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1.
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.  相似文献   

2.
A process for fabricating three-dimensional photonic crystals composed of SiO2–TiO2-based ceramics with a diamond structure was investigated. An epoxy structure having an inverse diamond configuration was fabricated by stereolithography, a rapid prototyping method. The epoxy structure was infiltrated with a ceramic slurry and then cold isostatically pressed. After sintering at 670°C for 5 h in air, the epoxy was burned off, leaving behind the desired structure of a ceramic photonic crystal. The calculated band diagram indicated that an absolute photonic band gap for all wave vectors existed. The measurement of transmission in the 〈100〉 direction from 10 to 20 GHz showed that a complete band gap formed at about 14.7–18.5 GHz. The magnitude of the maximum attenuation was as large as 30 dB at 17 GHz, which indicated that the fabricated structure worked well as a photonic crystal.  相似文献   

3.
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.  相似文献   

4.
Zirconolite (CaZrTi2O7) is a mineral that has a high containment capacity for actinides and lanthanides and is considered to be a good candidate for the immobilization of radioactive wastes. The glass–ceramic technique seems to be a very suitable and convenient method to produce zirconolite crystals by precipitating them in a specific glass matrix. In this study, development of a new zirconolite-based glass–ceramic belonging to SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O) system was investigated. The presence of PbO, together with B2O3 and K2O, allowed the preparation of a X-ray diffraction (XRD) amorphous glass with a relatively high concentration of ZrO2 and TiO2, which was successfully converted to a glass–ceramic containing 34 wt% of zirconolite after heating at 770°C for 4 h. Differential thermal analysis, XRD, scanning electron microscope, and energy dispersive X-ray spectroscopy were used to determine the crystallization conditions, identify the crystallized phases, determine their compositions and quantities and observe and analyze the microstructures. The zirconolite crystals showed a platelet morphology with a monoclinic structure characterized by a =1.246 nm, b =0.7193 nm, c =1.128 nm, and β=100.508°.  相似文献   

5.
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 τf=−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.  相似文献   

6.
Some K2O-Nb2O5-GeO2 glasses are prepared, and their crystallization behaviors are examined. 25K2O·25Nb2O5·50GeO2 glass with the glass transition temperature T g= 622°3C and crystallization onset temperature T x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K3,8Nb5Ge3O20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K2O·25Nb2O5·50GeO2 glass change steeply by heat treatment at T g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.  相似文献   

7.
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 (ɛr) of 16.2, a quality factor ( Q u× f ) of 66 000 at 4.97 GHz, and a temperature coefficient of resonant frequency (τf) of −60 ppm/°C, respectively. Addition of 4 wt% TiO2 improved the τf to −8.7 ppm/°C with an ɛr 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.  相似文献   

8.
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.  相似文献   

9.
A type of new low sintering temperature ceramic, Li2TiO3 ceramic, has been found. Although it is difficult for the Li2TiO3 compound to be sintered compactly at temperatures above 1000°C for the volatilization of Li2O, dense Li2TiO3 ceramics were obtained by conventional solid-state reaction method at the sintering temperature of 900°C with the addition of ZnO–B2O3 frit. The sintering behavior and microwave dielectric properties of Li2TiO3 ceramics with less ZnO–B2O3 frit (≤3.0 wt%) doping were investigated. The addition of ZnO–B2O3 frit can lower the sintering temperature of the Li2TiO3 ceramics, but it does not apparently degrade the microwave dielectric properties of the Li2TiO3 ceramics. Typically, the good microwave dielectric properties of ɛr=23.06, Q × f =32 275 GHz, τf = 35.79 ppm/°C were obtained for 2.5 wt% ZnO–B2O3 frit-doped Li2TiO3 ceramics sintered at 900°C for 2 h. The porosity was 0.08%. The Li2TiO3 ceramic system may be a promising candidate for low-temperature cofired ceramics applications.  相似文献   

10.
The response of ceramic superconductors and ceramic composites to compressive stresses at high temperatures has been examined. Monolithic YBa2Cu3O7-δ and composite YBa2Cu3O76/Ag were tested at constant true strain rates from 10-6 to 10-3 s-1 at temperatures from 800° to 950°C. Fine-grained monolithic YBa2Cu3O7-δ appears to have a regime of superplastic deformation between temperatures of 850° and 950°C at strain rates from 10-6 to 10-4 S-1. The addition of 20 vol% Ag to a coarser-grained material enhances the ductility of the ceramic and lowers the flow stress by a factor of 3 to 10. However, there is no evidence of superplasticity in the composite material in the range of temperature and strain rate where it was tested.  相似文献   

11.
The microstructure of ZrO2 fine particles produced by a novel synthesis method at 450° and 950°C has been studied. The fundamentals of the synthesis method, which involves both chemical and diffusion phenomena, are presented. The method is based on mass transport through the gaseous phase between metallic zirconium and Fe2O3 powder. The mass-transporting chemical species are zirconium and iron chlorides. This article focuses on the microstructure and structure of ZrO2 particles formed by the reaction between gaseous ZrCl4 and solid Fe2O3, which is a relevant reaction step that occurs during the synthesis process. The resulting ZrO2 crystals grown on Fe2O3 particles have been analyzed using transmission electron microscopy. Microstructural characterization has been complemented by X-ray diffractometry analysis. Tetragonal-ZrO2 is produced at 450°C and monoclinic-ZrO2 single crystals are produced at 950°C.  相似文献   

12.
A Nd-doped HfO2-Y2O3 ceramic having excellent transmittance was synthesized by HIPing, using high-purity powders (>99.99 wt%) of Y2O3, Nd2O3, and HfO2. The mixed powder compacts of these powders were sintered at 1650°C for 1 h under vacuum and HIPed at 1700°C for 3 h under 196 MPa of Ar. The specimen after HIPing consisted of uniform grains measuring about 30 μm and having pore-free structure. The optical transmittance of 1 at.% Nd-doped 2.6 mol% HfO2-Y2O3 ceramics ranging between visible and infrared wavelength was almost equivalent or superior to that of a Nd:Y2O3 single crystal grown by the Verneuil method.  相似文献   

13.
Chemical polishing and etching techniques were used to reveal the dislocation structures of sapphire and ruby crystals grown by the flame-fusion and flux techniques. The average density of edge dislocations lying in prism planes was 3.0 × 105 per cm2, which could be changed only slightly by chromium additions and annealing at 2000°C. An average basal dislocation density of 2 × 105 per cm2 decreased 35 to 80% on annealing. Crystal orientation (i.e., angle between the c axis and the growth axis) showed no effect on dislocation density but a pronounced effect on subboundary arrangement and density. The substructure of 0° crystals was more complex than that of 90° crystals; 60° crystals possessed a structure intermediate between 0° and 90°. Principal observations included (1) prismatic and basal slip on all as-grown crystals; (2) profuse basal slip, readily polygonized on annealing; (3) dislocation densities of flux crystals lower than those of Verneuil crystals; and (4) a rare form of basal twinning, composition plane , on all flux crystals.  相似文献   

14.
The electrical properties of Sr0.5Ba0.3TiO3 in the presence of Nb2O5 as a donor, 3Li2O · 2SiO2 as a sintering agent, and Bi2O3 as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr0.7Ba0.3TiO3+ 0.5 mol% Nb2O5+ 2 mol% 3Li2O · 2SiO2+ 0.2 mol% Bi2O3, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρj, 1010Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.  相似文献   

15.
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 m1/2, 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.  相似文献   

16.
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.  相似文献   

17.
BaCu(B2O5) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B2O5) phase was formed at 700°C and melted above 850°C. The BaCu(B2O5) ceramic sintered at 810°C had a dielectric constant (ɛr) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τf) of −32 ppm/°C. As the BaCu(B2O5) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B2O5) was added to the Ba(Zn1/3Nb2/3)O3 (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B2O5) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛr=35, and τf=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B2O5) ceramic sintered at 875°C for 2 h.  相似文献   

18.
Nanolaminates with a layered M N +1AX N crystal structure (with M: transition metal, A: group element, X: carbon or nitrogen, and N =1, 2, 3) offer great potential to toughen ceramic composites. A ternary Ti3AlC2 carbide containing ceramic composite was fabricated by three-dimensional printing of a TiC+TiO2 powder mixture and dextrin as a binder. Subsequent pressureless infiltration of the porous ceramic preform with an Al melt at 800°–1400°C in an inert atmosphere, followed by reaction of Al with TiC and TiO2 finally resulted in the formation of a dense multiphase composite of Ti3AlC2–TiAl3–Al2O3. A controlled flaw/strength technique was utilized to determine fracture resistance as a function of crack extension. Rising R -curve behavior with increasing crack extension was observed, confirming the operation of wake-toughening effects on the crack growth resistance. Observations of crack/microstructure interactions revealed that extensive crack deflection along the (0001) lamellar sheets of Ti3AlC2 was the mechanism responsible for the rising R -curve behavior.  相似文献   

19.
High dielectric constant and low loss ceramics with composition Ba2La3Ti3TaO15 have been prepared by a conventional solid-state ceramic route. This compound adopts A5B4O15 cation-deficient hexagonal perovskite structure. The dielectric properties of dense ceramics sintered in air at 1520°C have been characterized at microwave frequencies. It shows a relative dielectric constant of ∼45, quality factor Q u× f of ∼26 828 GHz and temperature variation of resonant frequency of −0.97 ppm/°C.  相似文献   

20.
Three distinctly different microstructures of silica (as quartz and crystobalite), alumina, enstatite, and celsian, were found to develop in a 60SiO2–20MgO–10Al2O3–10BaO glass ceramic. At 1010°C, growth of wormy fibrillar crystals was observed, indicating that crystal growth was diffusion controlled. At the intermediate temperature of 1080°C, a coarse cellular microstructure developed with multiple spherical particles nucleated on their surfaces and in the surrounding glass. At 1200°C, the glass crystallizes in a denderitic morphology but the dendrites were actually fragmented into multiple cube-shaped enstatite crystals, indicating a transition to interface-controlled growth. The crystals coarsen with time but maintain their order along the dendrite skeletons.  相似文献   

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