30-66 GHz millimetre wave phase shift keying modulator: performance and analysis

An octave bandwidth millimetre wave gallium arsenide (GaAs) MMIC modulator is demonstrated, wherein a novel method for directly producing QPSK/OQPSK/BPSK modulation directly from baseband IQ signals is presented. A key feature of the architecture is the absence of mixers. Expressions detailing the overall modulator error vector magnitude (EVM) performance with regard to its constituent components are derived. The dc power consumption of the circuit is less than 50 mW, and its 1 dB compression point is above 7 dBm. Frequency operation is between 30 and 66 GHz, over which QPSK EVM is below 15 , BPSK EVM 3 and data rates of greater than 500 MB/s are possible.     

Structural transformation during sintering and annealing of beta alumina

Changes in the phase content, microstructure and lattice parameter are observed in stabilized/ alumina specimens following extended sintering and annealing treatments. The resulting state is dependent on composition of the starting powder and on temperature and duration of heat treatment; the kinetics of transformation between and alumina are generally slow and certain/ ceramics remain in a metastable state even after a prolonged high temperature anneal. Following post-sinter heat treatment, splitting of X-ray diffraction peaks reveals a segregation of the phase into two components of differing lattice parameter. With sintering schedules of a long duration the splitting may even be present in the as-fired condition as recently reported by Harbach [1]. The splitting is attributed to a structural change resulting from the expulsion of Na₂O from supersaturated grains.     

毫米波单片集成低噪声放大器电路

基于0.25μm PHEMT(赝配高电子迁移率场效应晶体管)工艺,给出了一款毫米波MMIC(单片集成电路)低噪声放大器.放大器设计中采用了三级级联增加栅宽的电路结构,通过前级源极反馈电感的恰当选取获得了较好的输入驻波比和较低的噪声;采用直流偏置上加电阻电容网络,用来消除低频增益和振荡;三级电路通过电阻共用一组正负电源,使用方便,且电路性能较好:输入输出驻波比小于2.0,增益大于15dB,噪声系数小于3.0dB.1dB压缩点输出功率大于15dBm,芯片尺寸为1mm×2mm×0.1mm.这是国内报导的面积最小、性能最好的毫米波低噪声放大器.     

A comparative study of nickel and alumina sintering using spark plasma sintering (SPS)

Nickel and alumina powders with particle sizes of 4.3 μm and 108 nm, respectively, have been sintered in moulds of different sizes. Only the diameter (inner and/or outer) of moulds has been varied. The dimension modification is responsible for sample microstructure variation because set temperature, heating rate, dwell time and pressure were identical in all experiments. The influence of the die (or sample) dimension on the sample microstructure appears to depend strongly on the electric characteristics of the powder. The present paper is an attempt to correlate the bulk microstructure evolution with the die and sample size and the electrical current distribution within the system.     

Frequency-domain measurement of the millimeter wave indoor radiochannel

Coherent wideband frequency-domain measurements of the complex frequency response of millimeter wave indoor radio channels are discussed. In addition, results of measurements performed in a 2 GHz band centered around 58 GHz will be presented. It is shown that a 40 dB dynamic range and a 400 ns aliasing-free range are sufficient for a correct estimation of the rms delay spread from the measurement data     

Electroless deposition of copper onto alumina sub-micronic powders and sintering

This work has been initiated in order to find new routes for the preparation of 3D interconnected ceramic–metal composites, usually obtained by direct metal oxidation or liquid infiltration of a ceramic preform. The system under investigation is the alumina–copper system. The key point of this new route was the preparation of very fine composite powders, which allowed the material to be densified by natural sintering. The first part of the presentation deals with the preparation of the composite powders, which was achieved using an electroless process. Influence of experimental parameters controlling the deposition of a copper layer on a sub-micronic alumina powder was investigated. The second part deals with the processing of the composite. A statistical design methodology was applied to determine the main parameters controlling the shrinkage. For some conditions, a new in situ Cu–CuAlO₂ composite is formed, which presents an interesting microstructure with CuAlO₂ acicular grains.     

The mechanism of simultaneous sintering and phase transformation in alumina

The sintering behaviour of boehmitic alumina gels during the transformation to the stable phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis. The specimens for transmission electron microscopy were prepared from gel specimens, sintered to various predetermined temperatures, using an ion-beam thinning technique. The transmission electron microscope study and X-ray analysis have revealed a characteristic sintering behaviour which is associated with the to phase transition. The transformation to the phase occurs by a nucleation and growth process. During the growth process considerable redistribution of the fine porosity existing within the transition alumina matrix occurs, in the form of large elongated interconnected pores trapped within the nucleating grains. These pores grow rapidly to a size approximately one hundred times that of the grains. This process results in a rapid fall-off in sintering rate at the end of the transformation. A study of the/ interphase interface by transmission electron microscopy has led to the development of a model that accounts satisfactorily for the redistribution of the porosity.     

Forming and sintering of porous calcium-hexaaluminate ceramics with hydraulic alumina

We have developed a new eco-friendly fabrication process for porous ceramics using hydraulic alumina (HA) and water. In the present study, we fabricated porous calcium-hexaaluminate (CaAl₁₂O₁₉, CA6) ceramics using this new process. A boehmite gel 3-D network was formed by the hydration of HA in HA/CaCO₃ mixed slurry. The HA/CaCO₃ mixed slurry was hardened by the formation of this 3-D network. Even without the addition of an organic binder, green bodies containing the 3-D network demonstrated high compressive strength and retained their original shapes. Furthermore, the water acted as a fugitive material in the green bodies. Consequently, the open porosity of the CA6 ceramics could be controlled over a wide range of 42–62.7% by the addition of water (ratio of water to HA/CaCO₃ mixed powder: 0.4 to 1.6 by weight) without the use of organic fugitive materials. The results of evolved gas analysis-mass spectrometry measurements showed that the emissions from the hardened green body consisted mostly of water. Consequently, the new fabrication process for porous calcium-hexaaluminate ceramics was confirmed to be eco-friendly.     

Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.     

Effect of sintering on TiO2-impregnated alumina foams

The effect of sintering on the bulk properties, morphology and phase composition of ultralight Al₂O₃ foams impregnated with TiO₂ was investigated in comparison with pure alumina foam in the temperature range of 900–1600°C in air. Impregnation was carried out by immersion of pre-sintered alumina foam in a sol of titanium isopropoxide-acetylacetone complex. The changes of the foam linear shrinkage, effective density and porosity were studied along with morphological evolution and relationship between these properties was demonstrated. Titania impregnation increased the linear shrinkage (LS) during sintering by a maximum of 5% relative to pure alumina foams. The change of LS and weight loss of TiO₂/Al₂O₃ foams lead to a final density of 0.19 g/cm³ and porosity of 95%. The initial coating was found to develop a mosaic structure due to early shrinkage of the coating. After sintering at 1600°C the coating reacted with the underlying Al₂O₃ surface and became uniformly distributed. Finally, it was shown that the reacting TiO₂ layer formed the tialite (Al₂TiO₅) phase below 1400°C. This Tialite coating remained intact under 1200°C without stabilizers.     

烧结法制备氧化锆-氧化铝复合微滤膜

采用悬浮粒子烧结法，在两种不同结构的α－Ａｌ２Ｏ３支撑体上制备出０．１７μｍ左右的管式氧化锆复合微滤膜，研究制膜条件对ＺｒＯ２／α－Ａｌ２Ｏ３复合微滤膜的成膜质量的影响，结果发现，要得以无缺陷膜，干燥烧结的温度及时间的适当控制尤为重要，温度为ｂＴｓ烽结２ｈ成膜较好。采用气体泡压法测定膜的孔径分布，并以纯水通量表征膜的渗透性能。     

Pulse electric current sintering of alumina/nickel nanocomposites

Typical magnetically functionalized Al₂O₃/Ni nanocomposite materials were fabricated by pulse electric current sintering process. The rapid densification and various microstructures were accomplished with controlling PECS temperature. The PECSed Al₂O₃/Ni system exhibited peculiar mechanical and magnetic properties with microstructure developments. The mechanical properties of Al₂O₃/Ni nanocomposite were optimized as controlling microstructures, and fine metal dispersion into the Al₂O₃ matrix give to the various ferromagnetic properties. Electronic Publication     

High pressure/low temperature sintering of nanocrystalline alumina

Bulknanocrystalline α-Al₂O₃samples with a relative density >98% and a grain size < 50 nm have been produced by high pressure/low temperature sintering, using a toroidal-type high pressure apparatus. Nanocrystalline (n-) alumina powder with metastable γphase was used as the starting material. During sintering, the γphase transforms to αphase. The transformation temperature decreases from ~1075 °C at ambient pressure to about 460 °C at 8 GPa. Grain growth is limited by the low sintering temperature, and a multiplicity of nucleation events in the parent γ phase at very high pressure creates a nanoscale α grain size. The average grain size of the α-Al₂O₃ increases from 18 nm in the original powder to only about 49 nm in the sintered compact (98.2% dense). In addition, we found that high pressure could increase the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreased with sintering pressure under the same sintering temperature and time. Due to its high surface area, n-Al₂O₃ powder readily absorbs chemical species from the environment. Alumina hydrates, formed by the reaction of Al₂O₃ with chemisorbed OH⁻ species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al₂O₃. To control grain size of the transformed α phase, it is essential to eliminate the hydrates before sintering.     

Effect of surface roughness on grain growth and sintering of alumina

The production of ceramic bodies with less surface roughness is industrially important when one considers the aspect of final machining processes. Hence an attempt have been made to study the variation in surface roughness parameters (R _a, R _y, R _z) of alumina having three different kinds of roughness features at different sintering temperatures. Variation in surface roughness properties are also correlated with grain size. R _z shows significant difference between fine and intermediate surfaces, hence predicts small difference in their microstructural features. As a general trend, average grain size increases with increase in sintering temperature, but wide distribution of grains with enhanced non-uniform grain growth is observed when the surface is coarse. Hence, creation of fine surface in the green body is necessary for homogeneously distributed grains with controlled uniform grain growth. The final roughness and grain size of the sintered alumina depend on the initial surface roughness of the green body.     

毫米波系统内场测试仪方案设计

基于某毫米波系统实际项目,针对将故障隔离至内场可拆卸单元的要求,设计了基于CompactPCI总线的测试平台,具有高速度、高性能、高可靠、高可用、标准化、模块化的内场测试仪测控系统。在阐述内测仪功能的基础上提出了包括系统组成和软件设计在内的设计方案,并结合内测仪的工作过程,重点阐述了该测控软件中开关切换操作、测试报告管理、多线程技术与超时机制等关键环节的实现方法。     

Microwave and millimeter wave nondestructive testing: A succinct introduction

Microwave nondestructive techniques have a long history dating from the early 1950s, with a strong flurry of activities in the 1960s and 1990s. However, these techniques still are not widely known in the nondestructive testing (NDT) community and often are referred to as “emerging techniques” or “others.” It is only during the last two or three years that some NDT conferences have allocated a whole session or two solely to the topic of microwave NDT. For those involved in the research and development of microwave and millimeter-wave NDT techniques who have long appreciated the advantages of such techniques, this increased visibility is a welcome and encouraging change.     

Thermo-mechanical stability of porous alumina: effect of sintering parameters

Recently, we proposed a two-step heating schedule involving pulse electric current sintering (PECS), a kind of pressure assisted vacuum sintering, and subsequent post-heat treatment in air to fabricate porous alumina support, using commercially available alumina and carbon powders [J. Mater. Res. 18 (2003) 751]. During pressure assisted vacuum sintering, Al₂O₃–C system of low porosity was obtained and in second stage, i.e. during post-heat treatment in air, carbon particles present in the Al₂O₃–C system burnt out to form highly porous Al₂O₃ support. Following our previous brief study, the effects of sintering parameters such as temperature, pressure, and heating rate on the properties of the porous alumina were investigated. The porosity varied between 28 and 38% depending on the sintering parameters. As desired, the pore size distribution did not change with post-heat treatment temperature and hence the mechanical properties as well. It was concurred from this present study that the sintering parameters of PECS greatly influenced pore characteristics and other properties of porous compacts. We admit that the initial composition ratio of powder mixture (Al₂O₃:C) also plays important role on properties such as porosity, pore size, etc. which is beyond the scope of this present study.