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Annealed pyrolytic graphite (APG) is a material with thermal conductivity of about 1500 W/(m·K). This property may enable the usage of APG’s thermal potential to develop highly thermally conductive composites for devices requiring effective thermal management. In this paper, APG has been encapsulated in aluminum by brazing, and the thermal properties of Al-APG composite baseplates were measured. The results show that the thermal conductivity of the Al-APG composite baseplates is about 620 W/(m·K), which is four times higher than the pure aluminum plate (152 W/(m·K)).

An innovative porous filled heat exchanger is modeled to investigate the cooling effectiveness and temperature distribution at its base subject to a high heat flux. The effects of different nanofluid coolants (5% titanium dioxide (TiO2) in water, 1% alumina in water, 0.03% multi walled carbon nanotubes (MWCNT) in water, and 1% diamond in 40:60 ethylene glycol/water), different porous materials (copper and annealed pyrolytic graphite (APG)), and porosity values are investigated. The coolant enters from an inlet channel normal to the base, moves through the porous medium, and leaves the heat exchanger through two opposite exit channels parallel to the base. The effects of the inclination angle of the foam filled channel, inlet velocity value, and heat flux value are also studied. In addition, the effect of the inlet cross section is investigated by studying two different designs. One of the designs has a rectangular cross sectional inlet channel (extended all along the transverse direction) and the other design has a square one. The results indicate the importance of the utilization of a high conductive porous material. Utilization of APG porous matrix improves the cooling effectiveness at the base of the heat exchanger, for all studied coolants of pure water and water based nanofluids. The results also show that utilizing titanium dioxide nanofluids (TiO2) as coolant for both copper and APG porous matrices at low and high porosity structures, and for both square and rectangular inlet cross sections improves the cooling efficiency and temperature uniformity over the base. Investigation of the effect of inlet channel geometry, i.e., square and rectangular, indicates that employing a square cross section inlet channel would result in lower temperature values along the streamwise direction while higher temperature values are observed far from the center in transverse direction.

We have built an x-ray spectrometer in a von Hamos configuration based on a highly annealed pyrolytic graphite crystal. The spectrometer is designed to measure x-ray emission in the range of 2���5 keV. A spectral resolution E/��E of 4000 was achieved by recording the elastic peak of photons issued from the GALAXIES beamline at the SOLEIL synchrotron radiation facility.

New developments in the description and modeling of Highly Annealed Pyrolytic Graphite (HAPG) mosaic crystals have led to the possibility of designing optimized optical solutions for X-ray absorption fine structure (XAFS) spectroscopy. XAFS is a very versatile method that is usually divided into two sub methods: extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies, which need different experimental conditions concerning spectral resolving power, energetic bandwidth and number of detected photons. For facilitating XANES and EXAFS spectroscopies with laboratory- and von Hamos-based spectrometers, tailored optics were designed as well as optimized spectrometer components, i.e. an adequate microfocus X-ray source and a pixelated detector, were chosen. This is shown with a demonstration experiment on pure copper foil. In the XANES case a spectral resolving power of E/ΔE ≈ 4000 and an energy bandpass of around 300 eV were achieved with a measurement time of t = 7 min. For EXAFS, the tailored optic has an increased solid angle at moderate spectral resolving power in combination with a large energy bandpass of over 1 keV and a measurement time of t = 250 s for the given copper foil. These optimized solutions pave the way to perform XANES and EXAFS in the laboratory even for diluted samples with analyte concentrations of only a few weight percent or even less in a reasonable time frame of minutes to hours. Spectrometers, that already had an impact on research, especially catalysis research, therefore, made a huge leap in efficiency that prepares them to meet new challenges, not only as a standalone method, but also in combination with high-end synchrotron radiation facility-based XAFS experiments.

Next generation power modules demand increased heat extraction capability along with reduced weight and volume. In this paper, thermally annealed pyrolytic graphite (TPG) is analyzed and compared with conventional materials used in power modules for thermal management. Fundamental properties of TPG are explained and compared with commonly used materials in power module heat spreaders and substrates. The encapsulated TPG based heat spreader is manufactured and compared with bulk copper in simulation and experimental based analysis. The results show that encapsulated TPG based heat spreader achieves more than 50% reduction in thermal resistance along with 48% reduction in weight in the heat spreader layer.

Sodium borosilicate glass composites with up to 40 wt.% of annealed pyrolytic graphite (APG) have been fabricated through a sol-gel process. The APG was first encapsulated in a layer of mesoporous silica with a tunable thickness, found to substantially aid in dispersion and stability of the APG in a variety of alcohols. The silica coating on the APG provides an ideal interface between APG and a silica-based glass, achievable via a sol-gel process, as the aqueous precursors afford the opportunity to establish a uniform dispersion of the silica-coated APG and subsequently form a network around the dispersion. Multi-component gels of SiO 2 , B 2 O 3 , and Na 2 O with varying weight percentages of APG were produced. Gels were aged in sealed containers at an elevated temperature, significantly reducing the required gelation and aging times. Aged gels were heat treated and hot pressed under vacuum to yield fully dense, homogeneous glass free of phase separation or devitrification. Both unloaded and loaded sol-gel derived glasses have shown similar values for density and the coefficient of thermal expansion to that of a traditional melt-quenched glass of the same composition. Furthermore, both X-ray diffraction patterns and infrared spectra have shown similarities between the unloaded sol-gel derived glass and its melt-quenched counterpart. Loading of the sodium borosilicate glasses with silica-coated APG holds promise for enhanced thermal and electrical conductivity of the bulk material, as the high aspect ratio of the APG encourages formation of percolating networks within the glass matrix. Encapsulation of nanoparticles with silica provides a route for facile surface manipulation and obtaining stable, homogeneous dispersions in solution, while a multi-component sol-gel is attractive for other nanocomposite systems in which particle settling or agglomeration is a concern.

International audience; We present a proof-of-principle study demonstrating x-ray Raman Spectroscopy (XRS) from carbon samples at ambient conditions in conjunction with other common diagnostics to study warm dense matter, performed at the high energy density scientific instrument of the European x-ray Free Electron Laser (European XFEL). We obtain sufficient spectral resolution to identify the local structure and chemical bonding of diamond and graphite samples, using highly annealed pyrolytic graphite spectrometers. Due to the high crystal reflectivity and XFEL brightness, we obtain signal strengths that will enable accurate XRS measurements in upcoming pump-probe experiments with a high repetition-rate, where the samples will be pumped with high-power lasers. Molecular dynamics simulations based on density functional theory together with XRS simulations demonstrate the potential of this technique and show predictions for high-energy-density conditions. Our setup allows simultaneous implementation of several different diagnostic methods to reduce ambiguities in the analysis of the experimental results, which, for warm dense matter, often relies on simplifying model assumptions. The promising capabilities demonstrated here provide unprecedented insights into chemical and structural dynamics in warm dense matter states of light elements, including conditions similar to the interiors of planets, low-mass stars, and other celestial bodies.

Highly annealed pyrolytic graphite (HAPG) is an advanced type of pyrolytic graphite that, as a mosaic crystal, combines high integral reflectivity with a very low mosaicity of typically less than 0.1°. When used as dispersive X-ray optics, a high resolving power has been observed, rendering HAPG very suitable for applications in high-resolution X-ray spectroscopy, which conventionally relies on ideal crystals. For the design and modelling of HAPG crystals in applications requiring high spectral resolution, the diffraction properties must be known very accurately. To close this gap, a comprehensive characterization of HAPG crystals was performed that allows for modelling of the diffraction properties in different diffraction orders over a broad spectral range. The crystal properties under investigation are the mosaic spread, the peak reflectivity and the intrinsic reflection width. The investigations were carried out for different thickness crystal films, which were mounted adhesively on a substrate. It is shown that the diffraction properties are strongly correlated to the grade of adhesion, which depends crucially on the substrate material and its surface properties. The investigations were performed using monochromated tunable synchrotron radiation of high spectral purity with a high-precision experimental setup and calibrated detection devices at the electron storage ring BESSY II.

To study the properties of graphite at high temperatures and elevated pressures a method of fast heating by current pulse is presented. The experiment to obtain the dependencies of electrical resistance, input energy and specific heat versus temperature up to 5000 K for highly annealed pyrolytic graphite is shown. Specific heat up to 4600 K is nearly steady state, but it rises in the temperature range (4600–4800 K) before melting.

Sodium borosilicate (NBS) glass/annealed pyrolytic graphite (APG) fiber composites have been fabricated through a sol-gel process. A glass composition of 80wt% SiO2, 15wt% B2O3, and 5wt% Na2O was chosen, similar to commercial NBS glasses such as Pyrex. A variety of gels with increasing weight percentages of APG were produced and subsequently heat treated and sintered in a uniaxial hot-press. Sol-gel derived NBS glass composites have been prepared with up to 40wt% of APG homogeneously distributed throughout the glassy matrix. Increased loading of the sol-gel derived sodium borosilicate glass with SiO2-coated APG fibers has shown to decrease its coefficient of thermal expansion (CTE) below that of traditional melt-quenched NBS glasses of similar composition while keeping the bulk density relatively constant. The high aspect ratio of APG fibers encourages the formation of percolating networks within the glassy matrix, increasing the thermal conductivity of the composite material beyond that of a conventional NBS glass. The resulting thermal conductivity of the composites has been found to be directional, with a much higher conductivity value in the direction perpendicular to the axis of the hot press (“in-plane”) than in its parallel direction (“through-thickness”), indicating of orientation of the fiber axes perpendicular to the direction of applied pressure during processing.

The relative x-ray collection efficiency, spatial resolution, and spectral resolution of spherically-bent quartz, mica, and germanium crystals were compared with cylindrically and spherically-bent highly oriented pyrolytic graphite (HOPG) and spherically-bent highly annealed pyrolytic graphite (HAPG) mosaic crystals. The crystals were characterized using Mn-K-α 1 (5898.8 eV) and Mn-K-α 2 (5887.6 eV) x-rays generated from a Manson x-ray source. The HOPG and HAPG crystals had about (10–100)× higher x-ray collection efficiency than the quartz, mica, and germanium crystals. However, good spatial resolutions were obtained with the quartz (49 μm), mica (61 μm), and germanium (275 μm) crystals, while the HOPG and HAPG crystals provided no useful spatial resolving power. Deconvolution of the spectral broadening due to the Mn-K-α 1 intrinsic width (2.33 eV), source size (320 μm), and image plate detector resolution (63 μm) demonstrated the spectral resolutions ( E /Δ E) of the quartz (3800–6600), mica (4600), and germanium (3400–4500) crystals to be considerably higher than that of the HOPG (1200–2400) and HAPG (2500) crystals.

A tabletop, short-pulse laser-based hard X-ray (Kα) source equipped with an advanced X-ray optics and dedicated for high-resolution spectroscopy and time-resolved diffraction is described. Operation of the source together with a high-resolution spectrometer containing a large-aperture highly annealed pyrolytic graphite gave a resolution E/ΔE of ~1,800 for the spectral range around Kα line of Cu. The estimated total flux of the 8.05-keV photons was equal to 5.9 × 1010 ph/s in 4π sr. Performance boost of the source caused by X-ray optics relied on the significant increase in the Cu-Kα photon flux on both, the sample (4.7 × 106 ph/s) and the detector (3.4 × 103 ph/s). A spectral brightness of 1.4 × 107 ph/s/mm2/mrad2 was derived from the source parameters for the Kα line. Better performance due to high collecting power and reflectivity of the spectrometer enabled application of the cross-correlation technique with an Ni foil. An upper bound of emission duration of 323 ± 47 fs was obtained in this measurement. X-ray absorption near-edge spectroscopy on an Ni sample with an acquisition time of only 15 min confirmed the increased capability of the setup also for continuous spectrum (bremsstrahlung).

We present the first spectrally resolved measurements of x-rays scattered from cryogenic hydrogenjets in the single photon counting limit. The 120 Hz capabilities of the LCLS, together with a novelhydrogen jet design [J. B. Kimet al., Rev. Sci. Instrum. (these proceedings)], allow for the ability torecord a near background free spectrum. Such high-dynamic-range x-ray scattering measurementsenable a platform to study ultra-fast, laser-driven, heating dynamics of hydrogen plasmas. Thismeasurement has been achieved using two highly annealed pyrolytic graphite crystal spectrometers tospectrally resolve 5.5 keV x-rays elastically and inelastically scattered from cryogenic hydrogen andfocused on Cornell-SLAC pixel array detectors [S. Herrmannet al., Nucl. Instrum. Methods Phys.Res., Sect. A718, 550 (2013)].

International audience; The capability to efficiently transfer the heat away from high-powered electronic devices is a ceaseless challenge. More than ever, the aluminum or copper heat spreaders seem less suitable for maintaining the component sensitive temperature below manufacturer operating limits. Some emerging materials, such as Annealed Pyrolytic Graphite, are a new alternative to conventional solid conduction without the gravity dependence of a heat-pipe solution. Unfortunately, the ultrahigh performance rising of APG core is restricted to in-plane thermal conductivities which can be 200 times higher than its through-the-thickness conductivity. So a lower cross-plane thermal conductivity or a higher than anticipated interlayer thermal resistance would compromise APG-based materials as efficient heat spreaders. In order to analyze the sensitivity of these parameters on the effective thermal performances, an analytical model for predicting the temperature distribution over an APG flat-plate was developed. Its relevance was compared to numerical simulations and experiments for a set of boundary conditions.

Development of electronic devices and systems with increased capability and good reliability will require improved thermal management techniques. Technology improvements such as embedded heat pipes, integrated pumped fluid loops, and integrated high conductivity thermal spreaders such as annealed pyrolytic graphite provide advances that can enable more powerful devices in ever-decreasing package sizes. Although technology innovations provide one solution path, an alternative method that has not received much attention is simply optimized component placement. The present approach provides a fast method for determining optimized component placement that approaches a uniform distribution of heat flux. The result is improved thermal performance of electronic systems. A tool was developed which can optimally place any number of components within a rectangular domain. Optimized results were obtained for 18 uniform and 11 non-uniform components within 20 s and 7 s, respectively, using a 2.5GHz Core™ Duo processor. The approach presented in this paper is especially useful in situations where limited or no thermophysical and environmental conditions are readily available for the problem at hand and can be utilized in a variety of industries including microelectronics and satellite development.

The Orion high-resolution X-ray (OHREX) imaging spherically bent crystal spectrometer, operated with both image plates and CCD cameras, provides time-averaged plasma diagnostics through high-resolution spectroscopy with good signal-to-noise at the Orion laser facility. In order to provide time-resolved spectra, the OHREX will be outfitted with a streak camera, and in this case, even higher signal to noise will be desired. Using the OHREX’s sister instrument, the EBIT High-resolution X-ray (EBHiX) spectrometer, at the LLNL electron beam ion trap EBIT-I, we therefore compare the efficiency of a high-quality Ge (111) crystal (2d = 6.532 A) with that of a higher integrated reflectivity, but lower-resolution highly annealed pyrolytic graphite (HAPG) crystal (2d = 6.708 A) in the energy range 2408–2452 eV. We find that the HAPG provides overall more signal across the entire image; however, because of the much better focusing properties of the Ge crystal, the latter provides more signal within the central 100 μm of the spatial profile in the cross-dispersion direction and is thus more suitable for the narrow entrance window of the Livermore-built streak camera.

The capability to efficiently transfer the heat away from high powered electronic devices is a ceaseless challenge. More than ever, the aluminium or copper heat spreaders seem less suitable for maintaining the component sensitive temperature below manufacturer operating limits. Emerging materials, such as Annealed Pyrolytic Graphite (APG), propose a new alternative to conventional solid conduction without the gravity dependence of a heat-pipe solution. Unfortunately, the ultrahigh performance rising of APG core is restricted to in-plane thermal conductivities which can be 200 times higher than its through-the-thickness conductivity. So a lower-than-anticipated cross-plane thermal conductivity or a higher-than-anticipated interlayer thermal resistance would compromise APG-based material as efficient heat spreaders. In order to analyse the sensitivity of these parameters on the effective thermal performances, an analytical model for predicting the temperature distribution over an APG flat-plate was developed. To demonstrate its relevance, it was compared to numerical simulations for a set of boundary conditions. The comparison shows a high agreement between both calculations to predict the centroid and average temperatures of heating sources. The pertinence of the practical expression used for modelling APG flat-plates thermal behaviour appears quite relevant for early stage design, our concern.

Annealed pyrolytic graphite (APG) has many applications in the electronics industry due to its unusually high thermal conductivity in XY plane. A k·Core™ sample, manufactured by k Technology Corporation (kTC) composed of APG material with an aluminum encapsulant is evaluated and modeled by finite element analysis (FEA). The test vehicle has both 1-dimension and 2-dimension heat conduction classifications. The effect of 1-dimension versus 2-dimension is explained. Test conditions include edge cooling and thermal insulation (by plastic foam). Different combinations of test conditions (edge cooling and thermal insulation) can shift the measured temperature, but do not affect the thermal gradient. The thermal conductivity of the test sample obtained in the test is shown to be 540 W m −1 K −1 , which is 2.70 times the baseline aluminum sample (200 W m −1 K −1 ). These test results were correlated back to a finite element analysis model with a positive match. The APG “benefit” is defined to be the ratio of the temperature difference between the APG sample and the baseline sample over the temperature difference between the baseline sample and ambient. The benefit of the encapsulated APG is as high as 51.67%. The study provides for a better understanding of how this new type of thermal material can be applied in electronics cooling.

Ultrafast X-ray absorption spectroscopy (UXAS) offers the opportunity to investigate function-structure relationships of complex organic molecules or biological functional subunits without the need of crystallization. Of special interest from the viewpoint of structural biology is the region of K-edges of transition metals between 5 and 10 keV. Regardless of successful application of time-resolved diffraction techniques to investigations of crystal dynamics using synchrotron and laboratory based sources there are only very few examples for application of UXAS to revealing the structural dynamics in biomolecular systems. This is mainly caused by the lack of broadband ultrafast x-ray sources as well as of appropriate optics adapted to these sources. Due to the long-data-recording time in UXAS experiments the sample integrity is mainly determined by the average power of the pump pulses inducing the structural changes. Using a fixed energy of the pump pulse the latter one is determined by the repetition rate of the pump laser. In this paper we discuss the prospects of UXAS comparing fs laser plasma sources with different repetition rates in combination with tailor-made optics based on highly annealed pyrolytic graphite (HAPG).

It is known that thermal design engineers have difficulties in temperature control of GEO (Geostationary Equatorial Orbits) satellites, compared to other systems operating in ground applications due to harsh space environment. Electronic packaging is an essential discipline especially when high thermal loads together with other mechanical requirements need to be optimized. In most of the design, thermal management or cooling methods draw the main design path for the unit. In this study, there are eleven identical PCB's and cold plates in an electronic box. The main focus of this article is the thermal optimization of the cold plates to obtain a light and high conductive cold plate by considering all mechanical design requirements. In order to achieve this goal, the thermal unknowns in the design are reduced by improving mechanical design and production technique as much as possible. Thermal and structural analyses (Power Spectral Density- PSD) have been carried out to decide and check the mechanical requirements. All thermal and structural analyses have been carried out with finite element methods (FEM). The results show that 1.9 kg lighter cold plates can be achieved by using Annealed Pyrolytic Graphite (APG) with the same thermal performance of copper

We investigated the usability of anisotropic materials as inserts in aluminum-matrix-composite baseplates for typical high performance power semiconductor modules using finite-element simulations and transient plane source measurements. For simulations, several physical modules can be used, which are suitable for different thermal boundary conditions. By comparing different modules and options of heat transfer we found non-isothermal simulations to be closest to reality for temperature distribution at the surface of the heat sink. We optimized the geometry of the graphite inserts for best heat dissipation and based on these results evaluated the thermal resistance of a typical power module using calculation time optimized steady-state simulations. Here we investigated the influence of thermal contact conductance (TCC) between metal matrix and inserts on the heat dissipation. We found improved heat dissipation compared to the plain metal baseplate for a TCC of 200 kW/m2/K and above.To verify the simulations we evaluated cast composite baseplates with two different insert geometries and measured their averaged lateral thermal conductivity using a transient plane source (HotDisk) technique at room temperature. For the composite baseplate we achieved local improvements in heat dissipation compared to the plain metal baseplate.

Introduction As in the rest of the electronics world, the range of applications and functions of satellite systems is rapidly advancing. Applications such as phased array radars, satellite communication links, sensor systems, inter alia, combined with new packaging technologies and devices like multi-chip modules, high-density batteries, and high-speed processors have increased the technical challenges in packaging these systems. Combined with the quest for miniaturization, the power densities of electronic systems are rapidly increasing raising device temperatures to intolerable levels. Managing these high packaging densities and their corresponding high power dissipations requires the use of thermal systems capable of maintaining device temperatures below critical levels. Satellite systems have the additional the challenges of a harsh vacuum environment (no convection cooling) and requiring low mass solutions. A vital part of the space vehicle design, the thermal control subsystem must maintain all components at or below specified allowable operational temperature limits when exposed to external thermal environments. Alternative advanced thermal materials are needed to meet future spacecraft thermal radiator subsystem requirements. Encapsulated annealed pyrolytic graphite is a material system that can satisfy many of the packaging requirements of such systems. A novel, patented technique of encapsulating annealed pyrolytic graphite (APG) within a structural shell has been developed. Named k-Core, the encapsulated APG composite is an ideal thermal management material because of its high thermal conductivity (up to 1400. W/mK), low mass density (as low as 1.9 g/cm), high stiffness (up to 50 Msi), and the ability to have an engineered coefficient of thermal expansion. This material has been used in a variety of applications and form factors from large radiator panels to small electronic substrates. Presented herein is an overview of this material systems with examples of how it is used.

GST (Northrop Grumman Space Technology) has been developing and qualifying a high thermal conductivity thermal doubler to enhance high efficiency spacecraft equipment panel radiator heat rejection. Thermal doublers are high conductivity heat spreaders typically used in spacecraft applications to reduce electronic units operating temperatures, especially where the power dissipated per unit area by the electronics unit is excessively high (> 1 W/in). A typical low technology solution is the use of aluminum or graphite slabs (inserted between the unit and heat sink or radiator to space), for aluminum and graphite spacecraft equipment panels, respectively. The thermal doubler tested and reported in this paper is a hybrid material, composed of a core of very high thermal conductivity APG (annealed pyrolytic graphite) manufactured and patented by K-Technology which is then encapsulated within a structural aluminum shell (top and bottom layers, plus edge and central beams and via’s/buttons for structural support). The high conductivity APG material used in K-Technologies products is a unique form of pyrolytic graphite manufactured by the decomposition of a hydrocarbon gas in a high vacuum furnace (by Chemical Vapor Deposition CVD).

As more powerful space systems emerge, radiator technology is challenged to dissipate the waste heat at minimum weight. Through the integration of thin aluminum sheet or low mass carbon fiber composites and high conductivity annealed pyrolytic graphite, the development of a new class of high performance weight efficient radiators is feasible. A radiator concept based on k Technology Corporation's (kTC) high conductivity encapsulated annealed pyrolytic graphic materials (k-Core) has demonstrated this concept. APG is a highly crystalline material with a room temperature thermal conductivity of 1700 W/mK and density of 2.26 g/cm 3 . This design concept can manage high thermal flux systems, have small thermal distortions and be highly survivable. kTC's k-Core radiator concept outperformed a baseline aluminum/heat pipe design by a factor of five. The k-Core concept with 8 inch spaced heat pipes outperformed the baseline aluminum design with 4 inch heat pipe spacing by nearly a factor of three. This paper will present the completed work including the measured performance of these k-Core radiators.

For its primary mode of operation, the Burning Plasma Experiment (BPX) uses a double-null divertor for the removal of energy and charged particles from the plasma. Due to the high-power densities and the use of inertial cooling, high-thermal-conductivity annealed pyrolytic graphite (PG) and 1D carbon fiber composites are the two leading candidates for the plasma facing material. To maximize the power handling capability of the divertor, a tile thickness (normal to the plasma facing surface) greater than 30 mm, essentially infinitely thick for the burn durations being considered for BPX, is used and the separatrix is swept across the divertor surface to spread the heating over as large an area as possible. In the baseline divertor design concept, an arrangement of 546 modules forms a continuous surface, at the top and bottom, around the toroidal circumference of the vacuum vessel. Each divertor module consists of PG tiles housed in a metallic support frame which mounts to the vacuum vessel wall at four locations. The PG tiles have a c-direction thickness (normal to the PG tile laminations) of 6.35 mm and are oriented poloidally to minimize the eddy current forces induced during disruptions and to accommodate the thermally induced stresses during normal operation. >

A solar collector having the combined properties of high solar absorptance, low infrared emittance, and high thermal conductivity is needed for applications where solar energy is to be absorbed and transported for use in minisatellites. Such a solar collector may be used with a low temperature differential heat engine to provide power or with a thermal bus for thermal switching applications. One concept being considered for the solar collector is an Al2O3 cermet coating applied to a thermal conductivity enhanced polished aluminum substrate. The cermet coating provides high solar absorptance and the polished aluminum provides low infrared emittance. Annealed pyrolytic graphite embedded in the aluminum substrate provides enhanced thermal conductivity. The as-measured thermal performance of an annealed pyrolytic graphite thermal conductivity enhanced polished aluminum solar collector, coated with a cermet coating, will be presented.

The properties of two strongly bent Highly Annealed Pyrolytic Graphite (HAPG) crystals with different thicknesses of 40 μm and 100 μm are studied at all possible reflection orders using x-rays at 4.5 keV and 8 keV photon energies. Typical reflecting areas within 50% reflectivity drop boundaries have sizes of about ≤ 1 mm. These domains are mis-oriented by ≤ 1 minutes of arc to each other. The mosaicity was measured to be ~ 0.06° on a 1 × 1 mm2 scale, whereas it amounts to ~ 0.14° when the probed area becomes > 2 × 1 mm2. We find that the integrated reflectivity of the reflection (004) is in good agreement with the kinematical diffraction theory, while a maximum value of 2.3 mrad is achieved for 8 keV and reflection (002). The highest spectral resolution is obtained with an x-ray source of ≤ 50 μm size and a 40 μm thin graphite coating, which amounts to E/ΔE ≥ 1000 for 4.5 keV and 8 keV. In the case of 8 keV and reflection (008), the resolving power exceeds E/ΔE = 2000. In von-Hamos geometry, it was found that > 60% of the reflected photons are confined in a central 500 μm wide profile where high spectral resolution is pertained. Ray tracing simulations reveal that in order to pertain a certain resolution, a larger mosaicity would result in less contributing photons. Thus the efficiency of the crystal drops significantly when the mosaicity is increased and could not be increased by large crystal opening angles.

As assemblies of graphene sheets, carbon nanotubes, and fullerenes become components of new nanotechnologies, it is important to be able to predict the structures and properties of these systems. A problem has been that the level of quantum mechanics practical for such systems (density functional theory at the PBE level) cannot describe the London dispersion forces responsible for interaction of the graphene planes (thus graphite falls apart into graphene sheets). To provide a basis for describing these London interactions, we derive the quantum mechanics based force field for carbon (QMFF-Cx) by fitting to results from density functional theory calculations at the M06-2X level, which demonstrates accuracies for a broad class of molecules at short and medium range intermolecular distances. We carried out calculations on the dehydrogenated coronene (C24) dimer, emphasizing two geometries: parallel-displaced X (close to the observed structure in graphite crystal) and PD-Y (the lowest energy transition state for sliding graphene sheets with respect to each other). A third, eclipsed geometry is calculated to be much higher in energy. The QMFF-Cx force field leads to accurate predictions of available experimental mechanical and thermodynamics data of graphite (lattice vibrations, elastic constants, Poisson ratios, lattice modes, phonon dispersion curves, specific heat, and thermal expansion). This validates the use of M06-2X as a practical method for development of new first principles based generations of QMFF force fields.

Summary The non-faradaic differential electrode capacity of the basal plane of boronated stress-annealed pyrolytic graphite with 0.3 weight % boron has been studied in aqueous solution using an a.c. impedance bridge and compared to results previously obtained on the non-boronated material. On boronated graphite the potential of the minimum in the capacity-potential curve shifts negative by about 0.5 V relative to that for the non-boronated material. The direction of this shift is in agreement with the semiconductor interpretation proposed previously for the capacitance of the basal plane of the non-boronated material. The magnitude of the shift, however, is much larger and the capacity of the minimum for the boronated material much lower than estimated theoretically from the carrier concentration for the space charge region in the electrode phase. The potential of the minimum is essentially independent of pH over 14 units of pH for both the boronated and non-boronated material but the capacity-potential curves for the former deviate substantially from the near parabolic shape found for the latter, particularly at potentials anodic to the capacity minimum. Boronation apparently introduces surface chemical groups which contribute to the observed capacity at these anodic potentials. With scans to potentials more anodic than 1 V vs . RHE, pronounced hysteresis effects are observed in the capacity-potential curves for the boronated material, probably because of oxidation of the electrode surface.

Changes of ultrasonic transit times in compression‐annealed pyrolytic graphite (CAPG) were measured as a function of temperature (4–300°K at atmospheric pressure) and of pressure (0–20 kbar at 295°K). From the low‐temperature results, obtained for five independent acoustic modes, the temperature variations of all five elastic constants were calculated using the thermal expansion data of Bailey and Yates. From the high‐pressure data, taken for four independent modes, the changes of all five elastic constants and of the unit cell dimensions with pressure were calculated under the assumption that the small a‐axis compressibility is independent of pressure. The initial pressure derivatives of C33 and C44 are, respectively, about 25 and 35% lower than those obtained by Green et al. for CAPG. The pressure dependences of the volume and c‐axis compressibilities are in good agreement with measurements on natural graphite crystals by Lynch and Drickamer and by Bridgman, and in less satisfactory agreement with data of Kabalkina and Vereshchagin. The results support the contention that CAPG has bulk elastic properties very similar to those of single‐crystalline graphite.

Magnetic susceptibilities of compression annealed pyrolytic graphite intercalated with iodine monochloride (ICl) and of pyrolytic carbon substitutionally doped with boron (B) have been measured between 77 K and 300 K as functions of temperature and impurity concentration. The results are discussed in terms of McClure's theory based on the Slonczewski-Weiss band model. A numerical computation of the carrier concentration has led to an evaluation of the hole production rate of impurities; 67% for B and 5% for ICl. The low hole production rate of ICl seems to be ascribed to the clustering of intercalants taking place between carbon layers.

JET plans to install two pump limiter modules which can be used for belt-limiter, inner-wall and X-point discharges and, also, for 1–2 s as the main limiter. A design is presented which is compatible with two diagnostic systems and which allows access for remote-handling operations. The high heat-flux components are inertially cooled during a pulse. Heat is removed between discharges by radiation and pressure contacts to a water-cooled support structure. The pumping edge will be made of annealed pyrolytic graphite, which material has been shown to sustain heat loads of 45 MW/m 2 for 1–2 s. Exhaust efficiency has been estimated, for a 1-D edge model, using a Monte Carlo calculation of neutral gas transport. When the pump limiter is operated together with other wall components we expect an efficiency of ≈ 5% (2.5 × 10 21 particles / s ). As a main limiter the efficiency increases to about 10%.

Attenuation by coherent pair production in highly oriented compression annealed pyrolytic graphite has been used to polarize a 16 GeV bremsstrahlung beam. Graphite pieces of total volume 0.95 × 0.95 × 5.1 cm3 were assembled in water-cooled precision holders. Eighteen of these units were mounted in two independent coaxial assemblies. Each assembly provided precision angular orientation with respect to the beam direction, rotation of the graphite by 90° about the beam axis to rotate the plane of polarization, and removal from or insertion into the beam. One assembly, with 61 cm of graphite, served as a polarizer; the other, with 30.5 cm of graphite, was used as an analyzer. The complete device was placed between collimators in a sweeping magnet. Polarization of the attenuated beam, measured with the SLAC pair spectrometer, was 0.255 ± 0.020. This number, and the measured spectrum, are in qualitative accord with the calculations. The beam has yielded intensities greater than 4 × 108 equivalent quanta per pulse, and has been used in an experiment to measure asymmetries in pseudoscalar meson photoproduction.

The thermal conductivities of three specimens of well-oriented, annealed pyrolytic graphite have been measured over the temperature range 100–900°K. The conductivity parallel to the deposition plane is analysed in terms of mean free paths using the semi-continuum model of Komatsu and Nagamiya. An expression is derived for the phonon mean free path due to Umklapp scattering. A temperature independent phonon mean free path results from boundary scattering. The conductivity perpendicular to the deposition plane has been fitted to the Leibfried-Schlomann equation.

The elastic constants of pyrolytic graphite which has been highly ordered by annealing under compressive stress have been determined by ultrasonic, sonic resonance, and static test methods. Ultrasonic tests yielded the elastic constants c11, c12, c33, c44 = 1/s44, and the stress derivative of c33. The moduli 1/s11 and c44 were determined from the free flexural resonant vibrations of bars, and the shear modulus c44 also was determined from the fundamental torsional resonance of the bars and from the resonance of compound torsion oscillators composed of thin graphite disks with steel end pieces. Static tension, compression, and torsion tests on the pyrolytic graphite yielded a complete set of compliances (s11, s12, s13, s33, and s44). The following self‐consistent set of elastic constants (cij in units of 1011 dyn/cm2; sij in 10−11 cm2/dyn) has been deduced from the results: c11 = 106±2, c12 = 18±2, c33 = 3.65±0.10, c13 = 1.5±0.5, c44 = 0.018 to 0.035, 1/s11 = 102±3, 1/s33 = 3.65±0.10, s12 = −0.0016±0.0006,...

By means of a preferential etch technique in conjunction with transmission electron microscopy, a stress‐annealed pyrolytic graphite has been shown to be a compact of crystallites with La values typically lying in the range 1–40 μ. The boundaries defining these crystallites varied from the low‐angle tilt boundaries θ∼1′, found in Ticonderoga crystals, to high‐angle boundaries denoting misorientations of up to 30°. In contrast to the low over‐all boron concentration of the material, 1–2 ppm wt/wt, the concentration of this element residing in the intercrystallite boundaries was calculated at ∼103 ppm wt/wt. This provides confirmation of the premise that the lattice disorder existing across a boundary trace supports a vacancy concentration far in excess of that pertaining to more perfect regions of the graphite. It is postulated that at temperatures >2100°C this enhanced vacancy concentration will allow a dynamic interchange mechanism for crystallite growth, having an activation energy given by E* = ½(Efv +...

The anodic oxidation of 3000°C annealed pyrolytic graphite electrode was studied by the technique of potential sweep at linearly changing from 0.00V to+1.50V vs PHE, and by measuring the change of (00l) X-ray diffraction patterns, in 10M/l sulfuric acid electrolyte at 25°C.Experimental results are summerized as follows:(1) On the anodic potential sweep curves, two oxidation current peaks are observed as a small peak at about+1.2V and major current beyond+1.3V. The for, mer peak corresponds to the adsorption of oxygen on graphite surface and the latter to formation of graphite bisulfate lamellar compound prior to oxygen evolution.(2) On the cathodic sweep curves, the reduction current that appears as a peak between+1.30V and+1.00V corresponds to the decomposition of lamellar compound.

Summary The non-faradaic differential electrode capacity of the basal plane of high-pressure stress-annealed pyrolytic graphite has been examined in aqueous solution using an a.c. impedance bridge. The capacity has a near parabolic dependence on electrode potential with a minimum of ∼3 μF cm−2 in concentrated electrolytes and is pH independent. Studies in NaF solutions at concentrations in the range 0.9–10−5 M indicate that in the range of potential studied (+0.5 to −0.5 V vs. NHE) the minimum capacity is best attributed to the space charge region within the graphite and not to the diffuse ionic layer. Apparently the potential of zero charge of the electrolyte is outside the potential range of the measurements. The capacity value on the basal plane of the stress-annealed pyrolytic graphite compares favorably with that estimated from the carrier concentration using the presently available theory of the space charge layer in semiconductor electrodes.

The cathodic and anodic properties of the oxygen—peroxide couple have been studied on ordinary pyrolytic graphite, high-pressure annealed pyrolytic graphite and single crystal graphite in alkaline solutions using the rotating disk technique. Both the reduction of O2 and oxidation of peroxide are inhibited on the cleavage surface of the oriented graphite relative to the edge surface. This difference in behaviour is attributed to the lack of suitable adsorption sites on the cleavage surface for the reacting species and/or intermediates. The current/potential data, reaction orders, and stoichiometric number support the following mechanism: O2 → O2(ads); O2(ads) + HOH + e → HO2(ads) + OH−; 2HO2(ads) + OH− → HO2− + O2 + HOH, with the first and second steps rate-controlling for the cathodic reduction.

Bragg spectroscopy is a consolidated experimental method for high energy resolution X-ray measurements. However, this technique is limited to the detection of photons produced from point-like or well collimated sources and becomes quite inefficient for photons generated in extended and diffused ones; also, the possibility to perform simultaneous measurements of several X-ray lines is of great benefit when low rate signals are expected and individual angular scans require long exposure times. We present a prototype of a high resolution Highly Annealed Pyrolytic Graphite (HAPG) mosaic crystals based X-ray spectrometer, developed by the VOXES (high resolution VOn hamos X-ray spectrometer using HAPG for extended sources) collaboration at INFN Laboratories of Frascati, able to work with extended (millimetric) and isotropic sources. The spectrometer has been tested in the energy range 6–19 keV, with the aim to deliver a cost effective system having an energy resolution (FWHM) at the level of few eV, able to perform sub-eV precision measurements of single and multielement targets in a broad energy range. In this paper, the working principle of the VOXES spectrometer is presented, including a detailed description of the geometry, of the calibration methods and of the spectral fitting functions. Results of X-ray tracing simulations are compared to the experiment. The main results achieved by our spectrometer regard the peak resolutions. In particular, we report values as low as 4.02 ± 0.08 eV and 3.60 ± 0.05 eV for the Fe(Kα1,2) and Cu(Kα1,2) lines; for larger dynamic ranges, we report single shot spectra with Cu(Kα1,2) Ni(Kβ) and Zn(Kα1,2) lines with best resolution values of 5.15 ± 0.13 eV, 6.02 ± 0.24 eV and 6.20 ± 0.34 eV, respectively. Finally, we explored the capabilities of our spectrometer for energies above 15 keV, obtaining for Mo(Kα1,2) and Nb(Kβ) lines resolution values of 19.44 ± 0.75 eV and 39.90 ± 1.48 eV, respectively. The proposed spectrometer has possible applications in several fields, going from fundamental physics, synchrotron radiation and X-FEL applications, medicine and industry to hadronic physics experiments, where its performances make it a fundamental tool for a series of measurements like the energies of kaonic atoms transitions, allowing to extract fundamental parameters in the low energy QCD in the strangeness sector.

The oxygen reduction process occurring at the basal plane of stress-annealed pyrolytic graphite (SAPG) in acetonitrile containing 0.2 M tetrapropylammonium tetrafluoroborate has been studied by cyclic voltammetry. The first step is reported to be outer-sphere electron transfer to form the superoxide radical anion. The second step is reported to be the reduction of products formed by a following chemical reaction involving the superoxide radical anion and the SAPG surface. Support for this mechanism is based on a comparative study using mercury electrodes.

It is demonstrated that massive specimens of pyrolytic graphite heat-treated at 3600\ifmmode^\circ\else\textdegree\fi{}C may exhibit electrical characteristics comparable to those of the best single crystals presently available. Low-field magnetoresistance data taken perpendicularly to the $c$ direction of these specimens yield average carrier mobilities (1.3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ ${\mathrm{cm}}^{2}$/v-sec at 300\ifmmode^\circ\else\textdegree\fi{}K; up to 8\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{2}$/v-sec at 4.2\ifmmode^\circ\else\textdegree\fi{}K) in substantial agreement with Soule's and McClure's results for the layer planes of highly purified natural graphite. In conjunction with temperature dependence studies of the zero-field resistivity, it is found that the carrier density reproduces the theoretically predicted behavior down to 77\ifmmode^\circ\else\textdegree\fi{}K, but departs at lower temperatures. Plausible interpretations of this discrepancy and of the "knee" in the resistivity curve are briefly discussed.

A plasma limiter is used to control the plasma size in Tokamak fusion research devices and serves as the primary area of plasma wall interaction. Contact with the plasma produces very high heat fluxes, typically 500--5000 W/cm/sup 2/ for pulse lengths of 1--10 seconds. Wall temperatures can reach as high as 3000 K resulting in significant melting or wall erosion. Graphite and graphite composites have been chosen as the materials that best meet the requirements of this severe environment. Measurement of the thermophysical properties of these materials is critical to the modeling of the thermal response of the limiters during operation. The thermal diffusivities, thermal conductivities and thermal expansions of several candidate materials were measured. The categories of materials investigated in this study were pyrolytic graphite and annealed pyrolytic graphite; fine-grained graphites; anisotropic carbon/carbon fiber composites; two-directionally woven fibers with a graphitized pitch matrix; and carbon/carbon fiber composites with a four-directionally woven carbon fiber wave. The thermal conductivities of the carbon/carbon fiber composites were determined as a function of fiber orientation. Conductivities were measured using both the laser flash diffusivity technique and the thermal comparative method. The advantages and disadvantages of each of these methods are discussed and the datamore » compared. In addition, thermal expansion was measured as a function of fiber orientation using pushrod dilatometry techniques. 34 refs., 50 figs., 5 tabs.« less