Robert Heimann | Technische Universität Bergakademie Freiberg (original) (raw)

Papers by Robert Heimann

Research paper thumbnail of Born of fire: The pursuit of the ‘white gold’

Research paper thumbnail of Synthesis of Ceramic Layers for Solid Oxide Fuel Cells by Means of TPCVD

International Thermal Spray Conference

This paper investigates the potential of radio frequency thermal plasma chemical vapor deposition... more This paper investigates the potential of radio frequency thermal plasma chemical vapor deposition for producing Sr-doped La-Mn-perovskite and yttria-doped zirconia layers for solid-oxide fuel cells. Aqueous solutions were used as starting materials and were injected into the hot plasma core by means of an air-assist atomizer. Test results show how the microstructure, dopant distribution, and phase purity of the resulting layers depends both on process conditions and the material system. Paper includes a German-language abstract.

Research paper thumbnail of Correlation of Plasma Process Conditions, Phase Content, and Photocatalytic Performance of Induction Plasma-Sprayed Titania Coatings

Thermal Spray 2005: Proceedings from the International Thermal Spray Conference

Systematic variation of the induction plasma spray (IPS) conditions, i.e. (i) plasma power, (ii) ... more Systematic variation of the induction plasma spray (IPS) conditions, i.e. (i) plasma power, (ii) carrier gas flow rate, and (iii) powder feed rate was performed to deposit TiO2 coatings using statistical design of experiments (SDE) methodology. The microstructure, surface morphology, and anatase-rutile ratio of the coatings were studied by grazing-incidence X-ray diffraction (GIXD) and surface mapping with micro-probe Raman spectroscopy to determine the spatial phase distribution of anatase and rutile. The photocatalytic activity of the TiO2 coatings was tested by decomposition of 4-chlorophenol in aqueous solution in the dark and under UV irradiation. The rates of pH changes measured were compared with those of standard Degussa P-25 coatings. The photocatalytic activity of the samples shows a reasonable correlation with the phase content and the plasma processing conditions prevailing during coating deposition.

Research paper thumbnail of Radio-Frequency Suspension Plasma Spraying of Cobalt Spinel Anodes for Alkaline Water Electrolysis

Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 1998

Suspensions of cobalt spinel (Co3O4) powders (median size 6 µm and 0.3 µm) were r.f. plasma spray... more Suspensions of cobalt spinel (Co3O4) powders (median size 6 µm and 0.3 µm) were r.f. plasma sprayed to form electrocatalytically active anode layers for alkaline water electrolysis. Stable cobalt oxide suspensions of low viscosity and exceeding 50wt.% solid phase content have been processed. A spheroidization study revealed the formation of large spherical powder particles (- 30 + 80 µm). Cobalt oxide coatings were produced by this novel r.f.-Suspension Plasma Spraying (SPS) method. The porosity was controlled by optimizing the spray distance and the reactor pressure. The main disadvantage of the thermal plasma processing of cobalt spinel, i.e. the decomposition of the spinel phase into CoO, could not be prevented, not even with the application of an 80% oxygen plasma. With a relatively low power oxygen plasma post-treatment the deposited CoO layers can be oxidized to Co3O4 resulting in a promising improvement of the electrochemical performance of the anode layers.

Research paper thumbnail of Manufacture of thick chromium coatings by R.F. plasma spraying for PVD sputter targets

The production of plasma-sprayed thick chromium coatings on copper substrates for PVD sputter tar... more The production of plasma-sprayed thick chromium coatings on copper substrates for PVD sputter targets contains a number of technological challenges which were tried to be addressed using r.f. plasma spray technology. The efforts were focused on several objectives. The coatings should be as dense as possible. Their residual stresses should be low enough to avoid strong substrate bending and to prevent failure in adhesion or cohesion of the coating. The deposition efficiency during spraying should be as high as possible. Different spraying parameters were varied in order to find the optimum conditions for the spraying process. Two different powder grain sizes were used. First a spheroidization study was performed which gave evidence about the optimum chamber pressure, the optimum plasma power, and a manageable range of the powder feed rate. During spraying of the coatings the power feed was further optimized. Also the spraying distance, the substrate roughness and the substrate coolin...

Research paper thumbnail of In-vitro Investigation of Air Plasma-Sprayed Hydroxyapatite Coatings by Diffraction Techniques

Residual Stresses 2016, Dec 22, 2016

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC.

Research paper thumbnail of Weathering of ancient and medieval glasses—potential proxy for nuclear fuel waste glasses. A perennial challenge revisited

International Journal of Applied Glass Science, 2017

Ancient and medieval glasses that have survived the deleterious attack of the environment for mil... more Ancient and medieval glasses that have survived the deleterious attack of the environment for millennia have long since proposed as proxy to estimate and predict the corrosion mechanism of nuclear waste glasses. However, because both composition and environmental burial conditions vastly differ between hydrolytically less stable ancient glasses and modern advanced nuclear waste glasses, only semiquantitative conclusions can be drawn about the likely performance of the latter as long‐term stable immobilization matrices for high‐level radioactive nuclear waste. In this contribution, special emphasis has been devoted to the behavior of manganese, present as both iron decolorant and coloring ions in ancient Roman and medieval glasses. Study of the behavior of manganese in ancient glasses during weathering may provide some limited clues to the behavior of long‐lived radioactive technetium‐99. Knowledge of the corrosion kinetics of ancient glasses will allow, eventually, a reasonable pred...

Research paper thumbnail of Group V Metal Oxides (V, Nb and Ta) doping od Titania Coatings by Means of Suspension Plasma Spraying

Research paper thumbnail of D083 Depth-Resolved Strain Determination of Hydroxyapatite Coatings Using High Energy X-Ray Diffraction

Research paper thumbnail of The effect of deposition mechanism on the composition of surface films on silicon

Thin Solid Films, 1984

Surface films on Si(111) wafers were grown either by floating the sample on the surface of an HNO... more Surface films on Si(111) wafers were grown either by floating the sample on the surface of an HNO3HFCH3COOH solution with various HNO3:HFF ratios or by low temperature vapour deposition (LTV) from NO2HFH2O gas mixtures. From a combination of various surface analytical techniques as well as X-ray and electron diffraction it is concluded that the films formed by the floating method are composed of SiOx (x < 2) and those from LTV incorporate crystalline ammonium silico-fluoride and amorphous SiOx.

Research paper thumbnail of Thermal Induction Plasma Processes for the Synthesis of SOFC Materials

Materialwissenschaft und Werkstofftechnik, 2002

This work demonstrates the potential of two recently developed plasma spray methods to deposit Sr... more This work demonstrates the potential of two recently developed plasma spray methods to deposit Sr-doped La-(Mn,Fe,Co) perovskite (LSM, LSF, LSCF) cathodes, and yttria-doped zirconia (YSZ) electrolytes as components of solid oxide fuel cells (SOFC). These methods are Thermal Plasma Chemical Vapour Deposition (TPCVD) and Suspension Plasma Spraying (SPS). The microstructure obtained is determined by the method applied. TPCVD results in either globular or columnar coatings which were deposited with growth rates up to 30 lm/min. SPS with a deposition rate in excess of 100 lm/min yields coatings of layered microstructure. The homogeneity of the dopant distribution and the phase purity of YSZ is excellent. Process modifications will be necessary, however, to improve the properties in the case of perovskite coatings. Diese Arbeit demonstriert das Potenzial zweier junger Varianten des Plasmaspritzens, der durch ein thermisches Hochfrequenz-Plasma unterstu Ètzten chemischen Abscheidung aus der Dampfphase (Thermal Plasma Chemical Vapour Deposition, TPCVD) und des Suspensionsplasmaspritzens (SPS) zur Abscheidung von Sr-dotierten La-(Mn,Fe,Co)-Perowskit (LSM, LSF, LSCF)-Kathoden und Yttria-dotierten Zirconia (YSZ)-Elektrolyten fu Èr SOFC. Die Mikrostruktur der erzeugten Schichten ist abha Èngig vom eingesetzten Verfahren. So wurden mittels TPCVD globulare oder sta Èngelige Schichten mit Wachstumsraten bis 30 lm/min abgeschieden. Beim SPS entstehen bei Auftragsraten gro Èûer als 100 lm/min weitgehend lagige Schichten. Die Homogenita Èt der Dopandenverteilung sowie die Phasenreinheit der Schichten sind im Falle des YSZ hervorragend. Jedoch mu Èssen sie fu Èr die Perowskitschichten durch Prozessmodifikationen weiter verbessert werden.

Research paper thumbnail of Recent trends towards improved plasma-sprayed advanced bioceramic coatings on Ti6Al4V implants

Materialwissenschaft und Werkstofftechnik, 1999

In this contribution several recent trends are being described towards engineering of bioceramic ... more In this contribution several recent trends are being described towards engineering of bioceramic coatings applied by thermal spraying to titanium alloy substrates. (i) Optimization by statistical experimental design methodology of plasma spray parameters can be used to deposit coatings with low porosity, comparatively high adhesive strength and optimum crystallinity. (ii) Microstructural engineering of the hydroxyapatite powder by pre-spray annealing, i. e. sintering leads to a substantial decrease of the microporosity of the ceramic grains hence improving both adhesion strength and in vitro resorption resistance. (iii) Bond coats based on titania and titania/zirconia can be used to control the rate of heat transfer from the molten particles to the substrate and hence to improve the in vivo resorption resistance by suppressing the formation of amorphous calcium phosphate (ACP). (iv) Replacement of hydroxyapatite by specific compositions in the quaternary system CaO-TiO 2 -ZrO 2 -P 2 O 5 is a potential way to generate coatings with high chemical stability for long-term performance.

Research paper thumbnail of Chemical dehydrohalogenation of poly(ethylene-alt-chlorotrifluoroethylene)

Materials Chemistry and Physics, 2000

Chemical dehydrohalogenation of poly(ethylene-alt-chlorotrifluoroethylene) (PE-a-CTFE) has been s... more Chemical dehydrohalogenation of poly(ethylene-alt-chlorotrifluoroethylene) (PE-a-CTFE) has been studied as a possible route to carbyne. The reaction products have been characterized by infrared and 13 C solid-state nuclear magnetic resonance spectroscopy. The dehydrohalogenation of PEa -CTFE by potassium tert-butoxide in tetrahydrofuran has been shown to result in the formation of a halogen-substituted polyenyne structure with randomly distributed isolated triple bonds. The incomplete elimination of hydrogen halides can be explained by insufficient strength of the base, sterical hindrances caused by its bulkiness as well as by alternation defects in the original copolymer.

Research paper thumbnail of Hydroxyapatite coatings with a bond coat of biomedical implants by plasma projection

Journal of Materials Research, 1996

Research paper thumbnail of The morphology of hydrothermally grown strontium paracelsian

Journal of Crystal Growth, 1979

ABSTRACT

Research paper thumbnail of Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN), and La3Ga5.5Ta0.5O14 (LGT)

Journal of Crystal Growth, 1999

Mass densities, relative dielectric constants, and piezoelectric material parameters were determi... more Mass densities, relative dielectric constants, and piezoelectric material parameters were determined on single crystals of LGS, LGN, and LGT grown with high structural perfection by the Czochralski technique. Optimized values of the elastic sti!nesses and elastic compliances were derived by measuring and critically comparing the propagation velocities of both bulk (BAW) and surface acoustic waves (SAW).

Research paper thumbnail of Bildung von Mn-dotierten einkristallen von α-korund aus der gasphase

Journal of Crystal Growth, 1968

Eingegangen am 7. Dezember 1967 Red coloured single crystals of cc-A&O, have been grown by the hy... more Eingegangen am 7. Dezember 1967 Red coloured single crystals of cc-A&O, have been grown by the hydrolysis of the vapour resulted from the reaction of a melt of MnF, with solid y-AlaO at temperatures up to 950 "C in the presence of air. The red colour of the crystals depends on a Mn content of 0.9-l .7 'A. Emission spectrographic data have established the absence of other colouring ions. Some crystals show

Research paper thumbnail of Influence of surface films on the development of pits during etching of silicon

Journal of Crystal Growth, 1982

Thin surface films on dislocation-free silicon (111) wafers grown either by prepolishing in, or f... more Thin surface films on dislocation-free silicon (111) wafers grown either by prepolishing in, or floating on, HNO3-HF-CH3COOH solutions influence the development of pits during subsequent etching in conventional preferential etches (Sirtl and Secco etches). It is suggested that the etch action proceeds through holes in the film thus leading to a mechanism similar to localized corrosion in passivatable metals and alloys. Statistical analyses of the pit distributions further indicate that dislocations within the silicon do not contribute significantly to the pitting phenomenon.

Research paper thumbnail of Hydrothermal dissolution reactions of magnesium-aluminium-spinel in alkaline solutions

Journal of Crystal Growth, 1970

Single crystal spheres of spine1 of 4 mm diameter having a molar ratio of MgO/Al,03 1 : 5, were d... more Single crystal spheres of spine1 of 4 mm diameter having a molar ratio of MgO/Al,03 1 : 5, were dissolved in 0.1 M solutions of K,CO,, NaF, KF, Na3P0, and K,P04 under a pressure of 2 kbar and at temperatures between 390 "C and 630 "C. The final shape of the dissolved spheres are cubes. The reaction layers formed during dissolution varied in porosity and consisted of

Research paper thumbnail of Carbon allotropes: a suggested classification scheme based on valence orbital hybridization

Carbon, 1997

This letter is an attempt to group the inflationary number of members of carbon allotrope.A class... more This letter is an attempt to group the inflationary number of members of carbon allotrope.A classification scheme based on valence orbital hybridation is suggested.

Research paper thumbnail of Born of fire: The pursuit of the ‘white gold’

Research paper thumbnail of Synthesis of Ceramic Layers for Solid Oxide Fuel Cells by Means of TPCVD

International Thermal Spray Conference

This paper investigates the potential of radio frequency thermal plasma chemical vapor deposition... more This paper investigates the potential of radio frequency thermal plasma chemical vapor deposition for producing Sr-doped La-Mn-perovskite and yttria-doped zirconia layers for solid-oxide fuel cells. Aqueous solutions were used as starting materials and were injected into the hot plasma core by means of an air-assist atomizer. Test results show how the microstructure, dopant distribution, and phase purity of the resulting layers depends both on process conditions and the material system. Paper includes a German-language abstract.

Research paper thumbnail of Correlation of Plasma Process Conditions, Phase Content, and Photocatalytic Performance of Induction Plasma-Sprayed Titania Coatings

Thermal Spray 2005: Proceedings from the International Thermal Spray Conference

Systematic variation of the induction plasma spray (IPS) conditions, i.e. (i) plasma power, (ii) ... more Systematic variation of the induction plasma spray (IPS) conditions, i.e. (i) plasma power, (ii) carrier gas flow rate, and (iii) powder feed rate was performed to deposit TiO2 coatings using statistical design of experiments (SDE) methodology. The microstructure, surface morphology, and anatase-rutile ratio of the coatings were studied by grazing-incidence X-ray diffraction (GIXD) and surface mapping with micro-probe Raman spectroscopy to determine the spatial phase distribution of anatase and rutile. The photocatalytic activity of the TiO2 coatings was tested by decomposition of 4-chlorophenol in aqueous solution in the dark and under UV irradiation. The rates of pH changes measured were compared with those of standard Degussa P-25 coatings. The photocatalytic activity of the samples shows a reasonable correlation with the phase content and the plasma processing conditions prevailing during coating deposition.

Research paper thumbnail of Radio-Frequency Suspension Plasma Spraying of Cobalt Spinel Anodes for Alkaline Water Electrolysis

Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 1998

Suspensions of cobalt spinel (Co3O4) powders (median size 6 µm and 0.3 µm) were r.f. plasma spray... more Suspensions of cobalt spinel (Co3O4) powders (median size 6 µm and 0.3 µm) were r.f. plasma sprayed to form electrocatalytically active anode layers for alkaline water electrolysis. Stable cobalt oxide suspensions of low viscosity and exceeding 50wt.% solid phase content have been processed. A spheroidization study revealed the formation of large spherical powder particles (- 30 + 80 µm). Cobalt oxide coatings were produced by this novel r.f.-Suspension Plasma Spraying (SPS) method. The porosity was controlled by optimizing the spray distance and the reactor pressure. The main disadvantage of the thermal plasma processing of cobalt spinel, i.e. the decomposition of the spinel phase into CoO, could not be prevented, not even with the application of an 80% oxygen plasma. With a relatively low power oxygen plasma post-treatment the deposited CoO layers can be oxidized to Co3O4 resulting in a promising improvement of the electrochemical performance of the anode layers.

Research paper thumbnail of Manufacture of thick chromium coatings by R.F. plasma spraying for PVD sputter targets

The production of plasma-sprayed thick chromium coatings on copper substrates for PVD sputter tar... more The production of plasma-sprayed thick chromium coatings on copper substrates for PVD sputter targets contains a number of technological challenges which were tried to be addressed using r.f. plasma spray technology. The efforts were focused on several objectives. The coatings should be as dense as possible. Their residual stresses should be low enough to avoid strong substrate bending and to prevent failure in adhesion or cohesion of the coating. The deposition efficiency during spraying should be as high as possible. Different spraying parameters were varied in order to find the optimum conditions for the spraying process. Two different powder grain sizes were used. First a spheroidization study was performed which gave evidence about the optimum chamber pressure, the optimum plasma power, and a manageable range of the powder feed rate. During spraying of the coatings the power feed was further optimized. Also the spraying distance, the substrate roughness and the substrate coolin...

Research paper thumbnail of In-vitro Investigation of Air Plasma-Sprayed Hydroxyapatite Coatings by Diffraction Techniques

Residual Stresses 2016, Dec 22, 2016

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC.

Research paper thumbnail of Weathering of ancient and medieval glasses—potential proxy for nuclear fuel waste glasses. A perennial challenge revisited

International Journal of Applied Glass Science, 2017

Ancient and medieval glasses that have survived the deleterious attack of the environment for mil... more Ancient and medieval glasses that have survived the deleterious attack of the environment for millennia have long since proposed as proxy to estimate and predict the corrosion mechanism of nuclear waste glasses. However, because both composition and environmental burial conditions vastly differ between hydrolytically less stable ancient glasses and modern advanced nuclear waste glasses, only semiquantitative conclusions can be drawn about the likely performance of the latter as long‐term stable immobilization matrices for high‐level radioactive nuclear waste. In this contribution, special emphasis has been devoted to the behavior of manganese, present as both iron decolorant and coloring ions in ancient Roman and medieval glasses. Study of the behavior of manganese in ancient glasses during weathering may provide some limited clues to the behavior of long‐lived radioactive technetium‐99. Knowledge of the corrosion kinetics of ancient glasses will allow, eventually, a reasonable pred...

Research paper thumbnail of Group V Metal Oxides (V, Nb and Ta) doping od Titania Coatings by Means of Suspension Plasma Spraying

Research paper thumbnail of D083 Depth-Resolved Strain Determination of Hydroxyapatite Coatings Using High Energy X-Ray Diffraction

Research paper thumbnail of The effect of deposition mechanism on the composition of surface films on silicon

Thin Solid Films, 1984

Surface films on Si(111) wafers were grown either by floating the sample on the surface of an HNO... more Surface films on Si(111) wafers were grown either by floating the sample on the surface of an HNO3HFCH3COOH solution with various HNO3:HFF ratios or by low temperature vapour deposition (LTV) from NO2HFH2O gas mixtures. From a combination of various surface analytical techniques as well as X-ray and electron diffraction it is concluded that the films formed by the floating method are composed of SiOx (x < 2) and those from LTV incorporate crystalline ammonium silico-fluoride and amorphous SiOx.

Research paper thumbnail of Thermal Induction Plasma Processes for the Synthesis of SOFC Materials

Materialwissenschaft und Werkstofftechnik, 2002

This work demonstrates the potential of two recently developed plasma spray methods to deposit Sr... more This work demonstrates the potential of two recently developed plasma spray methods to deposit Sr-doped La-(Mn,Fe,Co) perovskite (LSM, LSF, LSCF) cathodes, and yttria-doped zirconia (YSZ) electrolytes as components of solid oxide fuel cells (SOFC). These methods are Thermal Plasma Chemical Vapour Deposition (TPCVD) and Suspension Plasma Spraying (SPS). The microstructure obtained is determined by the method applied. TPCVD results in either globular or columnar coatings which were deposited with growth rates up to 30 lm/min. SPS with a deposition rate in excess of 100 lm/min yields coatings of layered microstructure. The homogeneity of the dopant distribution and the phase purity of YSZ is excellent. Process modifications will be necessary, however, to improve the properties in the case of perovskite coatings. Diese Arbeit demonstriert das Potenzial zweier junger Varianten des Plasmaspritzens, der durch ein thermisches Hochfrequenz-Plasma unterstu Ètzten chemischen Abscheidung aus der Dampfphase (Thermal Plasma Chemical Vapour Deposition, TPCVD) und des Suspensionsplasmaspritzens (SPS) zur Abscheidung von Sr-dotierten La-(Mn,Fe,Co)-Perowskit (LSM, LSF, LSCF)-Kathoden und Yttria-dotierten Zirconia (YSZ)-Elektrolyten fu Èr SOFC. Die Mikrostruktur der erzeugten Schichten ist abha Èngig vom eingesetzten Verfahren. So wurden mittels TPCVD globulare oder sta Èngelige Schichten mit Wachstumsraten bis 30 lm/min abgeschieden. Beim SPS entstehen bei Auftragsraten gro Èûer als 100 lm/min weitgehend lagige Schichten. Die Homogenita Èt der Dopandenverteilung sowie die Phasenreinheit der Schichten sind im Falle des YSZ hervorragend. Jedoch mu Èssen sie fu Èr die Perowskitschichten durch Prozessmodifikationen weiter verbessert werden.

Research paper thumbnail of Recent trends towards improved plasma-sprayed advanced bioceramic coatings on Ti6Al4V implants

Materialwissenschaft und Werkstofftechnik, 1999

In this contribution several recent trends are being described towards engineering of bioceramic ... more In this contribution several recent trends are being described towards engineering of bioceramic coatings applied by thermal spraying to titanium alloy substrates. (i) Optimization by statistical experimental design methodology of plasma spray parameters can be used to deposit coatings with low porosity, comparatively high adhesive strength and optimum crystallinity. (ii) Microstructural engineering of the hydroxyapatite powder by pre-spray annealing, i. e. sintering leads to a substantial decrease of the microporosity of the ceramic grains hence improving both adhesion strength and in vitro resorption resistance. (iii) Bond coats based on titania and titania/zirconia can be used to control the rate of heat transfer from the molten particles to the substrate and hence to improve the in vivo resorption resistance by suppressing the formation of amorphous calcium phosphate (ACP). (iv) Replacement of hydroxyapatite by specific compositions in the quaternary system CaO-TiO 2 -ZrO 2 -P 2 O 5 is a potential way to generate coatings with high chemical stability for long-term performance.

Research paper thumbnail of Chemical dehydrohalogenation of poly(ethylene-alt-chlorotrifluoroethylene)

Materials Chemistry and Physics, 2000

Chemical dehydrohalogenation of poly(ethylene-alt-chlorotrifluoroethylene) (PE-a-CTFE) has been s... more Chemical dehydrohalogenation of poly(ethylene-alt-chlorotrifluoroethylene) (PE-a-CTFE) has been studied as a possible route to carbyne. The reaction products have been characterized by infrared and 13 C solid-state nuclear magnetic resonance spectroscopy. The dehydrohalogenation of PEa -CTFE by potassium tert-butoxide in tetrahydrofuran has been shown to result in the formation of a halogen-substituted polyenyne structure with randomly distributed isolated triple bonds. The incomplete elimination of hydrogen halides can be explained by insufficient strength of the base, sterical hindrances caused by its bulkiness as well as by alternation defects in the original copolymer.

Research paper thumbnail of Hydroxyapatite coatings with a bond coat of biomedical implants by plasma projection

Journal of Materials Research, 1996

Research paper thumbnail of The morphology of hydrothermally grown strontium paracelsian

Journal of Crystal Growth, 1979

ABSTRACT

Research paper thumbnail of Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN), and La3Ga5.5Ta0.5O14 (LGT)

Journal of Crystal Growth, 1999

Mass densities, relative dielectric constants, and piezoelectric material parameters were determi... more Mass densities, relative dielectric constants, and piezoelectric material parameters were determined on single crystals of LGS, LGN, and LGT grown with high structural perfection by the Czochralski technique. Optimized values of the elastic sti!nesses and elastic compliances were derived by measuring and critically comparing the propagation velocities of both bulk (BAW) and surface acoustic waves (SAW).

Research paper thumbnail of Bildung von Mn-dotierten einkristallen von α-korund aus der gasphase

Journal of Crystal Growth, 1968

Eingegangen am 7. Dezember 1967 Red coloured single crystals of cc-A&O, have been grown by the hy... more Eingegangen am 7. Dezember 1967 Red coloured single crystals of cc-A&O, have been grown by the hydrolysis of the vapour resulted from the reaction of a melt of MnF, with solid y-AlaO at temperatures up to 950 "C in the presence of air. The red colour of the crystals depends on a Mn content of 0.9-l .7 'A. Emission spectrographic data have established the absence of other colouring ions. Some crystals show

Research paper thumbnail of Influence of surface films on the development of pits during etching of silicon

Journal of Crystal Growth, 1982

Thin surface films on dislocation-free silicon (111) wafers grown either by prepolishing in, or f... more Thin surface films on dislocation-free silicon (111) wafers grown either by prepolishing in, or floating on, HNO3-HF-CH3COOH solutions influence the development of pits during subsequent etching in conventional preferential etches (Sirtl and Secco etches). It is suggested that the etch action proceeds through holes in the film thus leading to a mechanism similar to localized corrosion in passivatable metals and alloys. Statistical analyses of the pit distributions further indicate that dislocations within the silicon do not contribute significantly to the pitting phenomenon.

Research paper thumbnail of Hydrothermal dissolution reactions of magnesium-aluminium-spinel in alkaline solutions

Journal of Crystal Growth, 1970

Single crystal spheres of spine1 of 4 mm diameter having a molar ratio of MgO/Al,03 1 : 5, were d... more Single crystal spheres of spine1 of 4 mm diameter having a molar ratio of MgO/Al,03 1 : 5, were dissolved in 0.1 M solutions of K,CO,, NaF, KF, Na3P0, and K,P04 under a pressure of 2 kbar and at temperatures between 390 "C and 630 "C. The final shape of the dissolved spheres are cubes. The reaction layers formed during dissolution varied in porosity and consisted of

Research paper thumbnail of Carbon allotropes: a suggested classification scheme based on valence orbital hybridization

Carbon, 1997

This letter is an attempt to group the inflationary number of members of carbon allotrope.A class... more This letter is an attempt to group the inflationary number of members of carbon allotrope.A classification scheme based on valence orbital hybridation is suggested.