Low temperature sintering of Li2(Mg0.3Zn0.7)Ti3O8-0.12TiO2 microwave dielectric ceramics with controllable grain size

The effects of BaO-B₂O₃-SiO₂ (BBS) frit on sinterability, microstructure and microwave dielectric properties of Li₂(Mg_0.3Zn_0.7)Ti₃O₈-0.12TiO₂ (LMZT) ceramics were systematically investigated. BBS frit can effectively lower the sintering temperature of LMZT ceramics to below 900 °C. Suitable BBS frit addition can accelerate the growth of the LMZT grains while inhibit the abnormal grain growth at the same time. The LMZT ceramics with 2 wt% BBS frit sintered at 900 °C for 3 h show homogeneous microstructure composed of 5–10 μm grains and excellent dielectric properties: ε _r  = 24.1, Q × f = 21,980 GHz, τ _f  = ?4.1 ppm/ °C. It is compatible with Ag electrodes, which makes it a potential candidate material for low temperature co-fired ceramics technology application.     

Directional solidification and interface structure of Fe3O4-YIG eutectic

The directional solidification and interface structure of Fe₃O₄-Y₃Fe₅O₁₂ (yttrium iron garnet, YIG) eutectic grown in flowing air were investigated. The microstructure of the eutectic consisted of grains of broken and deformed lamellae. The orientation relationship between Fe₃O₄ and YIG is (0 1 1) Fe₃O₄(5 1 3) YIG, (1 1 1) Fe₃O₄(2 1 1) YIG and (2 1 1)Fe₃O₄(3 1 1) YIG. The two phases of the eutectic showed the same orientation relationship mentioned above, even when the growth rates were changed from 5–90 mm h^–1. High-resolution TEM revealed that a disorder of the garnet structure of YIG occurs along the interface with a width of two or three atomic layers.     

Low-temperature firing and microwave properties of TiO2 modified Li2ZnTi3O8 ceramics doped with B2O3

The low-temperature firing and microwave properties TiO₂ modified Li₂ZnTi₃O₈ ceramics with B₂O₃ addition have been developed. B₂O₃ addition could reduce the sintering temperature of Li₂ZnTi₃O₈ ceramics from 1,150 to 900 °C, which is attributed to the formation of liquid phases during the sintering process observed by SEM. The 1.5 wt% B₂O₃ doped Li₂ZnTi₃O₈ ceramics sintered at 900 °C have ε _r = 23.3, Q × f = 48,817 GHz, and resonant frequency τ _f  = ?15.35 × 10^?6/°C. Further, due to the compensating effect of rutile TiO₂ (τ_f  = 450 ppm/°C), the temperature coefficient of τ_f for with TiO₂ was adjusted to near zero value. The 1.5 wt% B₂O₃ doped Li₂ZnTi₃O₈ ceramics with 3 wt% TiO₂ dielectrics sintered at 900 °C exhibited the optimal microwave properties: ε_r = 25.9, Q × f = 46,487 GHz, and τ_f = ?0.35 ppm/°C.     

The effect of polyethylenimine on the microwave absorbing properties of a hybrid microwave absorber of Fe3O4/MWNTs

The hybrid microwave absorber of Fe₃O₄/multi-walled carbon nanotubes (MWNTs)modified with polyethylenimine (PEI) polymers was fabricated by chemical co-precipitation. The structure and morphology of hybrids are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron-microscopy (TEM). The effect of PEI on the distribution of Fe₃O₄ nanoparticles and the microwave absorbing properties of hybrid microwave absorber of Fe₃O₄/MWNTs were investigated. The TEM results show that Fe₃O₄ nanoparticles are attached homogeneously on MWNTs, which indicates that the adding of PEI is effective to control the distribution of Fe₃O₄ nanoparticles on the surface of MWNTs. The microwave absorbing properties results show that the maximum reflection loss (RL) of PEI modified Fe₃O₄/MWNTs hybrids is improved significantly, which is ?30.69 dB at 7.24 GHz and ?10 dB bandwidth is 1.84 GHz. However, the RL of the Fe₃O₄/MWNTs without PEI is ?21.96 dB at 7.02 GHz and ?10 dB bandwidth is 1.2 GHz.     

High piezoelectric coefficient of Ba0.85Ca0.15Ti0.90Zr0.10O3−Sr(Cu1/3Ta2/3)O3 ceramics

The (Ba_0.85Ca_0.15) (Zr_0.1Ti_0.9)O₃ ? x   wt% Sr(Cu_1/3Ta_2/3)O₃ [BCZT ? x  wt% SCT, x = 0–0.3] lead-free ceramics were prepared by the conventional solid-state method. The phase structure was investigated by X-ray diffraction. The results show that only a tetragonal phase is observed in these ceramics. The sintering behavior of BCZT ceramics is also improved by using SCT as a sintering aid. The ceramic with x  = 0.2 wt% SCT demonstrates good piezoelectric properties:d₃₃ ~ 577 pC/N and k_p ~ 59.1 %, Furthermore, the Curie temperature of the ceramics also increases and reaches 97 °C.     

Improved hydrogen storage properties of Mg-Li-Al-H composite system by milling with Fe2O3 powder

A sample of Fe₂O₃-doped 4MgH₂-Li₃AlH₆ composite was prepared by the ball milling technique, and the hydrogen storage properties were investigated for the first time. Results showed that the addition of Fe₂O₃ powder reduced the decomposition temperature and improved de/hydrogenation kinetics compared with undoped 4MgH₂-Li₃AlH₆. The onset decomposition temperature for the Fe₂O₃-doped 4MgH₂-Li₃AlH₆ composite decreased by 75 °C compared with that of the undoped composite. For the sorption kinetics, a hydrogen absorption capacity of 2.4 wt% was reached after 60 min in the 10 wt% Fe₂O₃-doped 4MgH₂-Li₃AlH₆ composite, whereas the neat composite absorbed 2.3 wt% hydrogen under the same conditions. For desorption kinetics, the Fe₂O₃-doped 4MgH₂-Li₃AlH₆ sample released 2.5 wt% hydrogen under 10 min of dehydrogenation, but the neat 4MgH₂-Li₃AlH₆ composite only desorbed 2.0 wt% hydrogen within the same period. The apparent activation energy calculated by Kissinger analysis for hydrogen desorption decreased to 112.9 kJ/mol after Fe₂O₃ was added compared with the undoped composite, which was 145.4 kJ/mol. The X-ray diffraction analysis shows the formation new phase of Li₂Fe₃O₄ in the doped sample after ball milling processes that could act as the real catalyst in the Fe₂O₃-doped 4MgH₂-Li₃AlH₆ composite.     

Effect of Ho3+ substitution on magnetic and microwave absorption properties of Sr(ZnZr)0.5Fe12O19 hexagonal ferrite nanoparticles

Sr_1?x Ho _x (ZnZr)_0.5Fe₁₁O₁₉ (x = 0.03, 0.06 and 0.09) hexaferrite nanocrystallites of average sizes in the range of 46–60 nm are synthesized by the citrate sol–gel method. Crystalline structure, morphology, magnetic properties, and microwave absorption properties of powders were studied via X-ray diffraction, field emission scanning electron microscope vibrating sample magnetometer, and vector network analyzer, respectively. The magnetic properties such as saturation magnetization (M _s ) and coercivity (H _c ) were calculated from hysteresis loops. The XRD patterns show that the main phase is M-type strontium hexaferrite without other impurity phases. Microwave absorption properties of hexaferrite (70 wt%)–acrylic resin (30 wt%) composites were measured by the standing-wave-ratio (SWR) method in the range from 12 to 20 GHz. Results showed that substitution of Ho³⁺ ions for Sr²⁺ ions in Sr(ZnZr)_0.5Fe₁₁O₁₉ resonance frequency moves to higher frequency. For samples with x = 0.03, a minimum reflection loss of ?42 dB was obtained at 16.6 GHz for a layer of 1.7 mm in thickness. It was concluded that the prepared composites could be good candidates for electromagnetic compatibility and other practical applications at high frequency.     

Enhanced electromagnetic wave absorption properties of polyaniline-coated Fe3O4/reduced graphene oxide nanocomposites

Fe₃O₄-reduced graphene oxide-polyaniline (Fe₃O₄–RGO–PANI) ternary electromagnetic wave absorbing materials were prepared by in situ polymerization of aniline monomer on the surface of Fe₃O₄–RGO nanocomposites. The morphology, structure and other physical properties of the nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, vibration sample magnetism, etc. The electromagnetic wave absorbing properties of composite materials were measured by using a vector network analyzer. The PANI–Fe₃O₄–RGO nanocomposites demonstrated that the maximum reflection loss was ?36.5 dB at 7.4 GHz with a thickness of 4.5 mm and the absorption bandwidth with the reflection loss below ?10 dB was up to 12.0 GHz with a thickness in the range of 2.5–5.0 mm, suggesting that the microwave absorption properties and the absorption bandwidth were greatly enhanced by coating with polyaniline (PANI). The strong absorption characteristics of PANI–Fe₃O₄–RGO ternary composites indicated their potential application as the electromagnetic wave absorbing material.     

Solvothermal synthesis of hollow glass microspheres/Fe3O4 composites as a lightweight microwave absorber

Hollow glass microspheres/Fe₃O₄ (HGMs/Fe₃O₄) composites with low density were successfully synthesized by solvothermal method at 160 °C. The morphology, composition, and microstructure of the samples were characterized by scanning electron microscopy and X-ray diffraction, respectively. The results show that the HGMs/Fe₃O₄ composites exhibit compact and continuous Fe₃O₄ particles coating on the surface of HGMs. The complex permeability and permittivity of HGMs/Fe₃O₄ composites obtained at 160 °C for different reaction times were measured in the frequency range of 1–18 GHz by vector network analysis. The microwave absorption properties were well-elucidated by the traditional coaxial line method. The as-prepared HGMs/Fe₃O₄ composites show excellent microwave absorption properties. When the thicknesses of these HGMs/Fe₃O₄ composites are more than 1.5 mm, they all exhibit strong absorption peaks (lower than ?10 dB). A possible mechanism of the improved microwave absorption properties was discussed.     

Preparation and microwave dielectric properties of multilayer Li1.075Nb0.625Ti0.45O3 co-fired ceramics with aqueous tape casting

Multilayer Li_1.075Nb_0.625Ti_0.45O₃ (LNT) ceramics for low temperature co-fired ceramic (LTCC) application were prepared by aqueous tape casting. The rheological test indicated that the aqueous slurries exhibited a typical shear-thinning behavior without thixotropy suitable for tape casting process. The green tapes with 2 wt% V₂O₅ addition have satisfactory mechanical properties. Scanning electron microscopy studies revealed that the green tapes have a smooth defect-free surface and that the sintered LNT ceramics have a fine grain microstructure with a uniform grain size. Therefore, aqueous tape casting is suitable for the manufacture of high performance multilayer LNT ceramics. Silver inner-electrode was sintered with LNT tapes at 900 °C, and no reaction been observed between LNT ceramic and sliver layers. The addition of V₂O₅ does not induce much degradation on the microwave dielectric properties. In the case of 2 wt% V₂O₅ addition, the ceramics show good microwave dielectric properties of ε _r  = 65, Q × f = 6,350 GHz. It represents that LNT ceramics could be promising for multilayer LTCC application.     

Low Temperature Solution Synthesis and Microwave Absorption Properties of Multiwalled Carbon Nanotubes/Fe3O4 Composites

Multiwalled carbon nanotubes (MWCNTs)/Fe₃O₄ nanocomposites were synthesized via a simple low temperature solution method. The phase structures and morphologies of the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the Fe₃O₄ spheres of about 150 nm were linked with MWCNTs. The microwave absorption properties of the MWCNTs/Fe₃O₄ nanocomposites were measured by vector network analysis (VNA). A wide region of microwave absorption was achieved due to dual magnetic and dielectric losses. When the matching thickness is 2 mm, the reflection loss (RL) of the sample exceeding ?10 dB was obtained at the frequency range of 9.9–12.4 GHz, with an optimal RL of ?29.8 dB at 11.04 GHz. A possible mechanism of the improved microwave absorption properties of the composites was discussed.     

Effect of TiO2 and Al2O3 doping on sintering behavior and microwave dielectric properties of Ba4(Sm0.5Nd0.5)28/3Ti18+xO54+2x ceramics

The effects of TiO₂ and Al₂O₃ doping on the phase formation, the microstructure and microwave dielectric properties of Ba_6?3x (Sm_1?y ,Nd _y )_8+2x Ti₁₈O₅₄ (x = 2/3 and y = 0.5; BSNT) ceramics were investigated. X-ray diffraction patterns showed that the main crystal phase of BSNT + xTiO₂ (x = 0–2) ceramics sintered at 1,280 and 1,300 °C for 5 h was Ba(Sm, Nd)₂Ti₄O₁₂, accompanied by a small number of second phases: Ba₂Ti₉O₂₀ and TiO₂ (x ≥ 1.0), while the new phase BaAl₂Ti₅O₁₄ appeared and the two phases Ba₂Ti₉O₂₀ and TiO₂ disappeared in BSNT ? 2TiO₂ ceramic doped with ≥2 wt% Al₂O₃ successively as identified by scanning electron microscopy and energy dispersive spectroscopy analysis. The TiO₂ and Al₂O₃ working as sintering aids conduced effectively to promote the densification and grain growth, and thus decreasing the sintering temperature, so when the amounts of TiO₂ was increased, Q × f and τ _f values increased continuously. The BSNT ? 2TiO₂ ceramics doped with y wt% Al₂O₃ decreased the density and dielectric constant, increased the Q × f value remarkably and the τ _f values was adjusted from 25.3 to ?7.3 ppm/ °C. When doped with 1.5 wt% Al₂O₃ sintered at 1,260 °C for 5 h, the ceramics obtained the excellent microwave dielectric properties: ε _r  = 74.3, Q × f = 11,928 GHz, and τ _f  = +5.39 ppm/ °C.     

Influence of Bi2O3 addition on structure and dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics

The structure and dielectric properties of perovskite Ag(Nb_0.8Ta_0.2)O₃ ceramics were explored. A small amount of Bi₂O₃ was used to modify the dielectric properties of the ceramics. The addition of Bi₂O₃ led the ceramics to a high densification and optimal dielectric properties. With the addition of 4.5 wt% Bi₂O₃, the permittivity of Ag(Nb_0.8Ta_0.2)O₃ ceramics increased from 470 to 733, the dielectric loss decreased from 62×10^?4 to 6.7×10^?4, and the temperature coefficient of capacitance, TCC, decreased from 2004 ppm/°C to ?50 ppm/°C. The high permittivity obtained was due to the high densification and weak Ta-O or Nb-O bond strength in the oxygen octahedron that results from the addition of Bi₂O₃.     

Microwave dielectric properties and low temperature sintering of Li2Zn(Ti1−xSnx)3O8 (x ≤ 0.20) ceramics with B2O3–CuO addition

In the present work, the influence of Sn substitution for Ti on the phase composition and microwave dielectric properties of the Li₂Zn(Ti_1?xSnx)₃O₈ (x ≤ 0.20) ceramics was studied. It was found that the Sn did not occupy the Ti site in the Li₂Zn(Ti_1?xSn_x)₃O₈ system but existed in the form of SnO₂ as a secondary phase. With the increase of Sn amount, the best microwave dielectric properties with ε_r = 23.3, Q × f = 71,000 GHz and TCF = ?21.7 ppm/°C were obtained in the Li₂Zn(Ti_0.9Sn_0.1)₃O₈ ceramic sintered at 1,120 °C. The sintering temperature of Li₂Zn(Ti_0.9Sn_0.1)₃O₈ ceramic can be can effectively lowered to below 960 °C by the addition of 0.4B₂O₃–0.6CuO and this materials is chemically compatible with silver. This makes the Li₂Zn(Ti_1?xSn_x)₃O₈ ceramics good candidates for low temperature co-fired ceramics technology.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Effect of Bi2O3 doping on the dielectric properties of medium-temperature sintering BaTiO3-based X8R ceramics

Medium-temperature sintering X8R ceramics were fabricated based on BaTiO₃-based ceramics with Bi₂O₃ additives. The effects of sintering aids Bi₂O₃ on crystalline structure and electrical properties of BaTiO₃-based ceramics were investigated. The sinterability of BaTiO₃ ceramics was significantly improved by adding Bi₂O₃, whose densification sintering temperature reduced from 1,260 to 1,130 °C. However, the dielectric constant (ε) of BaTiO₃-based ceramics doped with Bi₂O₃ was decreased dramatically. Both low ε phase Bi₄Ti₃O₁₂ and the decrease of the tetragonality (c/a ratio), which are demonstrated by XRD pattern, are resulted in the decrease of ε. The ε of samples doped with 5.5 wt% Bi₂O₃ was higher than the other doped samples. The substitution of Bi³⁺ for the Ba²⁺ in BaTiO₃ resulted in the increase of electrovalence (from +2 to +3) of A-site ion, so the attractive force between A and B (Ti⁴⁺) sites becomes stronger. Thus Ti⁴⁺’s polarization enhances, then ε was increased to some extent. The X8R BaTiO₃-based ceramics could be sintered at as low as 1,130 °C by doping 5.5 wt% Bi₂O₃ additives into the BaTiO₃-based ceramics, with a ε greater than 2,430 at 25 °C, dielectric loss lower than 1.3 % and temperature coefficient of capacitance <±15 % (?55–150 °C).     

Three-dimensional magnetic graphene oxide foam/Fe3O4 nanocomposite as an efficient absorbent for Cr(VI) removal

We developed a novel three-dimensional (3D) graphene oxide foam/Fe₃O₄ nanocomposite (GOF/Fe₃O₄) and evaluated its adsorption performance for Cr(IV) removal. The 3D free-standing graphene foam was firstly synthesized on nickel foam and then oxidized and magnetically functionalized with Fe₃O₄ nanoparticles to form GOF/Fe₃O₄. The GOF/Fe₃O₄ exhibited a very large surface area of 574.2 m²/g, a high saturation magnetization of 40.2 emu/g, and a maximum absorption capacity of 258.6 mg/g for Cr(IV) removal, which significantly outperformed the reported 2D graphene-based adsorbents and other conventional adsorbents. The present work may offer a way to prepare a range of 3D magnetic graphene-based adsorbents for application in effective removal of heavy metal ions.     

Study on properties of Ca2Zn4Ti15O36 ceramics with CaO-B2O3-SiO2 glass

The effects of CaO-B₂O₃-SiO₂ (CBS) glass additions on the sintering temperatures and dielectric properties of Ca₂Zn₄Ti₁₅O₃₆ ceramics have been investigated using X-ray diffraction, Scanning electron microscopy. A small amount of CBS glass addition to Ca₂Zn₄Ti₁₅O₃₆ ceramics can greatly decrease the sintering temperature to about 975 °C. It is revealed that the reduced sintering temperature is attributed to the formation of liquid phase. The major phases of the samples are Ca₂Zn₄Ti₁₅O₃₆ and rutile. The more glass added, the more rutile phase formed. The τ_f values shift towards negative first and then toward positive with the increasing glass additions. The Ca₂Zn₄Ti₁₅O₃₆ ceramics with 4 wt% glass addition sintered 1,050 °C for 2 h exhibit good microwave dielectric properties: Q·f values of 31,000 GHz and dielectric constant (ε _r ) of 44.7.     

Enhancing the microwave dielectric properties of Nd (Mg0.5Sn0.5)O3 ceramics by substituting Nd3+ with Ca2+

The microwave dielectric properties of Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were examined to evaluate their exploitation for mobile communication. Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with the sintering temperature. Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1,550 °C for 4 h had the following properties: a density of 6.86 g/cm³, a dielectric constant (ε_r) of 19.3, a quality factor (Q × f) of 99,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?65 ppm/°C.     

Hydrothermal synthesis of Fe3O4 and α-Fe2O3 nanocrystals as anode electrode materials for rechargeable Li-ion batteries

This paper describes a facile, economical and environment-friendly hydrothermal method of fabricating Fe₃O₄ and α-Fe₂O₃ nanoparticles at 180 °C for 12 h, respectively. The as-obtained products were characterized in detail. X-ray powder diffraction and transmission electron microscopy were used to investigate the products’ properties of crystal form, size, and morphology. The results showed the Fe₃O₄ and α-Fe₂O₃ nanocrystals’ diameter were about 5 and 20 nm, respectively. Moreover, the electrochemical performances of the Fe₃O₄ and α-Fe₂O₃ nanoparticles as anode materials for Li-ion batteries were also evaluated. The first-discharge capacities of Fe₃O₄ and α-Fe₂O₃ nanocrystals were 1,380 and 1,280 mAh g^?1, and stabled about 96 and 75 mAh g^?1 after 20 cycles, respectively. These materials offer substantial promise for developing alternative, high capacity negative electrodes for safer lithium batteries as energy storage and conversion materials.