Microwave Studies on Strontium Ferrite Based Absorbers

Single layer microwave absorbers based on strontium ferrite-epoxy composites have been fabricated and their reflection loss characteristics studied in the X-band (8–12.4 GHz) of microwave frequencies. Permittivity (_r – j_r) and permeability (_r – j_r) of Co and Ti added strontium ferrite SrCo _x Ti _x Fe_{12 – 2x} O₁₉ (x = 0.1 to 0.9 in steps of 0.2), have been measured. Thickness of the absorber is an important criterion influencing the absorption characteristics. Composites of 3 mm thickness are found to absorb over a reasonable range of X-band frequencies. A minimum reflection loss of –36.5 dB is observed for the composite with x = 0.3.     

Ni-Zn ferrite films synthesized from aqueous solution usable for sheet-type conducted noise suppressors in GHz range

Ni_0.2Zn_0.3Fe_2.5O₄ films (1–5 μm thick) were deposited by spin spray ferrite plating from an aqueous solutions onto polyimide sheets at 90^∘C. Their peel test and high-frequency permeability as well as noise suppression effects were investigated. The oxygen plasma treatment on polyimide sheet surface improved the film adhesion. There was not visible crack on the bended film surface for the Ni-Zn ferrite film thinner than 2 μm and was not peeled off even after the bending test of a million times. The films exhibited excellent high-frequency permeability profile and a natural resonance frequency (where the imaginary permeability reaches a maximum) f _r was 370 ± 30 MHz. The transmission loss increased with the film thickness, reaching the maximum Δ P _loss = 70% at 8 GHz for the 5 μm-thick film. The reflection loss in the measured frequency range was S ₁₁ < 10 % which is small enough for films to be used as the conducted noise suppressors. The value of Δ P _loss obtained for the 5-μm thick film was about 15% higher than that (Δ P _loss = 55 %) attained by the commercialized 50-μm thick noise suppressing sheet.     

Preparation, characterization and dielectric study of new ferroelectric relaxors crystallizing with the tetragonal tungsten bronze structural type

The replacement of sodium and niobium by lanthanum and iron in Pb₂NaNb₅O₁₅, have allowed to prepare the new solid state solution Pb₂Na_1−x La _x Nb_5−x Fe _x 0₁₅ (0 ≤ x ≤ 1), crystallising with the tungsten bronze structure. The latter synthesized, in air, by solid state reaction, has been studied by means of X-ray and dielectric measurements. The composition dependencies of ferroelectric—paraelectric transition temperature and the lattice parameters are reported. The dielectric constants ɛ′_r and ɛ″_r have been also investigated as a function of temperature in the range 20°–700°C, and as a function of frequency in the range 1 k–13 MHz. In the composition range 0 ≤ x ≤ 0.35, the materials exhibit a typical ferroelectric–paraelectric transition while, in the composition range 0.35 < x ≤ 1, a relaxor effect has been evidenced.     

Ni-Cu-Zn Ferrites for Low Temperature Firing: I. Ferrite Composition and its Effect on Sintering Behavior and Permeability

Ni-Cu-Zn ferrites of composition Ni_{1 − x − y}Cu_yZn_xFe₂O₄ with 0.4 ≰ x ≰ 0.6 and 0 ≰ y ≰ 0.25 were prepared by standard ceramic processing routes. The density of samples sintered at 900^∘C increases with copper concentration y. Dilatometry reveals a significant decrease of the temperature of maximum shrinkage with y. The permeability has maximum values of μ = 500–1000 for x = 0.6. The Curie temperature is sensitive to composition and changes form about 150^∘C for x = 0.6 to T_c > 250^∘C for x = 0.4, almost independent on the Cu-content. A small iron deficiency in Ni_0.20Cu_0.20Zn_{0.60 + z}Fe_{2 − z}O_{4 − (z/2)} with 0 ≰ z ≰ 0.06 significantly enhances the density of samples sintered at 900^∘C. The maximum shrinkage rate is shifted to T < 900^∘C. These compositions are therefore appropriate for application in low temperature co-firing processes. The permeability is reduced with z, hence a small z = 0.02 seems to be the optimum ferrite composition for high sintering activity and permeability.     

Fabrication,property and application of novel pyroelectric single crystals—PMN–PT

The pyroelectric properties and temperature stability of Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMN–xPT) single crystals (0.13 ≤ × ≤ 0.40) were investigated. The best choice for pyroelectric performance is [111]-oriented PMN–0.26PT single crystal whose figures of merit for voltage responsivity and detectivity are 0.11 m²/C and 15 × 10⁻⁵ Pa^−1/2, respectively. However, the [001]-oriented PMN–0.37PT single crystal has much better temperature stability, whose temperature coefficient of pyroelectric property is 0.5%/K in the range of 20 °C to 55 °C, and Curie temperature is high: 175 °C. We also found that PMN–xPT possessed low thermal diffusivity D ~ 4.4 × 10⁻⁷ m²/s, low volume specific heat C _v~ 2.5 × 10⁶ J/m³ K and tunable permittivity ε ~ (300–7000). The pyroelectric performances of PMN–xPT single crystals are superior to those of conventional pyroelectric materials and promising for IR device applications.     

Investigations of lanthanum doping on magnetic properties of nano cobalt ferrites

The magnetic properties of nano-crystallite cobalt lanthanum ferrite (CoLa_xFe_2-x O₄) with varied quantities of lanthanum (x = 0, 0.1, 0.15, 0.2, 0.25, 0.3) prepared by co-precipitation method have been studied by vibrating sample magnetometer (VSM) and LCR meter. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the size, structure, and morphology of the ferrite samples. The average crystallite size varied from 17.83 nm to 49.99 nm. All the samples, although, in nano range, show significant hysteresis. The saturation magnetization (M_s) values decreased from 60.57 emu/g to 30.15 emu/g. The remanence (M_R) fell from 10.85 emu/g to 6.39 emu/g. Doping with lanthanum La³⁺ ions modulates significantly the magnetic properties of cobalt spinel ferrites without sacrificing the ferromagnetic character.     

Electrical properties of Pb0.98Eu0.02(ZryTi1−y)0.995O3 prepared by sol–gel processing

A Pb_0.98Eu_0.02(Zr_yTi_1−y )_0.995O₃ compound series, with y = 0.60, 0.53 and 0.45 was prepared. PZT samples were synthesized by sol–gel technique. The crystallization and quality of the compounds were analyzed by powder X-ray diffraction and electron microscopy. The shape of the ε′(ω) vs T curves can be considered typical of a ferro-paraelectric transition. The ferro-paraelectric transition temperature for each composition was 348, 328 and 307°C, for the y = 0.45, 0.53 and 0.60, respectively. σ′(ω) is strongly influenced by short range processes. For the logσ′(ω) curves as function of temperature, there is evidence of a non dispersive dc-conductivity component for the high temperature region. The associated dc-activation energies are larger than those calculated for the ac region (at lower temperatures).     

Pyroelectric and dielectric properties of lead lanthanum zirconate titanate (Pb0.92La0.08)(Zr0.65Ti0.35)O3-P(VDF/TFE)(0.98/0.02) nanocomposites

PLZT-P(VDF/TFE) 0–3 composites with nanosized lead lanthanum zirconate titanate (Pb_0.92La_0.08)(Zr_0.65Ti_0.35)O₃ (PLZT 8/65/35) ceramic powders of volume fraction Φ up to 0.2 were fabricated using PLZT powders imbedded in a copolymer P(VDF/TFE)(0.98/0.02) matrix. The PLZT nanopowders were prepared by the sol-gel technique. The PLZT-P(VDF/TFE) composite samples were prepared from ceramic and polymer powders by the hot-pressing method. Dielectric response was studied in the frequency range from 100 Hz to 1 MHz and at temperatures from 100 to 450 K. The pyroelectric properties were studied by dynamic method with modulation frequency from 1 to 100 Hz. The dielectric response of the ceramics-polymer composite was found to be a combination of the responses of the pure polymer and the ceramics: (1) the addition of the PLZT ceramics increases the value of the dielectric permittivity ɛ′, (2) the composite shows the maximum of the permittivity coming from the PLZT ceramics, (3) the temperature dependences of the dielectric loss tgδ are characterized by the maximum attributed to the α-relaxation (glass transition) in the pure polymer. The pyroelectric coefficient of the composite increases from ∼20 μC/m²K in pure P(VDF/TFE) to ∼140 μC/m²K in the composites of Φ = 0.15.     

Low-temperature sintering and electrical properties of (1−x)Pb(Zr0.5Ti0.5)O3-xPb(Cu0.33Nb0.67)O3 ceramics

Electrical properties and sintering behaviors of (1 − x)Pb(Zr_0.5Ti_0.5)O₃-xPb(Cu_0.33Nb_0.67)O₃ ((1 − x)PZT-xPCN, 0.04 ≤ x ≤ 0.32) ceramics were investigated as a function of PCN content and sintering temperature. For the specimens sintered at 1050^∘C for 2 h, a single phase of perovskite structure was obtained up to x = 0.16, and the pyrochlore phase, Pb₂Nb₂O₇ was detected for further substitution. The dielectric constant (ε _r), electromechanical coupling factor (K_p) and the piezoelectric coefficient (d ₃₃) increased up to x = 0.08 and then decreased. These results were due to the coexistence of tetragonal and rhombohedral phases in the composition of x = 0.08. With an increasing of PCN content, Curie temperature (T_c) decreased and the dielectric loss (tanδ) increased. Typically, ε_r of 1636, K_p of 64% and d₃₃ of 473pC/N were obtained for the 0.92PZT-0.08PCN ceramics sintered at 950^∘C for 2 h.     

Ferroelectric properties of heterolayered lead zirconate titanate thin films

Heterolayered Pb(Zr_{1 − x} Ti _x )O₃ thin films consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ layers were successfully deposited via a multistep sol-gel route assisted by spin-coating. These heterolayered PZT films, when annealed at a temperature in the range of 600–700^∘C show (001)/(100) preferred orientation, demonstrate desired ferroelectric and dielectric properties. The most interesting ferroelectric and dielectric properties were obtained from the six-layered PZT thin film annealed at 650^∘C, which exhibits a remanent polarization of 47.7 μC/cm² and a dielectric permittivity of 1002 at 100 Hz. Reversible polarization constituents a considerably high contribution towards the ferroelectric hysteresis of the heterolayered PZT films, as shown by studies obtained from C-V and AC measurement.     

Dielectric Behavior and Magnetoelectric Effect in Ni0.75Co0.25Fe2O4 + Ba0.8Pb0.2TiO3 ME Composites

Magnetoelectric composites with ferrite + ferroelectric compositions xBa_0.8Pb_0.2TiO₃ + (1 – x) Ni_0.75Co_0.25Fe₂O₄ in which x varies as 0, 0.55, 0.70, 0.85 and 1.0 were prepared by ceramic method. X-ray analysis confirms single-phase formation in x = 0 and x = 1 compositions, whereas the presence of both phases is shown in x = 0.55, 0.70 and 0.85 compositions. Variation of dielectric constant () with temperature and frequency has been studied. All the samples have show linear magnetoelectric conversion in the presence of static magnetic field. Static magnetoelectric conversion factor, (dE/dH), was measured as a function of magnetic field in the samples with x = 0.55, 0.70 and 0.85 compositions. The maximum value of dE/dH was found to be 140 V/cm/Oe for x = 0.85 composition.     

Structural and ferroelectric characterization of BMZ–BF–PT ceramics

Bismuth containing crystalline solid solutions of [(BiMgZrO₃)_1 − y –(BiFeO₃) _y ] _x –(PbTiO₃)_1 − x , (BMZ–BF–PT) have been synthesized by high temperature solid-state reaction technique. The crystalline symmetry varied with the composition, indicating good solid state solubility of BMZ and BF with PT. X-ray diffraction (XRD) reveals that BMZ–PT and BMZ–BF–PT have a single-phase perovskite structure. The MPB of BMZ–PT lies in the region x = 0.55 to x = 0.6, which is supported by the transformation from rhombohedral to tetragonal phase. The SEM photographs exhibit the uniform distribution of grains in the matrix. Polarization–electric field hysteresis studies were carried out at Room Temperature for all the compositions up to an applied electric field of 3.5 kV/mm. It was found that with the increase of BiFeO₃ content in the composition the remnant polarization decreases and the ferroelectric loops get constricted. Variation of dielectric constant with temperature shows that there is an increase in Transition temperature (T _C) with the increase in applied frequency indicating a relaxor characteristic of the compound. Our results show that BMZ–BF–PT is a good low-lead (Pb) high-temperature ferroelectric ceramic.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Picturing the elephant: Giant piezoelectric activity and the monoclinic phases of relaxor-ferroelectric single crystals

Understanding the relationship between the structure and “giant” piezoelectric properties of relaxor ferroelectric solid solutions (1–x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ [PMN–xPT] and (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ [PZN–xPT] is an extremely difficult task. In this article, three main paradigms are reviewed. In the first, the monoclinic phases present at the morphotropic phase boundary (MPB) are responsible for the giant piezoelectric response in that they allow, or at least facilitate, polarization rotation. In the second, a strong polarization rotation effect is explained by the large piezoelectric shear coefficients of zero-field rhombohedral and orthorhombic phases due to the near degeneracy at the MPB and the intrinsic softness of the relaxor state; zero-field monoclinic symmetries are explained by residually distorted rhombohedral and orthorhombic phases in the presence of internal stresses and/or residual bias fields. In the third, the monoclinic “phases” are composed of very finely twinned rhombohedral or tetragonal domains. In this “adaptive phase” model, based on that for ferroelastic martensites, the large electric-field induced strains are extrinsic in nature and result from the progressive switching of the component “nano-twins”; the ease of polarization rotation is explained by a high domain wall mobility. These paradigms remain to be mutually reconciled. The article includes a thorough review of the history of PMN–xPT and PZN–xPT single crystals and, particularly, the most important work done over the last decade.     

Complex permeability of polycrystalline Mn-Zn and Ni-Zn ferrites

The complex permeability of ferrites is frequency dependent. The real part of the complex permeability deteriorates in a high frequency range and the imaginary part has a peak after starting the deterioration. This paper examines the possibility that the frequency characteristics for some ferrites can be approximately derived from a first-order linear differential equation for the magnetic field intensity and magnetic flux density. The first-order differential equation is expressed by the reciprocal of the complex permeability and provides first-order magnetic and electric circuits for ferrite cores. In contrast with the commonly used series R_s-L_s, circuit for the cores, obtained from B = (μ′ − jμ′)H, the first-order electric circuit derived consists of an inductance (L) and resistance connected in parallel. In this paper, it is demonstrated that the inductance L remains constant, whereas Ls decrease with the increase in frequency. In other words, the real part of the reciprocal of the complex permeability remains constant for an increase in frequency. In addition, it is found that the imaginary part of the reciprocal of the complex permeability is approximately proportional to the frequency. © 1998 Scripta Technica, Electr Eng Jpn, 123(2): 1–7, 1998     

Dielectric properties and morphotropic phase boundaries in the xPb(Zn1/3Nb2/3)O3−(1−x)Pb(Zr0.5Ti0.5O3 pseudo-binary system

Ceramics in the xPb(Zn_1/3Nb_2/3)O₃−(1−x)Pb(Zr_0.5Ti_0.5)O₃ [xPZN–(1−x)PZT] solid solution system are expected to display excellent dielectric, piezoelectric, and ferroelectric properties in compositions close to the morphotropic phase boundary (MPB). The dielectric behavior of ceramics with x = 0.1−0.6 has been characterized in order to identify the MPB compositions in this system. Combined with X-ray diffraction results, ferroelectric hysteresis measurements, and Raman reflectivity analysis, it was consistently shown that an MPB exists between x = 0.2 and x = 0.3 in this binary system. When x ≤ 0.2, the tetragonal phase dominates at ambient temperatures. In the range of x ≥ 0.3, the rhombohedral phase dominates. For this rhombohedral phase, electrical measurements reveal a profound frequency dispersion in the dielectric response when x ≥ 0.6, suggesting a transition from normal ferroelectric to relaxor ferroelectric between 0.5 ≤ x ≤ 0.6. Excellent piezoelectric properties were found in 0.3PZN–0.7PZT, the composition closest to the MPB with a rhombohedral structure. The results are summarized in a PZN–PZT binary phase diagram.     

Preparation and characterization of Z-type hexaferrites, Ba3(1−x)Sr3x Co2Fe24O41 with x = 0–0.5, via a two-step calcination with an intermediate wet milling

Z-type hexaferrites (Ba_{3(1 − x)}Sr_3x Co₂Fe₂₄O₄₁with x = 0–0.5, Z-hex) were prepared in a nearly phase pure state by a two-step calcination with an intermediate wet milling. The first calcination of the starting mixture comprising oxides or hydroxides at temperatures below 1100^∘C brought about a mixture of layer-structured M and Y-type hexaferrite phases, together with a spinel phase of Co ferrite. Z-hex was fully crystallized after the second calcination up to 1230^∘C. Wet milling between the two calcination steps was decisive for the phase purity. Emphasis was laid on the quantitative analyses of Z-hex, together with the evaluation of anisotropic growth of the crystallites. Sr addition stabilizes Z-hex, while decreases degree of anisotropy simultaneously.     

Dielectric spectroscopy and distribution of relaxation times of PMN-PSN ceramics

The complex dielectric permittivity of 0.05PMN–0.95PSN ceramic was measured in the frequency range from 20 Hz to 3 GHz. Two anomalies of complex dielectric permittivity were observed 398 and 286 K which can be attributed to the ferroelectric and antiferroelectric phase transitions respectively. From frequency dependences of the real and imaginary parts of dielectric permittivity the distribution of relaxation times f(τ) was calculated. The distribution of relaxation times data together with the dielectric dispersion data shows the competing interactions between these two phase transitions and appearance of the dipolar glass state below the antiferroelectric phase transition.     

Influence of substitution on dielectric diffuseness in Ba<Subscript>(1-x)</Subscript>Bi<Subscript>(2+2x/3)</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics prepared by chemical precursor decomposition method

Barium bismuth niobate, Ba_(1-x)Bi_(2+2x/3)Nb₂O₉ (BBN with x = 0.0, 0.1, 0.2, 0.3, 0.4) ceramic powders in the nanometer range were prepared by chemical precursor decomposition method (CPD). The single phase layered perovskite was prepared throughout the composition range studied. No intermediate phase was found during heat treatment at and above 600°C. The crystallite size and the particle size, obtained from XRD and TEM respectively, were in the range of 15–30 nm. The addition of Bi₂O₃ substantially improved the sinterability associated with high density (96%) which was otherwise difficult in the case of pure BaBi₂Nb₂O₉ (BBN x = 0.0). The sintering was done at 900°C for 4 h. The relative permittivity of BBN ceramics at both room temperature and in the vicinity of the temperature of maximum permittivity (T_m) has increased significantly with increase in bismuth content and loss is also decreased to a certain level of bismuth doping. T_m increased with increase in Bi₂O₃. The diffuseness (γ) in the phase transition was found to increase from 1.54 to 1.98 with the increase in Ba²⁺ substitution level from x = 0.0 to x = 0.3.     

Crystallization and dielectric properties of borate-based ferroelectric PbTiO3 glass-ceramics

Bulk crystallized PbTiO₃ (PT) was the major crystalline phase in borate-based glass ceramics with starting composition 39PbO–1BaO–25TiO₂–9.8Al₂O₃–1SiO₂–24.2B₂O₃ (mol%). Crystallization of PT starts at ≥520 °C and the crystal size and c/a ratio of PT were approximately the same for all samples heat treated between 520 and 700 °C. Two transient unidentified phases were induced when crystallized between 520 and 650 °C. The first at 520 °C followed by a second at 550 °C. At >650 °C, PbTi₃O₇ was present in addition to PbTiO₃. The electrical properties of glasses heat treated between 550–700 °C were investigated by fixed frequency capacitance measurements and impedance spectroscopy, IS. No permittivity maximum or Curie temperature, T _c, for the PT phase was observed from fixed frequency capacitance measurements for glasses heat treated below 700 °C, however IS data analysis using electric modulus (M″) spectroscopy revealed the PT phase to have T _c ∼480 and 440 °C for samples heat treated at ∼550 and 700 °C, respectively. The conductivity of the PT phase and the dc conductivity of the glass heat treated at 700 °C were significantly higher than those of glasses heat treated at lower temperatures suggesting there is significant compositional differences between the glass heat treated at 700 °C compared to those treated at ≤650 °C.