Influence of calcination temperature on the piezoelectric properties of Ag2O doped 0.94(K0.5Na0.5)NbO3–0.06LiNbO3 ceramics

The effects of calcination temperature on the bulk density, piezoelectric, and ferroelectric properties were investigated for the Ag₂O doped 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramics. The calcination temperatures were varied from 750 to 950 °C by 50 °C differences. An tetragonal XRD pattern, consistent with single-phase 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ was obtained after calcination at 850 °C for 2 h. And the experimental results showed that Ag₂O doped 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramics calcined at 850 °C had a remnant polarization P_r=24.5 μC/cm², bulk density=4.32 g/cm³, piezoelectric constant d₃₃=282 pC/N and electromechanical coefficient k_p=37.8%.     

Nano-sized BaSnO3 powder via a precursor route: Comparative study of sintering behaviour and mechanism of fine and coarse-grained powders

Preparation of a very fine BaSnO₃ powder by calcination of a barium tin 1,2-ethanediolato complex precursor and its sintering behaviour are described herein. A rate controlled calcination process to 820 °C leads to a nm-sized BaSnO₃ powder with a specific surface area of S = 15.1 m²/g (d_av. = 55 nm). The powder has a slightly larger cell parameter of a = 412.22(7) pm compared to the single crystal value, which decreases with increasing calcination temperature and reaches the reference value above 1000 °C. The sintering behaviour is compared between fine and coarse-grained BaSnO₃ powders. Corresponding powder compacts of the nano-sized BaSnO₃ achieve a relative density of 90% after sintering at 1600 °C for 1 h and at 1500 °C and a soaking time of 30 h, whereas coarse-grained powder compacts reach only 80% of the relative density at 1650 °C (10 h). Furthermore, the shrinkage mechanisms of fine and coarse-grained powder compacts have been investigated and are discussed.     

Synthesis and electrochemical properties of nickel oxide/carbon nanofiber composites

The electrospinning of polyacrylonitrile (PAN) with a polyaniline and graphene sol–gel mixture produced uniform, smooth fibers with an average diameter of 0.3 μm. These electrospun fibers were stabilized for 2 h at 200 °C and then carbonized at 800 °C for 5 h. Composites were prepared by depositing Ni(OH)₂ on the carbon nanofibers (CNFs) and calcining them at different temperatures. The composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The effect of the calcination temperatures on the electrochemical properties was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The specific capacitance (SC) was found to be highest (738 F g⁻¹) at a calcination temperature of 400 °C. The charge transfer resistance (R_p) decreased as the calcination temperature was increased. However, the electrical double layer capacitance (EDLC) increased with an increase in the calcination temperature. The EDLC increased from 0.144 F g⁻¹ at a calcination temperature of 100 °C to 485 F g⁻¹ at a calcination temperature of 500 °C.     

Preparation and characterization of LnMgAl11O19 (Ln=La,Nd, Gd) ceramic powders

Lanthanide hexaaluminate powders of LaMgAl₁₁O₁₉ (LMA), NdMgAl₁₁O₁₉ (NMA) and GdMgAl₁₁O₁₉ (GMA) were synthesized via the solid state reaction or sol–gel and calcination method. The LMA and NMA powders synthesized by the sol–gel and calcination method at 1600 °C for 8 h exhibit a single hexaaluminate phase with magnetoplumbite structure; however, the GMA powder synthesized by the sol–gel and calcination method at 1600 °C for 8 h contains both a hexaaluminate phase and a small amount of second phase GdAlO₃ with a perovskite structure. The powders synthesized by the solid state reaction method at 1500 °C for 4 h have a small particle size of 1–3 μm, and a large specific surface area and a good uniformity. The powders synthesized by the sol–gel and calcination method at 1600 °C for 8 h have a particle size of 5–20 μm, and exhibit to a certain extent agglomeration.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Electrochemical performance of La2NiO4+δ cathode for intermediate-temperature solid oxide fuel cells

The application of the La₂NiO_4+δ (LNO), one of the Ruddlesden–Popper series materials, as a cathode material for intermediate temperature solid oxide fuel cells is investigated in detail. LNO is synthesized via a complex method using ethylenediaminetetraacetic acid (EDTA) and citric acid. The effect of the calcination temperature of the LNO powder and the sintering temperature of the LNO cathode layer on the anode-supported cell, Ni–YSZ/YSZ/GDC/LNO, is characterized in view of the charge transfer resistance and the mass transfer resistance. Charge transfer resistance was not significantly affected by calcination and sintering temperature when the sintering temperature was not lower than the calcination temperature. Mass transfer resistance was primarily governed by the sintering temperature. The unit cell with the LNO cathode sintered at 1100 °C with 900 °C-calcined powder presented the lowest polarization resistance for all the measured temperatures and exhibited the highest fuel cell performances, with values of 1.25, 0.815, 0.485, and 0.263 W cm⁻² for temperatures of 800, 750, 700, and 650 °C, respectively.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells

Nanocrystalline SOFC cathode materials of perovskite family, La_1?xSr_xM_1?yCo_yO₃, where 0 < x ≤ 0.5, 0 < y ≤ 0.8 (M is transitional metal = Mn or Fe), have been synthesized at a relatively low temperature by combustion synthesis using alanine as a novel fuel. Detailed X-ray powder diffraction analyses show 47–96% phase purity in the as-synthesized powder and upon calcination at ～825 °C single-phase material is obtained wherein the nanocrystallinity (crystallite size ～19–24 nm) is retained. Densification studies of the materials are carried out within 900–1100 °C. The coefficient of thermal expansion (CTE) of these cathodes is measured. Electrical conductivity of the cathodes sintered at different temperatures are measured in the temperature range 700–900 °C and correlated with the density of the sintered materials. The electrochemical performances of Ni-YSZ anode-supported SOFC having YSZ electrolyte (～10 μm) with CGO interlayer (～15 μm) are studied with the developed cathodes in the temperature range 700–800 °C using H₂ as fuel and oxygen as oxidant. Highest current density of ～1.7 A/cm² is achieved during testing at 800 °C measured at 0.7 V with a cathode composition of La_0.5Sr_0.5Co_0.8Fe_0.2O₃. Precipitation of nanocrystalline grains over the core grains in porous microstructure of this cathode might be one of the reasons for such high cell performance.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Ultrahigh Q values and atmosphere-controlled sintering of Li2(1+x)Mg3ZrO6 microwave dielectric ceramics

Ultrahigh-Q Li_2(1+x)Mg₃ZrO₆ microwave dielectric ceramics were successfully prepared by means of atmosphere-controlled sintering through simultaneously adopting double crucibles and sacrificial powder. This technique played an effective role in suppressing the lithium volatilization and further promoting the formation of the liquid phase, as evidenced by the X-ray diffraction, microstructural observation and the density measurement. Both dense and even microstructure, and the suppression of detrimental secondary phases contributed to low-loss microwave dielectric ceramics with Q×f values of 150,000–300,000 GHz. Particularly, desirable microwave dielectric properties of ε_r=12.8, Q×f=307,319 GHz (@9.88 GHz), and τ_f=−35 ppm/°C were achieved in the x=0.06 sample as sintered at 1275 °C for 6 h.     

Thermal behavior of the nonstoichiometric lithium niobate powders synthesized via a combustion method

Lithium niobate (Li_xNb_1?xO_3+δ) powders with various compositions are prepared via combustion synthesis. The thermal properties, crystal structure, and surface morphology of the as-prepared lithium niobate powders are characterized by thermogravimetric and differential thermal analyses (TG/DTA), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). When the calcination temperature reached 900 °C, the secondary phases Li₃NbO₄ and LiNb₃O₈ were observed. The lithium concentration before 900 °C was 40–43%. The lattice parameters increased slightly with decreasing concentration of lithium ions. When the calcination temperature was higher than 900 °C, the major Li_0.91NbO₃ phase and the minor LiNbO₃ phase coexisted in the nonstoichiometric lithium niobate with 43% lithium content.     

Comparison between microwave and conventional calcination techniques in regard to reactivity and morphology of co-precipitated BaTiO3 powder,and the electrical and energy storage properties of the sintered samples

Barium titanate (BaTiO₃) nanopowders were synthesized by an aqueous co-precipitation method followed by calcination. Either 2.45 GHz microwaves or conventional heating was used in order to investigate the impact of these techniques on the synthesis time, microstructure, and electrical properties of the materials. The heating temperatures ranged from 620 °C to 810 °C. X-ray diffraction (XRD) revealed pure BaTiO₃ formation by microwave heating in a noticeably shorter time (five minutes) compared to conventional heating (3 h). Field emission scanning electron microscopy (FESEM) results confirmed that the microwave process led to nanocube formation, whereas in the conventional procedure, the particles tended to form spherical shapes. To evaluate the electrical properties, the samples heated at 620 °C were conventionally sintered at 1280 °C, 1330 °C, and 1380 °C. Higher dielectric, piezoelectric, and ferroelectric properties and more energy-saving efficiency (ε_r=1012, tan δ=0.035 d₃₃=85 pC/N, p_r=6.2 µC/cm² and η=48% respectively) were achieved in the microwave-heated BaTiO₃ sintered at 1380 °C compared to the conventionally heated BaTiO₃ (ε_r=824, tan δ=0.030 d₃₃=75 pC/N, p_r=5 µC/cm² and η=27%) demonstrating that microwave calcination substantially affects the final electrical properties.     

Dielectric and piezoelectric properties of low temperature synthesized iso-valent modified BT ceramics

Ba_(1?x)Y_xTiO₃ (where Y = Ca, Mg and Sr, x = 0.02, 0.04, 0.06 and 0.08) ferroelectric ceramic samples were synthesized in single perovskite phase by modified solid state reaction (MSSR) route. For single perovskite phase formation and dense grain morphology, 900 °C and 1300 °C were optimized as calcination and sintering temperatures. With the increase of substitutions% of Ca in BCT ceramic samples, the position of T_c increases slightly, whereas with the increase of Mg and Sr substitution% in BMT and BST systems, the position of T_c decreased but remains above RT. Decreased processing temperature with low temperature coefficient of capacitance made BCT ceramic samples useful for dielectric applications. Symmetric nature of the S–E loops indicated the increase of piezoelectric nature with the increase of Ca substitution% in BCT system.     

From platy kaolinite to aluminosilicate nanoroll via one-step delamination of kaolinite: Effect of the temperature of intercalation

Aluminosilicate nanorolls were prepared using a method of one-step delamination of kaolinite. In this method, cetyltrimethylammonium chloride (CTMACl) was intercalated into the interlayer space of methoxy-modified kaolinite, which resulted in the delamination and rolling of kaolinite layers. The reaction conditions of CTMACl intercalation significantly influenced the formation of nanorolls, as shown by characterizations using X-ray diffraction, electron microscopy, infrared spectroscopy, thermal analysis, and nitrogen adsorption. Overall, increasing the CTMACl-intercalation temperature helps to promote the transformation from platy kaolinite to nanorolls. The initial kaolinite particles were mostly transformed to nanorolls in the product prepared at 80 °C, whereas considerable kaolinite particles remained untransformed in the product prepared at 30 °C. At 80 °C, the obtained specific surface area (SSA) and the porous volume (V_por) values of the nanoroll product are nearly twice the values obtained at 30 °C and the tubular structure exhibits higher thermal persistence. The tubular morphology and the porosity of these nanorolls obtained at 80 °C, were largely retained after calcination at 600–800 °C. However, a calcination at 900 °C led to an obvious distortion of the nanorolls and a decrease in SSA and V_por values. The observed structural changes of the nanorolls under calcination generally resembled that of natural halloysite but occurred at lower temperatures because the prepared nanorolls were of lower structural order with thinner tube walls.     

Synthesis,characterization, thermal study,and crystal structure of a new layered alkaline earth metal sulfonate: Sr[C2H4(SO3)2]

A rare two-dimensional layered inorganic–organic hybrid material, strontium sulfonate Sr[C₂H₄(SO₃)₂] was synthesized by hydrothermal reaction of strontium chloride hexahydrate and ethane disulfonic acid at 160 °C. Single-crystal X-ray diffraction was utilized for structural determination. Data showed that the compound, crystallized in the monoclinic system and space group of C2/c. With cell parameters a = 8.3183(7) Å, b = 5.4416(5) Å and c = 14.9784(13) Å. The new material was extensively studied by DRIFT-infrared spectroscopy, thermogravimetric analysis, SEM and powder X-ray diffraction. Results show that the compound is thermally stable.     

Sol–gel synthesis of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The effect of calcination on Li ion conductivity of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolyte prepared by a sol–gel method is examined. The Li ion conductivity of LAGP increases with calcination temperature. After reaching maximum conductivity at 850 °C, the conductivity decreases with increase of the calcination temperature. The calcination holding time also strongly affects Li ion conductivity of LAGP. The conductivity increases with holding time until 12 h and then decreases. It is found that the control of crystallization rate is critical to obtain bulk LAGP with high Li ion conductivity. The highest bulk and total conductivities at 30 °C are 9.5×10⁻⁴ and 1.8×10⁻⁴ S cm⁻¹, respectively, obtained for the bulk LAGP calcined at 850 °C for 12 h.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Preparation of ultra-fine Sm0.2Ce0.8O1.9 powder by a novel solid state reaction and fabrication of dense Sm0.2Ce0.8O1.9 electrolyte film

A novel solid state reaction was adopted to prepare Sm_0.2Ce_0.8O_1.9 (SDC) powder. A mixed oxalate Sm_0.2Ce_0.8(C₂O₄)_1.5·2H₂O was synthesized by milling a mixture of cerium acetate hydrate, samarium acetate hydrate, and oxalic acid for 5 h at room temperature. An ultra-fine SDC powder with the primary particle size of 5.5 nm was obtained at 300 °C. The ultra-low temperature for the formation of SDC phase was due to the atomic level mixture of the Sm³⁺ and Ce⁴⁺ ions. The crystal sizes of SDC powders at 300 °C, 550 °C, 800 °C, and 1050 °C were 5.5 nm, 11.4 nm, 24.1 nm and 37.5 nm, respectively. The sintering curves showed that the powder calcined at lower temperature was easier to be sintered owning to its smaller particle size. A solid oxide electrolytic cell (SOEC), comprising porous La_0.8Sr_0.2Cu_0.1Fe_0.9O_3−δ (LSCF) for substrate, LSCF–SDC for active electrode, SDC for electrolyte, and LSCF–SDC for symmetric electrode, was fabricated by dip-coating and co-sintering techniques. An extremely dense SDC film with the thickness of 20 μm was obtained at only 1200 °C, which was about 100–300 °C lower than the literatures׳ reports. The designed SOEC was proved to work effectively for decomposing NO (3500 ppm, balanced in N₂), 80% NO can be decomposed at 600 °C.     

Microwave-enhanced catalytic degradation of phenol over nickel oxide

A mix-valenced nickel oxide, NiO_x, was prepared from nickel nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by sodium hypochlorite. Further, pure nickel oxide was obtained from the NiO_x by calcination at 300, 400 and 500 °C (labeled as C300, C400 and C500, respectively). They were characterized by thermogravimetry (TG), X-ray diffraction (XRD), nitrogen adsorption at −196 °C and temperature-programmed reduction (TPR). Their catalytic activities towards the degradation of phenol were further studied under continuous bubbling of air through the liquid phase. Also, the effects of pH, temperature and kinds of nickel oxide on the efficiency of the microwave-enhance catalytic degradation (MECD) of phenol have been investigated. The results indicated that the relative activity affected significantly with the oxidation state of nickel, surface area and surface acidity of nickel oxide, i.e., NiO_x (>+2 and S_BET = 201 m² g⁻¹) ≫ C300 (+2 and S_BET = 104 m² g⁻¹) > C400 (+2 and S_BET = 52 m² g⁻¹) > C500 (+2 and S_BET = 27 m² g⁻¹). The introduction of microwave irradiation could greatly shorten the time of phenol degradation.