Reducing local oscillator phase noise limitations on the frequencystability of passive frequency standards: tests of a new concept

We report on the experimental test of a new concept for reducing the limitation on short-term frequency stability of passive frequency standards due to local oscillator phase noise. This concept is general and can be applied to many passive frequency standards. Systems that use sinewave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f_m. This effect limits the fractional frequency stability to approximately σ_v(τ7)=0.9(f _m/ν₀) (S_φ(2f_m))^1/2τ^-1/2, where ν₀ is the carrier frequency and S_φ(2f _m) is the phase noise at twice the modulation frequency. (Contributions from higher even harmonics of the modulation generally can be neglected). This new concept uses notch filters at ±2f_m from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in σ_y(τ). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of less than 2×10^-14τ^-1/2 are possible if the atomic resonance has sufficient quality     

Mass sensitivities of shear horizontal waves in three-layer platesensors

Explicit velocity and mass sensitivity formulas for shear-horizontal (SH) plate wave sensors loaded symmetrically on both sides of a plate are presented. The sensor geometry is a composite plate which consists of a central isotropic plate sandwiched symmetrically between two identical layers of isotropic solids. It is demonstrated that if the side layers are considered as the mass loading, for the lowest SH mode (SSo) the sensitivity decreases by a factor of (1-(μ ₂/ρ₂)/(μ₁/ρ₁)) due to the elasticity and by a factor of (1+ρ₂h/ρ₁d)^-1 due to the inertia of the mass loading layers, where μ₁,μ₂ ; ρ₁, ρ₂ and 2d, h are the shear moduli, densities and thickness of the central plate and of the side layers, respectively. For higher order modes, the behavior of sensors which are operated near cutoff frequency is analyzed. The mass loading decreases the cutoff frequencies of the higher order modes and near the cutoff frequencies the mass sensitivities are very high but decrease dramatically as the mass loading increases. Specific examples are given for the case of a fused silica plate sandwiched between two thin lucite layers     

Lattice matching of D023 and D022 phases in Al-6at.%(Ti,V,Zr) systems

Composite quaternary alloys with 21–22 vol.% of Al₃M (M = Ti,V,Zr) were made by vacuum arc melting. Two inlermetallic phases were found in alloys: one is V rich D0₂₂-Al₃(Ti,V,Zr) and the other is Zr rich D0₂₃-Al₃(Ti,V,Zr). Measured lattice constants of precipitate phases were strongly dependent on the composition of transition elements in precipitates and generally obeyed Vegard's rule. The lattice misfits between Al₃M phases and the matrix did not change much in alloys while the lattice misfit between D0₂₂ and D0₂₃ Al₃M phases was found to decrease with an increase in Ti content implying that the interface between both phases became more smooth, hard to distinguish and separate. A geometrical model was made for the lattice matching between two, D0₂₂ and D0₂₃ Al₃M phases. Titanium has been found to act as a retarding element to separation into D0₂₃ and D0₂₂ phases and the pre-existing phase may have been L₁₂ or D0₂₃ phase.     

Conformity and structure of titanium oxide films grown by atomic layer deposition on silicon substrates

Conformity and phase structure of atomic layer deposited TiO₂ thin films grown on silicon substrates were studied. The films were grown using TiCl₄ and Ti(OC₂H₅)₄ as titanium precursors in the temperature range from 125 to 500 °C. In all cases perfect conformal growth was achieved on patterned substrates with elliptical holes of 7.5 μm depth and aspect ratio of about 1:40. Conformal growth was achieved with process parameters similar to those optimized for the growth on planar wafers. The dominant crystalline phase in the as-grown films was anatase, with some contribution from rutile at relatively higher temperatures. Annealing in the oxygen ambient resulted in (re)crystallization whereas the effect of annealing depended markedly on the precursors used in the deposition process. Compared to films grown from TiCl₄, the films grown from Ti(OC₂H₅)₄ were transformed into rutile in somewhat greater extent, whereas in terms of step coverage the films grown from Ti(OC₂H₅)₄ remained somewhat inferior compared to the films grown from TiCl₄.     

Raman scattering study on ferroelectric (Ba0.32Sr0.68)2Nb2O7 ceramics

Raman scattering technique was applied to examine the Ba-doping effect to the two low temperature phase transitions of Sr₂Nb₂O₇ (SN) in the temperature range from −190 to 600 °C. The line shape of Raman spectra can be well fitted by multidamped harmonic oscillator model. We did not observe any soft mode related to the two low temperature phase transitions corresponding to those of the pure SN. It is correlated to the disappearance of the incommensurate phase in (Ba_0.32Sr_0.68)₂Nb₂O₇ ceramics. However, the temperature dependence behavior of the three low frequency modes indicates another new structural phase transition around 270 °C. It is considered that the reduction of the interlayer interaction caused by partial replacement of Sr-site by Ba-site, whose ionic radius is larger than that of Sr, may be the reason for the disappearance of the incommensurate phase transition in (Ba_0.32Sr_0.68)₂Nb₂O₇ ceramics.     

High temperature stable GHz-range low-loss wide band transducersand filter using SiO2/LiNbO3, LiTaO3

GHz-range low-loss transducers and filters are required for communication systems, especially mobile telephone communication systems. Many types of low insertion-loss transducers and filters utilizing the high electromechanical coupling coefficient (K²) materials such as LiNbO₃ and LiTaO₃ have been developed. Unfortunately, these materials have large temperature coefficients of the frequency (TCF). In this paper, SAW substrates with high coupling coefficients and low propagation attenuations and small temperature coefficient of frequency in the GHz-range are theoretically and experimentally investigated. The experimental results show very low propagation loss of 0.02 dB/λ ₀ and larger K² than those of the substrates of LiNbO₃ and LiTaO₃ at the TCF of below -5 ppm/°C at 1~2 GHz-range. The low-loss filter results using internal reflection types of IDT show the insertion loss of about 2.9 dB at 1 GHz and 4.9 dB at 2 GHz under the TCF's of 0 and +20 ppm/°C. These materials are applicable for devices at GHz-range because SiO₂ thickness is very thin such as below 1 μm and the center frequency shift of the filter versus SiO₂ thickness is very small     

ISOLATION AND CHARACTERISATION OF C60(CF3)2

From the products of reactions of [60]fullerene with either K₂PtF₆ at 470 °C or AgF at 520 °C, we have isolated C₆₀(CF₃)₂, the simplest trifluoromethylfullerene, which gives a single ¹⁹F NMR line at -69.5 ppm. The HPLC retention time is less than that of C₆₀F₂ confirming the trend observed for other fluoro- vs. trifluoromethylfullerenes namely that the latter elute more rapidly. Other trifluoromethyl- containing species, C₆₀(CF₃)₄O, C₆₀F₅CF₃, C₆₀(CF₃)₄H₂, C₆₀(CF₃)₆H₂, and C₆₀(CF₃H)₃ were detected in the product mixture.     

Novel soft solution synthesis and characterization of submicromic LiCoVO4

A novel soft solution process has been used to prepare LiCoVO₄ by reacting Co(CH₃CH₂COO)₂, Li₂CO₃, NH₄VO₃ and citric acid. LiCoVO₄ powders were successfully prepared at as low as 450 °C in 4 h. Compared to the solid-state reaction processes, the soft solution process greatly reduced the temperature and the time for preparing LiCoVO₄. The inverse spinel structure and high crystallinity of the synthesized product has been confirmed by X-ray diffraction. Thermal analysis proves that the phase formation of the compound occurs at about 450 °C. The results of the IR investigations show that the band located at 820 cm⁻¹ corresponds to the stretching vibration mode of VO₄ tetrahedron with the A symmetry. SEM examination reveals a spherical grain distribution, the average particle size being typically lower than 1 μm. The quantitative result from ICP-AES analysis is Li_0.967Co_0.994VO₄.     

Mn掺杂Ni(OH)2的合成及电化学性能研究

本工作采用缓冲溶液法制备Mn掺杂Ni(OH)₂(Ni_1-xMn_x(OH)₂, x=0.1, 0.2, 0.3, 0.4), X射线衍射测试表明样品主要是β相, 有少量Mn₃O₄杂相; 循环伏安测试表明, x=0.2的材料还原峰积分面积最大、还原分支的峰电流最高; 恒流充放电测试表明, 在100 mA/g电流密度下, Ni_0.8Mn_0.2(OH)₂放电比容量最高, 其第20次循环放电比容量为271.8 mAh/g, 同等条件测试的商用β-Ni(OH)₂放电比容量为253.6 mAh/g; 在300、500 mA/g电流密度下, Ni_0.8Mn_0.2(OH)₂放电比容量仍保持最高, 分别为294.7、291.5 mAh/g, 而且Mn掺杂Ni(OH)₂的循环稳定性也优于商用β-Ni(OH)₂。Mn掺杂可改善镍电极的循环稳定性、降低镍电极成本, 具有广阔的应用前景。     

Consolidation behavior of L12 phase (Al + 12.5 at.% Cu)3Zr powder with nanocrystalline structure during CIP and subsequent sintering

The mechanically alloyed (Al + 12.5 at.% Cu)₃Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1₂ phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1₂ phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0₂₃. The optimum consolidation conditions for maintaining L1₂ phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1₂ and D0₂₃ phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.     

老化对水泥乳化沥青胶结料动态力学性能的影响

基于分数导数修正Burgers模型,建立了水泥乳化沥青胶结料（简称复合胶结料）的本构方程。结合不同材料配比的复合胶结料的老化试验及频率扫描试验,分析了老化时间及材料配比等因素对复合胶结料黏弹性力学参数的影响。研究表明,复合胶结料的存储模量与损耗模量均随老化时间的增加而增大,在老化时间为0~8 h时,存储模量及损耗模量增加较快,当老化时间超过8 h后,其增长趋势减缓。对于乳化沥青与水泥的质量比（m_A/m_C）为1.2和1.4的复合胶结料,在不同的加载频率下,其相位角均随老化时间的增加而减小。对于m_A/m_C为1.0的复合胶结料,其相位角随着老化时间的增加呈现先增加后减小的变化规律。随着老化时间的增加,复合胶结料趋向于弹性材料的力学性质。分数导数修正Burgers模型可以较好的描述老化后复合胶结料的黏弹性动态力学行为,模型参数弹性模量E₁、黏度η₁和分数导数r可以描述老化情况下材料黏弹特性的变化规律。     

Microstructure, crystallinity, and properties of low-pressure MOCVD-grown europium oxide films

A study of growth, structure, and properties of Eu₂O₃ thin films were carried out. Films were grown at 500–600 °C temperature range on Si(1 0 0) and fused quartz from the complex of Eu(acac)₃·Phen by low pressure metalorganic chemical vapor deposition technique which has been rarely used for Eu₂O₃ deposition. These films were polycrystalline. Depending on growth conditions and substrates employed, these films had also possessed a parasitic phase. This phase can be removed by post-deposition annealing in oxidizing ambient. Morphology of the films was characterized by well-packed spherical mounds. Optical measurements exhibited that the bandgap of pure Eu₂O₃ phase was 4.4 eV. High frequency 1 MHz capacitance–voltage (C–V) measurements showed that the dielectric constant of pure Eu₂O₃ film was about 12. Possible effects of cation and oxygen deficiency and parasitic phase on the optical and electrical properties of Eu₂O₃ films have been briefly discussed.     

Chemical vapour deposition of praseodymium oxide films on silicon: influence of temperature and oxygen pressure

Metal-organic chemical vapour deposition (MOCVD) of various phases in PrO_x system has been studied in relation with deposition temperature (450–750 °C) and oxygen partial pressure (0.027–100 Pa or 0.2–750 mTorr). Depositions were carried out by pulsed liquid injection MOCVD using Pr(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) precursor dissolved in toluene or monoglyme. By varying deposition temperature and oxygen partial pressure amorphous films or various crystalline PrO_x phases (Pr₂O₃, Pr₇O₁₂, Pr₆O₁₁) and their mixtures can be grown. The pure crystalline Pr₂O₃ phase grows only in a narrow range of partial oxygen pressure and temperature, while high oxygen pressure (40–100 Pa) always leads to the most stable Pr₆O₁₁ phase. The influence of annealing under vacuum at 750 °C on film phase composition was also studied. Near 90% step coverage conformity was achieved for PrO_x films on structured silicon substrates with aspect ratio 1:10. In air degradation of Pr₂O₃ films with transformation to Pr(OH)₃ was observed in contrast to Pr₆O₁₁ films.     

Stochastic nucleation and growth of islands on surfaces with the theory of non-classical nucleation

In order to study the irreversible growth and dynamics of islands on surfaces in metal-on (1 0 0) metal homoepitaxy at low temperatures and small coverage, a stochastic element in the theory of non-classical nucleation is introduced. The order parameter is the fractional change in the mass density σ=(ρ−ρ₀)/ρ₀ of the system. The local and non-local free energy densities are functionals of σ. The size distribution of the islands is extracted and compared with the experimental distribution and with the distribution obtained from other simulations. The time dependence of the average island size is also studied. The results are in relatively good agreement with the experimental results and with the results obtained from other simulations.     

Spectroscopic Evidence for Anionic Coordination Complexes of the Transition Metals with C70 and the Higher Fullerenes C76, C78, C82, C84, C86, C90, and C92

The carbonylate anions [M(CO)₅]^- (M = Mn, Re), [Co(CO)₄]^-, [CpFe(CO)₂]^-, and [CpM(CO)₃]^- (M = Mo, W) react with C₇₀ via single electron transfer processes to give, respectively, the corresponding 17-electron, metal-centered radicals Co(CO)₄, M(CO)₅ (M = Mn, Re), CpFe(CO)₂, and CpM(CO)₃ (M = Mo, W) in addition to the radical anion C₇₀^-. In secondary thermal or photochemical processes, the metal-centered radicals Co(CO)₄ and M(CO)₅ (M = Mn, Re) combine with the C₇₀^- to form the new η²-C₇₀ complexes [Co(CO)₃(η²-C₇₀)]^- and [M(CO)₄(η²-C₇₀)]^-. However, the metal-centered radicals CpM(CO)₃ (M = Mo, W) require photolysis to react with C₇₀^- to form [CpM(CO)₂(η²-C₇₀)]^-, whereas neither thermolysis nor photolysis induces reaction between CpFe(CO)₂ and C₇₀^-. The photochemical reaction of [Mn(CO)₅]^- with a mixture of higher fullerenes known to contain at least C₇₆, C₇₈, C₈₄, C₈₆, and C₉₀ resulted similarly in the formation of the higher fullerene complexes [Mn(CO)₄(η²-C_n)]^- (n = 76, 78, 80, 82, 84, 86, 88, 90, 92, 96, and 98), all identified using electrospray mass spectrometry.     

BaZrO3掺杂对(K0.49Na0.51)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3无铅压电陶瓷的结构与电性能的影响

采用固相反应法制备了(K_0.49Na_0.51)_0.98Li_0.02(Nb_0.77Ta_0.18Sb_0.05)O₃-xBaZrO₃ (NKNLST-xBZ, x = 0~0.020 mol)无铅压电陶瓷, 系统研究了BaZrO₃的掺杂量对陶瓷的压电、介电、机电和铁电性能的影响。结果表明: 随着BaZrO₃掺杂量x的增加, 陶瓷的晶体结构由正交相向四方相转变, 在x=0.005~0.008区间出现正交相与四方相两相共存的区域, 在此区域内陶瓷的晶粒变得细小且均匀, 介电损耗tanδ大幅降低, 压电常数d₃₃和平面机电耦合系数k_p增加。该体系陶瓷的介电常数ε T 33 /ε0则随着BaZrO₃的增加持续增加, 相变温度则向低温方向移动。当x=0.005时, 该组成陶瓷具有最佳的综合性能: 压电常数d33=372 pC/N, 平面机电耦合系数kp=47.2%, 介电损耗tanδ=3.1%, 以及较高的介电常数ε^T₃₃ /ε₀=1470和居里温度T_c=208℃。     

Characterization of A12O3, A12O3 + TiO2 Powder Mixture, and Coatings Prepared by Plasma Spraying

A1₂O₃ and its mixture with 3 wt% Ti0₂ powders were prepared by fusion and crushing methods. Al₂O₃ + 13 wt% TiO₂ and A1₂O₃ + 40 wt% TiO₂ powders were mixtures of fused AI₂O₃ + 3 wt% TiO₂ and 10 wt% and 37 wt% of TiO₂respectively. Chemical analysis of the powders showed that: approximately 1 wt% of impurities were present in each powder. True and tap densities were measured and are discussed for all powders. Powder size study showed narrow range of distribution of particle size in each powder. SEM study showed that the particles were in round and other irregular shapes in all the powders. A1₂O₃ was found to be in alpha phase and TiO₂ in rutile phase. All the powders were coated by plasma spraying at 16 kW. The density of the coatings were measured and are discussed. In the coatings, A1₂O₃ was in both alpha and gamma phases. In A1₂O₃ + 13 wt% TiO₂ and A1₂O₃ + 40 wt% TiO₂ coatings, the oxygen reduction from TiO₂ was observed.     

还原剂对Ni(OH)_2/还原氧化石墨烯复合材料结构及电化学性能的影响

为研究还原剂对Ni(OH)_2/还原氧化石墨烯(RGO)复合材料结构及电化学性能的影响,首先以氧化石墨烯(GO)和硝酸镍作前驱体,采用水热法制备了Ni(OH)_2/RGO复合材料;然后,利用XRD、SEM和Raman光谱仪表征了复合材料的结构和形貌,并采用循环伏安法、恒流充放电曲线和电化学阻抗谱研究了复合材料的电化学性能。结果表明:以(NH2)2CSO2作还原剂时,制备的β-Ni(OH)_2/RGO复合材料为RGO纳米片与Ni(OH)_2纳米片相互插层的结构;在电解液(6mol/L KOH溶液)中,0.2C放电倍率时β-Ni(OH)_2/RGO复合材料的比容量高达341.0mAh/g,10.0C放电倍率为时复合材料的比容量为242.2mAh/g,仍能保持β-Ni(OH)_2理论比容量的83.8%。所得结论表明制备的Ni(OH)_2/RGO复合材料显现出良好的电化学性能。     

用电化学噪声技术研究Q235钢在含氯盐模拟混凝土孔隙液中的腐蚀行为

采用电化学噪声技术(EN)和电化学阻抗谱(EIS)研究了Q235钢在0.5 mol/L NaCl的饱和Ca(OH)₂溶液(SCP)中的腐蚀过程,并对噪声数据进行时域分析与频域分析,对阻抗谱数据进行等效电路分析。采用SEM结合EDS和XRD研究了Q235钢的表面形貌和结构组成。结果表明,Q235钢在SCP溶液中的腐蚀过程可分为钝化膜的形成与破裂阶段(Ⅰ)、亚稳态点蚀阶段(Ⅱ)和Ca²⁺沉积和腐蚀产物形成阶段(Ⅲ)。在(Ⅰ)阶段,电流噪声的波动幅值较小,电流噪声标准偏差S_I、白噪声水平W_I较小、噪声电阻R_n较大;在(Ⅱ)阶段,电流噪声波动幅值较大,S_I、W_I呈现阶跃式增长,R_n显著降低;在(Ⅲ)阶段,电流噪声波动幅值增大到200 nA,S_I、W_I、R_n平稳波动。Q235钢在SCP溶液中腐蚀10 d后在其表面出现Fe₂O₃和弥散分布的CaCO₃晶体,此时阻抗谱中出现类Warburg阻抗,腐蚀反应受电荷转移和O₂扩散的联合控制。     

Deposition of Sm2O3 doped CeO2 thin films from Ce(DPM)4 and Sm(DPM)3 (DPM=2,2,6,6-tetramethyl-3,5-heptanedionato) by aerosol-assisted metal–organic chemical vapor deposition

Samarium-doped ceria (SDC) thin films were prepared from Sm(DPM)₃ (DPM = 2,2,6,6-tetramethyl-3,5-heptanedionato) and Ce(DPM)₄ using the aerosol-assisted metal–organic chemical vapor deposition method. -Al₂O₃ and NiO-YSZ (YSZ = Y₂O₃-stabilized ZrO₂) disks were chosen as substrates in order to investigate the difference in the growth process on the two substrates. Single cubic structure could be obtained on either -Al₂O₃ or NiO-YSZ substrates at deposition temperatures above 450 °C; the similar structure between YSZ and SDC results in matching growth compared with the deposition on -Al₂O₃ substrate. A typical columnar structure could be obtained at 650 °C on -Al₂O₃ substrate and a more uniform surface was produced on NiO-YSZ substrate at 500 °C. The composition of SDC film deposited at 450 °C is close to that of precursor solution (Sm : Ce = 1 : 4), higher or lower deposition temperature will both lead to sharp deviation from this elemental ratio. The different thermal properties of Sm(DPM)₃ and Ce(DPM)₄ may be the key reason for the variation in composition with the increase of deposition temperature.