RECENT ADVANCES IN NTC THICK FILM THERMISTOR PROPERTIES AND APPLICATIONS

An introduction to thermal sensors and thermistor materials is given in brief. After that novel electrical components such as thick film thermistors and thermal sensors based on them are described: Custom designed NTC thermistor pastes based on nickel manganite NiM2O4 micro/nanostructured powder were composed and new planar cell-based (segmented) constructions were printed on alumina. The thick film segmented thermistors were used in novel thermal sensors such as anemometers, water flow meters, gradient temperature sensor of the ground, and other applications. The advances achieved are the consequence of previous improvements of thermistor material based on nickel manganite and modified nickel manganite such as Cu0.2Ni0.5Zn1.0Mn1.3O4 and optimization of thick film thermistor geometries for sensor applications. The thermistor powders where produced by a solid state reaction of MnCO3, NiO, CuO, ZnO powders mixed in proper weight ratio. After calcination the obtained thermistor materials were milled in planetary ball mils, agate mills and finally sieved by 400 mesh sieve. The powders were characterized by XRD and SEM. The new thick film pastes where composed of the powders achieved, an organic vehicle and glass frit. The pastes were printed on alumina, dried and sintered and characterized again by XRD, SEM and electrical measurements. Different thick film thermistor constructions such as rectangular, sandwich, interdigitated and segmented were printed of new thermistor pastes. Their properties such as electrical resistance of the thermistor samples where mutually compared. The electrode effect was measured for all mentioned constructions and surface resistance was determined. It was used for modeling and realizations of high, medium and low ohmic thermistors with different power dissipation and heat loss. Finally all the results obtained lead to thermal sensors based on heat loss for measuring the air flow, water flow, temperature gradient and heat transfer from the air to the ground

Lead Telluride Doped with Au as a Very Promising Material for Thermoelectric Applications

PbTe single crystals doped with monovalent Au or Cu were grown using the Bridgman method. Far infrared reflectivity spectra were measured at room temperature for all samples and plasma minima were registered. These experimental spectra were numerically analyzed and optical parameters were calculated. All the samples of PbTe doped with Au or Cu were of the “n” type. The properties of these compositions were analyzed and compared with PbTe containing other dopants. The samples of PbTe doped with only 3.3 at% Au were the best among the PbTe + Au samples having the lowest plasma frequency and the highest mobility of free carriers-electrons, while PbTe doped with Cu was the opposite. Samples with the lowest Cu concentration of 0.23 at% Cu had the best properties. Thermal diffusivity and electronic transport properties of the same PbTe doped samples were also investigated using a photoacoustic (PA) method with the transmission detection configuration. The results obtained with the far infrared and photoacoustic characterization of PbTe doped samples were compared and discussed. Both methods confirmed that when PbTe was doped with 3.3 at% Au, thermoelectric and electrical properties of this doped semiconductor were both significantly improved, so Au as a dopant in PbTe could be used as a new high quality thermoelectric material

Characterisation of Mn0.63Zn0.37Fe2O4 powders after intensive milling and subsequent thermal treatment

Commercial Mn-Zn powder (Mn0.63Zn0.37Fe2O4, 93 wt. % and Fe2O3 7 wt. %) was milled 0.5, 1, 2 and 4 hours in a planetary ball mill. The goal was to observe intensive milling influences on oxidation and reduction processes that will happen during subsequent heating. Powders were characterized with XRD, SEM and particle seizer. Subsequent heating was monitored on TGA/DTA in an air atmosphere. After compaction of the milled powders, sintering was also performed in a dilatometric device. Sintered specimens were characterized micro structurally with SEM on a fresh breakage. Obtained differential TGA diagrams suggest intensive changes during prolonged milling of the oxidation kinetics on heating. Ferrite powders changed with milling as well as with second run heating were characterized to enable determination of the potentially best ratio of milling and heating to be applied to obtain the desired microstructure

The Response of a Heat Loss Flowmeter in a Water Pipe Under Changing Flow Conditions

A heat loss flowmeter for measuring the volume flow rate of water in real time (t) was formed using two thick film segmented thermistors, range constant voltage (RCV) power supply, acquisition card, PC, and software. The input water temperature T-w(t) was measured by a cold thermistor, while the water flow rate Q(t) was determined using the heat loss principle by measuring the self-heated thermistor current I(t). The flowmeter inertia, stability, and accuracy were measured and analyzed on a flowmeter prototype in real-time and real conditions on the water mains. The flowmeter response was measured for different durations of step input water flow-out functions and intervals between steps. An independent ultrasonic flowmeter was connected in series with the thermal flowmeter to measure the ordered input water flow functions. The realized intelligent functions in real time were: measuring the input water temperature T-w(t) and self-heating temperature T-s(t), auto-selection of RCV supply voltage U(Tw), determination and modeling of the water flow rate in real time Q(t), determination of the water volume V(t), and determination of the thermal gradient on the self-heating thermistor as a water flow indicator Delta Rs(t)