量子化霍尔电阻国家标准的研究

中国计量科学研究院在2003年建成了量子化霍尔电阻标准装置。成功研制了系统的核心器件—量子化霍尔器件。并通过对低温电流比较仪系统及运行过程的研究,发现了氦气气压波动引起的冻结磁通蠕动导致的附加噪声是限制系统不确定度的主要原因。进而提出了一种新型的气压滤波器,消除了这些干扰因素。还改进了电路动态特性,改善了前馈补偿环节,把匝数比提高了8倍,信噪比也因而提高了8倍。完成的量子化霍尔电阻标准装置的综合不确定度为2.4×10-10(k=1)。     

低温电流比较仪用于小电流测量的比例误差分析

小电流准确测量是电磁测量领域的热点和难点,低温电流比较仪作为目前最准确的电流比例量具,其电流分辨力高,在白噪声频带范围可达到甚至小于1fA/Hz1/2,电流增益准确性高.本文通过对基于SQUID的低温电流比较仪进行分析,低温电流比较仪比例误差小于10-11,可应用于nA量级电流的放大及准确测量.     

新型量子霍尔电阻样品计量应用研究

采用AsGa砷化镓系材料研制了多种结构的新型量子霍尔电阻样品,介绍了该样品的特点及应用领域。使用常温电流比较仪和低温电流比较仪,用过渡比对法对新型的量子霍尔电阻与标准量子霍尔电阻样品进行了双重测量比对验证,在实验室内测量结果的相对偏差小于4×10^-8,验证了新型的量子霍尔电阻样品和测量系统的准确度满足当前计量应用需求。     

二十一世纪中的计量测试技术--计量是可以溯源到标准量的测量

中国计量科学研究院张钟华院士负责用量子化霍尔效应建立国家电阻标准,他带领的研究组研制的低温电流比较仪的不确定度达到10^-10量级为世界第一,在世界电学领域享有极高声誉。在中国仪器仪表学会2005学术年会上张钟华院士就“二十一世纪中的计量测试技术”做了精彩演讲,博得与会代表热烈掌声。会后就同一问题,接受了本刊记者的采访。[编者按]     

中国计量科学研究院建成量子化霍尔电阻标准

国际计量委员会推荐，1990年1月1日起在世界范围内启用量子化霍尔电阻标准代替使用了几十年的电阻实物基准。中国计量科学研究院经过十几年的努力，在2003年建成了量子化霍尔电阻标准。课题组自主研制了能满足实际量值传递工作要求的量子化霍尔器件，并建成了高精度的低温电流比较仪，以把量子化霍尔电阻量值传递到日常检定工作中使用的十进电阻值。课题成果中有多项独创性的成就。目前所建量子化霍尔电阻标准的不确定度为10^-10量级，达到国际领先水平。     

中国计量科学研究院建成的量子化霍尔电阻标准

国际计量委员会推荐，1990年1月1日起在世界范围内启用量子化霍尔电阻标准代替使用了几十年的电阻实物基准。中国计量科学研究院经过十几年的努力，于2003年建成了量子化霍尔电阻标准。自主研制了能满足实际量值传递工作要求的量子化霍尔器件，并建成了高精度的低温电流比较仪，以把量子化霍尔电阻量值传递到日常检定工作中使用的十进位电阻。课题成果中有多项独创性的成就。目前所建量子化霍尔电阻标准的不确定度为10^-10量级，达到国际领先水平。     

中国计量科学研究院建成的量子化霍尔电阻标准

国际计量委员会推荐 ,1990年 1月 1日起在世界范围内启用量子化霍尔电阻标准代替使用了几十年的电阻实物基准。中国计量科学研究院经过十几年的努力 ,于 2 0 0 3年建成了量子化霍尔电阻标准。自主研制了能满足实际量值传递工作要求的量子化霍尔器件 ,并建成了高精度的低温电流比较仪 ,以把量子化霍尔电阻量值传递到日常检定工作中使用的十进位电阻。课题成果中有多项独创性的成就。目前所建量子化霍尔电阻标准的不确定度为 10 -10 量级 ,达到国际领先水平。     

一台直流比较仪式测温电桥

本文叙述一台基于直流比较仪原理的测温电桥。文中介绍了该电桥的总体线路结构和作用原理以及低频噪声不超过0.2微安匝的磁调制器的设计特点,并对采用主、辅比较仪实现开闭环调节的安匝自动平衡控制方案作了详细描述。实验表明,该跟踪系统稳定可靠。根据测试结果,该电桥在其基本量限内的测量误差不超过1.5×10~(-7)。最后,文章还提出一种进一步提高电桥比率精度的补偿线路,使电流比较仪可望达到1.5×10~(-8)量级的极限精度。     

非十进大功率电阻的测量

本文叙述了用电流比较仪电桥测量非十进大功率电阻的原理、电桥参数的选择、电路连线方法及测量步骤等,从而扩展了电流比较仪电桥的使用范围。     

精密工频电流-电压比例变换器的设计及校准方法研究

针对高精度、小量程的电流相关测量方法有限这一难题,根据有源阻抗的矢量电压合成消除互感器测量误差的方法设计一种精密工频电流-电压比例变换器。该变换器采用3只感应式比例单元实现范围较大的电流比例变换;采用双级电流互感器解决励磁电流引起的误差,减小电流的测试误差;其校准方法采用基于双通道电压比较仪和有源阻抗变换技术的溯源提高测试精度。通过实验证明该变换器能够实现10-6量级的电流-电压比例变换,其校准方法误差优于2×10-6,可测量最小2 m A的电流。     

Leakage current detection in cryogenic current comparator bridges

Several tests have been developed to locate leakage currents in cryogenic current comparator (CCC) resistance ratio bridges used at NIST to measure ratios of 1000 Ω/100 Ω, 6453.2 Ω/100 Ω, and 10 kΩ/100 Ω. The major advantage of the tests is that they can be performed in situ using the sensitivity of the CCC bridge. These test procedures have been used to reduce the leakage error uncertainty of CCC ratio measurements, linking working standards to the quantized Hall resistance (QHR) and to the NIST calculable capacitor experiment. CCC bridges require that the current which passes through a standard resistor must equal the current through the appropriate CCC winding to very high precision. This can be difficult to verify at or below 1 pA because a large number of possible leakage paths exist. Errors due to six important leakage current paths are given, and the calculated changes in the resistance ratio are compared with measurements made with a controlled leakage resistance in a 100 Ω/1 Ω CCC bridge     

Measurements of single electron transistor devices combined with aCCC: progress report

The BNM-LCIE is developing a current standard based on single electron transistor (SET) pumps. This paper gives an overview of the experimental set-up. It includes the circuit details, in particular the combination of miniature filters and homemade lines giving high attenuation in a wide frequency band, and a cryogenic current comparator (CCC) with winding ratio of 10000:1 for high accuracy amplification of the current. The results of the testing of the circuit and CCC are presented, together with the first measurements of the current through a SET carried out by means of a CCC. Coulomb blockade oscillations with an amplitude less than 200 fA have been observed with a signal to noise ratio approaching 100 and at bias voltages as small as a 100 nV     

Direct resistance comparisons from the QHR to100 M/spl Omega/ using a cryogenic current comparator

Measurements of room-temperature 100 M/spl Omega/ standard resistors and cryogenic thin-film resistors based directly on a quantized Hall resistance standard have been made with a cryogenic current comparator (CCC) bridge. This 15 496:2 ratio CCC attains a current sensitivity of 10.7 fA/Hz/sup 1/2/ in measurements of cryogenic thin-film resistors, without extensive shielding or filtering. A resistive primary winding helps the CCC maintain stability in the presence of external noise. The resistive-winding technique may be useful for the absolute measurement of small currents delivered by single-electron tunneling devices.     

The Cryo Current Comparator as a Calculable DC Ratio Standard

Cryo current comparators (CCC's) are known as current ratio standards with small ratio errors. These small errors are easily proven for windings with 1:1 ratios only. For ratios other than 1:1, say 1: N, the errors must be determined by a step-up calibration procedure based on 1:1 intercomparisons of all windings. This procedure becomes tedious as N gets larger than 1. In addition the uncertainty in the overall error is enhanced by each calibration step. This paper shows that by using a toroidal CCC design the step-up calibrations become unnecessary. Instead of determining the actual error, the special winding configuration has a maximum error which, in principle, can be determined by two simple 1:1 measurements. Therefore, the tedious calibration procedure and the overall uncertainty are reduced. High dc current ratios can now be established with this technique with errors smaller than 1 part in 1010.     

A sensitive magnetoresistive power amplifier

A small, sensitive, low noise, high gain power amplifier, using the anisotropic magnetoresistance effect in thin film permalloy, has been designed and its characteristics calculated. The minimum detectable input current is determined by Johnson noise and hence by input resistance and desired bandwidth. An example of theoretical performance is as follows. For an amplifier unit with approximate dimensions of300 times 300 times 2 mum and with input and load resistances of 50 Ω each the calculated noise at room temperature is equivalent to 10^-8A for a bandwidth (BW) of 1MHz or to 10^-7A for a BW of 100 MHz. At the 10^-8A input current level, the calculated power gain issim 600,000corresponding to a current gain of 775. Power gain decreases with input current asI^{-4/3}, reaching unity atI = 2.1 times 10^{-4}A. Hence, for a BW of 1 MHz, at room temperature, the input current operating range for both amplification and signal-to-noise ratio greater than one is fromI=10^{-8}A to2.1 times 10^{-4}A. To achieve high gain, the amplifier is configured so that the magnetization of the permalloy is biased to lie nominally along the hard axis, the sensing current in the permalloy makes an angle of 45° with the nominal magnetization direction, and the input current produces a magnetic field along the easy axis. This microsize, low noise, silicon compatible power amplifier will be useful in digital and FM applications and possibly as an amplifier for crosstie and bubble memories.     

Coupling capillary electrophoresis and high-field asymmetric waveform ion mobility spectrometry mass spectrometry for the analysis of complex lipopolysaccharides

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is a new technology for atmospheric pressure, room temperature separation of gas-phase ions. The FAIMS system acts as an ion filter that can continuously transmit one type of ion, independent of mass-to-charge ratio (m/z). Capillary electrophoresis-electrospray mass spectrometry (CE-MS) has been extensively used for the analysis of complex bacterial lipopolysaccharides (LPS). The coupling of FAIMS to CE-MS provides a sensitive technique for the characterization of these complex glycolipids, permitting the separation of trace-level LPS oligosaccharide glycoforms for subsequent structural characterization using tandem mass spectrometry. This was demonstrated for LPS from nontypeable Haemophilus influenzae strain 375 following O-deacylation with anhydrous hydrazine. This strain of H. influenzae can express a triheptosyl-containing glycoform to which four hexose residues are linked forming the outer-core region of the molecule. This has been referred to as the Hex4 glycoform. Glycoforms have been identified which differ in the number of phosphoethanolamine substituents in the inner-core. With the use of CE-FAIMS, isomeric Hex4 glycoforms containing two PEtn groups were separated and characterized by MS/MS. FAIMS provided a significant reduction in mass spectral noise, leading to improved detection limits ( approximately 70 amol of the major glycoform). The extracted mass spectrum showed that the apparent noise was virtually eliminated. In addition to the reduction of chemical background, the ion current was increased by as much as 7.5 times as a result of the atmospheric pressure ion-focusing effect provided by the FAIMS system. The linearity of response of the CE-FAIMS-MS system was also studied. The calibration curve is linear for approximately 3 orders of magnitude, over a range of 40 pg/microL to 10 ng/microL.     

基于NSGA-Ⅱ算法的变温热源内可逆简单MCBC的性能优化

基于前人所建立的变温热源内可逆简单等温加热修正的闭式布雷顿循环（modified closed Brayton cycle,MCBC）模型,分析了压气机压比等参数对循环性能的影响。分别以无因次功率和无因次生态学函数为优化目标,以压比和各个换热器的热导率分配为优化变量,对变温热源内可逆简单MCBC进行单目标优化。最后基于NSGA-Ⅱ算法,以无因次功率、热效率和无因次生态学函数为优化目标,以压比和各个换热器的热导率分配为优化变量,对变温热源内可逆简单MCBC进行多目标优化,并分析了相关参数的灵敏度。结果表明：常规燃烧室（regular combustion chamber,RCC）和收敛型燃烧室（converging combustion chamber,CCC）外侧流体的入口温比对单目标优化结果的影响存在着明显的相互关系;与单目标优化得到的结果相比,多目标优化得到的最优解对应的偏差指数更小,其中通过香农熵决策得到的偏差指数最小;优化变量变化±10%对最优无因次功率、最优热效率、最优无因次生态学函数及其对应的等温压降比的影响很小,其变化范围均不超过5%。     

An AC-bridge for low-frequency measurements of the quantized Hallresistance

A simple circuit which makes possible the use of a resistance-ratio bridge based on a cryogenic current comparator (CCC) with both AC and DC is described. The different sources of uncertainty associated with the use of AC in a CCC bridge are discussed. It is shown that they should have an effect which does not exceed a few parts in 10 ⁹ of the resistance-ratio being measured, if the frequency is limited to a few hertz. This analysis is confirmed by experimental results of resistance-ratio measurements between the quantized Hall resistance (QHR) and a 100 Ω resistance standard carried out at DC, 1, 2, and 4 Hz. These measurements are, to the author's knowledge, the first accurate DC measurements of the QHR. They demonstrate that the quantization of the Hall resistance, observed with AC and for the frequency range studied here, remains complete to within a few parts in 10⁹ or better     

Noise Characteristics of ZnO-Nanowire Photodetectors Prepared on ZnO:Ga/Glass Templates

In this paper, we report the fabrication of vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors on ZnO:Ga/glass templates. With 1 V applied bias, it was found that dark current density of the device was only 1.37times10^-7 A/cm². It was also found that UV-to-visible rejection ratio of the fabricated photodetector was around 1000 with a maximum quantum efficiency of 12.6%. It was also found that noise equivalent power and normalized detectivity of the ZnO nanowire photodetector were 5.73times10^-11 W and 6.17times10⁹ cmHz^0.5W^-1, respectively.     

Spaceborne estimate of atmospheric CO2 column by use of the differential absorption method: error analysis

For better knowledge of the carbon cycle, there is a need for spaceborne measurements of atmospheric CO2 concentration. Because the gradients are relatively small, the accuracy requirements are better than 1%. We analyze the feasibility of a CO2-weighted-column estimate, using the differential absorption technique, from high-resolution spectroscopic measurements in the 1.6- and 2-microm CO2 absorption bands. Several sources of uncertainty that can be neglected for other gases with less stringent accuracy requirements need to be assessed. We attempt a quantification of errors due to the radiometric noise, uncertainties in the temperature, humidity and surface pressure uncertainty, spectroscopic coefficients, and atmospheric scattering. Atmospheric scattering is the major source of error [5 parts per 10 (ppm) for a subvisual cirrus cloud with an assumed optical thickness of 0.03], and additional research is needed to properly assess the accuracy of correction methods. Spectroscopic data are currently a major source of uncertainty but can be improved with specific ground-based sunphotometry measurements. The other sources of error amount to several ppm, which is less than, but close to, the accuracy requirements. Fortunately, these errors are mostly random and will therefore be reduced by proper averaging.