Clinical-pathologic correlation: The impact of grossing at the bedside.

The unenlightened clinician may submit a skin specimen to the lab and expect an "answer." The experienced clinician knows that in performing skin biopsies, it is critical to select the most appropriate: 1) anatomic location for the biopsy1,2; 2) type of biopsy1,2; 3) depth and breadth of the biopsy; and 4) medium for hematoxylin and eosin staining (formalin) or direct immunofluorescence (Michel's Transport Medium or normal saline).2 Demographic information, anatomic location, clinical context, and differential diagnosis are all critical components of a properly completed requisition form.3-5 Proper biopsy design and appropriate grossing of the tissue at the bedside should be added to this list. In this article, we review the basics of gross pathologic examination and then provide four examples to demonstrate that optimal clinical-pathologic correlation requires the clinician consider the needs of the pathologist when tissue is presented to the lab.

Thermoreflectance Imaging of (Ultra)wide Band-Gap Devices with MoS2 Enhancement Coatings.

Measuring the maximum operating temperature within the channel of ultrawide band-gap transistors is critically important since the temperature dependence of the device reliability sets operational limits such as maximum operational power. Thermoreflectance imaging (TTI) is an optimal choice to measure the junction temperature due to its submicrometer spatial resolution and submicrosecond temporal resolution. Since TTI is an imaging technique, data acquisition is orders of magnitude faster than point measurement techniques such as Raman thermometry. Unfortunately, commercially available LED light sources used in thermoreflectance systems are limited to energies less than ∼3.9 eV, which is below the band gap of many ultrawide band-gap semiconductors (>4.0 eV). Therefore, the semiconductors are transparent to the probing light sources, prohibiting the application of TTI. To address this thermal imaging challenge, we utilize an MoS2 coating as a thermoreflectance enhancement coating that allows for the measurement of the surface temperature of (ultra)wide band-gap materials. The coating consists of a network of MoS2 nanoflakes with the c axis aligned normal to the surface and is easily removable via sonication. The method is validated using electrical and thermal characterization of GaN and AlGaN devices. We demonstrate that this coating does not measurably influence the electrical performance or the measured operating temperature. A maximum temperature rise of 49 K at 0.59 W was measured within the channel of the AlGaN device, which is over double the maximum temperature rise obtained by measuring the thermoreflectance of the gate metal. The importance of accurately measuring the peak operational temperature is discussed in the context of accelerated stress testing.

Thermal Transport across Metal/ß-Ga2O3 Interfaces.

In this work, we study the thermal transport at ß-Ga2O3/metal interfaces, which play important roles in heat dissipation and as electrical contacts in ß-Ga2O3 devices. A theoretical Landauer approach was used to model and elucidate the factors that impact the thermal transport at these interfaces. Experimental measurements using time-domain thermoreflectance (TDTR) provided data for the thermal boundary conductance (TBC) between ß-Ga2O3 and a range of metals used to create both Schottky and ohmic electrical contacts. From the modeling and experiments, the relation between the metal cutoff frequency and the corresponding TBC is observed. Moreover, the effect of the metal cutoff frequency on TBC is seen as the most significant factor followed by chemical reactions and defects between the metal and the ß-Ga2O3. Among all ß-Ga2O3/metal interfaces, for Schottky contacts, Ni/ß-Ga2O3 interfaces show the highest TBC, while for ohmic contacts, Cr/ß-Ga2O3 interfaces show the highest TBC. While there is a clear correlation between TBC and the phonon cutoff frequency of metal contacts, it is also important to control the chemical reactions and other defects at interfaces to maximize the TBC in this system.

Quantifying charge carrier localization in chemically doped semiconducting polymers.

Charge transport in semiconducting polymers ranges from localized (hopping-like) to delocalized (metal-like), yet no quantitative model exists to fully capture this transport spectrum and its dependency on charge carrier density. In this study, using an archetypal polymer-dopant system, we measure the temperature-dependent electrical conductivity, Seebeck coefficient and extent of oxidation. We then use these measurements to develop a semi-localized transport (SLoT) model, which captures both localized and delocalized transport contributions. By applying the SLoT model to published data, we demonstrate its broad utility. We are able to determine system-dependent parameters such as the maximum localization energy of the system, how this localization energy changes with doping, the amount of dopant required to achieve metal-like conductivity and the conductivity a system could have in the absence of localization effects. This proposed SLoT model improves our ability to predict and tailor electronic properties of doped semiconducting polymers.

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency.

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high-efficiency Na-doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.

Melt-Centrifuged (Bi,Sb)2 Te3 : Engineering Microstructure toward High Thermoelectric Efficiency.

Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κl ) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt-centrifugation to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κl compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid-fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)2 Te3 alloys. A segmented leg of melt-centrifuged Bi0.5 Sb1.5 Te3 and Bi0.3 Sb1.7 Te3 could produce a high device ZT exceeding 1.0 over the whole temperature range of 323-523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high-efficiency porous thermoelectric materials through an unconventional melt-centrifugation technique.

Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence.

Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97 Eu0.03 Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm-2 ) are particularly effective at reducing κL . When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.

Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics.

To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb1-xSb2x/3Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm-1 K-1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.

Solubility design leading to high figure of merit in low-cost Ce-CoSb3 skutterudites.

CoSb3-based filled skutterudite has emerged as one of the most viable candidates for thermoelectric applications in automotive industry. However, the scale-up commercialization of such materials is still a challenge due to the scarcity and cost of constituent elements. Here we study Ce, the most earth abundant and low-cost rare earth element as a single-filling element and demonstrate that, by solubility design using a phase diagram approach, the filling fraction limit (FFL) x in CexCo4Sb12 can be increased more than twice the amount reported previously (x=0.09). This ultra-high FFL (x=0.20) enables the optimization of carrier concentration such that no additional filling elements are needed to produce a state of the art n-type skutterudite material with a zT value of 1.3 at 850 K before nano-structuring. The earth abundance and low cost of Ce would potentially facilitate a widespread application of skutterudites.