12. Artikkelit / Articles

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Browsing 12. Artikkelit / Articles by Department "Department of Chemistry"

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  • 151Eu Mössbauer spectroscopy and x-ray-diffraction studies on the Pb2Ba2EuCu3O8+[delta] system
    (1994) Lindén, J.; Lippmaa, M.; Miettinen, J.; Tittonen, I.; Katila, T.; Karppinen, Maarit; Niinistö, L.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Simultaneous replacement of Sr by Ba and Y by Eu in the Pb-2213 system was found to yield single-phase Pb2Ba2EuCu3O8+δ samples, suitable for Eu151 Mössbauer measurements. The samples were synthesized by a solid-state reaction of metal oxides and carbonates under an inert atmosphere. An oxygen-rich sample corresponding to δ=1.79 was obtained by annealing the as-synthesized material (δ=0.16) in oxygen. The oxygen annealing increased the size of the lattice constants and led to an orthorhombic-to-tetragonal phase transition of the structure. The samples were also oriented in an 11.7-T magnetic field. The x-ray-diffraction spectra showed that the c axes tended to orient perpendicular to the applied field. The measured Mössbauer spectra exhibited an electric quadrupole interaction typical of the Eu site in high-Tc cuprates. The average orientation angles obtained from fittings of the Mössbauer spectra were in accordance with the results from the x-ray-diffraction measurements.
  • Analysis of UHMWPE wear particles produced in the simulation of hip and knee wear mechanisms with the RandomPOD system
    (2015) Saikko, Vesa; Vuorinen, Vesa; Revitzer, Hannu
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Anisotropic thermoelectric properties associated with dimensional crossover in quasi-one-dimensional SrNbO3.4+d(d~0.03)
    (2011) Kobayashi, W.; Hayashi, Y.; Matsushita, M.; Yamamoto, Y.; Terasaki, I.; Nakao, A.; Nakao, H.; Murakami, Y.; Moritomo, Y.; Yamauchi, H.; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have grown large single crystals of SrNbO3.4+d (d~0.03) with n=5 in the homologous series SrnNbnO3n+2 by using a traveling solvent floating-zone method and measured resistivity, thermopower, and thermal conductivity along all crystallographic axes. The thermoelectric properties are found to be highly anisotropic, which reflects a quasi-one-dimensional electronic structure. In particular, the thermopower along the b axis is −170 μV/K at 300 K, which is 1 order of magnitude higher than the −15 μV/K along the a axis and −25 μV/K along the c axis. A possible origin of the high anisotropy in the thermopower is discussed in terms of dimensional crossover associated with a structural modification.
  • Atomic layer deposition of Al-doped ZnO thin films
    (2013) Tynell, Tommi; Yamauchi, Hisao; Karppinen, Maarit; Okazaki, Ryuji; Terasaki, Ichiro
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Atomic layer deposition has been used to fabricate thin films of aluminum-doped ZnO by depositing interspersed layers of ZnO and Al 2O3 on borosilicate glass substrates. The growth characteristics of the films have been investigated through x-ray diffraction, x-ray reflection, and x-ray fluorescence measurements, and the efficacy of the Al doping has been evaluated through optical reflectivity and Seebeck coefficient measurements. The Al doping is found to affect the carrier density of ZnO up to a nominal Al dopant content of 5 at. %. At nominal Al doping levels of 10 at. % and higher, the structure of the films is found to be strongly affected by the Al 2O3 phase and no further carrier doping of ZnO is observed.
  • Ca-substitution and O-doping effects in superconducting Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z obtained from neutron diffraction refinements
    (1999) Karppinen, Maarit; Yamauchi, H.; Fujinami, K.; Nakane, T.; Peitola, K.; Rundlöf, H.; Tellgren, R.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Distinct calcium and oxygen doping effects were studied in the Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z (Cu−1212:P) system by means of neutron diffraction and superconducting quantum interference device experiments in the wide substitution ranges of 0<~x<~0.35 and 0
  • Characterization of magnetic properties of Sr2CuWO6 and Sr2CuMoO6
    (2014) Vasala, Sami; Saadaoui, Hassan; Morenzoni, Elvezio; Chmaissem, Omar; Chan, Ting-Shan; Chen, Jin-Ming; Hsu, Ying-Ya; Yamauchi, Hisao; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this work we examine the low-temperature magnetic properties of the two double-perovskite compounds Sr2CuWO6 and Sr2CuMoO6 using magnetic susceptibility, muon spin rotation and relaxation, and neutron powder diffraction measurements. Additionally, the most relevant spin exchange interaction constants are derived from ab initio electronic structure calculations, aided by x-ray absorption spectroscopy. The compounds exhibit quasi-two-dimensional magnetic properties, with broad maxima at Tmax = 83 and 95 K for Sr2CuWO6 and Sr2CuMoO6, respectively. However, three-dimensional long-range order takes place below TN = 24(1) and 28(2) K for Sr2CuWO6 and Sr2CuMoO6, respectively. Our results show that the low-dimensional magnetic correlations are mainly due to the significant next-nearest-neighbor interactions in the ab plane of the double-perovskite structure, whereas three-dimensional long-range magnetic order is caused by weaker next-nearest-neighbor interactions along the c axis. Next-nearest-neighbor interactions are also slightly frustrated by weaker nearest-neighbor interactions within the ab plane. Based on these results we predict the low-temperature magnetic structure in these compounds to be type-II antiferromagnetic order of the double-perovskite lattice.
  • Characterization of superconducting Bi2Sr2Can-1CunO4+2n phases with 57Fe Mössbauer spectroscopy
    (1990) Tittonen, I.; Hietaniemi, J.; Huttunen, J.; Lindén, J.; Katila, T.; Karlemo, T.; Karppinen, Maarit; Niinistö, L.; Ullakko, K.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In order to study the dependence of superconductivity on sample preparation and the influence of cationic doping, Bi2Sr2Can−1CunO4+2n samples were prepared using various sintering temperatures and nominal starting compositions. After each sintering the samples were characterized with x-ray-diffraction and magnetic-ac-susceptibility measurements. In all cases the corresponding doped57 samples were also made for Mössbauer measurements. The Mössbauer measurements usually show three quadrupole components arising from the different phases in the samples. The site assignment of Fe57 is discussed.
  • Coexistence of intrinsic and extrinsic magnetoresistance in the double-perovskite Sr[sub 2]Fe(Mo[sub 1-x]W[sub x])O[sub 6-w] system
    (2001) Lindén, J.; Yamamoto, T.; Nakamura, J.; Karppinen, Maarit; Yamauchi, H.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6−w system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1−xWx)O6−w samples with 0⩽x⩽1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN≈50 K for x⩾0.6. In samples with 0.7⩽x⩽0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Néel temperature. The peak was found to have its origin in the colossal magnetoresistanceeffect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMoO6−w. The coexistence of the two types of magnetoresistanceeffect is responsible for the enhancement of the overall magnetoresistance value. A slight enhancement in the magnetoresistance values around 300 K for the strongly W-substituted samples was found to be related to a second colossal magnetoresistance peak related to the para- to ferrimagnetic transition at Tc.
  • Competition between ferromagnetism and antiferromagnetism in the rutile Cr1-xVxO2 system
    (2016) Mustonen, Otto; Vasala, Sami; Chou, Ta-Lei; Chen, Jin-Ming; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present a comprehensive computational and experimental examination of the Cr1−xVxO2 (0 ≤ x ≤ 0.5) system. The entire series crystallizes in the rutile structure, but the compounds exhibit significantly different magnetic properties depending on x. Lattice parameter a increases linearly with x, but the c parameter is slightly reduced due to vanadium-vanadium bonding. The V-for-Cr substitution creates Cr3+-V5+ pairs; this leads to competition between ferromagnetic (Cr4+-Cr4+) and antiferromagnetic (Cr3+-Cr3+) interactions such that the materials change from ferromagnetic to antiferromagnetic with increasing x. Weak ferromagnetic interactions arising from Cr4+ are observed even in the seemingly antiferromagnetic phases with the exception of x = 0.5, which contains only Cr3+. Density functional theory calculations are performed, but they incorrectly predict the x = 0.5 phase to be a half-metal. This is caused by an incorrect prediction of the oxidation states of chromium and vanadium.
  • Competition between intragranular and intergranular tunneling magnetoresistance in polycrystalline Sr2FeMoO6
    (2006) Huang, Yun-Hui; Yamauchi, Hisao; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Polycrystalline Sr2FeMoO6 (SFMO) samples with various grain sizes and densities of Fe/Mo disorder defects were synthesized from sol-gel-derived precursors by means of a sample-encapsulation technique. The samples with perfect Fe/Mo ordering exhibited metallic transport behavior and a large magnetoresistance (MR) effect, whereas the samples with a high-disorder density and small grain size showed a metal-insulator transition and lower MR values. It is suggested that a competition between intragranular and intergranular tunneling MR effects exists in the SFMO system. To confirm this competition, homocomposite samples consisting of two or three single-phase SFMO components with different grain sizes were designed. Large enhancement in low-field magnetoresistance (LFMR) was observed in the homocomposites, which could be attributed to the enhanced intergranular effect. It is thus concluded that the intergranular effect is more important to the LFMR than the intragranular effect.
  • Control of hole distribution through isovalent R-cation substitution in Cu[sub 2]Ba[sub 2]RCu[sub 2]O[sub 8] superconductors
    (2007) Karppinen, Maarit; Abe, Y.; Grigoraviciute, I.; Chen, J. M.; Liu, R. S.; Yamauchi, H.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Superconductive Cu2Ba2RCu2O8 samples with R ranging from Gd to Tm have been synthesized through a sol-gel route. Both iodometric titration and CuL-edge x-rayabsorption near-edge structure (XANES)spectroscopy data indicate that the average Cu valence remains constant, whereas Tc increases with decreasing size of the R constituent. An explanation for this trend is revealed from O K-edge XANESspectra, which show that the smaller-for-larger R-cation substitution results in a shift of holes from the Cu2O2 charge reservoir to the superconductive CuO2 planes. Since Cu2Ba2RCu2O8 samples are underdoped, such a shift of holes raises the value of Tc.
  • Dissolution Control of Mg by Cellulose Acetate–Polyelectrolyte Membranes
    (2014) Yliniemi, Kirsi; Wilson, Benjamin. P.; Singer, Ferdinand; Höhn, Sarah; Kontturi, Eero; Virtanen, Sannakaisa
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Cellulose acetate (CA)-based membranes are used for Mg dissolution control: the permeability of the membrane is adjusted by additions of the polyelectrolyte, poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). Spin-coated films were characterized with FT-IR, and once exposed to an aqueous solution the film distends and starts acting as a membrane which controls the flow of ions and H2 gas. Electrochemical measurements (linear sweep voltammograms, open-circuit potential, and polarization) show that by altering the CA:PDMAEMA ratio the dissolution rate of Mg can be controlled. Such a control over Mg dissolution is crucial if Mg is to be considered as a viable, temporary biomedical implant material. Furthermore, the accumulation of corrosion products between the membrane and the sample diminishes the undesirable effects of high local pH and H2 formation which takes place during the corrosion process.
  • The effect of acetabular cup position on wear of a large-diameter metal-on-metal prosthesis studied with a hip joint simulator
    (2013) Saikko, Vesa; Ahlroos, Tiina; Revitzer, Hannu; Ryti, Oskari; Kuosmanen, Petri
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Effect of CoCr Counterface Roughness on the Wear of UHMWPE in the Noncyclic RandomPOD Simulation
    (2017) Saikko, Vesa; Vuorinen, Vesa; Revitzer, Hannu
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Electrochemical synthesis and properties of CoO[sub 2], the x=0 phase of the A[sub x]CoO[sub 2] systems (A=Li,Na)
    (2008) Motohashi, T.; Ono, T.; Katsumata, Y.; Kanno, R.; Karppinen, Maarit; Yamauchi, H.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Single-phase bulk samples of the “exotic” CoO2, the x=0 phase of the AxCoO2 systems (A=Li,Na), were successfully synthesized through electrochemical deintercalation of Li from pristine LiCoO2 samples. The samples of pure CoO2 were found to be essentially oxygen stoichiometric and possess a hexagonal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetism of CoO2 is featured with a temperature-independent susceptibility of the magnitude of 10−3emu/molOe, being essentially identical to that of a Li-doped phase, Li0.12CoO2. It is most likely that the CoO2 phase is a Pauli-paramagnetic metal with itinerant electrons.
  • Electronic phase diagram of the layered cobalt oxide system LixCoO2(0.0
    (2009) Motohashi, T.; Ono, T.; Sugimoto, Y.; Masubuchi, Y.; Kikkawa, S.; Kanno, R.; Karppinen, Maarit; Yamauchi, H.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0≤x≤1. Based on dc-magnetic-susceptibility data, combined with results of Co59 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x=0.35 and 0.40, which separates the Pauli-paramagnetic and Curie-Weiss metals. In the Pauli-paramagnetic regime (x≤0.35), the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x=0 end member is a noncorrelated metal in the whole temperature range studied. In the Curie-Weiss regime (x≥0.40), on the other hand, various phase transitions are observed. For x=0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static antiferromagnetic order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt≈175 K in samples with x=0.50 (=1/2) and 0.67 (=2/3), while only a tiny kink appears at T≈210 K in the sample with an intermediate Li composition, x=0.60. Thus, the phase diagram of the LixCoO2 system is complex and the electronic properties are sensitively influenced by the Li content (x).
  • Establishment of strongly overdoped states in the HgBa2Ca2Cu3O8+[delta] superconductor
    (1997) Fujinami, K.; Ito, T.; Suematsu, H.; Matsuura, K.; Karppinen, Maarit; Yamauchi, H.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Strongly overdoped HgBa2Ca2Cu3O8+δ (Hg-1223) samples with superconducting transitions as low as 97 K were successfully obtained by using HgO, Ca2CuO3, CuO, and mixtures of highly oxidized BaCuO2+ε (ε≈0.13) and/or BaO2 powders as starting materials for the high-pressure synthesis at 5 GPa and 950 °C. The overdoped state was confirmed by observing negative values for the Seebeck coefficient throughout the temperature range from Tc to 320 K in a thermoelectric power measurement. Also, both of the cell parameters a and c were found to decrease with decreasing Tc, reflecting, respectively, an increase in hole concentration in the CuO2 planes and the incorporation of oxygen into the HgOδ layer. The oxygen excess δ as determined by the Cu(+I)/Cu(+II) coulometric titration method, was ∼0.19 in the overdoped sample with Tc=107K. Subsequent reducing annealing in an Ar atmosphere at 280 °C increased the Tc to 131 K. Consistently, only positive Seebeck coefficient values were observed up to 320 K for the Ar-annealed sample. A clearly underdoped material with Tc=118K was obtained by annealing the same sample in Ar at a higher temperature (∼400°C). Finally, high-pressure synthesis starting from less-oxidized BaCuO2+ε (ε≈0.06) yielded a Hg-1223 material with δ≈0.10 and Tc=132K.
  • Europium substitution effects in superconducting YBa2Cu4O8 synthesized under one atmosphere oxygen pressure
    (1994) Lindén, J.; Lippmaa, M.; Miettinen, J.; Tittonen, I.; Katila, T.; Kareiva, A.; Karppinen, Maarit; Niinistö, L.; Valo, J.; Leskelä, M.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Y1−xEuxBa2Cu4O8 powder samples, with x=0, 0.25, 0.5, 0.75, and 1.0, were synthesized at ambient pressure using either an acetate-tartrate sol-gel method or a LiF flux process. The lattice parameters and purity of the samples were checked using X-ray diffraction. The superconducting transition was monitored by magnetic-susceptibility measurements. Replacing yttrium with europium increased the unit-cell volume, decreased the orthorhombicity (b/a) and the critical temperature. The hyperfine interactions at the europium site were studied by Eu151 Mössbauer spectroscopy. The complete quadrupole Hamiltonian of the 21.5-keV γ transition of Eu151 was successfully applied in the analyses of the Mössbauer spectra. The Mössbauer parameters obtained were found to resemble those measured for the EuBa2Cu3Cu3O7−δ (1:2:3) system. It was demonstrated that magnetic alignment of the crystallites could not be obtained with an 11.7-T field, contrary to the 1:2:3 and other high-Tc systems. The magnetic susceptibility for 1:2:4 single crystals appears to be isotropic.
  • Europium-based high-temperature superconductors studied by x-ray diffraction and 151Eu Mössbauer spectroscopy
    (1992) Lindén, J.; Hietaniemi, J.; Ikonen, E.; Lippmaa, M.; Tittonen, I.; Katila, T.; Karlemo, T.; Karppinen, Maarit; Niinistö, L.; Ullakko, K.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Isotropic powders and magnetically aligned crystallites of EuBa2Cu3O7−δ (1:2:3) and europium-doped Bi2Sr2CaCu2O8 (2:2:1:2) were studied by means of x-ray diffraction and Eu151 Mössbauer spectroscopy. The degree of crystallite orientation of the samples and the values of the lattice constants were determined by x-ray diffraction. The Mössbauer spectra were analyzed considering the full hyperfine Hamiltonian of the nuclear states of the 21.5-keV γ transition. The Mössbauer hyperfine parameters obtained from the superconducting and semiconducting phases are presented. A small change is seen in the Eu151 isomer shift when the oxygen deficiency δ of the 1:2:3 compound is varied. The shift can be explained by a decrease in the s-electron density due to lattice expansion. The changes in the oxidation state of the copper atoms with varying δ were determined from the Mössbauer data: The Cu(2) atoms retain their oxidation state, whereas the Cu(1) atoms adjust their valence according to the value of δ. In the 2:2:1:2 samples, the Eu concentration clearly affected the value of the electric-field gradient at the Eu nucleus. Using a standard procedure, magnetically aligned 2:2:1:2 samples were prepared. The preferred direction of the crystal c axis changed from parallel to perpendicular alignment with the external magnetic field, when the Eu concentration exceeded 20% of the Ca atoms.
  • Evidence for valence fluctuation of Fe in Sr[sub 2]FeMoO[sub 6-w] double perovskite
    (2000) Lindén, J.; Yamamoto, T.; Karppinen, Maarit; Yamauchi, H.; Pietari, T.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by 57Fe Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.