Rick Ubic

Low-loss dielectric ceramic materials and their properties

In addition to the constant demand of low-loss dielectric materials for wireless telecommunication, the recent progress in the Internet of Things (IoT), the Tactile Internet (fifth generation wireless systems), the Industrial Internet, satellite broadcasting and intelligent transport systems (ITS) has put more pressure on their development with modern component fabrication techniques. Oxide ceramics are critical for these applications, and a full understanding of their crystal chemistry is fundamental for future development. Properties of microwave ceramics depend on several parameters including their composition, the purity of starting materials, processing conditions and their ultimate densification/porosity. In this review the data for all reported low-loss microwave dielectric ceramic materials are collected and tabulated. The table of these materials gives the relative permittivity, quality factor, temperature variation of the resonant frequency, crystal structure, sintering temperature, measurement frequency and references. In addition, the methods commonly employed for measuring the microwave dielectric properties, important from the applications point of view, factors affecting the dielectric loss, methods to tailor the dielectric properties and materials for future applications, are briefly described. The data will be very useful for scientists, industrialists, engineers and students working on current and emerging applications of wireless communications.

Tunable bandgap in BiFeO3 nanoparticles: The role of microstrain and oxygen defects

We demonstrate a tunable bandgap from 2.32 eV to 2.09 eV in phase-pure BiFeO3 by controlling the particle size from 65 nm to 5 nm. Defect states due to oxygen and microstrain show a strong dependence on BiFeO3 particle size and have a significant effect on the shape of absorbance curves. Oxygen-defect induced microstrain and undercoordinated oxygen on the surface of BiFeO3 nanoparticles are demonstrated via HRTEM and XPS studies. Microstrain in the lattice leads to the reduction in rhombohedral distortion of BiFeO3 for particle sizes below 30 nm. The decrease in band gap with decreasing particle size is attributed to the competing effects of microstrain, oxygen defects, and Coulombic interactions.

Raman spectroscopy of B-site order-disorder in CaTiO3-based microwave ceramics

Abstract x CaTiO 3 –(1− x ) Sr(Mg 1/3 Nb 2/3 )O 3 (CTSMN) and y CaTiO 3 –(1− y ) NdAlO 3 (CTNA) were studied by Raman spectroscopy. A sharp Raman band at 825 cm −1 was observed in SrMg 1/3 Nb 2/3 O 3 , consistent with 1:2 long-range ordering of cations on the B-site. The intensity of this band decreased and its width increased with increasing x implying that the degree of order was reduced. A broad Raman band around 800 cm −1 , absent in both pure CaTiO 3 and NdAlO 3 , was found in CTNA solid solutions with the strongest intensity at x =0.5. The position and intensity of this band suggested that its origin was similar in nature to that observed in CTSMN. The occurrence of a band at ∼800 cm −1 in CTNA suggested a non-random distribution of Ti 4+ and Al 3+ on the B cation sites.

Microstructural Characterization of Next Generation Nuclear Graphites

This article reports the microstructural characteristics of various petroleum and pitch based nuclear graphites (IG-110, NBG-18, and PCEA) that are of interest to the next generation nuclear plant program. Bright-field transmission electron microscopy imaging was used to identify and understand the different features constituting the microstructure of nuclear graphite such as the filler particles, microcracks, binder phase, rosette-shaped quinoline insoluble (QI) particles, chaotic structures, and turbostratic graphite phase. The dimensions of microcracks were found to vary from a few nanometers to tens of microns. Furthermore, the microcracks were found to be filled with amorphous carbon of unknown origin. The pitch coke based graphite (NBG-18) was found to contain higher concentration of binder phase constituting QI particles as well as chaotic structures. The turbostratic graphite, present in all of the grades, was identified through their elliptical diffraction patterns. The difference in the microstructure has been analyzed in view of their processing conditions.

Raman spectroscopy and microwave properties of CaTiO3-based ceramics

xCaTiO3−(1−x)Sr(Mg1/3Nb2/3)O3 (CTSMN) and yCaTiO3−(1−y)NdAlO3 (CTNA) microwave ceramics have been studied by x-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. TEM and XRD revealed that all compositions underwent octahedral tilt transitions on cooling, generally resulting in an a−a−c+ tilt system. The exception was the NdAlO3 end member, which had the R3c space group, consistent with an a−a−a− tilt configuration. Sr(Mg1/3Nb2/3)O3 (SMN), (x=0) also exhibited +/−1/3{hkl} reflections in x-ray and electron diffraction patterns associated with 1:2 long-range ordering of the B-site cations. For x⩾0.2, no 1:2 ordered reflections were observed. The long-range B-site ordering in SMN gave rise to sharp Raman bands at 391 and 825 cm−1. The 391 cm−1 band disappeared for x⩾0.2 and the width of the 825 cm−1 band became broader as x increased. It was concluded that for samples with x⩾0.2, only short-range ordering remained which decreased in correlation length as x increased. In CT...

Unification of the negative electrocaloric effect in Bi1/2Na1/2TiO3-BaTiO3 solid solutions by Ba1/2Sr1/2TiO3 doping

The microscopic mechanisms of the negative electrocaloric effect (ECE) of the single-phase (1−x)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-xBa1/2Sr1/2TiO3 (BNT-BT-BST) perovskite solid solutions fabricated via the sol-gel technique are explored in this study. Dielectric and mechanical relaxation analyses are employed to investigate the ferroelectric and structural transitions of the samples. The electrocaloric properties of the samples were measured by thermodynamics Maxwell relations. The difference between the depolarization temperature (Td) and the maximum dielectric constant temperature (Tm) was found to decrease with increasing BST content. Doping with BST stabilized the ferroelectric phase along with unifying the EC temperature changes (ΔT) to only negative values. The origin of the uniform negative ECE of BNT-BT-BST is discussed.

Neutron and electron diffraction studies of La(Zn1/2Ti1/2)O3 perovskite

The crystallography and microwave dielectric properties of La(Zn(1/2)Ti(1/2))O(3) (LZT) ceramics prepared via the mixed-oxide route were investigated in this study. While samples were largely single phase, small amounts of ZnO impurity were detected in sintered pellets. Observed reflections in electron and neutron diffraction patterns indicate that the symmetry of LZT is P2(1)/n. The B site is ordered on {110} or pseudocubic {111}, but the presence of the pseudocubic 1/2(111) reflection is in itself insufficient to indicate the existence of such order. Rietveld refinements of the neutron diffraction data yield an excellent fit for such a model. The structure is highly twinned, with variants related through common {211} composition planes and 90 degrees rotations about <011>. The microwave dielectric properties measured were epsilon(r) = 34, Qf = 36,090 and tau(f) = -70 MK(-1).

Effect of Microstrain on the Magnetic Properties of BiFeO3 Nanoparticles

We report on size induced microstrain-dependent magnetic properties of BiFeO3 nanoparticles. The microstrain is found to be high (e > 0.3%) for smaller crystallite sizes (d < 30 nm), and shows a sharp decrease as the particle size increases. The presence of pseudo-cubic symmetry is evidenced for these nanoparticles. Raman spectral studies suggest straightening of the Fe-O-Fe bond angle accompanied by a decrease in FeO6 octahedral rotation for d < 65 nm. The magnetization shows a dip around 30 nm, half the size of spin cycloid length for BiFeO3, due to a decrease in rhombohedral distortion with crystallite size. We also observe a similar trend in the TN with respect to size indicating that the microstrain plays a significant role in controlling the magnetic property of BiFeO3.

Order–disorder transition in the (1−x)Li2TiO3–xMgO system (0 ≤ x ≤ 0.5)

The order–disorder phase transition of magnesium lithium titanate solid–solution (1−x)Li2TiO3–xMgO (0 ≤ x ≤ 0.5) ceramics prepared by conventional solid-state processing has been examined. The phase and structural analysis was carried out using electron diffraction, neutron diffraction and high-resolution transmission electron microscopy. Both electron and neutron diffraction results revealed the onset of an order-to-disorder transition at 0.3 < x < 0.4. Superlattice reflections found in certain regions of x = 0.2 samples and most areas of x = 0.3 samples were caused by a twin structure stabilized by Mg incorporation. Rietveld refinements of neutron diffraction data suggested a random distribution of Mg on the Li 4e sites and equal distribution of Mg on the two Ti 4e sites for x ≤ 0.3. As the Mg content continues to increase, the crystal symmetry transforms from monoclinic to cubic rocksalt. Consequently, the cation ordering on the 8f and 4d sites of the C2/c structure became corrupted and turned into short-range ordering on the 4a sites of a cubic structure with symmetry, resulting in diffuse scattering in electron diffraction patterns.

Metamorphic temperature investigation of coexisting calcite and dolomite marble––examples from Nikani Ghar marble and Nowshera Formation, Peshawar Basin, Pakistan

Using marble samples from the Nikani Ghar marble and Nowshera Formation from Northern Pakistan the determination of the temperature of metamorphism was undertaken with the help of calcite-dolomite solvus geothermometer. Two types of marbles, that is, calcite-dolomite marble and quartz-bearing calcite-dolomite marble were selected. Petrographic and scanning electron microscope analysis of dolomite samples indicated different grain sizes. X-ray diffraction technique indicated the calcites MgCO3 content up to 7.93 mol.%. Nikani Ghar marble samples have shown lower contents of MgCO3 as compared to samples from Nowshera Formation. The calcite-dolomite-quartz marble has also showed relatively lower MgCO3 content and hence rather low temperature (~500 °C). The temperature reached during peak metamorphism of the investigated marble occurrence, based on calcitedolomite solvus was 628 °C. Metamorphic temperatures derived from the present study were shown as a linear graph and values were in good agreement with the published literature.