Microstructural, XRD and electrical characterization of some thick film resistors

The microstructural and electrical characteristics (sheet resistivities, TCRs, and noise indices) of some 1 k/sq. and 10 k/sq. thick films were evaluated. The conductive phase was determined by X-ray diffraction (XRD) analysis. The microstructures of fired resistors were investigated by scanning electron microscopy (SEM) and analyzed by energy dispersive spectrometry (EDS). Some resistors were fired for a relatively long time at the highest temperature, i.e., 6 h at 850 °C, to allow the reactions in the material to reach equilibria. Sheet resistivities, temperature coefficients of resistivity, and noise indices of these resistors were compared with normally (10 min at 850 °C) fired resistors. After 6 h firing absolute temperature coefficient of resistivity (TCR) values of most resistors increased significantly, while sheet resistivities decreased. Complex impedance analysis showed that in most cases resistors with low noise indices showed nearly ideal resistor response while those with higher noise had a larger imaginary part.     

A new model for electron movement in a thick film resistor and its application to analysis of the structure and conduction mechanism in these resistors

A new model has been developed to explain the strain behaviour of thick film resistors. In this model, the electron moves in a meandering way through the resistor. By measuring the strain sensitivity and applying the meander model on it, information is obtained about the structure of the thick film resistor and the strain sensitivity of the micro-electron paths in the resistor. The latter can be a useful tool in testing models that describe conduction mechanisms in thick film resistors.In this research, measurements on the strain sensitivity of thick film resistors were made using a four-point-bending bridge and the four-point-resistance measurement was made using special probes. Long, narrow enamel-coated steel substrates were used to enhance the accuracy of the measurements.     

Preparation and characterization of Cu and Zn modified nickel manganite NTC powders and thick film thermistors

A simple ball milling/thermal treatment procedure was applied to obtain fine thermistor powders. Three different powder compositions were analyzed–Cu_0.2Ni_0.5Zn_1.0Mn_1.3O₄, Cu_0.25Ni_0.5Zn_1.0Mn_1.25O₄ and Cu_0.4Ni_0.5Mn_2.1O₄. XRD analysis showed that all three powder compositions had a cubic spinel structure. Correlation between the sintering temperature, structure and resulting electrical properties was analyzed on bulk samples. Thick film pastes were composed and segmented thick film thermistors were screen printed on alumina, dried and fired. SEM analysis revealed a typical dendrite structure with small grains and a developed surface area. Thick film sheet resistance was measured on a test matrix and the resistance decreased with increasing Cu content. The temperature dependence of sample resistance was measured in a climatic chamber enabling calculation of the material constant and activation energy. Aging of the obtained segmented thermistors was analyzed and the resistivity drift was 0.23% for the Cu_0.2Ni_0.5Zn_1.0Mn_1.3O₄ NTC thick film thermistor confirming greater stability of thermistors containing Zn and Cu that in combination with the determined good thermistor characteristics make them good candidates for temperature and heat loss sensor applications.     

Co-precipitation synthesis and characterization of NiO-Ce0.8Sm0.2O1.9 nanocomposite powders: effect of precipitation agents

NiO-Ce0.8Sm0.2O1.9 (NiO-SDC) nanocomposite powders applied as promising anode material for low-temperature solid oxide fuel cells (SOFCs) were synthesized by hydroxide co-precipitation method using NH3 x H2O, NaOH and NH3 x H2O + NaOH as precipitation agents. The crystal phases, morphologies and sintering behavior of the synthesized NiO-SDC nanocomposite powders were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and sintering experiments. The effect of precipitation agents on the synthesis of the NiO-SDC nanocomposite powders was discussed. Results show that different precipitation agents influence greatly the synthesis and characteristics of the NiO-SDC nanocomposite powders. The NiO-SDC nanocomposite powders synthesized with NH3 x H2O deviate from the original composition due to the loss of Ni. The loss of Ni is avoided and nano-sized NiO-SDC composite powders are synthesized, when NaOH and NH3 x H2O + NaOH are used as precipitation agents. The NiO-SDC nanocomposite powders can be synthesized at relatively low temperature using NH3 x H2O + NaOH as precipitation agent, and the synthesized NiO-SDC nanocomposite powders show good sintering characteristics.     

The effects of processing conditions on the resistivity and microstructure of ruthenate-based thick film resistors

Elevated temperature processing parameters affect the microstructure and electrical behaviour of thick film resistors on alumina substrates. Blended resistors (DuPont QS87 series) with a nominal sheet resistivity of 56 kΩ/□ and temperature coefficient (TCR) less than ±100 p.p.m K-1 were fired in a laboratory process that simulated production ramp rates and atmosphere. Resistances were measured in situ during firing in a three-factor, replicated experiment with two levels and centrepoints for peak temperature, firing time and probe current. Room temperature resistance values after firing show a strong correlation to temperature and time, which both increase resistance and flatten the R(T) curve around room temperature. In situ resistance during firing shows a weaker correlation, inverse with temperature because the thermally activated glass conduction has a greater share of the composite conduction at firing temperature. X-ray diffration (XRD) shows lead ruthenate, alumina, and zirconium silicate present in the resistors. The ruthenate lattice parameters increase with increasing firing temperature and time. Qualitative particle coarsening is observed with increasing firing temperature and time by transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDX) shows lead ruthenate, CuBi ruthenate and zirconium silicate crystallites dispersed in a lead silicate glass matrix, without much particle chaining. Resistance changes are attributed to increased separation of ruthenate particles by coarsening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low-temperature acetone-assisted hydrothermal synthesis and characterization of BiFeO3 powders

Well-crystallized pure perovskite bismuth ferrite (BiFeO₃) powders have been synthesized by a facile hydrothermal route at the temperature as low as 130 °C with the aid of acetone. In the synthesis, acetone played important roles in the low-temperature synthesis of pure BiFeO₃. The as-prepared BiFeO₃ powders mainly consisted of cubic particles with the size range from 50 to 200 nm. zero-field-cooled and field-cooled magnetization measurements indicated that pure BiFeO₃ powders showed a spin-glass transition below the freezing temperature. The as-prepared pure BiFeO₃ powders showed weak ferromagnetism and ferroelectricity simultaneously at room temperature. Moreover, the bismuth ferrite BiFeO₃ exhibit efficient photocatalytic activity under visible light irradiation.     

Fluctuation synthesis and characterization of Ti3SiC2 powders

 A novel fluctuation method for the synthesis of Ti₃SiC₂ powders was developed. The raw materials used in this process are Ti, Si, and graphite powders. Fluctuation synthesis utilized Si as in-situ liquid forming phase (additive), which was formed by heating the powder mixtures to 1300°C and using the heat released from the exothermic reaction for Ti₃SiC₂ formation. The result demonstrated that the reaction time for the formation of Ti₃SiC₂ was dramatically shortened using fluctuation method and the powders produced using this method contained more than twice amount of Ti₃SiC₂ compared to the solid reaction synthesized powders. The powders prepared by fluctuation method are fiber-like in morphology with dimensions of 0.8–2 μm in width and 5–10 μm in length. The growth direction of the fiber-like Ti₃SiC₂ particulate is {101^–1}*. The lattice parameters for Ti₃SiC₂ were determined by a trial-and-error method and are a=3.067 ? and c=17.645 ?. Received: 28 September 1998 / Reviewed and accepted: 1 October 1998     

Low temperature solid-phase synthesis of tetragonal BaTiO3 powders and its characterization

We report an alternative synthesis route for tetragonal and submicron barium titanate particles through a low temperature isothermal heat treatment of a precursor powder obtained from the BaO₂-TiO₂-C mixture utilizing combustion synthesis technique. Effort has been concentrated on the investigation and optimization of the synthesis process including combustion temperature and velocity, heat treatment procedure, and the characteristics of the powder obtained. It was shown that precursor powder prepared by the combustion method easily transformed to the tetragonal BaTiO₃ starting from 700 °C. Characterization of the products was performed by powder X-ray diffraction, scanning electron microscopy (SEM) and BET. From the results, it has been found that the formation of tetragonal BaTiO₃ powder at low temperature is conditioned by high chemical activity and specific characteristics of combustion products. The BaTiO₃ specimens were found to possess high dielectric constant and low dissipation factor at room temperature.     

Influence of the contacts and firing process on the properties of thick film resistors on alumina and dielectrics

This paper describes the behaviour of thick film ruthenium resistors when printed and fired onto standard 96% alumina substrates and onto alumina substrates covered with a dielectric film during long-term thermal ageing. It was observed that the behaviour of the resistors on alumina and dielectric depends upon the firing process of the resistors and the kind of terminations. An analysis of the effects of thermal ageing was made as well as some SEM observations of diffusion processes between the resistor and its terminations.     

Influence of pH on the synthesis and characterization of CuO powder for thick film room-temperature NH3 gas sensor

CuO films of 51 μm thickness have been fabricated from nanocrystalline powder, which has been synthesized by a sol–gel auto-combustion method at different pH values of the precursor solution. Studies reveal that the pH value of the precursor solution strongly affects the decomposition rate of the metal–citrate complex formed by precursors (cupric nitrate and citric acid). Structural characterization of the powder samples shows a considerable change in agglomeration behavior, crystallite size and strain with variation in pH value of the precursor solution. Studies show that high pH reaction conditions results in the production of highly porous CuO nanoparticles with lowest crystallite size of 27 nm. Thick films of the synthesized material show an extremely high response of 0.941 to few parts per million level of ammonia at room temperature as well as possesses good stability for a long period of time. The adsorption of ammonia on the sensor surface obeys Elovich equation and the reaction kinetics followed is of first order. The lowest potential barrier of 0.50 MΩ and highest rate constant of 0.0136 s⁻¹ have been found for ammonia adsorption on the sensor surface in case of film fabricated from CuO powder synthesized at high pH value of precursor.     

新型纳米TiO2膜电极的制备及应用研究

采用常压金属有机物化学气相沉积技术在钛板上沉积了TiO2膜电极,研究了其制备的重要工艺参数,进行了光化学、电化学特性的测试和表征,并应用于典型有机污染物苯酚的光电催化降解研究.结果表明,沉积温度是影响TiO2膜电极制备的重要因素.XRD图谱表明,沉积温度为500℃时负载的TiO3晶型结构为锐钛矿,600℃时出现金红石相.光电流测试表明,500℃时制备的TiO2膜的光电流最佳.以苯酚为污染物进行了光电催化降解实验,结果表明,其光电催化效果高于光催化、电催化降解效果,显示了较好的废水处理应用前景.     

Hydrothermal synthesis of ZnO nanorods in the presence of a surfactant

Controlling the dimensions, positioning, and shapes of semiconductor nanowires, nanorods, and nanobelts lies in the synthesis and understanding of their growth mechanism. Controlled growth and synthesis is required in the fabrication of nanodevices and nanosensors. Among methods utilized for one-dimensional nanostructure synthesis, the hydrothermal process--a simple and cost-effective technique involving a low process temperature--has emerged as a powerful tool for the fabrication of anisotropic nanomaterials. Under hydrothermal conditions, many starting materials can undergo quite unexpected reactions, which are often accompanied by the formation of nanoscopic morphologies that are not accessible by classical routes. Synthesized ZnO nanostructures from aqueous solutions are usually poor in terms of morphology and size control. To improve the growth conditions and the controllability of the process, the use of surfactants or organic solvents has been attempted. In the present work, ZnO nanorods were grown on templates with a pre-sputtered ZnO seed layer over oxidized Si (100) substrates, and polyvinyl pyrrolidone (PVP) was used as a surfactant. By varying the PVP concentration in the growth solution, we can control the diameter and density of ZnO nanorods. The optical property of ZnO nanorods is highly improved by PVP addition.     

Synthesis and characterization of nanosized Ti_3AlC_2 ceramic powder by elemental powders of Ti,Al and C in molten salt

A simple method to synthesize high-content ternary carbide Ti_3AlC_2 nanoparticles from Ti, Al, and C starting elemental powders without ball milling in NaCl–KCl molten salt was reported. The effects of mass ratio of the salt to starting materials, temperature, reaction time, and Al molar ratio on preparation of Ti_3AlC_2 were investigated. The Ti_3AlC_2 formation is dramatically influenced by temperature and mass ratio of the salt to raw materials: a higher temperature and higher mass ratio of the salt to raw materials are more preferable for Ti_3AlC_2 powder formation. Homogenous Ti_3AlC_2 powder with particle size of ■nm is synthesized by 3Ti/Al/2C starting elemental powders in NaCl–KCl molten salt at 900℃ for 10 h, 950℃ for 5h, or 1000℃ for 2h, respectively, when the mass ratio of the salt to 3Ti/Al/2C starting materials is 10:1.     

Metalorganic chemical vapor deposition of SrRuO3 thin film and its characterization

Synthesis of conducting oxide strontium ruthenate is carried out in a hot-wall tubular reactor, using Sr(C₁₁H₁₉O₂)₂/Ru(C₅H₅)₂/O₂ reaction system. Owing to a large difference in depositing efficiency between strontium and ruthenium precursors, the stoichiometric ratio of thin film is controlled in one cycle of two consecutive depositions at different temperatures. Thin films of SrRuO₃ single phase are synthesized in the subsequent 700°C annealing. Thin films of SrRuO₃ with extra ruthenium oxide can also be prepared by adjusting the molar ratio of RuO₂ and SrO layers. The deposition sequence of ruthenium oxide first, strontium oxide later is preferred. If the deposition sequence is reverse, the thin film is plagued with unreacted oxides even when the annealing temperature is raised to 800°C. The relative ease of preparing SrRuO₃ thin films, when RuO₂ is under SrO, is attributed to evaporation of ruthenium oxide in O₂ and diffusion in its open columnar microstructure. The sheet resistivity of thin film decreases with the ruthenium content. The room temperature resistivity of SrRuO₃ film of Ru/(Sr + Ru) = 0.5 is around 910 ohm-cm. The room-temperature resistivity of Ru/(Ru + Sr) = 0.53 decreases to 470 ohm-cm. The root mean square surface roughness of 700°C synthesized SrRuO₃ thin film is 22 nm, in a 2 × 2 m² area of film thickness 280 nm.     

Use of emanation thermal analysis in characterization of the influence of grinding on textural and structural properties of nanosized titania powders

It is shown that emanation thermal analysis (ETA) is a very sensitive method for studying the textural and structural changes undergone during thermal and/or mechanical treatment of TiO2. It is shown that the activation energy for the bulk diffusion of radon into titania samples annealed at 1100 °C dramatically increases by increasing the prior grinding time of the sample at room temperature. These results demonstrate that grinding of TiO2 prior to the annealing treatment considerably improves sintering and densification of the material.     

Biopolymer-based trimetallic nanocomposite synthesis,characterization and its application in the catalytic degradation of 4-nitrophenol

Journal of Materials Science: Materials in Electronics - Gum kondagogu (GK), a natural biopolymer was successfully employed in the synthesis of trimetallic (AgAuPd) nanocomposites and characterized...     

Sonochemical synthesis and characterization of cerium vanadate nanoparticles and investigation of its photocatalyst application

This research reports a facile sonochemical route for the synthesis of cerium vanadate (CeVO₄) nanoparticles with the aid of ammonium metavanadate, and cerium (III) nitrate hexahydrate as the starting reagents and water as the solvent. The structural, morphological and optical properties of as-obtained products were characterized by techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDX), and ultraviolet–visible (UV–vis) spectroscopy. The magnetic properties of as-prepared CeVO₄ nanoparticles were also investigated with vibrating sample magnetometer (VSM). To evaluate the photocatalyst properties of nanocrystalline CeVO₄, the photocatalytic degradation of methyl orange (MO) under ultraviolet light irradiation was carried out.