of orthorhombic and tetragonal phase in the samples at room temperature, homogenous microstructure with fine grain and high density.