Large eddy simulation of an ethylene–air turbulent premixed V-flame

AbstractLarge eddy simulation (LES) using a dynamic eddy viscosity subgrid scale stress model and a fast-chemistry combustion model without accounting for the finite-rate chemical kinetics is applied to study the ignition and propagation of a turbulent premixed V-flame. A progress variable c-equation is applied to describe the flame front propagation. The equations are solved two dimensionally by a projection-based fractional step method for low Mach number flows. The flow field with a stabilizing rod without reaction is first obtained as the initial field and ignition happens just upstream of the stabilizing rod. The shape of the flame is affected by the velocity field, and following the flame propagation, the vortices fade and move to locations along the flame front. The LES computed time-averaged velocity agrees well with data obtained from experiments

Crystal Structure, Infrared Spectra, and Microwave Dielectric Properties of Temperature-Stable Zircon-Type (Y,Bi)VO<inf>4</inf> Solid-Solution Ceramics

A series of (Bi 1-x Y x )VO 4 (0.4 ≤ x ≤ 1.0) ceramics were synthesized using the traditional solid-state reaction method. In the composition range of 0.4 ≤ x ≤ 1.0, a zircon-type solid solution was formed between 900 and 1550 °C. Combined with our previous work (scheelite monoclinic and zircon-type phases coexist in the range of x < 0.40), a pseudobinary phase diagram of BiVO 4 -YVO 4 is presented. As x decreased from 1.0 to 0.40, the microwave permittivity (ϵ r ) of (Bi 1-x Y x )VO 4 ceramics increased linearly from 11.03 to 30.9, coincident with an increase in the temperature coefficient of resonant frequency (TCF) from -61.3 to +103 ppm/°C. Excellent microwave dielectric properties were obtained for (Bi 0.3 Y 0.7 )VO 4 sintered at 1025 °C and (Bi 0.2 Y 0.8 )VO 4 sintered at 1075 °C with ϵ r ∼ 19.35, microwave quality factor (Qf) ∼ 25 760 GHz, and TCF ∼ +17.8 ppm/°C and ϵ r ∼ 16.3, Qf ∼ 31 100 GHz, and TCF ∼ -11.9 ppm/°C, respectively. Raman spectra, Shannon's additive rule, a classical oscillator model, and far-infrared spectra were employed to study the structure-property relations in detail. All evidence supported the premise that Bi-based vibrations dominate the dielectric permittivity in the microwave region

High Quality Factor, Ultralow Sintering Temperature Li6B4O9 Microwave Dielectric Ceramics with Ultralow Density for Antenna Substrates

Dense Li6B4O9microwave dielectric ceramics were synthesized at low temperature via solid-state reaction using Li2CO3and LiBO2. Optimum permittivity ∼ 5.95, quality factor ∼ 41 800 GHz and temperature coefficient of resonant frequency ∼ - 72 ppm/°C were obtained in ceramics sintered at 640 °C with a ultrasmall bulk density ∼2.003 g/cm3(∼95% relative density, the smallest among all the reported microwave dielectric ceramics). Li6B4O9ceramics were shown to be chemically compatible with silver electrodes but reacted with aluminum forming Li3AlB2O6and Li2AlBO4secondary phases. A prototype patch antenna was fabricated by tape casting and screen printing. The antenna resonated at 4.255 GHz with a bandwidth ∼279 MHz at -10 dB transmission loss (S11) in agreement with simulated results. The Li6B4O9microwave dielectric ceramic possesses similar microwave dielectric properties to the commercial materials but much lower density and could be a good candidate for both antenna substrate and low-temperature cofired ceramics technology

Novel water insoluble (NaxAg2-x) MoO4 (0 <= x <= 2) microwave dielectric ceramics with spinel structure sintered at 410 degrees

In the present work, a novel series of water insoluble ultra-low temperature firing (Na,Ag)2MoO4 microwave dielectrics were prepared via the traditional solid state reaction method. A spinel structured solid solution was formed in the full composition range in the (NaxAg2−x)MoO4 (0 ≤ x ≤ 2). As x increased from 0 to 2.0, cell volume decreased linearly from 9.32 Å to 9.10 Å. Sintering behavior were described using a so-called ‘bowing’ effect and densification was achieved below 420 °C for 0.5 ≤ x ≤ 1.2 with grain size, 1 to 5 μm. Optimum microwave dielectric properties were obtained for (Na1.2Ag0.8)MoO4 ceramics sintered at 410 °C with a permittivity ∼8.1, a microwave quality factor ∼44 800 GHz and the temperature coefficient of the resonant frequency ∼−82 ppm °C−1 at 13.9 GHz. Silver within the solid solution inhibited hydrolyzation of ceramics and also reduced their sintering temperature. Compared with the sintering temperatures of traditional microwave dielectric ceramic (Al2O3, >1400 °C) and normal low temperature co-fired ceramics (<960 °C), this system will save lots of energy during processing and accelerate developments of sustainable electronic materials and devices

Structure–property relationships of low sintering temperature scheelite-structured (1 − x)BiVO 4 –xLaNbO 4 microwave dielectric ceramics

A series of (1 − x)BiVO4–xLaNbO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared via a solid state reaction method. A scheelite-structured solid solution was formed for x ≤ 0.5 but for x > 0.5, tetragonal scheelite, monoclinic LaNbO4-type and La1/3NbO3 phases co-existed. As x increased from 0 to 0.1, the room temperature crystal structure gradually changed from monoclinic to tetragonal scheelite, associated with a decrease in the ferroelastic phase transition temperature from 255 °C (BiVO4) to room temperature or even below. High sintering temperatures were also found to accelerate this phase transition for compositions with x ≤ 0.08. Temperature independent high quality factor Qf >10 000 GHz in a wide temperature range 25–140 °C and high microwave permittivity εr ∼76.3 ± 0.5 was obtained for the x = 0.06 ceramic sintered at 800 °C. However, small changes in composition resulted in a change in the sign and magnitude of the temperature coefficient of resonant frequency (TCF) due to the proximity of the ferroelastic transition to room temperature. If TCF can be controlled and tuned through zero, then (1 − x)BiVO4–xLaNbO4 (0.0 ≤ x ≤ 1.0) is a strong candidate for microwave device applications

High permittivity and low loss microwave dielectrics suitable for 5G resonators and low temperature co-fired ceramic architecture

Bi 2 (Li 0.5 Ta 1.5 )O 7 + xBi 2 O 3 (x = 0, 0.01 and 0.02) ceramics were prepared using a solid state reaction method. All compositions were crystallized in a single Bi 2 (Li 0.5 Ta 1.5 )O 7 phase without secondary peaks in X-ray diffraction patterns. Bi 2 (Li 0.5 Ta 1.5 )O 7 ceramics were densified at 1025 °C with a permittivity (ϵ r ) of ∼ 65.1, Q f ∼ 15500 GHz (Q ∼ microwave quality factor; f ∼ resonant frequency; 16780 GHz when annealed in O 2 ) and the temperature coefficient of resonant frequency (TCF) was ∼ -17.5 ppm °C -1 . The sintering temperature was lowered to ∼920 °C by the addition of 2 mol% excess Bi 2 O 3 (ϵ r ∼ 64.1, a Q f ∼ 11200 GHz/11650 GHz when annealed in O 2 and at a TCF of ∼ -19 ppm °C -1 ) with compositions chemically compatible with Ag electrodes. Bi 2 (Li 0.5 Ta 1.5 )O 7 + xBi 2 O 3 are ideal for application as dielectric resonators in 5G mobile base station technology for which ceramics with 60 < ϵ r < 70, high Q f and close to zero TCF are commercially unavailable. They may additionally prove to be useful as high ϵ r and high Q f materials in low temperature co-fired ceramic (LTCC) technology

Temperature stable K0.5(Nd1−xBix)0.5MoO4 microwave dielectrics ceramics with ultra-low sintering temperature

K 0.5 (Nd 1-x Bi x ) 0.5 MoO 4 (0.2 ≤ x ≤ 0.7) ceramics were prepared via the solid-state reaction method. All ceramics densified below 720°C with a uniform microstructure. As x increased from 0.2 to 0.7, relative permittivity (e(open) r ) increased from 13.6 to 26.2 commensurate with an increase in temperature coefficient of resonant frequency (TCF) from - 31 ppm/°C to + 60 ppm/°C and a decrease in Qf value (Q = quality factor; f = resonant frequency) from 23 400 to 8620 GHz. Optimum TCF was obtained for x = 0.3 (-15 ppm/°C) and 0.4 (+4 ppm/°C) sintered at 660 and 620°C with e(open) r ~15.4, Q f ~19 650 GHz, and e(open) r ~17.3, Q f ~13 050 GHz, respectively. Ceramics in this novel solid solution are a candidate for ultra low temperature co-fired ceramic (ULTCC) technology

Crystal structure, impedance and broadband dielectric spectra of ordered scheelite-structured Bi(Sc1/3Mo2/3)O4 ceramic

Bi(Sc 1/3 Mo 2/3 )O 4 ceramics were prepared via solid state reaction method. It crystallized with an ordered scheelite-related structure (a = 16.9821(9) Å, b = 11.6097(3) Å, c = 5.3099(3) Å and β = 104.649(2)°) with a space group C12/C1, in which Bi 3+ , Sc 3+ and Mo 6+ are -8, -6 and -4 coordinated, respectively. Bi(Sc 1/3 Mo 2/3 )O 4 ceramics were densifiedat 915 °C, giving a permittivity (ε r ) 24.4, quality factor (Qf, Q = 1/dielectric loss, f = resonant frequency) ~48, 100 GHz and temperature coefficient of resonant frequency (TCF) -68 ppm/°C. Impedance spectroscopy revealed that there was only a bulk response for conductivity with activation energy (E a ) ~0.97 eV, suggesting the compound is electrically and chemically homogeneous. Wide band dielectric spectra were employed to study the dielectric response of Bi(Sc 1/3 Mo 2/3 )O 4 from 20 Hz to 30 THz. ε r was stable from 20 Hz to the GHz region, in which only ionic and electron displacive polarization contributed to the ε r

Measurements of J/psi Decays into 2(pi+pi-)eta and 3(pi+pi-)eta

Based on a sample of 5.8X 10^7 J/psi events taken with the BESII detector, the branching fractions of J/psi--> 2(pi+pi-)eta and J/psi-->3(pi+pi-)eta are measured for the first time to be (2.26+-0.08+-0.27)X10^{-3} and (7.24+-0.96+-1.11)X10^{-4}, respectively.Comment: 11 pages, 6 figure