Effects of sintering time on crystal structure,dielectric properties and conductivity of (Ca0.8Sr0.2)ZrO3 ceramics

(Ca_0.8Sr_0.2)ZrO₃ ceramics were prepared using solid-state reaction process, which were sintered at 1,480 °C for different sintering time (2, 4, 6, 8, 10, 12 h, respectively), their structures were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. Rietveld refinement for the (Ca_0.8Sr_0.2)ZrO₃ sintered for 10 h was carried out by powder XRD at room temperature and it crystallizes in orthorhombic space group Pnma [a = 5.77341(4) Å, b = 8.05569(6) Å, c = 5.63318(4) Å and V = 261.9920(30) Å³, Z = 4]. The (Ca_0.8Sr_0.2)ZrO₃ ceramics sintered at 1,480 °C for 2–12 h possessed a dielectric constant (ε _r) of 23.6–27.9, a quality factor (Q × f) of 2,160–21,460 GHz and a temperature coefficient of resonant frequency (τ _f ) from ?14 to +13.6 ppm/°C. The Arrhenius plot of the dc electrical conductivity changed significantly with increasing sintering time.     

Frequency and temperature dependence of electrical properties of barium and gadolinium substituted SrBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics

Barium strontium gadolinium bismuth niobate (Ba_0.1Sr_0.81Gd_0.06Bi₂Nb₂O₉, BSGBN) ceramics were prepared by using the conventional solid-state reaction method. The dielectric permittivity, modulus and impedance spectroscopy studies on BSGBN were investigated in the frequency range, 45 Hz–5 MHz and in the temperature range from room temperature (RT) to 570 °C. The dielectric anomaly with a broad peak was observed at 470 °C. Simultaneous substitution of Ba²⁺ and Gd³⁺ increases the transition temperature of SrBi₂Nb₂O₉ (SBN) from 392 to 470 °C. XRD studies in BSGBN revealed an orthorhombic structure with lattice parameters a = 5.4959 Å, b/a = 1.000, c = 25.0954 Å. Impedance and modulus plots were used as tools to analyse the sample behaviour as a function of frequency. Cole-Cole plots showed a non-Debye relaxation. Also, dc and ac conductivity measurements were performed on BSGBN. The electric impedance which describes the dielectric relaxation behaviour is fitted to the Kohlrausch exponential function. Near the phase transition temperature, a stretched exponential parameter β indicating the degree of distribution of the relaxation time has a small value.     

High dielectric permittivity and low loss of SrBi4Ti4O15 with PbO and V2O5 additions for RF and microwave applications

In this paper SrBi₄Ti₄O₁₅ (SBTi), a perovskite-type ceramic, with cation deficit A₅B₄O₁₅, was prepared by solid-state reaction method and PbO and V₂O₅ were added into SBTi (2, 5, 10 and 15 wt%). Samples were characterized through X-Ray Diffraction (XRD), Raman Spectroscopy and Scanning Electron Microscopy (SEM). Impedance Spectroscopy was carried out at room temperature. The analysis by XRD using the Rietveld refinement has confirmed the formation of single-phase compound with a crystalline tetragonal system (a = 3.8408 Å, b = 3.8408 Å and c = 41.0959 Å). A SEM shows globular grains (with addition of PbO) and crystal-shape ones (with additions of V₂O₅), from about 1 to 2 μm. The dielectric properties: dielectric permittivity (K′) and dielectric loss (tan δ) were measured at room temperature over a range of 100 Hz–40 MHz by complex impedance spectroscopy and in the microwave (MW) frequency region were studied. The study showed that these properties are strongly dependent on frequency and on the added level of the impurity. All the samples were analyzed taking into account to possible applications in radio frequency (RF) and MW devices.     

Preparation,characterization and dielectric properties of Ba5LnZnTa9O30 (Ln=La,Sm) ceramics

Ba₅LnZnTa₉O₃₀ (Ln=La, Sm) ceramics were prepared by high temperature solid-state reaction route. The samples were characterized by X-ray diffraction, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) methods. They belong to paraelectric phase of filled tetragonal TB structure at room temperature with unit cell a=12.5909(4) Å, c=3.9622(2) Å for Ba₅LaZnTa₉O₃₀; and a=12.5777(4) Å, c=3.9544(2) Å for Ba₅SmZnTa₉O₃₀. At 1 MHz, Ba₅LaZnTa₉O₃₀ has high dielectric constants of 89 with low dielectric loss 0.0067, and temperature coefficients of the dielectric constant (τ_ε) −811 ppm °C⁻¹; Ba₅LaZnTa₉O₃₀ has dielectric constants of 74 with low dielectric loss 0.0035, and τ_ε −474 ppm °C⁻¹.     

Modulus spectroscopy of lead potassium titanium niobate (Pb<Subscript>0.95</Subscript>K<Subscript>0.1</Subscript>Ti<Subscript>0.25</Subscript>Nb<Subscript>1.8</Subscript>O<Subscript>6</Subscript>) ceramics

The modulus Spectroscopy of Lead Potassium Titanium Niobate (Pb_0.95K_0.1Ti_0.25Nb_1.8O₆, PKTN) Ceramics was investigated in the frequency range from 45 Hz to 5 MHz and the temperature, from 30 to 600 °C. XRD analysis in PKTN indicated a orthorhombic structure with lattice parameters a = 18.0809 Å, b = 18.1909 Å and c = 3.6002 Å. The dielectric anomaly with a peak was observed at 510 °C. Variation of ε^I and ε^II with frequency at different temperatures exhibit high values, which reflects the effect of space charge polarization and/or conduction ion motion. The electrical relaxation in ionically conducting PKTN ceramic analyzed in terms of Impedance and Modulus formalism. The Cole–Cole plots of impedance were drawn at different temperatures. The dielectric modulus, which describes the dielectric relaxation behaviour is fitted to the Kohlrausch exponential function. Near the phase transition temperature, a stretched exponential parameter β indicating the degree of distribution of the relaxation time has a small value. From the AC conductivity measurements the activation energy near phase transition temperature (T _C°C) has been found to different from that of the above and below T _C. The temperature dependence of electrical modulus has been studied and results are discussed.     

Sintering characteristic,crystal structure and microwave dielectric properties of a novel thermally stable ultra-low-firing Na2BiMg2V3O12 ceramic

A novel ultra low-firing microwave dielectric ceramic with the composition of Na₂BiMg₂V₃O₁₂ was fabricated by a solid state reaction method. The phase structure, sintering behavior and microwave dielectric properties of ceramics were investigated. The ceramic can be well densified at 660 °C for 4 h. X-ray diffraction data show that Na₂BiMg₂V₃O₁₂ has a cubic garnet structure with lattice parameters of a = 12.4929 Å, V = 1,949.80 Å³, Z = 8 and ρ = 4.42 g/cm³. The microwave dielectric properties are strongly related to the density and morphology of the samples. The ceramic sintered 660 °C exhibits good microwave dielectric properties with ε_r = 23.2, Q × f = 3,700 GHz and τ _f  = 8.2 ppm/°C. These results indicate that Na₂BiMg₂V₃O₁₂ ceramic is a candidate for low temperature co-fired ceramics devices, such as chip multi-layer LC filter, microstrip bandpass filter, multilayer antenna.     

Synthesis and electrical studies of modified PbTiO3 ceramics: (Pb1−x Ca x ) (Mn0·05W0·05Ti0·90) O3

Modified ceramics (Pb_1−x Ca _x )[(Mn_0·5W_0·5)_0·10Ti_0·90]O₃ have been fabricated forx=0, 0·05, 0·10 and 0·15 by high temperature solid state reaction technique. XRD, SEM, DTA and electrical studies of the sample withx=0·10 have been performed. These studies show that the sample is homogeneous single phase perovskite type with tetragonal structure. The phase transition occurs at 330°C. Electrical behaviour of other samples have also been investigated as a function of frequency (1 kHz to 1 MHz) and temperature (26°C to 300°C). The samples withx=0·05 and 0·10 have low loss, low dielectric constant, and show negligible pyroelectric effect. The sample withx=0·15 has minimum values ofɛ and loss which are temperature independent up to about 200°C. It also shows good pyroelectric behaviour. Hence it may be of use in pyroelectric infrared sensors.     

Nd2Ba2CaCu2−xZnxTi3O14: a new family of tunable dielectric oxides showing a compositionally controlled tetragonal to cubic transition

Oxides of the formula Nd₂Ba₂CaCu_2−xZn_xTi₃O₁₄ (x=0.0, 0.5, 1.0, 1.5, 2.0) have been successfully prepared by high temperature ceramic route at 1025 °C, as well as by the low temperature citrate precursor route at 800 °C. The copper-rich compounds obtained by the ceramic route (x=0–1.0) are found to have a tetragonal structure, with the a and c parameters increasing with zinc substitution, from a=3.913(8) Å and c=19.63(1) Å for x=0, to a=3.925(1) Å and c=19.64(1) Å for x=1. The tetragonal distortion decreases with increase in zinc substitution and the oxides with x>1.5 were found to crystallize in the cubic structure with a=3.933(2) and a=3.937(1) Å for the x=1.5 and two compositions, respectively. The entire range of oxides (0≤x≤2) could be obtained as pure phases at 800 °C and crystallize in the cubic structure when prepared by the citrate precursor route. The room temperature dielectric constant (ε) at 100 kHz varies from 8 for the pure Cu composition and increased to 59 for the pure Zn analog. The dielectric constant depends on the synthetic route—the pure Zn compound Nd₂Ba₂CaZn₂Ti₃O₁₄ gave much lower dielectric constant (ε) of 23 at 100 kHz when prepared by the citrate precursor method (ε=59, solid state route).     

Effects of BaNb2O6 addition on microstructure and dielectric properties of BaTiO3 ceramics

(1 ? x)BaTiO₃–xBaNb₂O₆ [(1 ? x)BT–xBN] ceramics with x = 0, 0.005, 0.008, 0.01, 0.02, 0.03 were prepared by a conventional solid-state reaction route. The effect of BN addition on phase composition, microstructure and dielectric properties of BT-based ceramics were investigated by X-ray diffraction, scanning electron microscope and impedance spectroscopy. The results showed that a systematic structure change from the ferroelectric tetragonal phase to pseudo-cubic phase was observed near x = 0.01 at room temperature. It resulted in a considerable change of density, grain size and dielectric properties of the samples when BN was introduced. Meanwhile, it also lowered the sintering temperature of the ceramics. The dielectric constant peak and the variation rate of capacitance at Curie temperature are markedly depressed and broaden with increasing BN content. Especially, the ceramics with x = 0.008 and x = 0.01 showed good dielectric properties over the measured temperature range. Optimal dielectric properties of ε = 3,851, tanδ = 0.7 % at room temperature and Δε/ε₂₅ ≤ ±6.8 % (?55 to 125 °C) were obtained for the BT-based ceramics doped with 0.8 mol% BN, which was obviously superior to BaTiO₃ and BaNb₂O₆ ceramic, and it met the requirements of EIA X7R specifications.     

Microwave dielectric properties of low temperature firing (Li1/2Nd1/2)WO4 ceramic

A new kind of low temperature sintering ceramic with composition of (Li_1/2Nd_1/2)WO₄ were prepared by solid state reaction method. The phase and structure of the ceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy. The microwave dielectric properties of the ceramics were studied using a network analyzer. All the XRD patterns can be fully indexed as single-phase tetragonal structure (I41/n), with lattice parameters a = b = 5.25789 Å and c = 11.39124 Å. The ceramic sintered at 775 °C for 4 h exhibits a good microwave dielectric properties with permittivity about 16.1, Q × f about 4,210 GHz and TCF about 142 ppm/°C.     

Impedance spectroscopy of Ba<Subscript>3</Subscript>Sr<Subscript>2</Subscript>DyTi<Subscript>3</Subscript>V<Subscript>7</Subscript>O<Subscript>30</Subscript> ceramic

Polycrystalline sample of Ba₃Sr₂DyTi₃V₇O₃₀ was prepared at 950°C using a high-temperature solid-state reaction technique. X-ray structural analysis indicated the formation of a single-phase orthorhombic structure with lattice parameters: a = 12·2719 (39) Å, b = 8·9715(39) Å and c = 19·7812(39) Å. Microstructural study showed densely packed uniform distribution of grains over the surface of the sample. The a.c. impedance plots were used as tools to analyse the electrical response of the sample as a function of frequency at different temperatures (30–500°C). These plots revealed the presence of grain boundary effect, from 200·C onwards. Complex impedance analysis showed non-Debye type of dielectric relaxation. The Nyquist plots showed the negative temperature coefficient of resistance character of Ba₃Sr₂DyTi₃V₇O₃₀. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is the same, and hence the relaxation process may be attributed to the same type of charge carrier.     

Dielectric relaxation phenomena in the compound: LiCo<Subscript>3/5</Subscript>Mn<Subscript>1/5</Subscript>Cu<Subscript>1/5</Subscript>VO<Subscript>4</Subscript>

Solution-based chemical method has been used to produce LiCo_3/5Mn_1/5Cu_1/5VO₄ ceramics. The formation of the compound is checked by X-ray diffraction analysis and it reveals an orthorhombic unit cell structure with lattice parameters of a = 9.8262 Å, b = 3.0706 Å, c = 14.0789 Å. Field emission scanning electron micrograph indicates a polycrystalline texture of the material with grains of unequal sizes (~0.2 to 3 μm). Complex impedance spectroscopy technique is used to study the dielectric properties. Temperature dependence of dielectric constant (ε _r) at various frequencies exhibits the dielectric anomalies in ε _r at T _c (transition temperature) = 245, 255, 260 and 265 °C with (ε_r)_max. ~458, 311, 214 and 139 for 50, 100, 200 and 500 kHz, respectively. Frequency dependence of tangent loss at various temperatures shows the presence of dielectric relaxation in the material.     

Mutual phase stabilization of aluminium phosphate and titania in AlPO4-TiO2 binary system

Crystallization behaviour of amorphous aluminium phosphate (AlPO₄) and titania (TiO₂) in a mixed system of the two (5:1) has been reviewed in the light of our recent results. The polymorphous aluminium phosphate in such a binary system grows exclusively in a single phase over a temperature range 500–1150°C. The phase is reported to have a tridymite-like structure belonging to orthorhombic system with cell parametersa=9·638±0·0019,b=8·664±0·0017 andc=18·280±0036Å. Titania in the system preferentially retains its anatase phase morphology up to a temperature (950°C) well beyond its normal anatase → rutile transformation temperature showing a phenomenon of stabilization of this phase in such mixture. An interfacial reaction mechanism that can explain the observed phenomenon of mutual phase stabilization has been discussed and implications of this result towards the use of such technique for stabilization of various polymorphous compounds in a single phase has been pointed out.     

Synthesis of SrLaAlO4 fine ceramic powders by co-precipitation process

SrLaAlO₄ fine ceramic powders were synthesized by a co-precipitation process from the raw materials of La(NO₃)₃·6H₂O (99 %), Al(NO₃)₃·9H₂O (98–102 %) and SrCl₂·6H₂O (99 %). Effects of the precipitating pH value and thermal treatment temperature upon the phase constitution and microstructures of the product powders were determined together with the reaction mechanisms. The precipitating pH value indicated a key role dominating the reaction process. For the situation of precipitating pH = 7, SrLaAlO₄ phase began to form around 900 °C and became the major phase above 1,000 °C where small amount of secondary phases of La₂O₃, LaSrAl₃O₇, La₁₀Al₄O₂₁ were detected. With increasing the precipitating pH value to 8 and 9, SrLaAlO₄ major phase were obtained at 900 °C together with small amount of secondary phases of La₂O₃, LaSrAl₃O₇ and La(OH)₃, and the amount of secondary phases decreased with increasing thermal treatment temperature. The grain morphology of the product powders was also determined by the precipitating pH value and the thermal treatment temperature. SrLaAlO₄ fine ceramic powders with homogenous morphology and less secondary phases were obtained by heating the precursors with precipitating pH = 9 at 900 and 1,000 °C in air for 10 h. Densification of SrLaAlO₄ ceramics derived from the present powders could be achieved at 1,450 °C, and the optimal microwave dielectric properties were obtained as: ε _r  = 19.5, Qf = 56,500 GHz, τ _f  = ?32.8 ppm/°C.     

Crystal structure and thermal annealing behaviors of high d 33 Aurivillius-phase ceramics Li0.04Ce0.04Na(0.46−x/2)Bi(4.46+x/2)Ti(4−x)Sc x O15 with the Sc3+/Bi3+ co-substitution

Modified Aurivillius-type-structured piezoelectric ceramics, Li_0.04Ce_0.04Na_(0.46?x/2)Bi_(4.46+x/2)Ti_(4?x)Sc _x O₁₅ (LiCe–NBT–Sc?x, x = 0, 0.025, 0.075, 0.125, 0.15, 0.175) were synthesized by using conventional solid-state reaction process. Rietveld refinement for the x = 0.125 modified sample was carried out by using powder X-ray diffraction and LiCe–NBT–Sc?0.125 was confirmed to be a four-layer Aurivillius oxide with orthorhombic space group A2 ₁ am [a = 5.45814(7) Å, b = 5.43029(7) Å, c = 40.8547(4) Å and V = 1,210.902(26) Å³; Z = 4], at room temperature. The Sc³⁺/Bi³⁺ substitution led to an increase in Curie temperatures (T _c) and an enhancement in piezoelectric property, and the LiCe–NBT–Sc?0.125 ceramic with a T _c of 675 °C had a high piezoelectric activity (d ₃₃) of 32 pC/N. Variable thermal-annealing d ₃₃ and resistivity (ρ) of the LiCe–NBT–Sc?0.125 ceramic were investigated; the d ₃₃ of the O₂ annealed sample reached up to 37 pC/N, and its resistivity was about 6.8 × 10⁶ Ω cm at 575 °C and about 6.5 × 10⁵ Ω cm at 650 °C.     

Synthesis and characterization of γ-zirconium arsenate

The hydrothermal synthesis of a novel layered zirconium arsenate, Zr( AsO₄) (H₂AsO₄) · 2H₂O, with γ-type structure and preliminary data on its characterization by chemical and thermal analysis, X-ray diffraction (XRD) and ion exchange are reported. This XRD powder pattern is indexed on a monoclinic cell: a = 5.5699(2) Å, b = 6.8175(4) Å, c= 12.1203(6) Å and β= 103.15(1)°.     

The crystal structure of a new sodium zinc arsenate phase solved by “simulated annealing”

The framework structure of a new sodium zinc arsenate, produced by dehydration of the Na₆(ZnAsO₄)₆·8 H₂0 sodalite analog, has been solved by a simulated annealing method. This method utilizes typical tetrahedral-atom bonding schemes with possible space group and unit cell dimensions to randomly generate possible continuous frameworks and their calculated energies. The Rietveld refinement of the Na₆(ZnAsO₄)₆ solution phase was initially performed on room temperature X-ray data and then continued on low-temperature constant wavelength neutron data. This sodium zinc phosphate is a hexagonal polycrystalline material: space group P6₃ (no. 173), with a = b =9.005 (2) Å, c = 8.161 (2) Å, a = β = 90°, γ = 120°, V= 573.1 Å, and Z = 2, with R = 12.35% and R_w = 9.76% for 2756 (CW neutron) powder data points. This tridymite structural analog undergoes a room-temperature transformation to the open-framework sodalite structure.     

High dielectric constant and low-loss dielectric ceramics of Ba5LnZnNb9O30 (Ln = La,Nd and Sm)

Polycrystalline Ba₅LnZnNb₉O₃₀ (Ln = La, Nd and Sm) ceramics were prepared as single-phase materials through conventional solid-state ceramics route. The structure was characterized by X-ray diffraction methods and scanning electron microscopy (SEM) and the dielectric properties were measured from − 120 °C to 150 °C. All three compounds are paraelectric phases adopting the filled tetragonal tungsten–bronze (TB) structure at room temperature, and the Curie temperature (at 1 MHz) were − 60, − 25 and − 5 °C for Ba₅LaZnNb₉O₃₀, Ba₅NdZnNb₉O₃₀, and Ba₅SmZnNb₉O₃₀ respectively. At 1 MHz their dielectric constant (ε_r) varies from 258 to 310, dielectric loss (tanδ) from 0.0033 to 0.0068, and the temperature coefficients of the dielectric constant (τ_ε) from − 1220 to − 1390 ppm °C^− 1.     

Dielectric properties of Bi4(GeO4)3 and Bi4(SiO4)3

Dielectric constant, dielectric loss and conductivity of Bi₄(GeO₄)₃ and Bi₄(SiO₄)₃ single crystals have been measured as a function of frequency and in the temperature range from liquid nitrogen temperature to 400° C. The values of the static dielectric constant at room temperature are 16·4 and 13·7 for Bi₄(GeO₄)₃ and Bi₄(SiO₄)₃ respectively. The plots of log (σ) against reciprocal temperature at different frequencies of these crystals merge into a straight line beyond 250°C and the activation energies calculated in this region are found to be 0·95 eV and 1·2 eV for Bi₄(GeO₄)₃ and Bi₄(SiO₄)₃ respectively.     

Ferroelectric phase transition in Ba5RTi3Nb7O30 [R=Nd, Eu, Gd] ceramics

Polycrystalline samples of Ba₅RTi₃Nb₇O₃₀ [R=Nd, Eu, Gd], were prepared using high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the compounds shows the formation of single phase compounds (orthorhombic crystal system) at room temperature. Detailed studies of dielectric properties (ɛ, tanδ,σ) as a function of frequency (400 Hz to 10 kHz) and temperature (30° to 380°C) show that these compounds exhibit diffuse ferroelectric phase transition.