D. L. McElroy

Comparison of the Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of a High‐Purity Iron and an Armco Iron to 1000°C

The effects of temperature, purity, magnetic state, and crystal structure on the thermal conductivity, electrical resistivity, and Seebeck coefficient of iron were obtained from measurements on Armco iron (99.5% pure, ρ300/ρ4.2=11.0) and a high‐purity iron (99.95% pure, ρ300/ρ4.2=26.2). The most probable determinate errors of the measurements were thermal conductivity ±1.5%, electrical resistivity ±0.1%, and Seebeck coefficient ±0.9%; and larger absolute errors. Where theory permits, the thermophysical properties of iron are discussed in terms of contributing transport mechanisms. The thermal conductivity of iron can be calculated to ±1.5% between 0° and 910°C from electrical‐resistivity measurements and the lattice portion of the thermal conductivity determined in this study.

The thermal conductivity of AISI 304L stainless steel

A compilation and critical analysis of the thermal conductivity (γ) of AISI 304 stainless steel (SS) between 100 and 1707 K has been given in the literature. The author represented his “recommended” values of λ by an inflection in the A versus temperature relationship between 300 and 500 K. Because a physical mechanism had not been identified that would produce such a temperature dependence in γ of 304 SS, interest was generated in the possible existence of an as yet undiscovered phenomenon that might cause such an inflection. Consequently, experimental verification of the inflection was sought. The present paper presents recent measurements of λ, the electrical resistivity, and the absolute Seebeck coefficient of 304L SS from 300 to 1000 K and of the thermal diffusivity (α) from 297 to 423 K. The λ values computed from the a measurements were within ± 1.6% of the directly measured λ An inflection was not observed in the temperature dependence of λ between 300 and 500 K. After careful evaluation and because a physical mechanism still has not been identified which would produce such an inflection, the authors conclude that the inflection in the λ vs T relationship reported in the literature was caused by the data analysis technique.

Thermal Measurement of In-Situ and Thin-Specimen Aging of Experimental Polyisocyanurate Roof Insulation Foamed with Alternative Blowing Agents

This paper reports apparent thermal conductivity (k) values from field and laboratory aging tests on a set of industry-produced, experimental polyisocyanurate (PIR) laminated boardstock foamed with hydrochlorofluorocarbons (HCFCs) as alternatives to chlorofluorocarbon (CFC). The PIR boards were blown with five gases: CFC-11, HCFC-123, HCFC-14lb, and 50/50 and 65/35 blends of HCFC-123/HCFC-14lb. The k-values were determined from 0 to 50{degree}C (30 to 120{degree}F) using techniques that meet ASTM C 114 (Thin Heater Apparatus) and ASTM C 518 (Heat Flow Meter Apparatus). Results on laminate boards with facers provide an independent laboratory check on the increase in k observed for field exposure in the ORNL Roof Thermal Research Apparatus (RTRA). The observed laboratory increase in k was between 8% and 11% for a 240 day field exposure in the RTRA. A thin-specimen aging procedure established the long-term thermal resistance of gas-filled foams. Thin specimens were planed from the industry-produced boardstock foams and aged at 24 and 65{degree}C (75{degree}F and 150{degree}F) for up to 300 days. An exponential dependency of k with the quantity (diffusion coefficient X time){sup {1/2}}/ thickness, provide effective diffusion coefficients for air components into the foams and blowing agent out of the foams. The foams blown with alternative blowing agents exhibited k-values 3 to 16% (average 9.4%) above CFC-11 foams under similar conditions. Field exposures were conducted on specimens under single ply EPDM membranes in the RTRA for over 400 days. Hourly averages of panel temperature and heat flux were analyzed to obtain K as a function of mean temperature on a week by week basis. The relative performance of test specimens of HCFC-14B under a black and under a white membrane is reported. 29 refs., 10 figs., 10 tabs.

Apparent Thermal Conductivity Measurements by an Unguarded Technique

An unguarded longitudinal heat flow apparatus for measuring the apparent thermal conductivity (λ a ) of insulations was tested with mean specimen temper atures from 300 to 330°K on samples up to 0.91 m wide, 1.52 m long, and 0.15 m thick. Heat flow is provided by a horizontal electrically heated Nichrome screen that is sandwiched between test samples that are bounded by tempera ture controlled copper plates and 9 cm of mineral fiber insulation. A deter minate error analysis shows λ a measurement uncertainty to be less than ±1.7% for insulating materials as thin as 3 cm. Three-dimensional thermal modeling indicates negligible error in λ a due to edge loss for insulations up to 7.62 cm thick when the temperature difference across the sample is measured at the screen center. System repeatability and reproducibility were determined to be ±0.2%. Differences of λ a results from the screen tester and results from the National Bureau of Standards were 0.1% for a 10-kg/m3 Calibration Transfer Standard and 0.9% for 127-kg/m3 fibrous glass board (SRM 1450b). Measurements on fiber glass and rock wool batt insulations showed the dependence of λ a on density, temperature, temperature difference, plate emittance, and heat flow direction. Results obtained for λ a as a function of density at 24°C differed by less than 2% from values obtained with a guarded hot plate. These results demonstrate that this simple technique has the accuracy and sensitivity needed for useful λ a measurements on thermal insulating materials.

Analysis of Transient Heat Transfer Measurements on Porous Thermal Insulations

This paper presents the results on developing an appropriate heat transfer model for analyzing transient thermal measurements on porous insulation using the flat- screen tester at the Oak Ridge National Laboratory. A transient radiation and con duction model that employed the two-flux equations for describing radiation ex change was considered. The details of the finite-difference method used to solve the two-flux equations implicitly and the energy equation explicitly are presented Calculations for the screen temperature rise for two test insulations were made and compared to the measured results. The two test insulations were the National Bureau of Standards (NBS) fiberglass batt transfer standard and the NBS standard reference material 1450b fiberglass board. It was concluded that the heat transfer model was suitable to be used as a data-reduction model for deducing the thermophysical prop erties of porous insulation from transient flat-screen test data.

Physical Properties of Ni 3 Al Containing 24 and 25 Atomic Percent Aluminum

Thermal conductivity, electrical resistivity, Seebeck coefficient and thermal expansion data were obtained on well-annealed Ni 3 Al containing 24 and 25 at. % Al. The results span the temperature range 300 to 1000 K. The expansion coefficients did not vary with composition and increased with temperature, reaching values of aIout 17 × 10 −6 K −1 at 1000 K. The thermal conductivity and electrical resistivity changed rapidly with composition, and the thermal conductivity of 24 at. % Al is as much as 30% lower than that for stoichiometric Ni 3 A1. The electronic Lorenz function of Ni 3 Al was obtained by subtracting the estimated phonon conductivity component and found to be within about 5% of the Sommerfeld prediction from 300 to 1000 K. The electrical resistivity results for stoichiometric Ni 3 Al are influenced by the loss of ferromagnetic order at lower temperatures and are not adequately described by the Bloch-Gruneisen equation.

Thermal Transport Properties of Ordered and Disordered Ni3Fe

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 Matthiessens rule.

Evaluation of a commercial, portable, ambient‐temperature emissometer

The DevicesS an uncertainty of ±0.014 units was determined. These repeatability and accuracy values were confirmed by a round‐robin investigation using two working standards whose e’s were measured by three independent laboratories using model AE emissometers and by four laboratories that used absolute methods. The instrument was shown to measure the total hemispherical e, not the total normal e. Experimental difficulties were encountered during measurements on a very anisotropic material and in determinations of e’s of materials with transpare...

Non-Steady-State Behavior of Thermal Insulations:

RECOMMENDATIONS TO COMBAT the increasing cost of energy include energy conservation actions that emphasize the use of insulation to improve thermal performance in new and old buildings and in new and old industrial applications. Available data indicate that currently used thermal insulations exhibit a wide range of thermal, chemical, and mechanical properties. Not all of these properties have been measured and, in a few cases, agreement does not exist on how to determine their values. This paper describes the application of a new measurement technique to obtain property data to describe the transient behavior of thermal insulations.

Thermal expansion of epoxy-fiberglass composite specimens

The thermal expansion behavior of three epoxy-fiberglass composite specimens was measured from 20 to 120°C (70 to 250°F) using a fused quartz push-rod dilatometer. Billets produced by vacuum-impregnating layers of two types of fiberglass cloth with an epoxy were core-drilled to produce cylindrical specimens. These were used to study expansion perpendicular and parallel to the fiberglass layers. This type of composite is used to separate the copper conductors that form a helical field coil in the Advanced Toroidal Facility, a plasma physics experiment operated by the Fusion Energy Division at Oak Ridge National Laboratory. The coil is operated in a pulsed mode and expansion data were needed to assess cracking and joint stresses due to expansion of the copper-composite system. The dilatometer is held at a preselected temperature until steady state is indicated by stable length and temperature data. Before testing the composite specimens, a reliability check of the dilatometer was performed using a copper secondary standard. This indicated thermal expansion coefficient (α) values within ±2% of expected values from 20 to 200°C. The percentage expansion of the composite specimen perpendicular to the fiberglass layers exceeded 0.8% at 120°C, whereas that parallel to the fiberglass layers was about 0.16%. The expansion in the perpendicular direction was linear to about 70°C, with an α value of over 55×10−6 °C−1. Anomalous expansion behavior was noted above 70°C. The expansion in the direction parallel to the fiberglass layers corresponds to an α value of about 15×10−6 °C−1. The lower α values in the parallel direction are consistent with the restraining action of the fiberglass layers. The α values decreased with the specimen density and this is consistent with literature data on composite contraction from 20 to −195°C.