Fabrication of TiO2–SiO2 Photonic Crystals with Diamond Structure

Ceramic photonic crystals with diamond structure were fabricated using stereolithography and successive sintering. The green body of an epoxy resin incorporating 10 vol% TiO₂–SiO₂ was formed by stereolithography and then heated in air at 1100°–1400°C for 2 h. The sintered products maintained the diamond structure with a linear shrinkage ratio of about 57% and a porosity of 38%. The ceramic photonic crystal with eight unit cells showed a photonic band gap at the center frequency of 23.5 GHz. This fabrication method of three-dimensional (3D) ceramic photonic crystals is applicable to other 3D structural ceramics and does not require any molding techniques.     

Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO₂ ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO₂ paste (before sintering) was around 15 μm. After the SiO₂-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO₂ ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO₂ photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X 〈100〉 direction, between 490 and 630 GHz in the Γ– K 〈110〉 direction, and between 400 and 510 GHz in the Γ– L 〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.     

Band Gap Modification of Diamond Photonic Crystals by Changing the Volume Fraction of the Dielectric Lattice

Photonic crystals with a diamond structure of epoxy lattices in which TiO₂-based ceramic particles are dispersed were fabricated by stereolithography. The periodicity of the lattice was designed to reflect electromagnetic waves in the gigahertz range. The volume fraction (β) of the dielectric lattice medium was modified from 14% to 33% by changing the rod diameter of the lattice. The photonic band gap was observed along Γ-L 〈111〉, Γ-X 〈100〉, and Γ-K 〈110〉 directions and the complete photonic band gap was formed at over β= 20%. The width of the forbidden gap increased gradually when the β increased over 14%, and reached 2.4 GHz at β= 33%. These results agreed with the band calculation using the plane wave expansion method.     

Fabrication of Photonic Crystal with a Diamond Structure Having an Air Cavity Defect and its Microwave Properties

Three-dimensional photonic crystals with a diamond lattice structure consisting of 5 × 5 × 5 unit cells with the unit cell dimension of 15 mm were fabricated using TiO₂-based ceramic particles dispersed epoxy by stereolithography. The diamond lattice showed a perfect band gap between 14.3 and 15.8 GHz. An air cavity defect with a rectangular shape (15 mm × 45 mm × 15 mm) was introduced at the center of the crystal by extracting 3 unit cells in order to investigate the shape effect of the defect on the formation of localized defect modes of electromagnetic wave. When microwaves were radiated normal to the wide sides (45 mm × 15 mm) of the rectangular shape defect, a sharp localized mode appeared at the middle of the band gap. However, no localized mode was observed for incident waves normal to the smaller side (15 mm × 15 mm) because of the symmetry mismatching between internal eigenmodes in the defect cavity and incident plane waves. The mode analysis using a simple cavity model showed the penetration of the electric field of resonant modes about 2.4 mm into the host lattice.     

Fabrication and Microwave Properties of Three-Dimensional Photonic Crystals With a Diamond Structure Composed of Ceramic Spheres in Resin

Three-dimensional photonic crystals with a diamond structure composed of YSZ (3 mol% Y₂O₃-stabilized ZrO₂) spheres in a resin matrix were fabricated by using stereolithography. The lattice constant was 12 mm and the diameter of the spheres was 5 mm. These photonic crystals made of ceramic spheres showed complete photonic band gaps at around 12 GHz between the eighth and ninth bands. The propagation characteristics of microwaves agreed well with the calculated results using the plane wave expansion method.     

Microwave Absorption in Photonic Crystals Composed of SiC/Resin with a Diamond Structure

Three-dimensional photonic crystals were fabricated by infiltrating an epoxy mold with a SiC/polyester mixture. The epoxy molds with normal or inverse diamond structures were formed by stereolithography. The size of the mold was 45 mm × 18 mm × 18mm, and the lattice constant of the photonic crystals was 18 mm. The effects of the epoxy mold type, aspect ratio (the ratio of height and diameter of a diamond lattice rod), and number of sample units on the formation of photonic band gap (PBG) and microwave absorption ability along the 〈100〉 direction were studied. The attenuations of microwave transmission and reflection were measured through the photonic crystal samples at a frequency range of 3–12 GHz with a network analyzer. The results obtained suggest that the combination of the absorbing material SiC and diamond structure has a dual effect to form a PBG with a high absorption ability.     

Fabrication of Three-Dimensional Micro Photonic Crystals of Resin-Incorporating TiO2 Particles and their Terahertz Wave Properties

Three-dimensional (3D) photonic crystals with a diamond structure made from 40 vol% TiO₂–acrylate dielectric composites were formed by means of a CAD/CAM micro-stereolithography system. The lattice constant of the diamond unit cell was 500 μm and the forming accuracy was 10 μm. The photonic band gap in the Γ–X 〈100〉 direction measured by terahertz-time-domain spectroscopy appeared at 280–360 GHz, which agreed fairly well with the band gap calculated by the plane wave expansion method.     

Fabrication and Characterization of Three-Dimensional ZrO2-Toughened Al2O3 Ceramic Microdevices

Dense three-dimensional (3D) microdevices of ZrO₂-toughened Al₂O₃ (ZTA) were fabricated using microstereolithography and a subsequent sintering process. Using microstereolithography, 3D green bodies could be formed from a 40 vol% ZTA ceramic–resin paste. After sintering, the fabricated 3D devices are converted into dense ceramic devices without deformation. In this study, a gear (with a tooth edge of 25 μm) and a photonic crystal (with a lattice constant of 500 μm) were designed and fabricated. The dimensional accuracy of the fabrication process is within 20 μm and the sintering shrinkage is around 26% for these microdevices. The relative density of the sintered ZTA ceramics reached 96.5% of theoretical value. The measured hardness and toughness were about 14 GPa and 11 MPa m^1/2, respectively, in both the top and side surfaces. A band gap between 320 and 420 GHz was observed in the ZTA photonic crystal. The microstereolithography process can be easily applied to other ceramic materials and devices.     

Fabrication of three-dimensional ceramic photonic crystals and their electromagnetic properties

Fabrication process for three-dimensional ceramic photonic crystals with a diamond structure was investigated. A diamond structure composed of epoxy lattice including SiO₂–TiO₂ ceramic particles at 10 vol.% was fabricated as a precursor by stereolithography. After burning off the epoxy resin in air, the diamond structure of SiO₂–TiO₂ was successfully sintered at 1400 °C for 2.5 h. The linear shrinkage ratio was 50%. Cracks were not found in the sintered diamond structure. Photonic bandgap was observed at around 19 GHz.     

Fabrication of Metallodielectric Photonic Crystals with a Diamond Structure and their Microwave Properties

Three-dimensional (3D) metallodielectric photonic crystals with a diamond structure were fabricated in order to investigate the formation of stop bands and the absorption ability for microwaves with the dielectric absorbing media embedded into the 3D metal lattice. First, the metallic photonic crystals were prepared by filling the epoxy molds formed by stereolithography with a metal alloy having a low melting point of 70°C, followed by removal of the molds. The metallodielectric photonic crystals were then fabricated by infiltrating the porous metal crystal with a SiC/polyester mixture. The lattice constant of photonic crystals was 15 mm. The effects of different aspect ratios of diamond lattice rods, number of metallic lattice units along Γ-L 〈1 1 1〉, Γ-X 〈1 0 0〉, and Γ-K 〈1 1 0〉 directions, and metallodielectric samples along the Γ-X 〈1 0 0〉 direction on the formation of stop band and microwave absorption ability were investigated in the frequency range from 3 to 30 GHz. Metallodielectric photonic crystals formed showed good absorption ability. The measured transmission spectra of the metallic and metallodielectric crystals agreed well with the simulation of the transmission line modeling method.     

Microfabrication of Three-Dimensional Photonic Crystals of SiO2–Al2O3 Ceramics and Their Terahertz Wave Properties

Dense three-dimensional microphotonic crystals of SiO₂–Al₂O₃ ceramics were fabricated using microstereolithography and successive sintering process. The forming dimensional tolerance for a 50 vol% ceramic paste is 10 μm and sintering shrinkage is around 12%. Diamond-type photonic crystals with lattice constants of 500 and 125 μm were formed and sintered successfully. The band gaps of the samples were measured and compared with the theoretically calculated band diagram.     

Low-Temperature Sintering and Microwave Dielectric Properties of Li2MgSiO4 Ceramics

Development of a low-temperature sintered dielectric material derived from Li₂MgSiO₄ (LMS) for low-temperature cofired ceramic (LTCC) application is discussed in this paper. The LMS ceramics were prepared by the solid-state ceramic route. The calcination and sintering temperatures of LMS were optimized at 850°C/4 h and 1250°C/2 h, respectively, for the best density and dielectric properties. The crystal structure and microstructure of the ceramic were studied by the X-ray diffraction and scanning electron microscopic methods. The microwave dielectric properties of the ceramic were measured by the cavity perturbation method. The LMS sintered at 1250°C/2 h had ɛ_r=5.1 and tan δ=5.2 × 10⁻⁴ at 8 GHz. The sintering temperature of LMS is lowered from 1250°C/2 h to 850°C/2 h by the addition of both lithium borosilicate (LBS) and lithium magnesium zinc borosilicate (LMZBS) glasses. LMS mixed with 1 wt% LBS sintered at 925°C/2 h had ɛ_r=5.5 and tan δ=7 × 10⁻⁵ at 8 GHz. Two weight percent LMZBS mixed with LMS sintered at 875°C/2 h had ɛ_r=5.9 and tan δ=6.7 × 10⁻⁵ at 8 GHz.     

Electronic Structure and Bonding in Crystalline Y10[SiO4]6N2

The electronic structure and bonding of the complex ceramic crystal Y₁₀[SiO₄]₆N₂ is studied by a first-principles method. It is shown that this crystal is an insulator with a direct band gap of 1.3 eV. It has some unique properties related to the one-dimensional chain structure in the c -direction and the planar N-Y bonding in the x - y plane.     

Alumina/Epoxy Interpenetrating Phase Composite Coatings: I, Processing and Microstructural Development

Interpenetrating phase composite (IPC) coatings consisting of continuously connected Al₂O₃ and epoxy phases were fabricated. The ceramic phase was prepared by depositing an aqueous dispersion of Al₂O₃ (0.3 μm) containing orthophosphoric acid, H₃PO₄, (1–9.6 wt%, solid basis) and heating to create phosphate bonds between particles. The resulting ceramic coating was porous, which allowed the infiltration and curing of a second-phase epoxy resin. The effect of dispersion composition and thermal processing conditions on the phosphate bonding and ceramic microstructure was investigated. Reaction between Al₂O₃ and H₃PO₄ generated an aluminum phosphate layer on particle surfaces and between particles; this bonding phase was initially amorphous, but partially crystallized upon heating to 500°C. Flexural modulus measurements verified the formation of bonds between particles. The coating porosity (and hence epoxy content in the final IPC coating) decreased from ∼50% to 30% with increased H₃PO₄ loading. The addition of aluminum chloride, AlCl₃, enhanced bonding at low temperatures but did not change the porosity. Diffuse reflectance FTIR showed that a combination of UV and thermal curing steps was necessary for complete curing of the infiltrated epoxy phase. Al₂O₃/epoxy IPC coatings prepared by this method can range in thickness from 1 to 100 μm and have potential applications in wear resistance.     

Effect of Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.     

Glass-Free Zn2Te3O8 Microwave Ceramic for LTCC Applications

A Zn₂Te₃O₈ ceramic was investigated as a promising dielectric material for low-temperature co-fired ceramics (LTCC) applications. The Zn₂Te₃O₈ ceramic was synthesized using the solid-state reaction method by sintering in the temperature range 540°–600°C. The structure and microstructure of the compounds were investigated using X-ray diffraction (XRD) and scanning electron microscopy methods. The dielectric properties of the ceramics were studied in the frequency range 4–6 GHz. The Zn₂Te₃O₈ ceramic has a dielectric constant (ɛ_r) of 16.2, a quality factor ( Q _u× f ) of 66 000 at 4.97 GHz, and a temperature coefficient of resonant frequency (τ_f) of −60 ppm/°C, respectively. Addition of 4 wt% TiO₂ improved the τ_f to −8.7 ppm/°C with an ɛ_r of 19.3 and a Q _u× f of 27 000 at 5.14 GHz when sintered at 650°C. The chemical reactivity of the Zn₂Te₃O₈ ceramic with Ag and Al metal electrodes was also investigated.     

Study on radiation performances of dipole antenna with diamond‐structure EBG substrate fabricated by 3D printing technique

In order to improve antenna radiation performances, a dipole antenna with 3D diamond‐structure electromagnetic band‐gap (EBG) substrate is analyzed and its transmission performances are investigated. The diamond‐structure EBG substrate was fabricated with aluminum oxide ceramic by 3D printing technique and sintering process, and its band‐gap measured by a transmission‐reflection method was between 9 and 11.5 GHz, with a maximal loss of ?50 dB at 9.9 GHz. The radiation frequency of designed dipole antenna corresponded to the frequency of strongest reflection of the EBGs. Comparison of E‐plane radiation patterns of dipole antenna without substrate, dipole antenna with a common ceramic substrate and dipole antenna with EBGs showed that gain of dipole antenna with EBGs was 3 dB higher than that of dipole antenna with a common ceramic substrate and 5 dB higher than that of dipole antenna without substrate, thus the gain was improved by 3.2 times. Moreover, the radiation pattern of dipole antenna with EBGs was smoother, which showed that EBGs can effectively suppress propagation of surface waves. In addition, the main lobe width was reduced, thus antenna directivity was improved.     

Piezoelectric Multilayer Ceramic/Polymer Composite Transducer with 2–2 Connectivity

A multilayer piezoelectric ceramic/polymer composite with 2–2 connectivity was fabricated by thermoplastic green machining after co-extrusion. The multilayer ceramic body was composed of piezoelectrically active lead zirconate titanate (PZN)–lead zinc niobate (PZN)-lead zirconate titanate (PZT) layers and electrically conducting PZN–PZT/Ag layers. After co-extruding the thermoplastic body, which consisted of five piezoelectric layers interspersed with four conducting layers, it was computer numeric-controlled machined to create periodic channels within it. Following binder burnout and sintering, an 18 vol% array of 190 μm thin PZT slabs with a channel size of 880 μm was fabricated. The channels were filled with epoxy in order to fabricate a PZN–PZT/epoxy composite with 2–2 connectivity. The piezoelectric coefficient (effective d ₃₃) and hydrostatic figure of merit ( d _h× g _h) of the PZN–PZT/epoxy composite were 1200 pC/N and 20 130 × 10⁻¹⁵ m²/N, respectively. These excellent piezoelectric characteristics as well as the relatively simple fabrication procedure will contribute in widening the application range of the piezoelectric transducers.     

Synthesis, Characterization, and Microwave Dielectric Properties of ATiO3 (A=Co, Mn, Ni) Ceramics

The ATiO₃ (A=Co, Mn, Ni) dielectric ceramics have been synthesized by the conventional solid-state ceramic route. The structure and microstructure of these ceramic samples have been studied using powder X-ray diffraction and scanning electron microscopy. The microwave dielectric properties such as relative permittivity (ɛ_r), quality factor ( Q _u× f ), and coefficient of temperature variation of resonant frequency (τ_f) of the ceramics have been measured in the frequency range 4–6 GHz using resonance methods. The dielectric constant of ATiO₃ (A=Co, Mn, Ni) varies from 19 to 25 and τ_f close to −50 ppm/°C. The ceramics have high-quality factors ( Q _u× f ) of 62 500 GHz (at 5.42 GHz) for CoTiO₃, 15 200 GHz (at 5.22 GHz) for MnTiO₃, and 13 900 GHz (at 5.24 GHz) for NiTiO₃, respectively.     

Effects of Thermal Annealing on Formation of Micro Porous Titanium Oxide by the Sol–Gel Method

We investigated the structural and optical properties of microporous titanium oxide (TiO₂) fabricated by the sol–gel method using templates of colloidal crystals with polystyrene spheres when the annealing temperature was changed between 600° and 1000°C. From X-ray diffraction patterns and SEM images, the rutile TiO₂ annealed at a high temperature did not form periodic porous bodies, while the anatase TiO₂ annealed at lower than 800°C formed periodic porous bodies. The porous TiO₂ obtained acts as an air-sphere/TiO₂ photonic crystal with an FCC structure. It is suggested that TiO₂ sol annealed at a lower temperature do not lead to phase transition from the anatase phase to the rutile phase to obtain the air-sphere/TiO₂ photonic crystal by the sol–gel method using templates of colloidal crystals.