Dielectric properties and electrical conduction in yttrium iron garnet (YIG)

The dielectric properties (dielectric constant and loss) of a single crystal of yttrium iron garnet (Y₃Fe₅O₁₂) were measured in the temperature range 77–725 K and in the frequency range 100 Hz-1 MHz. AC conductivity was derived from dielectric constant and loss. DC conductivity was measured in the temperature range 30–725 K. Thermoelectric power (TEP) was measured from 77–800 K. On the basis of the results, conduction in this garnet is interpreted as due to small polarons. The nature of conduction at different temperature ranges is discussed in the light of existing reports on defect formation.     

The magnetic and dielectric properties of microwave sintered yttrium iron garnet (YIG)

Yttrium iron garnet (YIG) material is widely used in microwave devices. Experiments show that microwave sintering (MS) treated YIG materials possess excellent properties with a saturation magnetization of 14.60 emu/g and coercive force 34.82 Oe. In the frequency range of 1 MHz–1.8 GHz, the relative dielectric constant is from 6.5 to 7.0, the line-width is 105 Oe, dielectric loss less than 0.09 and magnetic loss less than 0.7. Furthermore, the sintering time and temperature were significantly reduced from 20 h and 1300 °C for the conventional sintering (CS) process to 2 h and 900 °C for MS technique, respectively.     

Low-temperature synthesis and microstructure-property study of single-phase yttrium iron garnet (YIG) nanocrystals via a rapid chemical coprecipitation

Single-phase yttrium iron garnet (Y₃Fe₅O₁₂, YIG) nanocrystals have been synthesized via a rapid chemical coprecipitation process with reverse strike operations, followed by calcining the precipitates at the temperature around 750 °C. The formation of YIG nanocrystals from the amorphous precipitates and their microstructural features and magnetic properties were investigated by FT-IR, XRD, TG-DSC, FESEM, TEM and VSM. It has been found that the as-obtained precipitates could be thermally activated to directly form the crystalline phases of garnet structure around 650 °C, including cubic YIG and minor tetragonal YIG but no trace of YFeO₃, which was often involved during the synthesis of YIG or doped-YIG when a chemical coprecipitation method was used. The calcinations could make the tetragonal YIG entirely transform into the cubic phase at 750 °C and allow the crystallites of the latter to grow from ∼22 nm to ∼50 nm in size almost linearly as a function of the temperature ranging from 650 °C to 900 °C. Moreover, the room temperature saturation magnetization of the samples after calcinations at various temperatures showed a nonlinear increase from 0.24 emu g⁻¹ to 24.54 emu g⁻¹, which should be associated with the alignments of atomic magnetic moments in the materials from completely-disordered to partially-ordered firstly and further to completely-ordered and, in the last stage, mainly with the growing YIG nanocrystals.     

Using lead sulphate instead of lead oxide in flux growth of yttrium iron garnet(YIG) crystals

Crystal growth of YIG from fluxes containing lead sulphate in place of lead oxide in the usual lead oxide-lead fluoride-boron oxide flux system has been tried. Lead sulphate decomposes during crystal growth giving lead oxide and sulphur trioxide. Due to the influence of sulphur trioxide in the system the yield of crystals almost doubles. There is no change either in the morphology of the crystals or their lattice parameter. It is possible that solubility of YIG is different in the new flux and the changed solubility causes the increase in yield of crystals.     

Observation of a cubical-like microstructure of strontium iron garnet and yttrium iron garnet prepared via sol-gel technique

This is our initial response towards preparation of nano-inductors garnet for high operating frequencies strontium iron garnet (Sr3Fe5O12) denoted as SrIG and yttrium iron garnet (Y3Fe5O12) denoted as YIG. The garnet nano crystals were prepared by novel sol-gel technique. The phase and crystal structure of the prepared samples were identified by using X-ray diffraction analysis. SEM images were done to reveal the surface morphology of the samples. Raman spectra was taken for yttrium iron garnet (Y3Fe5O12). The magnetic properties of the samples namely initial permeability (micro), relative loss factor (RLF) and quality factor (Q-Factor) were done by using LCR meter. From the XRD profile, both of the Y3Fe5O12 and Sr3Fe5O12 samples showed single phase garnet and crystallization had completely occurred at 900 degrees C for the SrIG and 950 degrees C for the YIG samples. The YIG sample showed extremely low RLF value (0.0082) and high density 4.623 g/cm3. Interesting however is the high Q factor (20-60) shown by the Sr3Fe5O12 sample from 20-100 MHz. This high performance magnetic property is attributed to the homogenous and cubical-like microstructure. The YIG particles were used as magnetic feeder for EM transmitter. It was observed that YIG magnetic feeder with the EM transmitter gave 39% higher magnetic field than without YIG magnetic feeder.     

Effect of alloying on aqueous corrosion and mechanical behaviour of iron aluminide Fe3Al

Passivity-inducing elements have been added to iron aluminide, Fe₃Al, to tackle their poor room-temperature ductility problem. The effect of alloying on the aqueous corrosion and mechanical behaviour of iron aluminides has been examined. It was found that the corrosion behaviour of intermetallic Fe₃Al-5M (M=Cr, Mo, Ta and Ti) was superior compared to that of binary Fe₃Al in electrolytes of pH 4 (H₂SO₄) and pH 8 (NaOH). The relative corrosion behaviour of these intermetallics in these electrolytes was comparable. The possible reasons for passivity enhancement have been discussed. Fe₃Al-5M₁ (M₁=Mo, Ta, V, Nb and Si) intermetallics could not be processed thermomechanically at 1000 °C because they cracked during deformation processing. The Fe₃Al-5Cr and Fe₃Al-5Ti intermetallics could be processed up to 80% deformation at 1000 °C by rolling into thin strips. These intermetallics exhibited improved room-temperature ductilities but poor yield strengths. The improvement in ductility has been attributed to passivity and microstructural effects. The low yield strengths of these intermetallics are poorly understood.     

Densification and microstructure development in the reaction sintering process of yttrium iron garnet

Factors affecting the densification and microstructure development in the reaction sintering process (RSP) of yttrium iron garnet were investigated. Three different powder mixtures were used: Fe₂O₃/Y₂O₃, Fe₂O₃/YFeO₃ (1100 ° C calcined), and Fe₂O₃/YFeO₃ (1200 ° C calcined). The conventionally prepared garnet powder was also adopted as a reference material. It was found that the RSP using Fe₂O₃-YFeO₃ systems has a beneficial effect on densification from the dilatation occurring along with the reaction of garnet formation. On the other hand, it has a detrimental effect due to the local contraction induced by the reaction in the Fe₂O₃-Y₂O₃ system. The densification rate and ultimate density achievable are also affected by the YFeO₃ powder adopted in RSP. A high grain-growth rate was obtained for garnet when the 1200 ° C calcined YFeO₃ powder was used. This leads to a high densification rate at low temperature. However, the densification ability deteriorates at temperatures above 1425 ° C due to the trap of pores in the fast-grown grains. Conversely, the grain-growth rate in RSP with 1100 ° C-calcined YFeO₃ was moderate, and although it gives a slower densification rate at low temperature, the ultimate density can be raised to 99% theoretical density at 1450 ° C.     

Sol-gel processing and sintering of yttrium aluminum garnet (YAG) powders

Gels of yttrium aluminum garnet (YAG) with the stoichiometric composition 3Y₂O₃·5AI₂O₃, were prepared by a sol-gel technique and dried by supercritical extraction with CO₂. Powders were produced by lightly grinding the dried gels. Crystallization of the powder occurred at 900°C and within the limits of detection, the X-ray diffraction pattern of the crystallized material was identical to that of the stoichiometric composition. Powder compacts with a green density of 0.50 of the theoretical were sintered to nearly full density in O₂ during constant heating rate sintering at 5 °C min^–1 to 1600 °C. This is better than the density obtained with powders from a similar gel dried conventionally (by evaporation of the liquid) and considerably better than that obtained with powders prepared by solid state reaction. The room temperature flexural strength and fracture toughness of the material fabricated from the supercritically dried gels were 190 MPa and 2.2 MPa.m^1/2, respectively. These strength and fracture toughness values are higher than those reported in other studies for YAG produced by the sintering route.     

Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y₂O₃, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO₃ and Al₂O₃ will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y₂O₃-77 mol% Al₂O₃ and 1702±7° C. YAlO₃ (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO₃ formed during metastable solidification transforms to YAG in the presence of Al₂O₃ at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.     

An investigation of the synthesis,consolidation and mechanical behaviour of Al 6061 nanocomposites reinforced by TiC via mechanical alloying

Nanostructured Al 6061–x wt.% TiC (x = 0.5, 1.0, 1.5 and 2.0 wt.%) composites were synthesised by mechanical alloying with a milling time of 30 h. The milled powders were consolidated by cold uniaxial compaction followed by sintering at various temperatures (723, 798 and 873 K). The uniform distribution and dispersion of TiC particles in the Al 6061 matrix was confirmed by characterising these nanocomposite powders by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The mechanical properties, specifically the green compressive strength and hardness, were tested. A maximum hardness of 1180 MPa was obtained for the Al 6061–2 wt.% TiC nanocomposite sintered at 873 K, which was approximately four times higher than that of the Al 6061 microcrystalline material. A maximum green compressive strength of 233 MPa was obtained when 2 wt.% TiC was added. The effect of reinforcement on the densification was studied and reported in terms of the relative density, sinterability, green compressive strength, compressibility and Vickers hardness of the nanocomposites. The compressibility curves of the developed nanocomposite powders were also plotted and investigated using the Heckel, Panelli and Ambrosio Filho and Ge equations.     

Optical absorption and nonlinear transmission of tetrahedral V (d) in yttrium aluminum garnet

The saturation of the optical absorption in V³⁺ : YAG crystal is investigated. The absorption cross section of tetrahedral V³⁺ at 1.08 μm is estimated to be 8.2±2.5x10^-18 cm². Q-switching and passive mode-locking for a number of solid state lasers with wavelengths at 747 nm, 780 nm, 1.06 μm and 1.34 μm have been obtained with a V³⁺ :YAG saturable absorber.     

The response of yttrium aluminum garnet (YAG) grains and grain boundaries to nanoindentation

A systematic investigation on the mechanical response of the interior grain and grain boundary of yttrium aluminum garnet is carried out using nanoindentation and microstructure characterization. The nano-hardness is calculated, while the residual indent is analyzed in terms of surface profiles, plastic deformation and elastic recovery. The results show that the nano-hardness at the center of grain is higher than that in the grain boundary region. All indents display an anisotropic elastic recovery, mainly occurring in the loading direction. The grain boundary has a better plasticity than the center of grain, which is probably due to a weaker inter-atomic bonding force at the grain boundary rather than the formation of glassy phase at intergranular region. In addition, indentation size effect in the center of grain and at grain boundary region are determined by the analysis of nano-hardness using Meyer’s law. A weaker indentation size effect in the grain boundary region is observed due to a smaller elastic recovery.     

The alloying behaviour and mechanical properties of polycrystalline Ni3Al (γ′ phase) with ternary additions

A survey of the results from a variety of techniques has shown that the substitution behaviour of alloying additions is primarily determined by electronic considerations. Thus, Si, Ti, V, Mn, Nb, Hf and Ta substitute for aluminium; Co and Cu substitute for nickel and Cr, Fe, W, and Mo substitute for both species. A number of conclusions have been drawn from an analysis of compression test data, the most significant of which was that the mechanical properties of γ′ depended on both the substitution behaviour of the alloying addition and the degree of non-stoichiometry. Considerable strengthening is only obtained when (1) the alloying addition substitutes for aluminium and has a large size misfit parameter, and (2) the alloy is aluminium-rich or stoichiometric.     

Effect of powder processing and alloying additions (Al/ZrB2) on the microstructure,mechanical and electrical properties of Cu

The present work elucidates the effect of powder processing conditions (milling/mixing) and conductive alloying element (Al: aluminium) and ceramic (ZrB₂: zirconium diboride) reinforcement addition on the densification, microstructure and electrical conductivity of copper (Cu) processed via hot pressing route. Disregard of alloying element/reinforcement/content or powders preparation method, the density of Cu materials varied between 92.16 and 99.76% ρ_th (theoretical density) after hot pressing at a low temperature of 500 °C. In case of Cu-Al alloys, the powder processing method significantly influenced its microstructure and conductivity. Particularly the Cu-Al alloys processed using mixed powders consisted of various phases Cu, α-Cu, γ₁ (Cu₉Al₄), δ (Cu₃Al₂), ζ₁ (Cu₄Al₃), η₂ (CuAl) and θ (CuAl₂) and the Cu alloys prepared using milled powders composed of either only α-Cu or α-Cu and γ₁ (Cu₉Al₄) phases (depending on the Al content). Whereas, only Cu and ZrB₂ phases were observed with the Cu-ZrB₂ composites processed using either milled or mixed powers. In case of Cu-Al alloys, the hardness (0.88–3.41 GPa) and strength (540.30–1120.18 MPa) of Cu increased with the addition of Al. Interestingly, the hardness (0.88–2.55 GPa) and strength (508.50–970.60 MPa) of Cu increased upto 5 wt% ZrB₂ and then they lowered with further addition of ZrB₂. In particular, the hardness and strength of Cu-ZrB₂ composites are lower than Cu-Al alloys reflecting the effectiveness of solid solution strengthening in the Cu alloys as compared to dispersion strengthening mechanism in Cu composite. The pure Cu prepared using milled powders exhibited low conductivity (75.70% IACS) than Cu processed using as-received/un-milled powders (97.00% IACS). Also, the Cu-ZrB₂ composites measured with better electrical conductivity than Cu-Al alloys. Depending on the milling conditions and alloying/reinforcement amount, the conductivity of Cu-ZrB₂ composites varied between 44.10 and 88.70% IACS.     

Conduction mechanism and magnetic behavior of dysprosium strontium iron garnet (DySrIG) nanocrystals

Garnet nanoparticles Dy_2.8Sr_0.2Fe₅O₁₂ (DySrIG) were prepared by citrate auto-combustion method and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and differential thermal analysis (DTA). The suitable formation of this garnet in single phase was at 1100 °C with crystallite of size 95 nm. The Curie temperature of DySrIG is obtained at 610 K. The effective magnetic moment μ_eff was calculated experimentally and theoretically and they are compatible with each other. The dielectric constant ?′ increases from order 10² at room temperature to 10⁴ at 850 K passing by four transition temperatures. The temperature dependence of the resistivity of DySrIG at different frequencies (f) 100 kHz ≤ f ≤ 5 MHz indicates the presence of 5 transition temperatures which are slightly different from those of ?′ data. The resistivity data are frequency independent at f < 1 MHz. The transition height is decreased by increasing the temperature from ≈5 MΩ cm at 320 K and 200 kHz to ≈20 Ω cm at 700 K. Accordingly, Dy_2.8Sr_0.2Fe₅O₁₂ (DySrIG) is recommended for the use in phase shifter, circulators and microwave applications.     

Microstructure and thermoelectric performance evaluation of p-type (Bi,Sb)2Te3 materials synthesized using mechanical alloying and spark plasma sintering process

Journal of Materials Science: Materials in Electronics - In this paper, three p-type thermoelectric compounds, namely Bi0.5Sb1.5Te3, Bi0.3Sb1.7Te3, and Bi0.2Sb1.8Te3 were manufactured by mechanical...