Specific Heat Power Research Articles

Role of Bi/Te co-dopants on the thermoelectric properties of SnSe polycrystals: an experimental and theoretical investigation

A sustainable solution to the energy crisis may be found in thermoelectric materials and generators, capable of transforming thermal energy into electrical energy or vice versa. SnSe is one of the emerging thermoelectric materials with distinctive properties. The main advantages of this compound are earth-abundant, inexpensive, non-toxic and it is also known for its high thermoelectric performance. Here we prepared Bi/Te co-doped SnSe polycrystals; whereas, Bi and Te are added with different compositions such as (x = 0.0,0.02,0.04,0.06 and y = 0.03) in (Sn1-xBixSe1-YTeY) matrix by using the solid-state reaction method. XRD data confirms the samples belong to the orthorhombic crystal system with the Pnma space group. DFT calculations were used to see structural stability and electronic properties for pure and doped SnSe samples. Temperature-dependent semiconducting behavior of the samples has been demonstrated by electrical resistivity. The Seebeck coefficient, correlated with carrier concentration and mobility, validates the p-type behavior for the pristine samples and the n-type behavior for co-doped samples. The dominant behavior of phonon scattering has been demonstrated by thermal conductivity analysis. After co-doping there is decrement in total thermal conductivity was observed which 1.3 times lower than SnSe. A theoretical calculation was used to validate experimental results to estimate electrical properties, Seebeck coefficient, specific heat capacity, thermal conductivity, and power factor using Quantum espresso code with Boltzmann transport Equation. 4% Bi-doped sample displayed a significant increment in electrical conductivity and an enhanced Seebeck coefficient, which led to the power factor enhancement of approximately 2.0 times in contrast to the pristine sample and enhanced ZT of about 0.055 which is 3.43 times higher than the pristine SnSe.Graphical abstract

Process steam generation from low-grade waste heat in a direct- contact adsorption heat pump based on superhydrophobic surface-modified Zeolite 13X

ABSTRACT Adsorption heat pump (AdHP) is a potential technology to recover and storage low-temperature waste heat. This paper presents an experimental research on direct low-pressure process steam generation from hot water based on superhydrophobic surface-modified zeolite 13X. The measured water contact angle of the modified zeolite is larger than 150°, which restricts free water entering the pores inside the zeolite particles. Cyclic experiments are conducted for performance evaluation. High-temperature steam (205–229°C, 0.1–0.3 MPa) is generated from hot water (72°C) in the AdHP driven by dry air (130°C). The temperature of generated steam is promoted with increasing preset steam pressure, among which the temperature is raised by about 23°C at 0.3 MPa. As the boiling point of water rises simultaneously, the sensible heat demand is increased. Thus, the mass of the steam is accordingly decreased. GTL (gross temperature lift) reaches 130°C. When the preset pressure is increased, COPh (coefficient of performance for heating) and SHP (specific heat power) for direct steam generation are decreased by 36% and 39%, respectively. One feasible option to elevate these two indicators is to increase the inlet water to near boiling water to match the change in the preset pressure. However, the GTL will be decreased based on the option. Balance should be made based on the choice of design for practical application.

Antiferromagnetism of CeCd0.67As2 existing deep inside the narrow gap semiconducting state

Single crystals of $R$Cd$_{0.67}$As$_2$ ($R$ = La and Ce) have been synthesized by high temperature ternary melt and their physical properties have been explored by means of magnetization, specific heat, electrical resistivity, Hall coefficient, and thermoelectric power measurements. $R$Cd$_{0.67}$As$_2$ compounds indicate a (structural) phase transition at high temperatures, accompanied by a remarkable increase of the electrical resistivity with an extremely low carrier concentration. CeCd$_{0.67}$As$_2$ exhibits a large magnetic anisotropy and an antiferromagnetic (AFM) order below $T_{N} = 4$~K. Magnetic susceptibility curves, together with magnetization isotherms and specific heat, are analyzed by the point charge model of crystalline electric field (CEF). In the paramagnetic state, the observed magnetic properties can be well explained by the CEF effects, implying that the 4$f$ moments remain localized. Electrical resistivity measurements, together with Hall resistivity and thermoelectric power, also suggest highly localized 4$f$ electrons, where Kondo contributions are negligible. The low temperature physical properties manifest strong magnetic field dependencies. For $H \perp c$, $T_{N}$ shifts to lower temperature as magnetic field increases, and eventually disappears at $H_{c} \sim 60$~ kOe. Inside the AFM state, three metamagnetic transitions are clearly evidenced from the magnetization isotherms. The RKKY interaction may be responsible for the AFM ordering in CeCd$_{0.67}$As$_2$, however it would have to be mediated by extremely low charge carriers. Although the AFM ordering temperature in CeCd$_{0.67}$As$_2$ can be continuously suppressed to zero, no AFM quantum phase transition is expected due to the lack of conduction electron clouds to screen the 4$f$ moments.

Investigation of Structural, Mechanical, Optoelectronic, and Thermoelectric Properties of BaXF3 (X = Co, Ir) Fluoro-Perovskites: Promising Materials for Optoelectronic and Thermoelectric Applications.

Coded within Wien2K, we carry out DFT-based calculations for investigations of the structural, elastic, optoelectronic, and thermoelectric properties of BaXF3 (X = Co, Ir) fluoro-perovskites. The Birch-Murnaghan fit to the energy-vs-volume data and formation energy shows that these fluoro-perovskites are structurally stable. The phonon calculation confirms the thermodynamic stability, while the relation between elastic constants such as C 11 - C 12 > 0, C 11 > 0, C 11 + 2C 12 > 0, and B > 0 validates the mechanical stability of the compounds. BaIrF3 exhibits a strong ability to endure compressive and shear stresses. BaCoF3 shows a weaker capacity of withstanding changes in volume, attributed to a lower bulk modulus. Demonstrating a higher G-modulus of rigidity than the BaIrF3, BaCoF3 demonstrates stronger resistance to change the shape and both compounds are found to be anisotropic and brittle. The determined band structure profiles reveal that both BaCoF3 and BaIrF3 demonstrate a metallic nature. In addition, the metallic nature of BaCoF3 and BaIrF3 is reinforced by the density-of-states (DOS) study, where Co and F atoms contribute significantly to the total DOS in the valence band in the case of BaCoF3, while that of BaIrF3 is predominated by the Ba and F atoms. The computed values of ε1(0) for BaCoF3 and BaIrF3 are approximately 30 and 19, respectively, which are in line with Penn's model. The researched materials are confirmed to be strong contenders for optoelectronics by the lack of absorption in the visible range. For their potential use in thermoelectric device applications, thermoelectric parameters such as temperature-dependent Seebeck coefficient, specific heat capacity, thermal conductivity, power factor, and figure of merit are also investigated, which show that these materials are thermally stable and promising for applications in thermoelectric devices.

Experimental analysis of the fixed flat-plate solar collector with Sn-Al2O3 selective absorber and gravity water flow

To improve solar collector efficiency, a variety of designs and materials have been introduced into production practice. Studies describing solar collector specifics, therefore, are particularly valuable to the scientific community as they contribute to the overall body of knowledge and constant improvement the in the scientific field. In that regard, the study presented in this paper analyses the thermal performance of the fixed flat-plate collector with an Sn-Al2O3 selective absorber. The fixed flat-plate collector design utilizes gravity water flow in an open loop system. A two-month study was conducted to perform the analysis. The experiment was based on measurements of water flow in the fixed flat-plate col-lector, the water temperature at the fixed flat-plate collector inlet and outlet, and solar radiation intensity on a horizontal surface. Results for three randomly selected measurement days have shown that fixed flat-plate collector can achieve relatively satisfactory values for average daily specific heat power, thermal efficiency, and inlet-outlet water temperature gradient, respectively: June 29 (381.78 W/m2, 60.67%, and 9.06?C), June 30 (364.33 W/m2, 59.43%, and 7.46?C), and July 15 (373.06 W/m2, 59.85%, and 8.69?C). Apart from the relatively good measurement results, this type of solar collector does not require circulating pumps for operation, which brings a double advantage: energy saving and energy production.

Ferromagnetic CeSi1.2Ga0.8 alloy: Study on magnetocaloric and thermoelectric properties

We present the magnetic, specific heat, electrical resistivity, and thermoelectric power measurements for the intermetallic compound CeSi1.2Ga0.8. This composition belongs to the series CeSi2-xGax, which exhibits diverse magnetic orderings and different crystallographic structures depending on the substitution level x. It has been previously suggested that for 0.0 < x < 1.3 the compounds crystallize in the α-ThSi2 type structure and for 0.7 < x < 1.3 they order ferromagnetically below 14 K. Our complementary studies for x = 0.8 reveal that it is a complex system due to the Kondo scattering competing with the RKKY interactions. The transition from the paramagnetic to the ferromagnetic state takes place at TC = 12.5(1) K. Moreover, the Seebeck coefficient and electrical resistivity show not only an anomaly related to the magnetic transition, but also a broad maximum at ∼90 K related to the crystal field effect. The case of x = 0.8 is of special interest as it is at the crossing of the characteristic temperatures, i.e. Tord (magnetic ordering temperature) ∼ TK (Kondo temperature) ∼10 K. Additionally, ab initio electronic structure calculations were performed allowing us to determine phase stability, (revealing two stable phases - α-GdSi2-type and α-ThSi2-type structures) as well as spin and orbital moment decomposition on Ce-states for both considered structures.

Ab-initio study about the electronic, optical and thermoelectric nature of α-, β-, and γ-phases of CdS semiconductor: using the accurate m-BJ approach

In this work the first principles-based calculations with the FP-LAPW (full potential linear augmented plane wave method) are employed to investigate structural, thermal and optoelectronic properties of the three different phases of CdS binary compound. Our electronic band structure calculations display a direct type band transition with a gap value (Γv − Γc) equal to 0.9 eV, and 1.2 eV for β and γ phases respectively. Due to inexistence of VBM (valence band maxima) and the CBM (conduction band minima) located at the same Γ-point an indirect band transition was predicted in case of α-phase. Our calculated T-DOS and P-DOS visualizes to be shifted successively towards higher values along the energy axis. The σ s−p bonding character in the V/B is responsible for the instigation of relatively localized states S-p, Cd-d and Cd-p states close to Fermi energy. We also computed and discussed the important optical constants like the complex dielectric constant components, electron energy loss functions, reflectivity spectra, the absorption coefficients, refractive indices, real component of optical conductivity and the extinction coefficient. The inter band contribution resulting due to the corresponding optical nature was also studied and discussed in detail for these three phases. The temperature dependent thermoelectric parameters were studied to explore the thermoelectric behaviour. The thermoelectric parameters like thermal conductivity, Seebeck coefficient, specific heat capacity, Power factor, Electrical conductivity, Susceptibility and figure of merit of the three phases are investigated for their possible thermoelectric applications. The present work could be concluded as a theoretical qualitative type calculation related to optoelectronic and thermoelectric nature of the three studied phases and their efficient device application.

Influence of Pr substitution on the physical properties of the Ce1−xPrxCoGe3 system: Combined experimental and first-principles study

We present the results of our investigations of physical properties for the novel Ce$_{1-x}$Pr$_x$CoGe$_3$ system performed with a number of experimental methods: magnetic susceptibility, specific heat, electrical resistivity, magnetoresistance, and thermoelectric power. Moreover, the electronic structure was studied by means of photoelectron spectroscopy measurements and first-principles calculations. All investigated compositions of the Ce$_{1-x}$Pr$_x$CoGe$_3$ series crystallize in the tetragonal BaNiSn$_3$-type structure. The lattice parameters and unit cell volumes decrease with increasing Pr concentration. On the basis of the measurements taken, a preliminary magnetic phase diagram was created. A continuous suppression of the long-range magnetic ordering was observed with increase of Pr concentration. The critical Pr concentration for magnetic moment ordering was determined from linear extrapolation of the ordering temperature $versus$ $x$ to the lowest temperatures ($T = 0$ K) and is equal to about 0.66. Based on the first-principles calculations we show how the substitution of Pr for Ce affects the electronic structure and magnetic properties of the considered alloys. Within a single model we take into account the magnetic ordering, fully-relativistic effects, and Hubbard U repulsion on Ce and Pr. The impact of Hubbard U on the results of calculations is also discussed. We present the valence-band analysis, Mulliken electronic population analysis, and calculated electronic specific heat coefficients. For CeCoGe$_3$ it is found that the $++--$ configuration of magnetic moments on Ce is slightly more stable than the $+-+-$ one, and also that the calculated value of total magnetic moment on Ce (including spin and orbital parts) is in good agreement with the measurements.

Crystal electric field effects and heavy-fermion behavior in cubic PrCu2Au3 compound

We report the crystal structure as well as the magnetic, electronic and transport properties of PrCu2Au3 which crystallizes in the MgCu4Sn-type structure with the cubic space group F4¯3m (No. 216). In this structure, the Pr3+ ion sits on a lattice site with the cubic Td symmetry which dictates the crystal electric field (CEF) splitting of the Hund’s 3H4 ground multiplet into four levels. The sample has been investigated by measurements of temperature dependences of specific heat, Cp(T), electrical resistivity, ρ(T), magnetic susceptibility, χ(T), isothermal magnetization, M(B,T), thermal conductivity, κT(T) and thermoelectric power, S(T). χ(T) follows a Curie-Weiss behavior above 100 K with an effective magnetic moment, μeff = 3.3 μB/Pr which is comparable to a calculated value of 3.58 μB/Pr for a free Pr3+ ion and a Weiss temperature, θCW = 15.70 K indicating a dominant ferromagnetic interaction. From the analysis of M(B,T) and the 4f-electron derived specific heat (C4f(T)) results, the CEF ground state is a Γ5 magnetic triplet and the CEF energy scheme is presented. The Sommerfeld coefficient, γ estimated from the low-temperature C4f(T) is 304 mJ/(mol K2) indicating a mass enhancement of the conduction electrons at low temperatures. Further analysis of the heavy fermion properties based on the Sommerfeld-Wilson ratio supports the formation of a heavy fermion state in PrCu2Au3. However, the absence of an enhancement in S/T at low temperatures possibly suggests a weak correlation between the conduction electrons and the 4f-electrons at the Fermi energy. The thermal transport properties suggest that PrCu2Au3 is likely a compensated metal.

Promising thermoelectric properties of heavy-fermion semimetal Pr3Os4Ge13

Here we report the magnetic, electronic and thermal transport properties of the heavy-fermion semimetal Pr3Os4Ge13 with a cage-like structure by means of magnetic susceptibility, χ(T), isothermal magnetization, M(B, T), electrical resistivity, ρ(B, T), Hall coefficient, RH(T), specific heat, Cp(T), thermal conductivity, κ(T) and thermoelectric power, S(T). ρ(T) and RH(T) show semimetallic features in a manner that mimics a thermal activated behaviour with an activation energy, Δ/kB = 6.5 K indicating the opening of a small energy gap in the material. At low temperatures, a Sommerfeld coefficient, γ = 128 mJ (mol−1 K−2) observed indicates a mass enhancement of the quasiparticles at low temperatures which bears witness to a heavy fermion state in Pr3Os4Ge13. A Wilson–Sommerfeld ratio, WR and the dimensionless ratio, S/γT of 1.01 and 0.62 ± 0.018 observed, respectively, are in good agreement with the Fermi liquid scenario. S(T) and RH(T) reveal hole dominated transport (p-type material) with a relatively large room temperature value of S(T) = 32.85 ± 0.98 μV K−1. A room temperature low value of κ(T) = 1.61 W K−1 m−1 leads to a thermoelectric figure of merit, ZT = 0.03 ± 0.001 which is comparable to values achieved in several clathrates around the same temperature. Features from S(T) and ρ(T) favour the realization of a higher ZT value at elevated temperatures.

Correction of Errors in DSC Measurements Using Detailed Modeling of Thermal Phenomena in Calorimeter-Sample System

Measurements of quantities by differential scanning calorimetry (DSC) are influenced by intrinsic factors given by the construction limits of particular DSC devices. The position of temperature sensors outside the samples causes the produced experimental outputs to suffer from signal delay or partial signal consumption. Differences in sample mass and heating rate belong to other typical sources of errors that cannot be fully eliminated even by the experimental calibration. In this article, a computational method involving detailed modeling of thermal phenomena in the calorimeter-sample system is used for the correction of inaccuracies in DSC measurements. The applied model is able to extract pure data related to the sample itself, which is free of distortion. In a practical application of the method, lime hydrate samples of various mass are analyzed at first experimentally, while several different heating rates are used, and the influence of varying parameters on the obtained outputs is recorded. The experimental data are then reconstructed using computational modeling of thermal phenomena in the calorimeter-sample system. The corrected specific heat power values are found to be up to 9% higher and can be attributed to the heat capacity of inner parts of the device that consumes a part of the heat. The highest agreement between experimental and computational outputs is achieved when a sample filling one half of the crucible's volume is heated at 7 K · min <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Such a parameter choice shows a low-temperature shift (<; 5 °C), and the specific heat power difference is only 0.09 mW · mg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (3.4%).

Energy efficiency of a flat-plate solar collector using thermally treated graphene-based nanofluids: Experimental study

A covalent functionalization approach was utilized for the preparation of highly dispersed pentaethylene glycol-thermally treated graphene-water as the absorbing material inside a flat-plate solar collector. Four mass fractions of nanofluids were prepared (0.025, 0.05, 0.075, and 0.1 wt% pentaethylene glycol-thermally treated graphene-water). Graphene nanoparticles were characterized by energy dispersive X-ray analysis with a scanning electron microscope. Measurements of the thermophysical properties were subsequently carried out for the nanosuspensions. The raw investigation data were collected from an indoor flat-plate solar collector test setup. The experimental procedure included different sets of variables such as input temperatures of 303, 313, and 323 K; fluid mass flow rate of 0.00833, 0.01667, and 0.025 kg s−1; and heat flow density of 500, 750, and 1000 W m−2. The thermophysical tests of pentaethylene glycol-thermally treated graphene-water nanofluids showed a proportional increase against weight concentrations, while the specific heat power was reduced. The tests showed an increment in energy efficiency by increasing the fluid mass flow rate and heat input. By comparison, the thermal efficiency decreased with the increasing temperature of the fluid supply. Relative to the base fluid, the energy efficiency of pentaethylene glycol-thermally treated graphene/water-based flat-plate solar collector increased to 10.6%, 11%, and 13.1% at the three fluid mass flow rates. In conclusion, an exponential form was used to derive the thermal effectiveness of flat-plate solar collector based on the experimental data.

Heat generation in a Couette-Taylor flow multicylinder system

The paper presents the results of an experimental study of energy production in the Couette-Taylor heat generator with independent rotation of cylinders: the system is applied to solving the problem of direct conversion of wind energy into thermal energy. The system consists of two nested multicylinder rotors. The regimes for two counter-rotating rotors are studied. The study is focused on the rotor drag torque and the heat power of the generator as a function of the relative angular velocity of two rotors at a fixed viscosity of the working liquid or as a function of the working liquid viscosity at a steady relative angular velocity of two rotors. Representing of this multicylinder design of the heat generator to a form of a single equivalent annular channel between two rotating cylinders allows generalization of experimental data for the law of the drag torque and specific heat power as a function of the Reynolds number. This generalization offers a possibility of developing engineering methods for calculating the thermal parameters of various systems for fluid heating.

Experimental study of heat and mass recovery on steam generation in an adsorption heat pump with composite zeolite-CaCl2

High-temperature steam generation in adsorption heat pump with composite zeolite-CaCl2 has been experimentally investigated by introducing heat and mass recovery. A direct contact heat exchange method is utilized to enhance the overall heat transfer rate. Composite zeolite (CZ) is prepared by immersing zeolite particle in CaCl2 solution at mass concentration of 40%. Cyclic experiment consists of two processes: regeneration with 130 °C dry air and steam generation with 80 °C hot water. The heat and mass recovery with steam below 90 °C is added before generation process. Mass of generated steam above 200 °C is increased by 9.12%. The effective time for steam generation is dramatically extended by 18.33%. Adsorption and thermal equilibriums inside the packed bed are achieved more quickly with heat and mass recovery. Gross temperature lift for one-stage adsorption heat pump is above 100 °C. Coefficient of performance for heating (COPh) and specific heat power (SHP) are elevated by 9.55% and 9.23%, respectively. The increase in mass of generated steam contributes to the improvement of COPh. Meanwhile, only a short time used for heat and mass recovery enables the rise in SHP.

Comprehensive studies of the transformation between antiferromagnetic CeCoGe3 and heavy fermion CeFeGe3 compounds

We report extensive studies of a series of alloys CeCo1−xFexGe3 in which suppression of the antiferromagnetic order in CeCoGe3 with temperature decreasing to 0 K was observed as a result of substitution of Co with Fe. Unusual temperature dependences were observed of a number of parameters characterizing these compounds: magnetic susceptibility, specific heat, electrical resistivity, magnetoresistance and thermoelectric power, which may be indication towards non-Fermi liquid behaviour connected with quantum critical point (QCP) or can result from a quantum Griffiths phase (QGP) formation. The three AFM transitions found previously for a single crystal CeCoGe3 compound were also detected in our high quality polycrystalline samples, including the case of x>0. All measurements, especially those of magnetocaloric effect (MCE), suggest that for samples with x≥0.1 the contribution of ferromagnetic component, known already for x=0 from the neutron diffraction experiments, increases significantly, with the AFM contribution still present at low temperatures. Finally, the use of different complementary methods enabled construction of a comprehensive magnetic phase diagram of CeCo1−xFexGe3 system.

Thermoelectric polymer composite yarns and an energy harvesting wearable textile

A thermoelectric wearable is proposed based on an innovative 3D wearable design, with p- type polymer impregnated cotton yarns and n-type polymer impregnated cotton yarns, where the ratio of the p-/n-type yarn cross-sections is optimised to maximise the thermoelectric conversion efficiency. The thermoelectric yarns are embroidered through a double-layer cotton fabric, using plain-stitch, and connected via painted silver electrodag patches. The first stage of this study involved the characterisation of P3HT-, PEDOT:PSS- and PCBM-cotton fabric composite samples, in terms of their fibre volume fraction, specific heat capacity, thermal conductivity, electrical conductivity, Seebeck coefficient, Z and power factors. In this initial assessment, P3HT was selected as the p-type polymer compared to PEDOT:PSS, and PCBM was selected as the n-type polymer. A small, 3D wearable prototype was fabricated and tested: it exhibited a Seebeck coefficient of 380 μV K−1 but a relatively high resistance. It was estimated that scaling up this thermoelectric wearable to body coverage would produce about 1.5–2 μW for a temperature difference of 40 °C applied on the thermoelectric device.

“MIL-101(Cr)–methanol” as working pair for adsorption heat transformation cycles: Adsorbent shaping, adsorption equilibrium and dynamics

Adsorption Heat Transformation (AHT) is one of the most promising solutions for reducing the consumption of fossil fuels and effective environmental protection. The working pair “adsorbent – adsorbate” is a key factor affecting the performance of AHT cycle. This paper addresses the investigation of the adsorption of methanol vapor on MIL-101(Cr), which belongs to a family of porous crystalline solids, Metal – Organic Frameworks. MIL-101(Cr) is shaped with polyvinyl alcohol (PVA) as a binder to form grains. The equilibrium of methanol adsorption on the grains of MIL-101(Cr) is studied and the potential of the MIL-101(Cr) – methanol working pair is estimated for various AHT cycles. The dynamics of methanol adsorption is explored under conditions of a new cycle for upgrading temperature of ambient heat. The main findings of this study are: (i) the addition of PVA does not affect methanol adsorption equilibrium; (ii) the amount of methanol exchanged under typical conditions of the cooling and ambient heat amplification cycles varies from 0.27 to 0.31 g/g; (iii) under conditions of the heat amplification cycle the methanol adsorption on the loose grains of 0.8–1.8 mm size, occurs under the “grain size insensitive mode” when the dynamics of adsorption in the adsorbent beds with the same thickness does not depend on the size of MIL grains. For the desorption runs, the poor mass transfer decelerates the process for the grains of 1.6–1.8 mm size; (iv) the specific useful heat and heating power for heat amplification cycle equal 385 kJ/kg and 0.65–1.95 kW/kg, respectively. The high values of specific heat and heating power illustrate an encouraging potential of the “MIL-101(Cr) – methanol” pair for the ambient heat amplification cycle.

Diesel fuel combustion under steam gasification conditions

By the example of diesel fuel, the combustion of liquid hydrocarbon fuel in a burner device with a forced controlled steam supply is experimentally studied. Spraying of liquid fuel is provided as a result of its interaction with a high-speed jet of superheated steam, without fuel contact with the atomizer. This method of dispersion has significant technological advantages in the use of substandard liquid fuels. These are associated with the prevention of clogging of the atomizer and fuel supply channels, which improves the performance and reliability of the burner device. The composition of combustion products and the specific heat power are studied in a wide range of changes of the operating parameters of the burner device (steam flow rate and fuel consumption). The regimes under which high fuel combustion completeness is ensured at low content of harmful emissions in gaseous reaction products have been found.

Magnetism and the spin state in cubic perovskite CaCoO3 synthesized under high pressure

Cubic $\mathrm{SrCo}{\mathrm{O}}_{3}$ with an intermediate spin state can only be stabilized by high pressure and high temperature (HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metal perovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubic to orthorhombic as seen in the perovskite family of $\mathrm{Ca}M{\mathrm{O}}_{3}$ ($M={M}^{4+}$ of transition metals, $\mathrm{G}{\mathrm{e}}^{4+}, \mathrm{S}{\mathrm{n}}^{4+}$, and $\mathrm{Z}{\mathrm{r}}^{4+}$) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curie temperature as the crossover to the localized electronic state is approached. We report a successful synthesis of the perovskite $\mathrm{CaCo}{\mathrm{O}}_{3}$ with a HPHT treatment. Surprisingly, $\mathrm{CaCo}{\mathrm{O}}_{3}$ crystallizes in a simple cubic structure that remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has been thoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermal conductivity, and thermoelectric power. Metallic $\mathrm{CaCo}{\mathrm{O}}_{3}$ undergoes two successive magnetic transitions at 86 K and 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state ${t}^{4}{e}^{1}$ of $\mathrm{C}{\mathrm{o}}^{4+}$ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state ${t}^{3}{e}^{2}$ increases for $Tg100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The shortest Co-O bond length in cubic $\mathrm{CaCo}{\mathrm{O}}_{3}$ is responsible for delocalizing electrons in the ${\ensuremath{\pi}}^{*}$-band and itinerant-electron ferromagnetism at $Tl54\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. A comprehensive comparison between $\mathrm{SrCo}{\mathrm{O}}_{3}$ and $\mathrm{CaCo}{\mathrm{O}}_{3}$ and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ bands would make $\mathrm{CaCo}{\mathrm{O}}_{3}$ suitable to study the Hund's coupling effect in a metal.

Results of operational verification of vertical ground heat exchangers

This article is devoted to verification of VGHEs (Vertical Ground Heat Exchangers) used as a low-temperature energy source for heat pumps. The aim and objective of the study was to obtain practical knowledge of temperatures of rock mass and heat transfer fluids which affect the life cycle of the low temperature sources and the effectiveness of heat pump systems. The objective was also to gain knowledge of specific thermal energy and power extracted by these exchangers which are relevant to the evaluation of frequently used configurations of VGHEs. The results of the operational tests of two active boreholes with different arrangements, namely a single U exchanger (A), a double U exchanger (B) – and the reference (inactive) borehole (R). The tests were performed in the years 2012–2014 and involved 2 heating seasons and 2 exchanger stagnation periods. Results of the tests showed that exchanger B is more effective than exchanger A in terms of temperatures and particularly specific heat power and energies. The average specific thermal power in the heating periods were 23.11Wm−1 and 26.57Wm−1 for exchanger B and 16.88Wm−1 and 20.66Wm−1 for exchanger A. Total specific energies transferred from the massif during the heating periods were higher by 6.05kWhm−1 and 6.94kWhm−1 at exchanger B than at exchanger A.