Joachim Sonntag - Academia.edu (original) (raw)

Papers by Joachim Sonntag

Research paper thumbnail of Electron Theory of Metals - Answers to Unsolved Problems/Questions

Journal of Materials Science Research, Dec 13, 2023

Research paper thumbnail of Comment on “Effective medium theory based modeling of the thermoelectric properties of composites: comparison between predictions and experiments in the glass–crystal composite system Si10As15Te75–Bi0.4Sb1.6Te3” by J.-B. Vaney et al., J. Mater. Chem. C, 2015, 3, 11090

Journal of Materials Chemistry C, 2016

Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to... more Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to predict the thermopower of Si10As15Te75–Bi0.4Sb1.6Te3.

Research paper thumbnail of The effect of the band edges on the Seebeck coefficient

Journal of Physics: Condensed Matter, May 21, 2010

The classical thermopower formulae generally applied for the calculation of the Seebeck coefficie... more The classical thermopower formulae generally applied for the calculation of the Seebeck coefficient S are argued to be incomplete. S can be separated into two different contributions, a scattering term, S(0), and a thermodynamic term, ΔS, representing the additional change of the electrochemical potential μ with temperature T caused by 'non-scattering' effects, for instance, the band edge shift with T. On the basis of this separation into S(0) and ΔS, it is shown that shifts of the band edges with T lead to an additional contribution to the classical thermopower formulae. This separation provides the basis for an interpretation of positive thermopowers measured for many metals. Positive thermopower is expected if the energy of the conduction band edge increases with T and if this effect overcompensates for the influence of the energy dependent conductivity, σ(E). Using experimental thermopower data, the band edge shifts are determined for a series of liquid normal metals.

Research paper thumbnail of Temperature Dependence of the Electrical Resistivity of Semiconducting CrAl Solid Solutions at High Temperatures

Physica Status Solidi B-basic Solid State Physics, May 1, 1978

Research paper thumbnail of Disordered electronic systems: Concentration dependence of the dc conductivity in amorphous transition-metal–metalloid alloys (metallic regime)

Physical review, Aug 15, 1989

In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the ... more In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the electrical resistivity p can be approximated by d lnp= a*de, where a" is constant for a given alloy and g=x /(1x). N and T stand for a transition metal with completely and incompletely occupied d bands, respectively, and M stands for a metalloid element. If, in the alloy, phase separation is realized, there is electron redistribution between the two phases A and 8. For aN , M"alloys this can be described by dn =-Pnd g with g=xs /X", where n is the electron density in the conduction band (CB) formed by the A phase. X~and X& are the fractions of the A and 8 phases having the average concentrations x"and x~, respectively. P depends on the average potential difference between the A and 8 phases. 8 is the phase with the deeper average potential. Part of the electrons in the 8 phase occupies the valence band (VB) formed by the 8 phase. Another part occupies trap states (as far as available below EF), leading to electron localization. The electron redistribution leads to long-range electron-density fluctuations expressed by 5n =11+()(no n);no is the total s and p valence-electron concentration. Under certain conditions both CB and VB can contribute to the electronic transport.dn =Pn dg is expected to apply also to a T, "M, alloys , where the electron redistribution can enclose part of the d electrons as well. Positive Hall coeScients are expected, when both the VB has "hole" conductivity, and this contribution dominates compared with those of the CB. Activation of electrons from the 8 to the A phase with increasing temperature can lead to a negative temperature coefticient of p.

Research paper thumbnail of Composites to Produce a Material with Zero Absolute Thermopower S = 0 or a Thermopower Switch between S = 0 and S ≠ 0

Materials, Sep 24, 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Research paper thumbnail of Electronic Transport in Alloys with Phase Separation (Composites)

Open Journal of Composite Materials, 2019

A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT ... more A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT S T ρκ = , where S, ρ and κ are the electronic transport coefficients, Seebeck coefficient, electrical resistivity and thermal conductiviy, respectively. T is the absolute temperature. Large values for ZT have been realized in nanostructured materials such as superlattices, quantum dots, nanocomposites, and nanowires. In order to achieve further progress, (1) a fundamental understanding of the carrier transport in nanocomposites is necessary, and (2) effective experimental methods for designing, producing and measuring new material compositions with nanocomposite-structures are to be applied. During the last decades, a series of formulas has been derived for calculation of the electronic transport coefficients in composites and disordered alloys. Along the way, some puzzling phenomenons have been solved as why there are simple metals with positive thermopower? and what is the reason for the phenomenon of the "Giant Hall effect"? and what is the reason for the fact that amorphous composites can exist at all? In the present review article, (1), formulas will be presented for calculation of () 1 σ ρ = , κ , S, and R in composites. R, the Hall coefficient, provides additional informations about the type of the dominant electronic carriers and their densities. It will be shown that these formulas can also be applied successfully for calculation of S, ρ , κ and R in nanocomposites if certain conditions are taken into account. Regarding point (2) we shall show that the combinatorial development of materials can provide unfeasible results if applied noncritically.

Research paper thumbnail of The Origin of the Giant Hall Effect in Metal-Insulator Composites

Open Journal of Composite Materials, 2016

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I co... more Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 10 4 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by       B A dn n d − = ⋅ ⋅ υ β υ [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. A υ and B υ are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

Research paper thumbnail of High-throughput characterization of the Seebeck coefficient of a-(Cr1 −xSix)1 −yOythin film materials libraries as verification of the extended thermopower formula

Journal of Physics: Condensed Matter, Jun 13, 2011

In a previous paper (Sonntag 2010 J. Phys.: Condens. Matter 22 235501) the classical thermopower ... more In a previous paper (Sonntag 2010 J. Phys.: Condens. Matter 22 235501) the classical thermopower formula has been argued to be incomplete, because it only takes into account the scattering properties of the carriers, but not the temperature dependence of the electrochemical potential μ caused by variation of the carrier density and/or band edge shift with temperature T. This argument is now checked experimentally by high-throughput measurements of the thermopower (Seebeck coefficient) S of a-(Cr(1-x)Si(x))(1-y)O(y) thin film materials libraries. The concentration dependences of S differ depending on whether the measurements are done with the complete film (where x ranges continuously from x≈0.3 to 0.8; y≈0.1-0.2) or with the separated pieces (each piece with another average value of x). These differences are especially large if, in addition, an oxygen gradient is present.

Research paper thumbnail of Abnormal Thermal Conductivity of Chromium-Aluminium Alloys in the Temperature Range 5 to 100 K

De Gruyter eBooks, Dec 31, 1978

G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of ... more G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of chromium-aluminium alloys is measured. Within the range of A1 concentrations from 20 to 25 atyo in the temperature range of near 35 K, a maximum of thermal conductivity is found. I n this range of Al concentrations an anomalously high electrical resistivity has been measured by Chakrabarti and Beck. For interpreting this abnormal behaviour of thermal conductivity, two possibilities are discussed. In the present state of investigation, however, a well defined interpretation is not possible. An Chrom-Aluminium-Legierungen wird die WarmeleltfLhigkeit gemessen. Dabei zeigt sick im Konzentrationsbereich von 20 bis 25 Atyo A1 im Temperaturbereich um 35 K ein Maximum der Warmeleitfahigkeit. I n diesem Konzentrationsbereich wurden von Chakrabarti und Beck aiiomal hohe elektrische Widerstande gemessen. Zur Deutung dieses anomalen Verhaltens der Warmeleitfahigkeit werden zwei Moglichkeiten diskutiert. Eiue eindeutige Interpretation kann im derzeitigen Stadium der Untersnchungen noch nicht gefunden werden.

Research paper thumbnail of A thermoelectric sensor system and manufacturing processes

Research paper thumbnail of Composites to Produce a Material with Zero Absolute Thermopower S = 0 or a Thermopower Switch between S = 0 and S ≠ 0

Materials, 2021

From the theory of two-phase composites it is concluded that in the concentration dependence of t... more From the theory of two-phase composites it is concluded that in the concentration dependence of the Seebeck coefficient S a kink can occur precisely at S=0 absolute if the two phases have different kinds of carriers, electrons and holes, and if the phase grains are spherical without preferred orientations and arranged in a symmetrical fashion. This feature, indeed found to be realized in amorphous Cr1−xSix thin films deposited by ion beam sputtering from Cr-Si alloy targets, can be applied to make reference standards for S=0 at room temperature and even at higher temperatures. Additionally, it may be used to design a thermopower switch between S=0 and S≠0. It is also concluded that the structure realized in any alloy during solidification does not only depend on the diffusion mobility of the atoms and on the existence of a (relative) minimum in the Gibbs’ free energy. It depends also on the fact whether this structure is compatible with the demand that (spatial) continuity of the en...

Research paper thumbnail of Electronic Transport in Alloys with Phase Separation (Composites)

Open Journal of Composite Materials, 2019

A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT ... more A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT S T ρκ = , where S, ρ and κ are the electronic transport coefficients, Seebeck coefficient, electrical resistivity and thermal conductiviy, respectively. T is the absolute temperature. Large values for ZT have been realized in nanostructured materials such as superlattices, quantum dots, nanocomposites, and nanowires. In order to achieve further progress, (1) a fundamental understanding of the carrier transport in nanocomposites is necessary, and (2) effective experimental methods for designing, producing and measuring new material compositions with nanocomposite-structures are to be applied. During the last decades, a series of formulas has been derived for calculation of the electronic transport coefficients in composites and disordered alloys. Along the way, some puzzling phenomenons have been solved as why there are simple metals with positive thermopower? and what is the reason for the phenomenon of the "Giant Hall effect"? and what is the reason for the fact that amorphous composites can exist at all? In the present review article, (1), formulas will be presented for calculation of () 1 σ ρ = , κ , S, and R in composites. R, the Hall coefficient, provides additional informations about the type of the dominant electronic carriers and their densities. It will be shown that these formulas can also be applied successfully for calculation of S, ρ , κ and R in nanocomposites if certain conditions are taken into account. Regarding point (2) we shall show that the combinatorial development of materials can provide unfeasible results if applied noncritically.

Research paper thumbnail of Disordered electronic systems: Concentration dependence of the dc conductivity in amorphous transition-metal–metalloid alloys (metallic regime)

Physical Review B, 1989

In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the ... more In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the electrical resistivity p can be approximated by d lnp= a*de, where a" is constant for a given alloy and g=x /(1x). N and T stand for a transition metal with completely and incompletely occupied d bands, respectively, and M stands for a metalloid element. If, in the alloy, phase separation is realized, there is electron redistribution between the two phases A and 8. For aN , M"alloys this can be described by dn =-Pnd g with g=xs /X", where n is the electron density in the conduction band (CB) formed by the A phase. X~and X& are the fractions of the A and 8 phases having the average concentrations x"and x~, respectively. P depends on the average potential difference between the A and 8 phases. 8 is the phase with the deeper average potential. Part of the electrons in the 8 phase occupies the valence band (VB) formed by the 8 phase. Another part occupies trap states (as far as available below EF), leading to electron localization. The electron redistribution leads to long-range electron-density fluctuations expressed by 5n =11+()(no n);no is the total s and p valence-electron concentration. Under certain conditions both CB and VB can contribute to the electronic transport.dn =Pn dg is expected to apply also to a T, "M, alloys , where the electron redistribution can enclose part of the d electrons as well. Positive Hall coeScients are expected, when both the VB has "hole" conductivity, and this contribution dominates compared with those of the CB. Activation of electrons from the 8 to the A phase with increasing temperature can lead to a negative temperature coefticient of p.

Research paper thumbnail of Comment on “Effective medium theory based modeling of the thermoelectric properties of composites: comparison between predictions and experiments in the glass–crystal composite system Si10As15Te75–Bi0.4Sb1.6Te3” by J.-B. Vaney et al., J. Mater. Chem. C, 2015, 3, 11090

Journal of Materials Chemistry C, 2016

Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to... more Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to predict the thermopower of Si10As15Te75–Bi0.4Sb1.6Te3.

Research paper thumbnail of The Origin of the Giant Hall Effect in Metal-Insulator Composites

Open Journal of Composite Materials, 2016

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I co... more Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 10 4 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by       B A dn n d − = ⋅ ⋅ υ β υ [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. A υ and B υ are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

Research paper thumbnail of Electrical and structural properties of concentrated Cr(Al) thin films

Thin Solid Films, 1988

Abstract Both thin films and bulk materials of concentrated disordered Cr(Al) alloys show a sharp... more Abstract Both thin films and bulk materials of concentrated disordered Cr(Al) alloys show a sharp maximum in the room temperature resistivity ϱ at 24 at.% Al which is correlated with a sharp minimum in the temperature coefficient of resistivity (TCR). In thin films (thickness less than 1 μm) under the influence of oxygen adsorbed on the surface the values of ϱ and TCR change drastically during annealing to temperatures at or above 900 K. The reason for this is a diffusion of aluminium atoms to the surface. Mooij's rule which is extended substantially to higher values of ϱ and the TCR is used to characterize this effect. After annealing, a new phase with a tetragonal structure is found.

Research paper thumbnail of Deposition of Cr-Si thin films by reactive plasmatron-magnetron sputtering

Thin Solid Films, 1985

The sputtering of metallic Cr-Si targets enables substantial enhancement of the resistivity of th... more The sputtering of metallic Cr-Si targets enables substantial enhancement of the resistivity of the prepared films by the addition of oxygen. Results are presented for the dependence of the oxygen concentration co in the films on the oxygen pressure which is a process-dependent parameter. The dependence of the resistivity on co is strongly characteristic of the material and is only weakly influenced by technological parameters. Knowledge of these functions leads to conclusions regarding the sheet resistance attainable and the engineering requirements with respect to the reproducibility needed in practice. Investigations of the microstructure of the films with increasing co reveal an increasing proportion of SiOx which agrees qualitatively with both the pronounced increase in resistance close to the percolation threshold and the increasing negative temperature coefficient TCR0.

Research paper thumbnail of Disordered electronic systems. II. Phase separation and the metal-insulator transition in metal-metalloid alloys

Physical Review B, 2005

The electronic transport in the phase separated regime is determined by both the different local ... more The electronic transport in the phase separated regime is determined by both the different local band structure in the phases ͑called phases A and B͒ and electron redistribution (electron transfer) to the phase with the deeper average potential ͑phase B͒. Equations for the dependence of the electronic conductivity on metalloid concentration x are derived. In amorphous metal-metalloid alloys the metal-insulator transition ͑M-I transition͒ characterized by the transition from Ͼ 0 to = 0 at temperature T =0 at x = x c takes place in the phase separated regime. The M-I transition in S 1−x M x alloys is determined by the conduction band ͑phase A͒, whereas in N 1−x M x , and in many T 1−x M x alloys, it is determined by the valence band ͑phase B͒ ͑N and T stand for a transition metal with completely and incompletely occupied d band, respectively, S for a simple metal as Al, Ga, In,. . ., and M for a metalloid element as Si or Ge͒. ͑1͒ Granular structure, ͑2͒ rapid decrease of the average metal grain size with increasing x, and ͑3͒ relatively small x c are characteristic features for S 1−x M x thin films deposited under extreme deposition conditions and are caused by the fact that a considerable part of electrons transferred occupy surface states leading to charged phase boundaries. The fractal structure found in Al 1−x Ge x alloys after annealing is related with the formation of a maximum of phase boundary faces for acceptance of the transferred electrons. For strong scattering in a single phase, there are a minimum metallic conductivity min Ӎ͑c * /6͒͑e 2 / h͒͑1/d͒ and mobility edges at density of states 4c * m / h 2 d, where c * =1/4 ͑d is the average atomic distance. e and m are the elementary charge and effective mass of the electrons, respectively, and ប = h /2 is Plancks constant͒.

Research paper thumbnail of Abnormal thermal conductivity of chromium-aluminium alloys in the temperature range 5 to 100 K

Physica Status Solidi (a), 1978

G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of ... more G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of chromium-aluminium alloys is measured. Within the range of A1 concentrations from 20 to 25 atyo in the temperature range of near 35 K, a maximum of thermal conductivity is found. I n this range of Al concentrations an anomalously high electrical resistivity has been measured by Chakrabarti and Beck. For interpreting this abnormal behaviour of thermal conductivity, two possibilities are discussed. In the present state of investigation, however, a well defined interpretation is not possible. An Chrom-Aluminium-Legierungen wird die WarmeleltfLhigkeit gemessen. Dabei zeigt sick im Konzentrationsbereich von 20 bis 25 Atyo A1 im Temperaturbereich um 35 K ein Maximum der Warmeleitfahigkeit. I n diesem Konzentrationsbereich wurden von Chakrabarti und Beck aiiomal hohe elektrische Widerstande gemessen. Zur Deutung dieses anomalen Verhaltens der Warmeleitfahigkeit werden zwei Moglichkeiten diskutiert. Eiue eindeutige Interpretation kann im derzeitigen Stadium der Untersnchungen noch nicht gefunden werden.

Research paper thumbnail of Electron Theory of Metals - Answers to Unsolved Problems/Questions

Journal of Materials Science Research, Dec 13, 2023

Research paper thumbnail of Comment on “Effective medium theory based modeling of the thermoelectric properties of composites: comparison between predictions and experiments in the glass–crystal composite system Si10As15Te75–Bi0.4Sb1.6Te3” by J.-B. Vaney et al., J. Mater. Chem. C, 2015, 3, 11090

Journal of Materials Chemistry C, 2016

Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to... more Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to predict the thermopower of Si10As15Te75–Bi0.4Sb1.6Te3.

Research paper thumbnail of The effect of the band edges on the Seebeck coefficient

Journal of Physics: Condensed Matter, May 21, 2010

The classical thermopower formulae generally applied for the calculation of the Seebeck coefficie... more The classical thermopower formulae generally applied for the calculation of the Seebeck coefficient S are argued to be incomplete. S can be separated into two different contributions, a scattering term, S(0), and a thermodynamic term, ΔS, representing the additional change of the electrochemical potential μ with temperature T caused by 'non-scattering' effects, for instance, the band edge shift with T. On the basis of this separation into S(0) and ΔS, it is shown that shifts of the band edges with T lead to an additional contribution to the classical thermopower formulae. This separation provides the basis for an interpretation of positive thermopowers measured for many metals. Positive thermopower is expected if the energy of the conduction band edge increases with T and if this effect overcompensates for the influence of the energy dependent conductivity, σ(E). Using experimental thermopower data, the band edge shifts are determined for a series of liquid normal metals.

Research paper thumbnail of Temperature Dependence of the Electrical Resistivity of Semiconducting CrAl Solid Solutions at High Temperatures

Physica Status Solidi B-basic Solid State Physics, May 1, 1978

Research paper thumbnail of Disordered electronic systems: Concentration dependence of the dc conductivity in amorphous transition-metal–metalloid alloys (metallic regime)

Physical review, Aug 15, 1989

In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the ... more In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the electrical resistivity p can be approximated by d lnp= a*de, where a" is constant for a given alloy and g=x /(1x). N and T stand for a transition metal with completely and incompletely occupied d bands, respectively, and M stands for a metalloid element. If, in the alloy, phase separation is realized, there is electron redistribution between the two phases A and 8. For aN , M"alloys this can be described by dn =-Pnd g with g=xs /X", where n is the electron density in the conduction band (CB) formed by the A phase. X~and X& are the fractions of the A and 8 phases having the average concentrations x"and x~, respectively. P depends on the average potential difference between the A and 8 phases. 8 is the phase with the deeper average potential. Part of the electrons in the 8 phase occupies the valence band (VB) formed by the 8 phase. Another part occupies trap states (as far as available below EF), leading to electron localization. The electron redistribution leads to long-range electron-density fluctuations expressed by 5n =11+()(no n);no is the total s and p valence-electron concentration. Under certain conditions both CB and VB can contribute to the electronic transport.dn =Pn dg is expected to apply also to a T, "M, alloys , where the electron redistribution can enclose part of the d electrons as well. Positive Hall coeScients are expected, when both the VB has "hole" conductivity, and this contribution dominates compared with those of the CB. Activation of electrons from the 8 to the A phase with increasing temperature can lead to a negative temperature coefticient of p.

Research paper thumbnail of Composites to Produce a Material with Zero Absolute Thermopower S = 0 or a Thermopower Switch between S = 0 and S ≠ 0

Materials, Sep 24, 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Research paper thumbnail of Electronic Transport in Alloys with Phase Separation (Composites)

Open Journal of Composite Materials, 2019

A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT ... more A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT S T ρκ = , where S, ρ and κ are the electronic transport coefficients, Seebeck coefficient, electrical resistivity and thermal conductiviy, respectively. T is the absolute temperature. Large values for ZT have been realized in nanostructured materials such as superlattices, quantum dots, nanocomposites, and nanowires. In order to achieve further progress, (1) a fundamental understanding of the carrier transport in nanocomposites is necessary, and (2) effective experimental methods for designing, producing and measuring new material compositions with nanocomposite-structures are to be applied. During the last decades, a series of formulas has been derived for calculation of the electronic transport coefficients in composites and disordered alloys. Along the way, some puzzling phenomenons have been solved as why there are simple metals with positive thermopower? and what is the reason for the phenomenon of the "Giant Hall effect"? and what is the reason for the fact that amorphous composites can exist at all? In the present review article, (1), formulas will be presented for calculation of () 1 σ ρ = , κ , S, and R in composites. R, the Hall coefficient, provides additional informations about the type of the dominant electronic carriers and their densities. It will be shown that these formulas can also be applied successfully for calculation of S, ρ , κ and R in nanocomposites if certain conditions are taken into account. Regarding point (2) we shall show that the combinatorial development of materials can provide unfeasible results if applied noncritically.

Research paper thumbnail of The Origin of the Giant Hall Effect in Metal-Insulator Composites

Open Journal of Composite Materials, 2016

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I co... more Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 10 4 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by       B A dn n d − = ⋅ ⋅ υ β υ [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. A υ and B υ are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

Research paper thumbnail of High-throughput characterization of the Seebeck coefficient of a-(Cr1 −xSix)1 −yOythin film materials libraries as verification of the extended thermopower formula

Journal of Physics: Condensed Matter, Jun 13, 2011

In a previous paper (Sonntag 2010 J. Phys.: Condens. Matter 22 235501) the classical thermopower ... more In a previous paper (Sonntag 2010 J. Phys.: Condens. Matter 22 235501) the classical thermopower formula has been argued to be incomplete, because it only takes into account the scattering properties of the carriers, but not the temperature dependence of the electrochemical potential μ caused by variation of the carrier density and/or band edge shift with temperature T. This argument is now checked experimentally by high-throughput measurements of the thermopower (Seebeck coefficient) S of a-(Cr(1-x)Si(x))(1-y)O(y) thin film materials libraries. The concentration dependences of S differ depending on whether the measurements are done with the complete film (where x ranges continuously from x≈0.3 to 0.8; y≈0.1-0.2) or with the separated pieces (each piece with another average value of x). These differences are especially large if, in addition, an oxygen gradient is present.

Research paper thumbnail of Abnormal Thermal Conductivity of Chromium-Aluminium Alloys in the Temperature Range 5 to 100 K

De Gruyter eBooks, Dec 31, 1978

G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of ... more G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of chromium-aluminium alloys is measured. Within the range of A1 concentrations from 20 to 25 atyo in the temperature range of near 35 K, a maximum of thermal conductivity is found. I n this range of Al concentrations an anomalously high electrical resistivity has been measured by Chakrabarti and Beck. For interpreting this abnormal behaviour of thermal conductivity, two possibilities are discussed. In the present state of investigation, however, a well defined interpretation is not possible. An Chrom-Aluminium-Legierungen wird die WarmeleltfLhigkeit gemessen. Dabei zeigt sick im Konzentrationsbereich von 20 bis 25 Atyo A1 im Temperaturbereich um 35 K ein Maximum der Warmeleitfahigkeit. I n diesem Konzentrationsbereich wurden von Chakrabarti und Beck aiiomal hohe elektrische Widerstande gemessen. Zur Deutung dieses anomalen Verhaltens der Warmeleitfahigkeit werden zwei Moglichkeiten diskutiert. Eiue eindeutige Interpretation kann im derzeitigen Stadium der Untersnchungen noch nicht gefunden werden.

Research paper thumbnail of A thermoelectric sensor system and manufacturing processes

Research paper thumbnail of Composites to Produce a Material with Zero Absolute Thermopower S = 0 or a Thermopower Switch between S = 0 and S ≠ 0

Materials, 2021

From the theory of two-phase composites it is concluded that in the concentration dependence of t... more From the theory of two-phase composites it is concluded that in the concentration dependence of the Seebeck coefficient S a kink can occur precisely at S=0 absolute if the two phases have different kinds of carriers, electrons and holes, and if the phase grains are spherical without preferred orientations and arranged in a symmetrical fashion. This feature, indeed found to be realized in amorphous Cr1−xSix thin films deposited by ion beam sputtering from Cr-Si alloy targets, can be applied to make reference standards for S=0 at room temperature and even at higher temperatures. Additionally, it may be used to design a thermopower switch between S=0 and S≠0. It is also concluded that the structure realized in any alloy during solidification does not only depend on the diffusion mobility of the atoms and on the existence of a (relative) minimum in the Gibbs’ free energy. It depends also on the fact whether this structure is compatible with the demand that (spatial) continuity of the en...

Research paper thumbnail of Electronic Transport in Alloys with Phase Separation (Composites)

Open Journal of Composite Materials, 2019

A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT ... more A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT S T ρκ = , where S, ρ and κ are the electronic transport coefficients, Seebeck coefficient, electrical resistivity and thermal conductiviy, respectively. T is the absolute temperature. Large values for ZT have been realized in nanostructured materials such as superlattices, quantum dots, nanocomposites, and nanowires. In order to achieve further progress, (1) a fundamental understanding of the carrier transport in nanocomposites is necessary, and (2) effective experimental methods for designing, producing and measuring new material compositions with nanocomposite-structures are to be applied. During the last decades, a series of formulas has been derived for calculation of the electronic transport coefficients in composites and disordered alloys. Along the way, some puzzling phenomenons have been solved as why there are simple metals with positive thermopower? and what is the reason for the phenomenon of the "Giant Hall effect"? and what is the reason for the fact that amorphous composites can exist at all? In the present review article, (1), formulas will be presented for calculation of () 1 σ ρ = , κ , S, and R in composites. R, the Hall coefficient, provides additional informations about the type of the dominant electronic carriers and their densities. It will be shown that these formulas can also be applied successfully for calculation of S, ρ , κ and R in nanocomposites if certain conditions are taken into account. Regarding point (2) we shall show that the combinatorial development of materials can provide unfeasible results if applied noncritically.

Research paper thumbnail of Disordered electronic systems: Concentration dependence of the dc conductivity in amorphous transition-metal–metalloid alloys (metallic regime)

Physical Review B, 1989

In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the ... more In the metallic regime of several a-Nl "M and a-T& M alloys, the concentration dependence of the electrical resistivity p can be approximated by d lnp= a*de, where a" is constant for a given alloy and g=x /(1x). N and T stand for a transition metal with completely and incompletely occupied d bands, respectively, and M stands for a metalloid element. If, in the alloy, phase separation is realized, there is electron redistribution between the two phases A and 8. For aN , M"alloys this can be described by dn =-Pnd g with g=xs /X", where n is the electron density in the conduction band (CB) formed by the A phase. X~and X& are the fractions of the A and 8 phases having the average concentrations x"and x~, respectively. P depends on the average potential difference between the A and 8 phases. 8 is the phase with the deeper average potential. Part of the electrons in the 8 phase occupies the valence band (VB) formed by the 8 phase. Another part occupies trap states (as far as available below EF), leading to electron localization. The electron redistribution leads to long-range electron-density fluctuations expressed by 5n =11+()(no n);no is the total s and p valence-electron concentration. Under certain conditions both CB and VB can contribute to the electronic transport.dn =Pn dg is expected to apply also to a T, "M, alloys , where the electron redistribution can enclose part of the d electrons as well. Positive Hall coeScients are expected, when both the VB has "hole" conductivity, and this contribution dominates compared with those of the CB. Activation of electrons from the 8 to the A phase with increasing temperature can lead to a negative temperature coefticient of p.

Research paper thumbnail of Comment on “Effective medium theory based modeling of the thermoelectric properties of composites: comparison between predictions and experiments in the glass–crystal composite system Si10As15Te75–Bi0.4Sb1.6Te3” by J.-B. Vaney et al., J. Mater. Chem. C, 2015, 3, 11090

Journal of Materials Chemistry C, 2016

Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to... more Vaney et al. found that the thermopower formula for composites derived in ref. 2 clearly fails to predict the thermopower of Si10As15Te75–Bi0.4Sb1.6Te3.

Research paper thumbnail of The Origin of the Giant Hall Effect in Metal-Insulator Composites

Open Journal of Composite Materials, 2016

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I co... more Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 10 4 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by       B A dn n d − = ⋅ ⋅ υ β υ [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. A υ and B υ are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

Research paper thumbnail of Electrical and structural properties of concentrated Cr(Al) thin films

Thin Solid Films, 1988

Abstract Both thin films and bulk materials of concentrated disordered Cr(Al) alloys show a sharp... more Abstract Both thin films and bulk materials of concentrated disordered Cr(Al) alloys show a sharp maximum in the room temperature resistivity ϱ at 24 at.% Al which is correlated with a sharp minimum in the temperature coefficient of resistivity (TCR). In thin films (thickness less than 1 μm) under the influence of oxygen adsorbed on the surface the values of ϱ and TCR change drastically during annealing to temperatures at or above 900 K. The reason for this is a diffusion of aluminium atoms to the surface. Mooij's rule which is extended substantially to higher values of ϱ and the TCR is used to characterize this effect. After annealing, a new phase with a tetragonal structure is found.

Research paper thumbnail of Deposition of Cr-Si thin films by reactive plasmatron-magnetron sputtering

Thin Solid Films, 1985

The sputtering of metallic Cr-Si targets enables substantial enhancement of the resistivity of th... more The sputtering of metallic Cr-Si targets enables substantial enhancement of the resistivity of the prepared films by the addition of oxygen. Results are presented for the dependence of the oxygen concentration co in the films on the oxygen pressure which is a process-dependent parameter. The dependence of the resistivity on co is strongly characteristic of the material and is only weakly influenced by technological parameters. Knowledge of these functions leads to conclusions regarding the sheet resistance attainable and the engineering requirements with respect to the reproducibility needed in practice. Investigations of the microstructure of the films with increasing co reveal an increasing proportion of SiOx which agrees qualitatively with both the pronounced increase in resistance close to the percolation threshold and the increasing negative temperature coefficient TCR0.

Research paper thumbnail of Disordered electronic systems. II. Phase separation and the metal-insulator transition in metal-metalloid alloys

Physical Review B, 2005

The electronic transport in the phase separated regime is determined by both the different local ... more The electronic transport in the phase separated regime is determined by both the different local band structure in the phases ͑called phases A and B͒ and electron redistribution (electron transfer) to the phase with the deeper average potential ͑phase B͒. Equations for the dependence of the electronic conductivity on metalloid concentration x are derived. In amorphous metal-metalloid alloys the metal-insulator transition ͑M-I transition͒ characterized by the transition from Ͼ 0 to = 0 at temperature T =0 at x = x c takes place in the phase separated regime. The M-I transition in S 1−x M x alloys is determined by the conduction band ͑phase A͒, whereas in N 1−x M x , and in many T 1−x M x alloys, it is determined by the valence band ͑phase B͒ ͑N and T stand for a transition metal with completely and incompletely occupied d band, respectively, S for a simple metal as Al, Ga, In,. . ., and M for a metalloid element as Si or Ge͒. ͑1͒ Granular structure, ͑2͒ rapid decrease of the average metal grain size with increasing x, and ͑3͒ relatively small x c are characteristic features for S 1−x M x thin films deposited under extreme deposition conditions and are caused by the fact that a considerable part of electrons transferred occupy surface states leading to charged phase boundaries. The fractal structure found in Al 1−x Ge x alloys after annealing is related with the formation of a maximum of phase boundary faces for acceptance of the transferred electrons. For strong scattering in a single phase, there are a minimum metallic conductivity min Ӎ͑c * /6͒͑e 2 / h͒͑1/d͒ and mobility edges at density of states 4c * m / h 2 d, where c * =1/4 ͑d is the average atomic distance. e and m are the elementary charge and effective mass of the electrons, respectively, and ប = h /2 is Plancks constant͒.

Research paper thumbnail of Abnormal thermal conductivity of chromium-aluminium alloys in the temperature range 5 to 100 K

Physica Status Solidi (a), 1978

G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of ... more G. POMPE (a), J. SONNTAG (b), A. BIEDERMANN~) (a), and s. SCHIEFER~) (a) Thermal conductivity of chromium-aluminium alloys is measured. Within the range of A1 concentrations from 20 to 25 atyo in the temperature range of near 35 K, a maximum of thermal conductivity is found. I n this range of Al concentrations an anomalously high electrical resistivity has been measured by Chakrabarti and Beck. For interpreting this abnormal behaviour of thermal conductivity, two possibilities are discussed. In the present state of investigation, however, a well defined interpretation is not possible. An Chrom-Aluminium-Legierungen wird die WarmeleltfLhigkeit gemessen. Dabei zeigt sick im Konzentrationsbereich von 20 bis 25 Atyo A1 im Temperaturbereich um 35 K ein Maximum der Warmeleitfahigkeit. I n diesem Konzentrationsbereich wurden von Chakrabarti und Beck aiiomal hohe elektrische Widerstande gemessen. Zur Deutung dieses anomalen Verhaltens der Warmeleitfahigkeit werden zwei Moglichkeiten diskutiert. Eiue eindeutige Interpretation kann im derzeitigen Stadium der Untersnchungen noch nicht gefunden werden.