Microscale Freeform Integration by Directed Self Assembly

Most solid freeform fabrication (SFF) manufacturing processes assemble uniform components such as powder particles or polymer chains to produce desired geometries. Their capacity for producing highly functional parts (integrated actuation, sensing, and electronics) will dramatically increase when multiple materials and functional subcomponents can be automatically integrated. This paper addresses criteria for a system that integrates multiple materials and components through computer-controlled self-assembly. It builds complex systems from layers of self-assembled micro-components. The paper will address implementation methods, present a concept demonstration, and consider its application to micro-thermoelectric systems. This manufacturing process can be enhanced further through integration with mature additive processes.Mechanical Engineerin

Europium Clustering and Glassy Magnetic Behavior in Inorganic Clathrate-VIII Eu8Ga16Ge30

The temperature- and field-dependent, electrical and thermal properties of inorganic clathrate-VIII Eu8Ga16Ge30 were investigated. The type VIII clathrates were obtained from the melt of elements as reported previously. Specifically, the electrical resistivity data show hysteretic magnetoresistance at low temperatures, and the Seebeck coefficient and Hall data indicate magnetic interactions that affect the electronic structure in this material. Heat capacity and thermal conductivity data corroborate these findings and reveal the complex behavior due to Eu2+ magnetic ordering and clustering from approximately 13 to 4 K. Moreover, the low-frequency dynamic response indicates Eu8Ga16Ge30 to be a glassy magnetic system. In addition to advancing our fundamental understanding of the physical properties of this material, our results can be used to further the research for potential applications of interest in the fields of magnetocalorics or thermoelectrics

Origin of the magnetic anomaly and tunneling effect of europium on the ferromagnetic ordering in Eu8-xSrxGa16Ge30 (x = 0,4) type-I clathrates.

Systematic dc magnetization studies using the Banerjee criterion, Kouvel-Fisher, and magnetocaloric effect methods provide physical insights into the origin of themagnetic anomaly and the tunneling effect of europium on the ferromagnetic ordering in Eu8Ga16Ge30 type-I clathrates.We showthat Eu8Ga16Ge30 undergoes a second-order magnetic transition (SOMT) at TC ∼ 35 K, resulting from the magnetic interaction between the Eu2+ ions at the Eu2 sites, followed by a secondary magnetic transition at TL ∼10 K (indicated as amagnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu2+ ions at the Eu1 and Eu2 sites. The critical exponent β = 0.388 is close to that predicted from the three-dimensional Heisenberg model (β = 0.365), while the critical exponent γ = 0.956 is close to that predicted from the mean-field model (γ = 1). The substitution of Sr2+ for Eu2+ retains the SOMT but largely reduces the transition temperatures (TC ∼ 15 K and TL ∼ 5 K), with the critical exponents β = 0.521 and γ = 0.917 close to those predicted from the mean-field model (β = 0.5 and γ = 1). These results point to the important fact that the tunneling of Eu2+ between the four equivalent sites in the tetrakaidecahedral cage tends to prevent the occurrence of a long-range ferromagnetic ordering in the type-I clathrate materials

A synthetic approach for enhanced thermoelectric properties of PEDOT:PSS bulk composites

The thermoelectric properties of PEDOT:PSS/Bi 0.5 Sb 1.5 Te 3 polymer/inorganic bulk composites with different Bi 0.5 Sb 1.5 Te 3 content were investigated. The composites were prepared at various concentrations of Bi 0.5 Sb 1.5 Te 3 by a solution-phase process before grinding to fine powders in liquid N 2 for hot pressing into bulk polymer composite materials. The measured transport properties are well described within a theoretical model for effective media involving a tunneling mechanism induced by thermal voltage fluctuations. Our results present a strategy for the preparation of bulk polymer composites and demonstrate an avenue for optimization of the thermoelectric properties of PEDOT:PSS/Bi 0.5 Sb 1.5 Te 3 bulk composites. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4933254] Recently, polymers have been considered for thermoelectric applications primarily because of their unique combination of properties that are atypical of inorganic material, namely, mechanical flexibility, low cost, low temperature and low cost processing, and general non-toxicity

Synthesis, bottom up assembly and thermoelectric properties of Sb-doped PbS nanocrystal building blocks

The precise engineering of thermoelectric materials using nanocrystals as their building blocks has proven to be an excellent strategy to increase energy conversion efficiency. Here we present a synthetic route to produce Sb-doped PbS colloidal nanoparticles. These nanoparticles are then consolidated into nanocrystalline PbS:Sb using spark plasma sintering. We demonstrate that the introduction of Sb significantly influences the size, geometry, crystal lattice and especially the carrier concentration of PbS. The increase of charge carrier concentration achieved with the introduction of Sb translates into an increase of the electrical and thermal conductivities and a decrease of the Seebeck coefficient. Overall, PbS:Sb nanomaterial were characterized by two-fold higher thermoelectric figures of merit than undoped PbS

Magnetic and Electronic Properties of Eu₄Sr₄Ga₁₆Ge₃₀

Magnetization, static and ac magnetic susceptibility, nuclear forward scattering, and electrical resistivity measurements have been performed on polycrystalline Eu4Sr4Ga16Ge30, a type I clathrate that has divalent strontium and europium ions encapsulated within a Ga-Ge framework. These data are compared with those of type I clathrates Eu8Ga16Ge30 and Eu6Sr2Ga16Ge30. The ferromagnetic ordering of these Eu-containing clathrates is substantially altered by the incorporation of strontium, as compared to Eu8Ga16Ge30. Ferromagnetism, accompanied by a relatively large negative magnetoresistance, is observed below 15 and 20 K in Eu4Sr4Ga16Ge30 and Eu6Sr2Ga16Ge30, respectively. An effective magnetic moment of 7.83 µB per Eu ion is observed above 30 K for Eu4Sr4Ga16Ge30, a moment which is close to the free-ion moment of 7.94 µB per europium(II) ion

Direct Experimental Evidence for Atomic Tunneling of Europium in Crystalline Eu\u3csub\u3e8\u3c/sub\u3eGa\u3csub\u3e16\u3c/sub\u3eGe\u3csub\u3e30\u3c/sub\u3e

Mössbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, ∼450  MHz, tunneling of magnetic europium between four equivalent sites in Eu8Ga16Ge30, a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 Å. The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials

Enhanced cryogenic magnetocaloric effect in Eu8Ga16Ge30 clathrate nanocrystals

We observe an enhanced magnetic entropy change ( D S M ) at cryogenic temperatures (T < 20 K) in Eu 8 Ga 16 Ge 30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm, D S M is enhanced at low temperatures, reaching the highest value ( 10 J/kg K) at 5 K for a field change of 5 T. For all samples investigated, there is a cross-over tem- perature ( 25 K) in D S M (T) above which D S M decreases with crystal size, opposite to that observed at low temperatures. A careful analysis of the magnetic and magnetocaloric data reveals that as the crystal size decreases the magnetic interaction between Eu 2 þ ions on the Eu2 site gov- erning the primary ferromagnetic transition at 35 K becomes gradually weaker, in effect, altering the interaction between Eu 2 þ ions occupying the Eu1 and Eu2 sites responsible for the secondary ferromagnetic transition at 15 K. As a result, we have observed a strong change in magnetization and the enhancement of D S M at low temperature