NATIONAL INSTITUTE of Materials Physics 105 bis Atomistilor Str, 077125 Magurele, CP MG-7, Romania Magnetite in Glassy Matrix V. Sandu, M. S. Nicolescu, V. Kuncser, I. Ivan, National Institute of Materials Physics-Bucharest, Magurele, 077125, Romania E. Sandu „Horia Hulubei“ National Institute of Physics and Nuclear Engineering, Magurele, 077125, Romania Acknowledgments: Romanian National Authority for Science, project 72.151/2008
Glass ceramic as composite material • Is glass ceramic a composite? • Composite materials are materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or microscopic scale. • A nanocomposite is as a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm), or structures having nano- scale repeat distances between the different phases that make up the material. It is more usually taken to mean the solid combination of a bulk matrix and nano- dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite will differ markedly from that of the component materials. • Recognized as composites: – Ceramic-matrix nanocomposites – Metal-matrix nanocomposites – Polymer-matrix nanocomposites • But not glass ceramic : vitreous matrix containing dispersed nanograined crystalline phases. • Crystalline grains nucleate and growth: - during cooling down of the molten composition - as a result of different thermal treatments
Magnetic Glass Ceramics • Glass-ceramic containing magnetic nano-crystallite (iron oxides, barium, and strontium hexaferrites, etc) grown within the vitreous matrix. • Features: - very fine single-or multi-domain magnetic particles - superparamagnetic behavior - dipol-dipol interaction prevents grain aglomeration • Very flexible and cheap process that depends on: - appropriate choice of the ingredients - particular thermal excursion - stable ferro fluids with a large number of carrier due to the lack of sedimentation; - magnetic drug transport and targeting; - magnetofectia; - biodetection and magnetic separation; - Hyperthermia: incorporation of the magnetic particles via endocytosis; - MRI contrast agents; - Component of glass-ceramics substitutes of bones. For in vivo medical applications only iron oxides, i.e., Magnetite (Fe 3 O 4 ) or maghemite ( -Fe 2 O 3 ) can be used
Preparation of glass-ceramics with high content of magnetite by direct crystallization Magnetite: Fe 3 O 4 -Inverse spinel structure: Fe 3+ A Tetrahedral positions Fe 3+ B & Fe 2+ B octahedral positions -Fe 3+ A & Fe 3+ B are antiferromagnetically ordered and the ferrimagnetism comes from Fe 2+ B spins -Continuous exchange of electrons between Fe 2+ and Fe 3+ in the octahedral positions -Verwey transition at T V 120 K -Curie temperature T c = 858 K Still under dispute: Charge & orbital ordering Multiferroicity
Samples preparation Basically: borosilicatic glass with constant B 2 O 3 and Na 2 O content and small amounts of either one of Al 2 O 3 , and nucleators (Cr 2 O 3 , P 2 O 5 ). Variation of the ratio SiO 2 /Fe 2 O 3 . The batches were melt into alumina crucible for 2.5-3 hours in the temperature range 1400-1500 C. -The melts were poured onto a steel sheet and the resulting slabs were immediately transferred to an annealer operating at 560 C for 2-4 hours. - -Cooling down 450 C (10 C/hour) -Cooling down to 300 C (20 C/hour) and then inertially to 25 C. Sample Starting oxide composition (%w) code SiO 2 B 2 O 3 Na 2 O Fe 2 O 3 Cr 2 O 3 Al 2 O 3 P 2 O 5 C1 47 28.6 6.4 17.5 0.5 - P1 46.5 28.6 6.4 17.5 - - 1 C2 36.5 28.6 6.4 24.5 0.5 3.5 P2 39.5 28.6 6.4 24.5 - - 1
Why these compositions? 1. The silica tetrahedra network may also include other tetrahedral or triangular groups (B, Na) and the intermediate (F 2 O 3 , Al 2 O 3 ) 2. Cr and P have nucleating oxides with high solubility at high T but low solubility and high diffusivity at low T: • Cr in glass melts has two valence states Cr 2+ and Cr 6+ . The latter is stabilized by the presence of alkaline oxides (Na). Cr 6+ has a intense field q /r = 17.2 with a strong ordering effect on O ions which promote the separation of crystalline nuclei at low T • P 5+ is glass formating at high T. In tetrahedral coordination the charge difference between Si 4+ and P 5+ leads to separation not as P 2 O 5 but in combination with alkaline metal • AlO 4 stabillizes PO 4 tetrahedra due to the special bonds, so, increases the stability of glass relative to recrystallization. Fe 3 O 4 crystallizes on these nucleii
Thermal expansion investigation Sample T ir T G T sr T D C1 C1 464.8 495.9 509.5 553.6 P 1 425.5 461.2 474 572.4 C2 438.0 465.5 476.9 513.5 P2 419.2 464.4 479.1 546.8 Glassy temperature vs. composition and nucleators Expansion constant vs composition and nucleators.
X-ray Difrraction • Intensive process of recrystallization occurs in all samples • Attests to the efficiency of Cr 2 O 3 and P 2 O 5 as a nucleating agents. • unique crystalline phase: magnetite. • minor phase: F 2 O 3 , (in C1). Cr 2 O 3 P 2 O 5 FTIR-data Grain size as extracted from the peak (311)
Morphology C1 C2
P2 P1
Mössbauer Spectroscopy Two magnetic components: magnetically ordered component: sextet central paramagnetic component: doublet. Hyperfine parameters Fe coordination: Octahedral Fe 2.5+ H hfo = 46 T Tetrahedral Fe 3+ H hft = 49 T Ideal Magnetite R = N(Oct)/N(Tetra) = 2 Sextets: magnetically ordered Fe C-1: R = 2.1 & H hfo = 45.9 T almost ideal magnetite. C-2: R = 1.7 & H hfo = 46.1 T octahedral postion equally underoccupied P-1: R = 1.7 & H hfo = 45.8 T octahedral postions equally underoccupied well structured magnetite. P-2: R = 2 The paramagnetic doublets: Fe ions dispersed in the glassy matrix. C-1: 16% of the total Fe ions C-2: 10% as Fe 3+ . P-1: 25 %: 22% Fe 3+ , octahedral coordination, 3% is Fe 2+ . P-2: 41 %: 32 % Fe 3+ , 9 % Fe + , >2, tetrahedral coordination
dc-Magnetization Cr 2 O 3 -multi domain P 2 O 5 -single domain - AFM ordering of Fe 3+ and Fe 2+ in O - FM ordering of Fe 3+ in T J. Wang et al., Mat. Chem. Physi. 13 6 (2009) - domains R = 1.7, d = 33 nm R = 1.7, d = 79 nm Tetrahedrally dominant tetrahedrally dominant R = 2.0, d = 26 nm R = 2.1, d = 121 nm Slightly octahedrally dominant
C1 ac- susceptibility • The peaks shift with but not enough to attribute of activated processes. Not T B but T V . • ” increases with : rotation of the magnetic moments and the change of the ionic order within walls friction • Low T shoulder ??? C2 • Slower dependence • ” decreases with • The shoulder at low T evolves to a peak (21.12 K at 30 Hz) • A second shoulder is present at high temperatures (85 K at 30 Hz). dc- susceptibility
P1 ac- susceptibility • Sharp Verwey transition T V = 122 K • negligible dependence of • Low T peak in ” • ” decreases on average with • Is it related to R = 1.7??? (electronic processes between Fe 3+ and Fe 2+ ) P2 • Broad ” • ” increases • Single domain nanoparticles dc- susceptibility
Conclusions 1. At high Fe content, borosilicatic glass can be crystallized with the formation of magnetite containing glass ceramic composite 2. Cr 2 O 3 and P 2 O 5 are good nucleators for magnetite crystallization but between 12 and 44 % of Fe ions remain dispersed in the glassy matrix as paramagnetic ions. 3. Cr 2 O 3 - promotes large grains and leaves small amunt of Fe ions within glass solution. However it leads to structural unbalanced occupation of the tetrahedral and octahedral sites. Specifically, at low Fe content is supported the underoccupation of the tetrahedral sites whereas at high Fe content the octahedral sites are Fe-deficient. 4. P 2 O 5 promotes small crystallites agglomerated in large almost spherical grains with a well defined Verwey transition, but with lower values of the magnetization due to a reduced contribution of the Fe ions to the growth of crystalline, hence magnetically ordered, phase. An important amount of Fe ions is left dispersed in the glassy matrix. When the amount of Fe is increased, it results a structurally ideal magnetite but with a huge amount (41% ) of paramagnetic Fe. 5. The magnetic response is complex and depends on the degree of vacancies in the structure of magnetite as well as on the location of these vacancies. However, the problem is more complex and a clear response would require also the analysis of the role of the structural and ferroelastic domains in the dynamic behavior. 6. The absolute value of the magnetization is the result of participation of Fe ions to the formation of magnetite phase, therefore, the samples with low content of paramagnetic phase display the highest specific magnetization.
Thank you
Location of our magnetite
Recommend
More recommend
