Structural and electrical properties of FeMg0.7Cr0.6Co0.7−xAlxO4 (0 ≤ x ≤ 0.3) thick film NTC thermistors

The microstructure and electrical properties of FeMg_0.7Cr_0.6Co_0.7−xAl_xO₄ (x = 0, 0.1, 0.2, and 0.3) thick film negative temperature coefficient (NTC) thermistors, fabricated by screen printing, were studied. The sintered FeMg_0.7Cr_0.6Co_0.7−xAl_xO₄ bodies were the solid solutions of the constituent oxides along with a small amount of MgAl₂O₄. The electrical resistivity increased with sintering temperature mainly because of an increase in the amount of insulating MgAl₂O₄ resulting from the partial decomposition of the solid solutions during sintering. In addition, the added Al₂O₃ led to a significant increase in the resistivity. The effect of the sintering temperature and Al₂O₃ on the electrical properties of the NTC thermistors was discussed.     

Bulk nucleated fine grained mono-mineral glass-ceramics from low-silica fly ash

Cr₂O₃-nucleated fine grained mono-mineral glass-ceramics of augite were produced from low-silica fly ash and additives of SiO₂, Al₂O₃ and MgCO₃, via two steps of heat treatment for nucleation and crystal growth. The starting glass approached the composition of CaMg_0.75Al_0.4Fe_0.1Si_1.75O₆, derived from CaMg_0.75Al_0.5Si_1.75O₆, which belongs to diopside – Ca-Tschermak solid solutions. The influence of Cr₂O₃ (up to 0.75 wt.%) on the development of crystalline phases, the properties and the microstructure of the resultant glass-ceramics crystallized at different temperatures was investigated.     

Enhanced Electrical Resistivity of Al2O3 Addition Modified Na0.5Bi2.5Nb2O9 High‐Temperature Piezoceramics

Enhanced electrical resistivity about over two orders of magnitude for Na_0.5Bi_2.5Nb₂O₉ (NBNO) high‐temperature piezoceramics potentially used in piezoelectric sensors is achieved by the introduction of highly insulating Al₂O₃ addition. Possible pyrochlore secondary phases of two types of grains: irregular polyhedral and platelike, forming at the grain boundaries, have significant blocking effect on the electric current paths and thus lead to improved electrical resistivity. Al₂O₃‐modified NBNO ceramics show higher d₃₃ values up to 16.5 pC/N than that of pure NBNO ceramics (12.4 pC/N) because higher electric field is applied during poling process owing to enhanced electrical resistivity, whereas the T_c values keep almost constant. A compromise between high T_c, good polarizability and enhanced electrical resistivity has been achieved by suitable addition of Al₂O₃, which would shed light on how to obtain novel Aurivillius phase ferroelectrics for practical applications at high temperature.     

Effect of additives on the electrical properties of a (Ba0.92Sr0.08)TiO3-based positive temperature coefficient resistor

In the present work, the effects of the additives (BN, Y₂O₃+Nb₂O₅, Sb₂O₃+Nb₂O₅, and Cr) on the electrical properties of (Ba_0.92Sr_0.08)TiO₃-based positive temperature coefficient resistor (PTCR) were investigated. It was noted that the addition of BN lowered the sintering temperature of the material system studied. A proper amount of BN could enhance the PTC effect, but the room-temperature resistivity would increase with increasing the content of BN. The addition of double donor Sb₂O₃+Nb₂O₅ could lead to a lower room-temperature resistivity than that of Y₂O₃+Nb₂O₅. Metal Cr was also added to further reduce the resistivity. The low room-temperature resistivity and PTC behavior was achieved by controlling both the metallic Cr content and the process of reducing sintering and oxidation treatment.     

Glass-ceramics with internally crystallized pyrochlore for the immobilization of uranium wastes

In this work, pyrochlore glass-ceramics (GCs)for uranium immobilization were synthesized by a pretreatment and melting-heat treatment method. The effects of different uranium contents on the phase composition, microstructure, uranium valence and chemical durability of the glass-ceramics were systematically investigated. With increasing uranium content, pyrochlore was always the dominant phase internally crystallized in the glass. CaAl₂Si₂O₈, UTi₂O₆ and U₃O₈were gradually observed as minor phases in the glass-ceramics. The growth of pyrochlore enriched with uranium at different uranium contents and the overall distribution of uranium in the glass-ceramics were revealed by SEM. The uranium valence states in the samples were confirmed by XPS to be a mixture of tetravalent and hexavalent states. In addition, the GC3 sample with a high uranium waste content of 16.96 wt% exhibited a low uranium leaching rate.     

An investigation of angle-dependent optical properties of multi-layer structure pigments formed by metal-oxide-coated mica

The angle-dependent pigments with mica/TiO₂/Fe₂O₃, mica/TiO₂/Cr₂O₃ and mica/TiO₂/Co₂O₃ three-layer structure and with mica/TiO₂/Cr₂O₃/Fe₂O₃, mica/TiO₂/SiO₂/Fe₂O₃ and mica/TiO₂/SiO₂/Co₂O₃ four-layer structure were both prepared by the conventional wet chemical method. The obtained pigments were formed by precipitating different ions, such as Cr³⁺, Fe³⁺, Co²⁺ and SiO₃²⁻ on the pretreated substrates, mica titanium. The comparative optical effects of three-layer and four-layer structure pigments are investigated in detail by using the five angles' spectrophotometer. The results show that the angle-dependent effect and reflection rate of the pigment were improved by the increasing coated layers from three to four-layer structure. The possible mechanism was also discussed in this paper.     

Effects of B2O3 substitution for Al2O3 on the crystallization and properties of translucent mica glass-ceramics

The effects of substituting Al₂O₃ with B₂O₃ on the structure, crystallization, mechanical properties, thermal properties and optical properties of translucent mica glass-ceramics were thoroughly investigated. The results demonstrated that the addition of 0.5 wt% B₂O₃ was optimum for glass precipitation, which increases the crystallinity of glass-ceramics and provides good translucency. When the content of B₂O₃ was greater than 0.5 wt%, both crystallinity and translucency decreased noticeably. The replacement of B₂O₃ for Al₂O₃ had no influence on the type of crystal phase and the precipitation of tetrasilicic fluoromica with non-stoichiometric ratio, while it did have an effect on the crystallinity and structure. The crystal sizes of glass-ceramics were in the nanoscale range and the transmittance test results indicated that they exhibit excellent translucency.     

Effects of Er3+ and/or Cr3+ doping on crystallization activation energy and fluorescence properties of transparent ZnGa2O4 glass-ceramics

Er³⁺ and/or Cr³⁺ doped transparent ZnGa₂O₄ glass-ceramics were successfully obtained by one-step heat treatment. The results showed that Er³⁺ ions can enrich around ZnGa₂O₄ crystal to reduce the crystallization activation energy and promote the growth of ZnGa₂O₄ crystal. Cr³⁺ ions may successfully occupy the Ga³⁺ sites in the ZnGa₂O₄ lattice but will increase crystallization activation energy and inhibit the growth of the ZnGa₂O₄ crystal. Before and after crystallization, the coordination-field intensity of Cr³⁺ ions increased from 2.17 to 2.86, resulting in the peak position of its emission spectra moving from 850 to 688 nm. By excitation at 378 nm, the precursor glass co-doped with Er³⁺ and Cr³⁺ ions only showed the characteristic emission peaks belonging to Er³⁺ ions. After heat treatment, the characteristic emission peaks belonging to Er³⁺ and Cr³⁺ ions existed simultaneously, and the emission color changed from green to yellow. By excitation at 980 nm, there were only characteristic emission peaks belonging to Er³⁺ ions of the Er³⁺/Cr³⁺ co-doped glasses before and after heat treatment. The results showed that the Er³⁺ and/or Cr³⁺ doped ZnGa₂O₄ glass-ceramics have adjustable luminescence ability and show potential application value in the field of luminescence display.     

Boroaluminosilicate glass-ceramics containing mullite-type Cr3+:Al4B2O9 nanocrystals with broadband near-infrared luminescence

Multi-component silicate glass is an ideal matrix for fabricating glass-ceramics because of its excellent physical-chemical stability and high optical transmittance. In this paper, a series of Cr³⁺ doped multi-component silicate glasses were designed for the preparation of glass-ceramics that crystalizes mullite-type Cr³⁺:Al₄B₂O₉ nanocrystals. When excited at 450 nm, the obtained GCs exhibit a broadband NIR luminescence band covering a spectral region from 650 to 1200 nm. Two different crystallographic sites of Cr³⁺ in Al₄B₂O₉ nanocrystal are considered to account for the observed broadband luminescence. Due to the controllable size and uniformly dispersion of precipitated nanoparticles, this boroaluminosilicate glass-ceramic could find potential applications as monolithic near-infrared light sources in solid-state light emitting devices.     

Effect of Fe2O3 content on the electrical resistivity of aluminous porcelain applied to electrical insulators

In general, the presence of Fe₂O₃ is claimed to reduce the insulating behavior of electrical porcelains, even if a rigorous proof has not yet been reported. The purpose of this study was to evaluate the real influence of Fe₂O₃ content on the electrical resistivity of a standard aluminous porcelain widely used in insulators. The electrical resistivity for the composition with 3 wt% Fe₂O₃ was higher than those found for standard aluminous porcelain, which was discussed in terms of the concentration of glassy and mullite phases. A reduction in the electrical resistivity was only observed in porcelain samples containing over 3 wt% Fe₂O₃. The presence of hematite phase was considered responsible for this reduction. These results suggest that low-cost raw materials with greater Fe₂O₃ content should not extensively affect the insulating properties and could therefore be used in manufacturing aluminous electrical porcelain.     

Processing of transparent glass-ceramics by nanocrystallisation of LaF3

Transparent glass-ceramics have been prepared by heat-treating oxyfluoride glasses in the Na₂O–Al₂O₃–SiO₂–LaF₃ system. The nanocrystallisation of LaF₃ was achieved by controlling time and temperature parameters. Glasses and glass-ceramics were characterised by dilatometry, DTA, XRD and TEM. The mean crystal size (<20 nm) and the crystal fraction increase with the temperature of heat treatment, while they reach a maximum at about 20 h at a temperature close to T_g. The crystallisation of phases containing glass modifier elements as well as F anions leads to the increase in the viscosity of the remaining glass matrix. Phase separation occurs in glass-ceramics depending on the glass composition which affects nanocrystallisation.     

The direct experimental observation and solidification mechanism of heavy metals in blast furnace slag glass-ceramics

Glass-ceramics with high performance and high crystallinity was produced using rare earth blast furnace slag (REBFS) as the primary raw material, along with composite nucleating agents Fe₂O₃ and Cr₂O₃. The migration pattern, distribution characteristics, and solidification properties of heavy metals in glass-ceramics were investigated by the controlled heat treatment. The findings indicated that the heat treatment temperature has a significant effect on the distribution and color of heavy metals in glass-ceramic. In the crystallized glass-ceramic, spinel acts as the crystal core for heavy metal solidification, Cr, Cu, and Mn can exist stably in the spinel by the solid solution. The glass-ceramic was expected to be used as a decorative building material.     

The effect of Cr2O3 as a nucleating agent in iron-rich glass-ceramics

In this work, the effect of Cr₂O₃ as a nucleating agent, in iron rich glasses has been investigated by means of DTA, XRD and density measurements. By Cr₂O₃ addition, from 0·4 to 1·0 wt%, a lowering of the crystallisation peak temperature resulted in the DTA trace, the maximum effect corresponding to 0·7 wt%. By evaluating the degree of crystallisation of the glass at 0·7 wt% Cr₂O₃, the highest efficiency in the nucleation process also corresponds. The optimum values for the nucleation and crystallisation time and temperature, determined for 0·7 wt% Cr₂O₃ addition, have been 70 min at 630°C and 30  min at 800°C. The crystalline phases formed at different thermal treatment temperatures of the parent glass have been investigated by XRD; the spinel is the only phase after the nucleation; pyroxene is the major phase after the crystallisation. The results of this study have highlighted that a small percentage of Cr₂O₃ strongly affects the spinel formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallisation of high iron content glasses. ©     

Crystallization characteristics and physico-chemical properties of indium-containing lithium iron aluminosilicate glasses

The crystallization characteristics, microhardness and chemical durability of glasses based on the Li₂O–Al₂O₃–Fe₂O₃ (In₂O₃)–SiO₂ system were investigated. The present work aims to study the effect of In₂O₃/Fe₂O₃ replacements on the crystallization and the resulting microstructure of the glass-ceramics. Very fine grained microstructure was mostly developed by In₂O₃/Fe₂O₃ replacements in the glass-ceramics. The crystalline phases formed after controlled heat-treatment of the glasses were β-eucryptite ss, lithium meta- and di-silicate as well as two forms of pyroxene phases (LiFeSi₂O₆ and LiInSi₂O₆). The microhardness values of the resulting glass-ceramics were decreased with In₂O₃/Fe₂O₃ replacements while the chemical durability was improved. The obtained data were correlated to the type of the crystalline phases formed and the resulting microstructure. Promising glass-ceramic materials of fine microstructure and good chemical durability could be obtained.     

Nanosized cordierite–sapphirine–spinel glass-ceramics from natural raw materials

Inexpensive nanosized sintered cordierite glass-ceramic was prepared from quartz sand, kaolin, and magnesite. The addition of nucleation catalysts, such as TiO_2, Cr₂O_3, and admixed TiO₂–Cr₂O₃, was tested in the cordierite base glass. Cordierite, sapphirine, spinel, magnesium aluminium silicate, and cristobalite were developed using the crystallisation process. These glass-ceramics have ultra-fine grain sizes with nanorounded crystals measuring less than 200 nm, particularly in the Cr₂O₃-containing samples. Due to its different crystalline phases, the new glass-ceramics varied in hardness from 6374 to 8139 MPa and had coefficients of thermal expansion (CTE) from 0.83 to 6.89×10⁻⁶ °C⁻¹. In glass-ceramic samples, the spinel and sapphirine imparted high CTE (from 6.89 to 5.31×10⁻⁶ °C⁻¹) and hardness values (from 8139 to 7894 MPa), whereas cordierite provided lower CTE (from 0.83 to 2.66×10⁻⁶ °C⁻¹) and hardness values (from 7453 to 6374 MPa).     

Effect of ZnO content on the physical,mechanical and chemical properties of glass-ceramics in the CaO–SiO2–Al2O3 system

The objective of this study was to evaluate the effect of ZnO content on the physical, mechanical and chemical properties of CaO–Al₂O₃–SiO₂ (CAS) glass-ceramics produced from Colombian wastes, such as fly ash, granulated blast furnace slag and glass cullet. The CaO/SiO₂ molar ratio of the mixtures was held constant (0.36). ZnO was added to the mixtures in proportions of 4, 7 and 10 wt%. The glass-ceramics were produced by the controlled crystallization of a parent glass. The values of crystallization temperature (T_p) show a fall up to 7 wt% and then shoots up with 10 wt% concentration of ZnO, but in general, ZnO addition lowers the temperature required for the formation of crystalline phases. In general, anorthite (CaAl₂Si₂O₈) is the main phase observed in all heat treated samples, in addition to albite (Na(AlSi₃O₈)) and labradorite (Na_0.45 Ca_0.55 Al_1.55 Si_2.45 O₈). The crystalline phases hardystonite (Ca₂ZnSi₂O₇) and willemite (Zn₂SiO₄) were also identified in the samples with 7 and 10 wt% ZnO. The densities of the glass-ceramics were between 2658 and 2848 kg/m³, and it was found that ZnO helps to increase the density of glass-ceramics. The elastic modulus was in the 100–105 GPa range, the fracture toughness was between 0.45 and 0.64 MPa m^1/2, and the Vickers microhardness was between 632 and 653 MPa. With regards to the durability, the weight loss of the glass-ceramics immersed in alkaline solution (NaOH) did not exceed 1.5 wt% after immersion for 6 h at 80 °C. The results of this study confirm that the vitrification process is a favorable option to utilize these industrial wastes.     

AC Impedance Spectroscopy of CaF2‐doped AlN Ceramics

The electrical conductivity of CaF₂‐doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF₂‐doped AlN ceramics were sintered with a second additive, Al₂O₃, added to control the electrical conductivity. The effects of calcium fluoride (CaF₂) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF₂ additive have no secondary phases at grain boundaries. Addition of Al₂O₃ caused the formation of amorphous phases at grain boundaries. Addition of Al₂O₃ to CaF₂‐doped AlN ceramics at temperatures 200°C–500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al₂O_3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.     

Preparation of glass-ceramics with diopside as the main crystalline phase from low and medium titanium-bearing blast furnace slag

Because of the low content of TiO₂, the utilization of low and medium Ti-bearing blast furnace slag is a difficult problem. In this study, glass-ceramics with diopside as the main crystalline phase was made by using TiO₂ in the blast furnace slag as the nucleating agent and adding additional elements of TiO₂, Fe₂O₃ and Cr₂O_3. The effects of these three nucleating agents on the phase composition and structural properties of the prepared glass-ceramics were evaluated by DSC, XRD, and SEM to determine their optimal dosage. Research results show that the suitable mass percentages of the TiO₂, Fe₂O₃ and Cr₂O₃ crystal nucleation agents of the glass-ceramics are 7.69%, 0.96%, and 2.88%, respectively. The prepared glass-ceramics have excellent physical and chemical properties, e.g., a bending strength of 114.74 MPa, a bulk density of 2.77 g/cm³, a water absorption rate of 0, and a mass-loss rate of only 0.085%.     

Grain boundary blocking effects in Sm/Yb-doped AlN ceramics

The effect of microstructural changes on the electrical and thermal properties of AlN ceramics is studied in terms of cation size and nature of sintering aids (i.e. Sm₂O₃ and Yb₂O₃) in AlN ceramics. It is revealed that the addition of Yb₂O₃ to Sm-bearing AlN ceramics results in 80 % reduction of thermal conductivity with an increase of the grain boundary resistivity that is one order of magnitude larger than for the sample without Yb₂O₃. Additionally, the grain boundary/grain resistivity ratio is significantly increased, when the Sm₂O₃ sintering aid is employed instead of Yb₂O₃, for which the secondary phases at the grain boundaries and the triple junctions are responsible for the increase in the electrical resistivity. The microstructural investigations confirm the tendency of the secondary phase to segregate at the triple junctions in Sm-containing AlN ceramics while it is grain boundaries that are favored as segregation site in the case of Yb.     

Influence of Cr2O3 on highly nonlinear properties and low leakage current of ZnO–Bi2O3 varistor ceramics

ZnO–Bi₂O₃–Sb₂O₃–Co₂O₃–MnO₂–xCr₂O₃ (ZBSCM–xCr₂O₃, 0≤x≤0.6 mol%) varistors were fabricated through the conventional solid state method, and the effects of Cr₂O₃ on the microstructures and electrical properties were investigated. Results showed that the secondary phases CrBi₁₈O₃₀ and Co₂Cr_0.5Sb_0.5O₄ emerged when x ranges from 0.2 to 0.4. In these compositions, Cr₂O₃ acted as a donor and decreased the electrical properties of ZBSCM. For samples with x=0.5, the secondary phases transformed to MnCr₂O₄ and the electrical properties increased significantly: the nonlinear coefficient α sharply increased up to 80.71 and the barrier height ϕ_b reached 3.88 eV. This indicates that the donor effect of Cr₂O₃ disappeared. In addition, with the increase of Cr₂O₃, the average grain size of ZnO decreased from 7.48 μm to 5.46 μm, which in turn resulted in an increase of breakdown voltage E_1mA from 216.17 V/mm to 362.50 V/mm. Besides, all the samples showed the low value of leakage current of lower than 0.1 μA. This varistor might be a promising candidate for highly effective applications.