Interfacial Structure of Disordered-Ordered Domain Boundaries in Lanthanum-Doped Barium Magnesium Niobate

Microstructural studies on the interfacial boundaries of disordered and 1:2 ordered domains in lanthanum-doped (Ba_0.975La_0.025)(Mg_0.34Nb_0.66)O₃ were performed using high-resolution transmission electron microscopy and synchrotron powder X-ray diffractometry. Disordered and 1:2 ordered domains both coexisted in a single grain. Each domain occupied its own region, and the interfaces were atomically sharp and coherent. The wavelength of the superlattice modulation in the 1:2 ordered domain was ∼0.71 nm. The transition from the 1:2 ordered region to the disordered one could be differentiated at the interface by using the superlattice modulations in the 1:2 ordered domain. From these observations, a deducible interfacial model also was presented.     

Atomic Resolution Transmission Electron Microscopy of the Microstructure of Ordered Ba(Cd1/3Ta2/3)O3 Perovskite Ceramics

Microstructures of ordered Ba(Cd_1/3Ta_2/3)O₃ perovskite dielectric ceramics with and without a boron additive have been observed by atomic resolution transmission electron microscopy (TEM). The selected area electron diffraction and lattice image show a well-ordered structure with hexagonal symmetry (lattice constants of a ∼5.8 Å and c ∼7.1 Å) in the ordered Ba(Cd_1/3Ta_2/3)O₃ with a boron additive, which is similar to those in ordered Ba(Zn_1/3Ta_2/3)O₃ and Ba(Mg_1/3Ta_2/3)O₃ ceramics. Ordered domains with a twin crystallographic relationship and high-density domain interfaces induced by ordering were observed in the ordered Ba(Cd_1/3Ta_2/3)O₃ without a boron additive sintered at a relatively high temperature. Atomic resolution TEM further revealed the conservative twin boundaries along (001) and (110) planes and non-conservative antiphase boundaries with a projected displacement vector of the type [001] in the ordered Ba(Cd_1/3Ta_2/3)O₃ without a boron additive. Finally, the energetics of different domain interfaces are discussed with the interfacial structures in ordered Ba(Cd_1/3Ta_2/3)O₃ ceramics revealed by an electron microscope.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

Direct Compositional Analysis of Ordered Domains in Pure and La-Doped Pb(Mg1/3Nb2/3)O3 by Analytical Electron Microscopy Techniques

The ordered domain structures in Pb(Mg_1/3Nb_2/3)O₃(PMN) and Pb_{1– x} La _x (Mg_1/3Nb_2/3)O₃ are identified using high-resolution transmission electron microscopy (HRTEM) and nanobeam diffractometry (NBD). The chemical compositions in the ordered domains and in the disordered matrices are also acquired using energy-dispersive spectroscopy (EDS). The best matching computer-simulated HRTEM image has a Mg²⁺/Nb²⁺ ratio of return ½. There is no obvious chemical composition difference between the ordered domains and the disordered matrices. The number of the normalized total positive valence electrons remains almost constant in the ordered domains and in the disordered matrices for all the samples. The reason for the growth of the ordered domains in La-doped PMN also is discussed.     

Microstructural Changes in Lanthanum-Doped Barium Magnesium Niobate

Observations of microstructural changes in (Ba_0.95La_0.05)-(Mg_0.35Nb_0.65)O₃ and (Ba_0.925La_0.075)(Mg_0.36Nb_0.64)O₃ (BLMN) were carried out using high-resolution transmission electron microscopy (HRTEM) and synchrotron powder X-ray diffractometry (XRD). In both samples, not only 1:1 and 1:2 ordered domains coexisted in a single grain, but also the intermediate phase, whose structure had a superlattice modulation of 1.42 nm, which was equivalent to 6 times the unit cube of disordered perovskite found on the nanoscale. The ordered 1:2 domains gradually transformed to 1:1 ordered structure through the formation of an intermediate superlattice structure that comprised 6 × 6 × 6 cubic unit cells with different chemical orderings of B-site ions in B-site lattices. Also, the features of thin plates could be detected by XRD patterns and HRTEM. When the thicknesses were very thin, about several atomic distances, stacking faults occurred on (111) planes. However, when their thicknesses were >50 nm, the thin plates existed as a transition phase with their own structure. They were coherent with the matrix and continuously decomposed into the matrix phase by the lateral migration of the interfaces.     

Cation Ordering and Domain Boundaries in Ca[(Mg1/3Ta2/3)1−xTix]O3 Microwave Dielectric Ceramics

Cation ordering and domain boundaries in perovskite Ca[(Mg_1/3Ta_2/3)_{1− x} Ti _x ]O₃ ( x =0.1, 0.2, 0.3) microwave dielectric ceramics were investigated by high-resolution transmission electron microscopy (HRTEM) and Rietveld analysis. The variation of ordering structure with Ti substitution was revealed together with the formation mechanism of ordering domains. When x =0.1, the ceramics were composed of 1:2 and 1:1 ordered domains and a disordered matrix. The 1:2 cation ordering could still exist until x =0.2 but the 1:1 ordering disappeared. Neither 1:2 nor 1:1 cation ordering could exist at x =0.3. The space charge model was used to explain the cation ordering change from 1:2 to 1:1 and then to disorder. A comparison between the space charge model and random layer model was also conducted. HRTEM observations showed an antiphase boundary inclined to the (111) _c plane with a projected displacement vector in the 〈001〉 _c direction and ferroelastic domain boundaries parallel to the 〈100〉 _c direction.     

Microstructural Characteristics of Strontium Magnesium Niobate

Microstructural studies were conducted on the domain boundaries in Sr(Mg_1/3Nb_2/3)O₃ (SMN) complex perovskite compound using X-ray diffractometry and transmission electron microscopy. Both the 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in SMN. SMN had a 1:2 ordered monoclinic unit cell, which was distorted by the antiphase tilting of oxygen octahedra. Two types of domain boundaries were found: the antiphase boundaries (APBs), which are not confined crystallographically, and the ferroelastic domain boundaries, which were parallel to the crystallographic planes. SMN had the superlattice reflections of type ±⅙[111] and ±½[111] in the electron diffraction patterns, which cannot be indexed in terms of the 1:2 ordered trigonal phase with only a hexagonal unit cell. The presence of the ferroelastic domains that contained both the 1:2 ordered and the antiphase tilting had been verified by a high-resolution transmission electron microscopy lattice image. The structure of SMN was well explained by a model proposed by other researchers. The formation of the 1:2 ordered domains preceded the ferroelastic domains. Normally, the growth of the ferroelastic domain is not affected by APBs, but it is interrupted by them when the driving force for growth is insufficient, resulting in the stoppage of the domains at APBs.     

Microstructure and Grain Boundary Structure of Na+-Diffused (Sr,Ca)TiO3 Capacitor-Varistor Ceramics

The microstructure of (Sr,Ca)TiO₃ capacitor-varistor materials has been investigated by employing electron microscopy techniques (TEM, STEM, HREM, EDX, and EPA). The material is found to contain (Sr,Ca)TiO₃ grains (∼30 μm) having perovskite crystal structure with domains, a Na⁺-diffused layer at the grain boundaries which is dependent on thermal diffusion conditions, and multiple-grain junctions in which the Ti _n O_2n–1 Magneli phase coexists with an amorphous intergranular phase. In addition, wider grain boundaries (10–30 nm), thin grain boundaries (∼1 nm), and clean grain boundaries which are free from intergranular phase were observed, and the effects of different grain boundaries on the diffusion of Na⁺are discussed.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.     

Effect of Sintering Additives on Superplastic Deformation of Nano-Sized β-Silicon Nitride Ceramics

The effect of sintering additives on superplastic deformation of nano-sized β-Si₃N₄ ceramics has been studied by compression tests at 1500°C. The sintering additives were (i) Y₂O₃+Al₂O₃; (ii) Y₂O₃+MgO; and (iii) Y₂O₃. Nano-sized Si₃N₄ ceramics with different sintering additives had similar microstructures. For the first two sintering additives, the stress exponents were determined to be ∼2 at a lower stress region and ∼1 at a higher stress region, where the strain rate was dependent on sintering additives only at the higher stress region, and was independent at the lower stress region. Nano-ceramics with Y₂O₃ additives had only one region, which had a stress exponent of ∼1 within the stress range that we studied. The results could be explained by the different deformation mechanisms at the higher and lower stress regions and the influence of viscosity of liquid phase on the transition stress.     

Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3-La(Mg2/3Nb1/3)O3 System

A complete range of perovskite solid solutions can be formed in the (1 − x )Ba(Mg_1/3Nb_2/3)O₃- x La(Mg_2/3Nb_1/3)O₃ (BMN-LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.     

Phase Relations and Ordering in the System Erbia-Hafnia

The system erbia-hafnia was investigated using thermal expansion measurements and room-temperature X-ray diffraction of quenched and annealed samples. The range of existence of the solid solutions based on HfO₂ and Er₂O₃ was determined by the precision lattice parameter method. Three compounds with hexagonal structure are reported: the first is an ordered fluorite-related phase with the ideal formula Er₄Hf₃O₁₂ and decomposes with increasing temperature at ∼1500°C into a solid solution of the fluorite type; the second is formed at ∼55 mol% Er₂O₃, corresponding closely to the formula Er₅Hf₂O_11.5, and undergoes a transformation at ∼1650°C into cubic solid solutions to fluorite and C type. The third compound is formed within the concentration range between 60 and 90 mol% Er₂O₃. This compound, Er₆HfO₁₁, decomposes at ∼1700°C into a cubic solid solution of the C type. A proposed phase diagram for the system erbia-hafnia is presented.     

Fracture Strength and Microstructure of β-SiAlON with Hafnia Addition

The influence of HfO₂ addition on the fracture strength and microstructure of ß-SiAlON ceramics sintered from Si₃N₄ and Al₂O₃ powders was investigated. The strength was increased by the addition of HfO₂, from ∼500 MPa to 700 MPa, and was almost constant from ambient temperature to 1300°C. Monoclinic HfO₂ grains that were distributed in the SiAlON grain boundaries had a flat shape (∼20 nm thick) and were surrounded by an amorphous phase. The aluminum concentration in ß-SiAlON in the samples with an Al₂O₃ starting composition of 15 wt% was decreased by the addition of HfO₂. The amount of secondary phases was very small at grain boundaries between the SiAlON grains; amorphous phases were observed infrequently at the triple points but were very small (∼20 nm). The effects of HfO₂ addition on the mechanism of the microstructure development and fracture strength are discussed.     

Abnormal Grain Growth During Rapid Annealing of Sol–Gel Alumina Thin Film Deposited on Ni-Based Superalloy

A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol–gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al₂O₃ grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al₂O₃ grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al₂O₃ layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: III. Effect of the Muffling Environment

The effect of different muffling environments on the structure and dielectric losses of Ba(Zn_1/3Nb_2/3)O₃ (BZN) microwave ceramics was investigated. The microwave dielectric losses of stoichiometric BZN pellets heated in ZnO-rich environments were severely degraded (e.g. Q × f ∼15 000 in ZnO powder) compared with samples muffled in their own powder ( Q × f ∼80 000). Structure analyses and gravimetric measurements confirmed that the ceramics muffled in ZnO powder or vapor absorb excess ZnO to form non-stoichiometric solid solutions with reduced cation order and Q . By using starting compositions in the (1− x )BZN−( x )BaNb_4/5O₃ binary ( x =0.04), the stoichiometry can be tailored to ensure that after the uptake of ZnO, the ceramics remain well ordered and are located in a high Q region of the system. For example, ZnO-vapor-protected (0.96)BZN−(0.04)BaNb_4/5O₃ reached a very high Q × f (∼1 05 000) after sintering at 1400°C for 5 h.     

Synthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors

Powders of composition Ba_0.65Sr_0.35TiO₃ were prepared from catecholate precursor phases, BaTi(C₆H₄O₂)₃ and SrTi (C₆H₄O₂)₃. The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000.     

Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing

The synthesis of ultrafine cerium dioxide (CeO₂) powders via mechanochemical reaction and subsequent calcination was studied. Anhydrous CeCl₃ and NaOH powders, along with NaCl diluent, were mechanically milled. A solid-state displacement reaction—CeCl₃+ 3NaOH → Ce(OH)₃+ 3NaCl—was induced during milling in a steady-state manner. Calcination of the as-milled powder in air at 500°C resulted in the formation of CeO₂ nanoparticles in the NaCl matrix. A simple washing process to remove the NaCl yielded CeO₂ particles ∼10 nm in size. The particle size was controlled in the range of ∼10–500 nm by changing the calcination temperature.