Microwave dielectric properties of Li3A3Te2O12 (A = Y,Yb) garnets for low temperature cofired ceramic technologies

Te⁶⁺-containing microwave dielectric ceramics Li₃A₃Te₂O₁₂ (A = Y, Yb) with low firing temperatures were prepared using the solid-state reaction method. Li₃Y₃Te₂O₁₂ and Li₃Yb₃Te₂O₁₂ can be obtained as garnet single-phase ceramics with low sintering temperatures of 940 ℃ and 950 ℃, respectively. The cations, such as Li⁺, Te⁶⁺, and Y³⁺/Yb³⁺, fully occupied the tetrahedral, octahedral, and dodecahedral sites of garnet structure, respectively. Li₃A₃Te₂O₁₂ (A = Y, Yb) ceramics exhibit ε_r ～ 7.83 ± 0.2 and 5.94 ± 0.2, Q × f ～ 47,800 ± 500 GHz and 41,800 ± 500 GHz, and τ_f ～ –47 ± 3.0 ppm/°C and –76 ± 3.0 ppm/°C, respectively. The full width at half-maximum (FWHM) values of the A_1g Raman mode correlate negatively with the Q × f values. Moreover, Li₃A₃Te₂O₁₂(A = Y, Yb) ceramics possess good chemical compatibility with the Ag electrode, making them promising candidates for low-temperature cofired ceramics (LTCC) technologies.     

Composition tunable manganese-doped magnetite microwave absorber composites for radio frequency identification communication

The performance of radio frequency identification (RFID) tags deteriorates in transmitting/receiving characteristics when the tags are mounted on conductive substances such as metals due to the interference from the metal reflected electromagnetic waves. In this work, the manganese-doped magnetite (Mn_XFe_3?XO₄ (0 ≤ X ≤ 1)) nanoparticles with different manganese contents are synthesized by the one-pot sol-gel method for the application of microwave absorber. The effects of Mn doping on the morphology, microstructure, dielectric and magnetic properties, and microwave absorption ability of Mn_XFe_3?XO₄ nanoparticles are investigated. The saturation magnetization and dielectric and magnetic losses of Mn_XFe_3?XO₄ nanoparticles enhance as the Mn doping increases, and achieve the maximum when X = 0.50, and decrease as the Mn doping further continues to augment. At the optimized value of X = 0.50, the reflection and transmission losses are measured to be 17.51 and 3.88 dB, respectively. Furthermore, the nanoparticles are mixed with polytetrafluoroethylene powder and hot-pressed into a composite mat to demonstrate its application as microwave absorber. When the composite mat is inserted between the RFID tag and metal surface, the RFID tag recovers the read count by 86%, which is due to the high absorption ability (80.77%) of the composite mat. Therefore, the composition tunable Mn_XFe_3?XO₄ nanoparticles can be a potential candidate for the application of microwave absorber in wireless communication and electronic industries.     

Dielectric behaviour of BZT-BCT nano pseudobinary system near TCP-assisted morphotopic phase boundary

The present study investigates the comparative dielectric studies in nano pseudobinary system synthesized by conventional and hybrid microwave sintering. The material was formed near Tri Critical Point (TCP)-assisted Morphotropic Phase Boundary (MPB) region from equal weight percentage of Lead Zirconate Titanate (BZT) and Barium Calcium Titanate (BCT) employing high energy ball milling. In the temperature dependent LCR study, a diffused phase transition (DPT) behaviour was identified for both conventional and hybrid microwave sintered samples. In each case, modified Currie-Weiss relation indicates an anomaly in the paraelectric state. Further, another approach using macroscopic composition-fluctuation model also exhibits the anomalous behaviour. Universal dielectric response (UDR) demonstrates that such behaviour is created by a temperature assisted relaxation phenomena by the formation of oxygen vacancies. Beyond 260 °C, microwave sintering creates more oxygen vacancies across grain boundary region as compared to the conventional sintering and in turn plays the key role for enhanced dielectric permittivity.     

Microwave-sintering enhanced photovoltaic conversion in polycrystalline Nd-doped BiFeO3

This work reports remarkable photovoltaic effects in the microwave-sintered lead-free perovskite (Bi_0.93Nd_0.07)FeO₃ (B7NFO) ceramics with indium tin oxide (ITO) thin film under 405 nm blue irradiation. Maximal power conversion efficiency (PCE) ~1.2% and external quantum efficiency (EQE) ~17% can be obtained at low irradiation intensity. Optical absorption and photoluminescence emission suggest direct bandgap (E_g) in the range of 2.11–2.21 eV in B7NFO ceramics. Grain boundaries exhibit a lower electrical potential and higher electrical conductivity, and can act as conduction channels for the photo-generated charge carriers. High-resolution transmission electron microscopy (HR-TEM) suggests that field-enhanced photovoltaic effects are associated with large and highly ordered polar nano-regions (PNRs) in the matrix. Microwave-sintering process corresponds to a lower activation energy (433 kJ/mol) of grain growth than the conventional-sintering process (604 kJ/mol). This study demonstrates that the microwave-sintering process can be an effective and cost-less technique for energy-harvesting polycrystalline materials.     

Ilmenite-type MgTiO3 ceramics by complex (Mn1/2W1/2)4+ cation co-substitution producing improved microwave characteristics

In this article, the (Mn_1/2W_1/2)⁴⁺ complex cation co-doped ilmenite MgTiO₃ ceramics with improved microwave characteristics were synthesized. The correlations between the crystal structural evolution induced by ionic substitution and the microwave characteristics were investigated using the structural analysis and the P–V–L bond theory. Theoretically, the Mg–O bond should have a larger value of covalency than Ti–O bond, attributing to the distribution of densities of states, where the s and p states of the Mg atom overlap with those of the O atom. This conclusion fits well the bond theory estimation. The dielectric constant is dominated predominantly by the average bond covalency, which is intrinsically caused by the increase of doping contents. Moreover, the structural stability declines slightly with the increase of (Mn_1/2W_1/2) contents. From the perspective of structural evolution, this dielectric performance is also reflected by the variations of the Raman shift and the FWHM value of A_g⁵ mode. The actual Q × f value, however, experiences a great enhancement at x = 0.010, which benefits generally from the uniformity of the grain size and the inhibition of reduction of Ti⁴⁺ valence. The excellent microwave characteristics of MgTi_0.99(Mn_1/2W_1/2)_0.01O₃ ceramics were achieved: a ε_r of 18.74, a Q × f value of 160992 GHz and a τ_f of ?58.2 ppm/°C, when sintered at 1325 °C.     

Magnetodielectric response of composites based on a natural garnet and spinel ferrites for sub-GHz wireless applications

A natural garnet with excellent dielectric properties was mixed with different weight percentages of two spinel ferrites, Ni_0.5Zn_0.5Fe₂O₄ (NZO) and LiFe₅O₈ (LFO) to tailor its magnetodilectric properties. 3 wt% B₂O₃ was added to enhance the density of the composites. X-ray diffraction study revealed the decomposition of the mineral into hematite and cordierite and vibrational spectroscopic analysis confirmed the non-reactivity of decomposed mineral with spinel ferrites. Microstructural analysis shows well densified and almost tightly packed grains for garnet-Ni_0.5Zn_0.5Fe₂O₄ (G-N) and garnet-LiFe₅O₈ (G-L) composites. The optimised dielectric and magnetic properties of 0.5 G-0.5 NZO are ε_r = 4.1, μ_r = 1.8, tan δ_ε = 0.02, tan δ_μ = 0.49, whereas that of 0.5 G–0.5 LFO are ε_r = 4.4, μ_r = 1.4, tan δ_ε = 0.001, tan δ_μ = 0.05 at 1 GHz. Due to the moderate permittivity of garnet, a better impedance matching compared to magnetodielectric composites based on high-permittivity dielectric counterparts is observed. Hence, the present study indicates that G-N and G-L composites are potential candidates for sub-gigahertz wireless applications.     

Ultralight and superelastic polyvinyl alcohol/SiC nanofiber/reduced graphene oxide hybrid foams with excellent thermal insulation and microwave absorption properties

Being in the strategic direction of next-generation absorbers, multifunctional microwave absorbing materials possess great application value in military and commercial fields. However, the stringent requirements for performance necessitate the combination of multiple functions in such type of composites, which is still a challenge. This work aims to develop a foam-type absorber composed of multi-dimensional organic and inorganic materials, in which reduced graphene oxide sheets and polyvinyl alcohol membranes serve as the framework and crosslinker to form a three-dimensional skeleton. Meanwhile, SiC nanofibers as a reinforcing component can effectively suppress the over-stacking of reduced graphene oxide and enhance the conductivity and mechanical strength of cell walls. Among the remarkable microwave absorbing properties of the obtained foam, there are the ultra-light (9.85 mg cm^-3), broadband (7.04 GHz), and strong absorption (reflection loss of -61.02 dB), all combined in the ultra-thin (2.5 mm). In addition, the foam possesses superelastic and excellent heat-insulating characteristics that ensure shock resistance, heat preservation, and infrared stealth. The remarkable versatility benefits from the porous structure, as well as from the synergistic effect of multi-dimensional organic and inorganic constituents of the foam. Therefore this study lays the foundation for the design of new-generation microwave absorbers with broad application potential.     

Microwave-Assisted,Base-Catalyzed Synthesis of Fatty Acid Methyl Esters from Seeds and Fish Oil Supplements

Growing health awareness has resulted in the increased use of dietary supplements derived from plants and marine sources, leaving consumers unsure of their best options. There were three objectives of the present study. The first was to design and evaluate an efficient derivatization procedure. The second was to perform a comparative analysis of liquid oils and their corresponding capsules of hemp, chia, and flax seeds. The final objective was to determine the fatty acid (FA) composition of six fish oil products and compare it to the one provided on the label. For the FA profiling, we implemented two efficient, one-step, sustainable methods with high percentage recovery for the synthesis of FA methyl esters (FAME), which use base catalysis and microwave-assisted heating. Our results found no difference in nutritional value between liquid oils and capsules of the seed supplements, with flaxseed and chia offering a higher, beneficial n-3:n-6 ratio compared to hemp oil. Four of the fish oils analyzed contained significantly less eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) than their reported label, and the other two not only agreed with the manufacturers' declaration but were able to fulfill the daily adequate intake (AI) with fewer capsules.     

Microwave dielectric properties of silico-carnotite Ca3M2Si3O12 (M= Yb,Y) ceramics synthesized via high energy ball milling

The Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics with orthorhombic silico-carnotite structure were fabricated via high-energy ball milling and solid-state reaction route. Dense Ca₃Yb₂Si₃O₁₂ and Ca₃Y₂Si₃O₁₂ ceramics sintered at 1260 °C and 1240 °C revealed promising microwave dielectric properties with ε_r = 9.2 and 8.7, Q×f = 56,400 GHz and 29,094 GHz, τ_f = −77.5 ppm/°C and −76.8 ppm/°C, respectively. The connection between crystal structure and Q×f values of Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics was discussed with respect to the packing fraction, and their intrinsic microwave dielectric properties were examined using the infrared reflectivity spectra analysis. The thermal stability of Ca₃Yb₂Si₃O₁₂ was improved successfully by forming 0.91Ca₃Yb₂Si₃O₁₂‐0.09CaTiO₃ composite ceramics with τ_f = +2.9 ppm/°C, ε_r = 12.93 and Q×f = 26,729 GHz.