BNT Ceramics Research Articles

Semiconducting tailoring and electrical properties of A-site Co substituted Bi0·5Na0·5TiO3-δ ferroelectric ceramics

The new (1-x) Bi0·5Na0·5TiO3-xCoTiO3-δ ferroelectric semiconductor ceramics were prepared successfully by the solid-state method. The single perovskite structures and long-range polarity order of the (1-x) Bi0·5Na0·5TiO3-xCoTiO3-δ semiconductor ceramics were observed by X-ray diffraction (XRD) and Raman spectra, respectively. The typical relaxation behavior was founded in all the ceramics. The perfect ferroelectric loops and outstanding optical properties were achieved in the (1-x) Bi0·5Na0·5TiO3-xCoTiO3-δ ceramics. A very small optical bandgap (Eg = 1.44 eV) was obtained in the 0.94Bi0·5Na0·5TiO3-0.06CoTiO3-δ ceramic, which is approximately 55% lower than that of pure BNT (Eg = 3.2 eV). The Jsc and Voc of poled 0.94Bi0·5Na0·5 [Ti4+0.71, Ti3+0.29]O2.71-0.06Co [Ti4+0.71, Ti3+0.29]O2.71 ceramic can reach ~ 1.00 nA/cm2 and 0.13 V, respectively. The activation energies of the grains and grain boundaries were calculated to be 0.48 and 0.52 eV, respectively. These results show that the Co substation BNT ceramics highlight a promising candidate for multi-functional photoelectric applications.

Simultaneously achieved high energy-storage density and efficiency in BaTiO3–Bi(Ni2/3Ta1/3)O3 lead-free relaxor ferroelectrics

The development of Pb-free energy-storage materials with excellent stability has attracted attention in recent times due to the energy and environmental crises. In this study, a series of (1 − x)BaTiO3–xBi(Ni2/3Ta1/3)O3 [(1 − x)BT–xBNT, x = 0.08, 0.1, 0.12, 0.14, and 0.16] relaxor ferroelectric ceramics was designed and synthesized. A highly recoverable energy-storage density (Wrec) of 2.63 J/cm3 and energy-storage efficiency (η) of 90% are obtained simultaneously in 0.84BT–0.16BNT ceramics. This excellent energy-storage characteristic may be attributed to the enhanced relaxor characteristic and its dense microstructure. In addition, excellent stability of energy-storage properties (variation in the Wrec < 4% over 20–100 °C, Wrec < 13.1% over 1 to 200 Hz and Wrec < 3% after 104 cycles) can also be observed. These results indicate that the 0.84BT–0.16BNT ceramics can be considered as potential Pb-free candidate materials for applications in advanced energy-storage applications.

Physical properties and sinterability of pure and iron-doped bismuth sodium titanate ceramics

Pure (BNT) and iron-doped bismuth sodium titanate (Fe-BNT) ceramics were produced according to the formula Bi0.5Na0.5Ti1−xFexO3−0.5x, where x = 0 to 0.1. The addition of Fe2O3 enables decreasing the sintering temperature to 900 °C in comparison with 1075 °C for pure BNT, whilst also achieving lower porosities and greater densities. This is attributed to oxygen vacancy generation arising from substitution of Fe3+ onto the Ti4+ site of the BNT perovskite structure, and the resulting increase in mass transport that this enables during sintering. X-ray diffraction (XRD) analysis of Fe-BNT samples shows single-phase BNT with no secondary phases for all studied Fe contents, confirming complete solid solution of Fe. Rietveld refinement of XRD data revealed a pseudocubic perovskite symmetry (Pm-3m), and unit cell lengths increased with increasing Fe content. Scanning electron microscopy (SEM) showed that average grain size increases with increasing Fe content from an average grain size of ~ 0.5 μm in (x = 0) pure BNT to ~ 5 μm in (x = 0.1) Fe-doped BNT. Increasing Fe content also led to decreasing porosity, with relative density increasing to a maximum > 97% of its theoretical value at x = 0.07 to 0.1. The addition of Fe to BNT ceramics significantly affects electrical properties, reducing the remnant polarization, coercive field, strain and desirable ferroelectric properties compared with those of pure densified BNT. At room temperature, a high relative permittivity (ɛ′) of 1050 (x = 0.07) at an applied frequency of 1 kHz and a lower loss factor (tanδ) of 0.006 (x = 0.1) at an applied frequency of 300 kHz were observed by comparison with pure BNT ceramics.

High thermally stable dielectric permittivity, polarization enhancement and electrostrictive properties in Zr-substituted bismuth sodium titanate lead-free ferroelectric ceramics

Zr-substituted (Bi0.5Na0.5)(Ti1−xZrx)O3 (BNTZ) with x = 0.02–0.4 ceramics were prepared using solid-state reaction technique and their structural, dielectric and ferroelectric properties were investigated systematically. X-ray diffraction results suggest that Zr4+ ion can enter the (Bi0.5Na0.5)TiO3 (BNT) crystal lattices at a limited value and a secondary phase appears in BNTZ ceramics when the x exceeds 0.1. Temperature-dependent dielectric permittivities of BNTZ ceramics show only one dielectric peak from room temperature to 500 °C for each composition, and this dielectric peak becomes broad gradually as x increases from 0.02 to 0.4. The permittivity of x = 0.4 varies less than ±10% between room temperature and 300 °C, indicating a superior thermal stability of the permittivity. Polarization enhancement is revealed by the polarization-electric field hysteresis loops and highest ferroelectric properties are obtained in x = 0.04. The electric-field-induced strains of x = 0.04 show a monotonous increase as temperature increases from 30 °C to 150 °C. At 80 kV/cm, a high strain level of 0.268% is achieved. Our results suggest that the introduction of Zr4+ ion could effectively tailor the dielectric and ferroelectric properties of BNT ceramics and x = 0.4 composition could find potential application in high temperature capacitor devices.

Structural, electrical, and weak ferromagnetic-to-antiferromagnetic nature of Ni and La co-doped BaTiO3 by sol–gel combustion route

We report on structural, electrical, and weak ferromagnetic-to-antiferromagnetic possessions of Ba(1−x)LaxNi0.01Ti0.99O3 (BLNT) samples with x = 0.00, 0.25, 0.50, and 0.75%. The X-ray diffraction (XRD) and Raman spectra show that the sol–gel combustion route synthesized samples are successfully incorporated with La3+ ions into BaNi0.01Ti0.99O3 (BNT) lattice. The EPR signals at g = 1.967 and g = 2.003 are attributed to the V(Ba) and V(Ti) defects, respectively. For x = 0.75%, V(Ba) signal disappears due to the subtle charge-compensation mechanism present in the BLNT sample. The BNT ceramics exhibit a weak ferromagnetic nature; upon La ion doping, the weak ferromagnetic (WFM) tends to exhibit strong antiferromagnetic (AFM) nature in BLNT. This magnetic phase transition can be described in the stipulation of the change in the oxidation state. Escalating in the dielectric constant and ac conductivity was due to the higher doping concentration of La3+ ion in BNT ceramics. BLNT clearly shows that as La3+ ion-doping concentration increases, the diameter of the impedance semicircle decreases due to the increased conducting nature of the samples. The ferroelectric property was deliberately investigated by a P–E hysteresis loop and its saturation polarization was found to decrease with increasing concentrations of La3+ ion doping, and the observed results are discussed.

Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5)TiO3–BaTiO3–Bi(Li0.5Ta0.5)O3 lead-free ceramics

Exploring lead-free materials to substitute the commercialized PZT-based compounds is in great demand for the development of miniaturized and nonhazardous actuator devices. Here, a new ternary lead-free (1-x)(Bi0.5Na0.5)1-yBayTiO3-xBi(Li0.5Ta0.5)O3 (BNBTy-xBLT) solid solution system is designed to optimize the electrostrain performance of the well-studied BNT ceramics, and the effects of BLT and Ba substitutions on phase evolution as well as electrical performances of BNT ceramics are systematically investigated. All the compositions prepared by solid-state sintering process exhibit similar pseudocubic phase with rhombohedral/tetragonal distortions in nanoregions. Through optimizing the BLT and Ba contents, the composition with 6.9 mol% Ba and 0.8 mol% BLT exhibits the maximum unipolar strain of ∼0.39% (equivalent normalized strain d33*∼654 pm/V, 60 kV/cm). Moreover, the origin of the excellent electrostrain response is emphatically analyzed from microscopic (domain structure evolution) and macroscopic (TF-R shifting) perspectives, which results from the electric-field-induced reversible relaxor-ferroelectric phase transition. These results suggest the composition-optimized BNT-BT-BLT system as a potential alternate to lead-based systems in actuator applications.

Effect of Microwave Sintering on the Properties of 0.95(Ca0.88Sr0.12)TiO₃⁻0.05(Bi0.5Na0.5)TiO₃ Ceramics.

Perovskite ceramics are a common microwave dielectric material, but the development and application of this material has been limited by the high, positive resonance frequency temperature coefficient and sintering temperature. Therefore, adjusting the temperature coefficient of the resonance frequency and reducing the sintering temperature have become important research directions. In this work, 0.95(Ca0.88Sr0.12)TiO3–0.05(Bi0.5Na0.5)TiO3 ceramics (referred to as 0.95CST-0.05BNT) were prepared by standard solid-state reaction and microwave sintering. Microwave sintering greatly shortened the sintering period and holding time. Moreover, the 0.95CST–0.05BNT ceramics showed more uniform grain size distribution, and microwave sintering reduced energy consumption in the experiment. Therefore, the temperature coefficient of the resonance frequency of MWS ceramics was reduced by 119 × 10−6 /℃. All of the ceramics, which were sintered at 1300 °C for 40 minutes, showed optimal microwave dielectric properties: εr = 187.6, Q × f = 8958 GHz, and τf = +520 × 10−6 /°C.

Structure Analysis and Ferroelectric Response of Bi0.5Na0.5TiO₃ Nanopowder Synthesized by Sol-Gel Method.

In this work, bismuth sodium titanate, Bi0.5±xNa0.5±yTiO₃ (BNT, x, y = -0.05-0.08) nanopowders were produced using the low-temperature sol-gel technique. The effects of deficient and excess amounts of Bi and Na on BNT structure were systemically examined through X-ray powder diffraction (XRD), energy dispersive analysis (EDS) and scanning electron microscope (SEM). The optimized composition of the BNT nanopowder was pelletized and sintered at different temperatures (950°C-1150 °C). Highly dense ceramics possessing pure perovskite phase was observed for the sample sintered at an optimum sintering temperature (1100 °C). The ferroelectric properties were found to increase with an increase in sintering temperature up to 1100 °C and then decrease. This study justifies that Bi and Na non-stoichiometry (proper excess), processing and sintering temperatures play important role in the successful synthesis of BNT ceramics.

Phase transition, electrical properties and large strain response in lead-free (1-x-y)BNT-xBKT-yKNN ceramics

Lead-free piezoelectric composition ceramics of (1-x-y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yK0.5Na0.5NbO3; (1-x-y)BNT-xBKT-yKNN (with 0.18≤x ≤ 0.26 and 0≤y ≤ 0.07) were fabricated using the solid-state combustion method. The structure exhibited co-existing rhombohedral and tetragonal phases for all samples. Increasing of x and y contents, the average grain size decreased from 1.42 μm to 1.02 μm and 1.25 μm to 0.70 μm, respectively. The diffuseness exponent (γ) of the ceramics was between 1.817 and 1.939 for x composition and between 1.766 and 1.999 for y composition indicating that the (1-x-y)BNT-xBKT-yKNN solid solutions had diffuse phase transition behavior. The change in the P-E loops of the ceramics indicated that the long-range ferroelectric order of the samples was disturbed and turned to the polar nano-regions (PNRs) with increased x and y contents. The polarization hysteresis loop transformed from well saturated typical ferroelectric, to pinched, and then to the relaxor state with increased x and y content. The addition of BKT and KNN contents significantly enhances the field-induced strain in BNT ceramics. The largest Smax of 0.25% corresponded to a high-field effective d33* of 509 pm/V, which was found in the composition of 0.79BNT-0.20BKT-0.01KNN.

Structural, dielectric and AC-conductivity studies of Gd doped lead-free Bi0.5Na0.5TiO3 ceramics

ABSTRACTLead–free (Bi1-xGdx)0.5Na0.5TiO3 (BGNT; x = 0.00 to 0.06) ceramics synthesized by a solid state reaction method. The effect of Gd on the structural, microstructural and dielectric properties of BNT ceramics was reported. X-ray diffraction pattern shows the pure perovskite rhombohedral phase without any secondary phase. The microstructural analysis revealed that the reduction in the grain size observed with increasing the Gd concentration. The x = 0.06 sample exhibited the improved dielectric properties such as higher dielectric constant (ϵr = 809) and lower dielectric loss (tanδ = 0.201), measured at 1 MHz. The frequency dependent AC-conductivity (σac) of BGNT ceramics was analyzed using Jonscher's power law.

Mechanical bending strength of (Bi0.5Na0.5) TiO3-based lead-Free piezoelectric ceramics

(Bi0.5Na0.5)TiO3 [BNT] is expected as one of candidate lead-free materials because these ceramics show relatively good high-power piezoelectric properties. In this study, we tried to understand the bending strength and fracture behavior of the BNT-based ceramics. To measure the bending strength, a three-point bending test on the basis of JIS was conducted using 12.0 × 4.0 × 1.0 mm3 specimens. An average bending strength, σA, of pure BNT ceramics sintered at 1100 °C for 2, 12 and 24 h were 217, 195 and 187 MPa, respectively. It is cleared that the σA increased with decreasing the sintering time, (grain size and pore size). We also investigated the bending strength of Nb2O5 doped BNT ceramics [BNT-Nb x, x = 0.05 ∼ 1.5 wt%] and MnCO3 doped BNT ceramics [BNT-Mn x, x = 0.5 and 1.0 wt%]. Values of the σA of BNT-Nb 0.5 and BNT-Mn 0.5 were 222, and 188 MPa, respectively. It is clarified that soft dopants (Nb) can improve the bending strength of BNT-based ceramics. Additionally, hot-pressed BNT [HP-BNT] were sintered at 1050 °C for 5 h, and the σA of HP-BNT was 245 MPa.

Effect of Ce on structural and dielectric properties of lead-free (Bi 0.5 Na 0.5 )TiO 3 ceramics

Abstract The structural, microstructural and dielectric properties of the Ce doped Bi 0.5 Na 0.5 TiO 3 (BNTC) lead-free piezoelectric ceramics prepared by a conventional solid state reaction process have been investigated. Rietveld refinement of the BNTC ceramics shown that the volume of the unit cell increased with the concentration of Ce up to x = 0.015 and decreases further with the excess Ce doping (≥ x = 0.025). The average grain size of the BNTC ceramics found to be increased linearly up to x = 0.015 and higher concentration of Ce inhibited the grain growth. The dielectric response of the Ce doped BNT ceramics improved significantly and the optimum dielectric properties ( e r = 672 and tan δ = 0.206 at 1 MHz) were obtained for the composition of x = 0.025. For the low concentration of x , the Ce 3+ was substituted in Ti 4+ site, which enhanced the diffuseness coefficient ( γ = 1.83). Conversely, the γ value was found to be decreased when x ≥ 0.025, which is due to the substitution of Ce 3+ in Na + site instead of Ti 4+ site and weakens the relaxor behavior. The temperature dependent of AC-conductivity of BNTC ceramics was analyzed using Arrhenius's law and the variable range hopping (VRH) model and the corresponding density of states ( N ( E F )), hopping length ( R H ), and hopping energies ( W H ) were calculated. The obtained results indicate that BNTC ceramics are suitable for promising industrial high performance piezoelectric applications.

Effect of Nb and Mn Substitution on Bi&lt;sub&gt;0.5&lt;/sub&gt;Na&lt;sub&gt;0.5&lt;/sub&gt;TiO&lt;sub&gt;3 &lt;/sub&gt;Lead-Free Piezoceramics with Enhanced Electrical Properties

The perovskite oxide Bi0.5Na0.5MnxTi1-xO3, Bi0.5Na0.5NbxTi1-xO3, Bi0.5Na0.5 (Mn0.5Nb0.5)xTi1-xO3 and Bi0.5Na0.5TiO3 ceramics (x=0.25%) were prepared via the conventional solid state reaction method. The role of Mn as an acceptor, Nb as a donor and (Mn0.5Nb0.5) substitution at B site in BNT lead-free piezoceramics was investigated. The (Mn0.5Nb0.5) substitution led to the inhibited of reduction of Ti4+ to Ti3+ and gave rise to large defect-dipole clusters containing highly localized electrons which should be responsible for the increase of Tc and Td. The ferroelectric properties and field-induced strains were both improved by Mn-acceptor and (Mn0.5Nb0.5) co-doped. The fatigue-resistant properties of Nb doped BNT ceramics were comparable to BNT ceramics, Mn doped ceramics were found to have significantly improved fatigue-resistant properties, while almost no profound fatigue was observed in BNT-MnNb ceramics after switching over 106 cycles at room temperature.

The preparation of lead-free bismuth sodium titanate ceramics via the solid state combustion technique

ABSTRACTLow calcined temperature, high density and good dielectric properties of BNT ceramics successfully prepared by the solid state combustion technique and glycine was used as fuel to reduce the reaction temperature. The effects of calcination temperature (500–900°C) and sintering temperature (1100–1150°C) on phase formation, microstructure and dielectric properties of (Bi0.5Na0.5)TiO3 (BNT) ceramics were investigated. The phase purity, crystal structure and microstructure of the samples were tested using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the pure rhombohedral perovskite structure of BNT powders were observed from the sample calcined at 650°C, which is lower than the solid state reaction technique by ∼200°C. The purity phase was detected in all ceramic samples. The microstructure of the BNT powders exhibited an almost-spherical morphology and a highly agglomerated form. The average grain size of the ceramics was increased with increase sintering temperature. The TF-R and Tm were slightly increased with the increase of sintering temperature. The highest density and the best dielectric properties was obtained by sample sintered at temperature of 1120°C. The solid state combustion technique is a new alternative for reduction of calcined temperature, improvement of densification and modify of dielectric properties to prepared BNT ceramics.

Quenching effects for piezoelectric properties on lead-free (Bi1/2Na1/2)TiO3 ceramics

Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 (BNT), were fabricated by a quenching procedure after sintering, and then their electrical properties were investigated with the aim to increase their depolarization temperature Td. From the measurement of the temperature dependence of dielectric properties, Td increased with increasing quench temperature. The Td of a BNT sample quenched from 1100 °C was 223 °C, which was almost 50 °C higher than that prepared by the ordinary cooling process. From the measurement of P–E hysteresis loops, both the remanent polarization Pr and the coercive field Ec of BNT samples prepared by ordinary firing were almost the same as those quenched from 1100 °C. Additionally, from the measurements by a resonance–antiresonance method, the electromechanical coupling factor k33 of ordinarily fired BNT was 0.45, and that of the quenched BNT was 0.46. From these results, it is clarified that the quenching procedure is an effective way to increase the Td of BNT ceramics without deteriorating ferroelectric and piezoelectric properties.

High temperature dielectric, ferroelectric and piezoelectric properties of Mn-modified BiFeO3-BaTiO3 lead-free ceramics

Lead-free and high-temperature 0.71BiFeO3-0.29BaTiO3 ceramics with Mn modification (BF-BT-x %Mn) were fabricated by conventional solid-state reaction method, and the high temperature dielectric, ferroelectric and piezoelectric properties were investigated. All compositions exhibited pseudo-cubic phases. Mn modification improved the electrical properties of BF-BT solid solutions at both room and high temperature. The Curie temperature T C, depolarization temperature T d, dielectric constant e r, dielectric loss tanδ, piezoelectric constant d 33, electromechanical coupling factor k p, remnant polarization P r of BF-BT-1.2 %Mn ceramics were 506, 500 °C, 556, 0.04, 169 pC N−1, 0.373, 31.4 μC cm−2, respectively. The e r, tanδ, and k p of BF-BT-1.2 %Mn ceramics were stable with the increase of temperature until 500 °C. The coercive field E c of BF-BT-1.2 %Mn ceramics was nearly 30 kV cm−1 at 200 °C, which was much larger than those of PZT, BS-PT,BNT and KNN ceramics. The high field strain coefficient d*33 reached as large as 525 pm V−1 at 200 °C. These results showed that the BF-BT-x %Mn ceramics were promising candidate for high temperature piezoelectric applications.

Properties and Phase Shift Structure of (1-x)BT-(x)BNT Ceramics

In the research, the properties of barium titanate - bismuth sodium titanate [(1-x)BaTiO3-(x)Bi0.5Na0.5TiO3: (1-x)BT-(x)BNT] ceramics prepared by conventional mixed oxide method with various molecular weight of BNT or x between 0.0 and 0.3 were investigated. The optimum condition for calcined powders of x = 0.0 was found at 900 °C for 2 h, and x = 0.1 - 0.3 were found at 850 °C for 2h. The calcined powders were pressed and sintered at 1000 – 1200 °C for 2h. The phase structure was examined by x-ray diffraction (XRD). The microstructure was examined by scanning electron microscopy (SEM). Density of sintered samples was measured by Archimedes method with distilled water as the fluid medium. It was found that, all various x of (1-x)BT-(x)BNT ceramics XRD patterns display the tetragonality increased with increasing sintering temperature. All the peaks shift to higher angles when increasing x value indicating the decrease of lattice parameter “a” and increase of lattice parameter “c”. The average grains size of (1-x)BT-(x)BNT ceramics was increased with increasing sintering temperature. The highest density was 5.53 g/cm3 and was obtained from the sample sintered at 1200 °C.

Structure and electromechanical properties in Bi0.5Na0.5TiO3-based lead-free piezoceramics with calculated end-member Bi(Ni0.5Ti0.5)O3

The structure of end-member Bi(Ni0.5Ti0.5)O3 was calculated by first-principles method and solid solution of the (1−x)BNT–xBi(Ni0.5Ti0.5)O3 ((1−x)BNT–xBNiT) system was prepared using solid state reaction. Effect of BNiT addition to BNT ceramics on the phase structure, strain behavior, dielectric, ferroelectric, and piezoelectric properties was systematically investigated. The phase diagram was obtained and the relationship between the phase structure and electromechanical properties has been clarified. In comparison to other systems, results in this study indicated a relationship between the morphotropic phase boundary (MPB) composition and tolerance factor and the calculated tetragonality of the end-member Bi(Me′0.5Me″0.5)O3, and the t value corresponding to the formation of MPB located in a quite narrow range of 0.975–0.976 in the (1−x)BNT–xBi(Me′0.5Me″0.5)O3 systems with low tolerance factor end-members. Thus, this work do not only supplement for BNT-based piezoceramics, but also provide a guideline to predict the approximate MPB region quickly for designing new lead-free piezoelectric ceramics.

Hydrothermal Synthesis of Sodium Potassium Bismuth Titanates

(Bi0.5Na0.5)TiO3 (BNT) is an excellent candidate lead-free piezoelectric material system. This study prepares BNT ceramics using the hydrothermal method at 180°C with an NaOH concentration of 12 M. The effect of potassium content on the crystal structure of the ceramics is studied. A single phase of [Bi0.5(Na1−xKx)0.5]TiO3 ceramics is obtained using the process applied to obtain BNT. The phase structure transformed from rhombohedral to tetragonal at x = 0.3, as evidenced by the disappearance of the cubic phase. The surface morphology of the materials is affected by K substitution.

Microstructure and microwave dielectric properties of Ba4.2Nd9.2Ti18−xSnxO54(x = 0, 0.25, 0.5, 1, 1.5, 2) ceramics

Ba4.2Nd9.2Ti18−xSnxO54(x = 0, 0.25, 0.5, 1, 1.5, 2) microwave dielectric ceramics with high Q × f value were prepared by conventional solid state route. The microstructure and microwave dielectric properties of Ba4.2Nd9.2Ti18−xSnxO54(x = 0, 0.25, 0.5, 1, 1.5, 2)(BNTS) ceramics were systematically investigated. XRD patterns showed that there was only a single BaNd2Ti5O14 phase identified in all samples and no second phase was found. SnO2 substitution increases the lattice parameters of BNT ceramic. Moreover, the bulk density of BNT ceramics increases as the x increases. Both the permittivity and τf value decrease when the Sn concentration increases. The SnO2 substitution has a big influence on Q × f value. When x = 0.5, the BNTS ceramic gains the highest Q × f value. Afterwards, the Q × f value decreases sharply when x value increases. Excellent microwave dielectric properties were achieved in Ba4.2Nd9.2Ti18−xSnxO54(x = 0.5) ceramics sintered at 1340 °C for 2 h: er = 80.6, Q × f = 9177 GHz, τf = 61 ppm/°C.