After annealed, type K (Chromel vs Alumel) thermocouples were heated above 200 °C, their temperature measurements were in error up to 1.3%, as determined by comparison calibrations to working standard 90% Pt–10% Rh/Pt thermocouples or platinum resistance thermometers. Reannealing the type K thermocouples removed the errors. The errors were due to changes in the thermal emf vs temperature relationship of the type K thermocouples, which from previous work of others can be attributed to short‐ranged ordering of the Chromel thermoelements. Reannealing the thermocouples removed the errors because the order–disorder transformation is reversible; that is, short‐ranged ordering of the Ni and Cr atoms of the Chromel alloy occurs between 200° and 600 °C, and disordering occurs above 600 °C. The traveling gradient method was used to determine the effects of heat treatment on the thermal emf of type K thermocouples, to investigate the kinetics of ordering of Chromel, and to determine the amount of order produced by heat treatments. Though the order–disorder transformation could not be stopped, our results demonstrate that there exists an optimum amount of order in the Chromel thermoelement to yield a repeatable thermal emf vs temperature relationship for a type K thermocouple in a specific application. When the experimental conditions of the application were controlled carefully, isothermal heat treatment to produce the optimum order in the Chromel thermoelements and calibration of the type K thermocouples prior to use essentially eliminated the temperature measurement errors due to order. The optimum amount of order depended on the application and was determined experimentally. Variations from the optimized heating or cooling cycles, changes in the depth of thermocouple immersion during use, or modification of the temperature gradient on the thermocouple caused temperature measurement errors of up to 1% with ordered and calibrated type K thermocouples.

Temperature measurement errors with type K (Chromel vs Alumel) thermocouples due to short‐ranged ordering in Chromel

Exploring thermoelectric effects and Wiedemann–Franz violation in magnetic nanostructures via micromachined thermal platforms

Les auteurs etudient l'influence des traitements thermiques sur la valeur du pouvoir thermo-electrique de l'alliage cuivre–fer, mesuree a la temperature ambiante. Apres avoir analyse l'influence des conditions de trempe, ils montrent que le pouvoir thermoelectrique depend essentiellement de la quantite de fer en solution solide dans le cuivre, independamment de la quantite et de la repartition de la phase precipitee.



An investigation is made of the influence of thermal treatments on the value of thermoelectric power of copper–iron alloys, measured at room temperature. First the influence of quenching conditions is studied; then it is shown that the thermoelectric power mainly depends on the iron concentration in copper solid solution and is independent of precipitated phase repartition and volume.

Influence des traitements thermiques sur le pouvoir thermoélectrique des alliages cuivre–fer

The heat capacity, accurate to about ± 1%, of Ni3Fe has been measured from 300 to 1670 K using pulse calorimetry, with heatingrates from 20 to 60 K s−1. The specimen was heat treated in the calorimeter to induce various degrees of long-range order, varying in Bragg-Williams parameter from S = 0 to S = 0.96. The heat capacity was measured for various initial values of S. The rapid heating allowed measurements on the ordered structures of Ni3Fe at temperatures where they were metastable. Below 750 K, the heat capacity-temperature curve was lowered as Sincreased. The Curie transformation for a high S value was 70 K above that for a disordered condition (S = 0). Also, the order-disorder transformation was suppressed to 230 K above the equilibrium transformation temperature using a heating rate of 60 K s−1.



Die Warmekapazitat des Ni3Fe wurde auf ungefahr ± 1% genau von 300 bis 1670 K mittels eines Impulskalorimeters bei Heizgeschwindigkeiten von 20 bis 60 K s−1 gemessen. Das Probestuck wurde im Kalorimeter mit Warme behandelt, um verschiedene im Bragg-Williams-Parameter von S = 0 bis S = 0,96 variierende Stufen der Fernordnung herbeizufuhren. Die Warmekapazitat wurde fur verschiedene S−Anfangswerte der Warmekapazitat gemessen. Das schnelle Erhitzen ermoglichte Messungen der geordneten Strukturen des Ni3Fe bei Temperaturen in denen sie metastabil waren. Unter 750 K wurde die Temperaturkurve der Warmekapazitat abgesenkt, wenn S sich erhohte. Die Curie-Transformation eines hohen S-Wertes war 70 K uber der einer ungeordneten Kondition (S = 0). Auch wurde die Ordnung-Unordnungstransformation bis 230 K uber der Gleichgewichtsubergangstemperatur mit Hilfe einer Heizgeschwindigkeit von 60 K s−1 unterdruckt.

The heat capacity of Ni3Fe: experimental data from 300 to 1670 K†

Permalloy nanoparticles containing bacterial cellulose hydrogel obtained after reduction was compressed into a xerogel flexible sheet by hot pressing at 60 C at different pressures. The permalloy nanoparticles with an ordered structure have a bimodal size distribution centered around 25 nm and 190 nm. The smaller nanoparticles are superparamagnetic while the larger particles are ferromagnetic at room temperature. The sheets have a room temperature magnetisation of 20 emu/g and a coercivity of 32 Oe. The electrical conductivity of the flexible sheets increases with hot pressing pressure from 7 S/cm to 40 S/cm at room temperature.

Flexible Bacterial Cellulose Permalloy nanocomposite xerogel sheets size scalable magnetic actuator cum electrical conductor

Thermal conductivity λ, electrical resistivity ρ, and Seebeck coefficient S measurements have been made from 80 to 400 K on a Ni3Fe alloy (74.77‐at. % Ni and 25.33‐at. % Fe) in both disordered and highly ordered states. The effect of lattice disorder is to lower λ and S and increase ρ. Although λ and ρ for the three states studied differ by about 50%, the Lorenz ratios λρ/T are the same to within the combined uncertainty of the measurements. This ratio is near the Sommerfeld value Lo from 200 to 400 K, and the positive deviation from Lo below 200 K indicates a significant lattice component to λ in all states of disorder. The thermal conductivities of the three states of Ni3Fe exhibit peaks between 142 and 175 K. The unusual relative shift of the temperatures of the maximum conductivities is related directly to the large deviation of the electrical resistivity differences from Matthiessen's rule.

R. S. Graves

Papers

Thermal Transport Properties of Ordered and Disordered Ni3Fe