Enhanced negative thermal expansion of (KMg)3+-substituted Fe2Mo3O12 ceramics

Herein, we report the solid-state synthesis of (KMg)_xFe_2-xMo₃O₁₂ (0 = x ≤ 1.5) ceramics. Phase composition, crystal structure, morphology, phase transition and thermal expansion behavior of the (KMg)_xFe_2-xMo₃O₁₂ ceramics were investigated by XRD, Raman, XPS, HRTEM, EDX, SEM, TMA and high-temperature XRD. Results indicate that (KMg)³⁺ dual-cations have successfully replaced Fe³⁺ in Fe₂Mo₃O₁₂ ceramics and single-phase monoclinic (KMg)_xFe_2-xMo₃O₁₂ ceramics were prepared for 0.25 = x ≤ 1. (KMg)³⁺ introduction can increase the density of (KMg)_xFe_2-xMo₃O₁₂ ceramics and effectively improve their negative thermal expansion (NTE) performance. In addition, the phase transition temperature (Tc) of Fe₂Mo₃O₁₂ was reduced from 508.1 °C to room temperature with the increase of (KMg)³⁺-substitution. Monoclinic KMgFeMo₃O₁₂ ceramics was observed to show stronger NTE in a wider temperature range of 30–700 °C for the first time. Its corresponding coefficient of thermal expansion (CTE) is as high as ?17.21 × 10^?6 °C^?1. The distortion of [FeO₆/MgO₆] polyhedra in (KMg)_xFe_2-xMo₃O₁₂ caused by (KMg)³⁺-substitution contributed to the stronger NTE.     

Double cation (KMg)3+ co-substitution to regulate negative thermal expansion property of ln2W3O12

To improve the negative thermal expansion (NTE) performance of ln₂W₃O₁₂, a novel series of NTE (KMg)_xln_2-xW₃O₁₂ ceramics were fabricated via the solid-state method. The effects of (KMg)³⁺ substitution on the phase composition, microstructure and thermal expansion property of the ln₂W₃O₁₂ ceramics were characterized using X-ray diffraction (XRD), Raman spectrometer (Raman), X-ray photoelectron spectrometer (XPS), scanning electron microscopy (SEM), transmission electron microscope (TEM) and thermal mechanical analyzer (TMA). Results indicate that (KMg)³⁺ can partially replace In³⁺ in In₂W₃O₁₂ and form a new phase K_xMg_xln_2-xW₃O₁₂ with monoclinic symmetry. For x = 0.5, pure monoclinic (KMg)_0.5ln_1.5W₃O₁₂ ceramics is prepared and shows strong NTE. Its coefficient of thermal expansion is −7.89 × 10⁻⁶ °C⁻¹ in 30–700 °C, in addition, no phase transition was observed over the entire testing temperature range. These research results indicate that double cations co-substitution is an effective strategy to improve the NTE property of ln₂W₃O₁₂ through crystal structure modulation. This strategy could be extended to the performance modulation of other NTE materials.     

复合氧化物材料的负热膨胀机理

介绍了相转变、桥氧原子的横向热振动、刚性多面体的旋转耦合、固体内压转变、相界面弯曲、阳离子迁移等六种模式的负热膨胀机理。并对其应用前景和发展趋势进行了预测     

Electrospinning synthesis and negative thermal expansion of one-dimensional Sc2Mo3O12 nanofibers

Orthorhombic Sc₂(MoO₄)₃ nanofibers have been prepared by ethylene glycol assisted electrospinning method. The effects of annealing temperature, precursor concentration, spinning distance and solvent on the preparation of Sc₂(MoO₄)₃ nanofibers were characterized by XRD, SEM, HRTEM, EDX and high-temperature XRD. XRD analysis shows as-prepared nanofibers are amorphous. Orthorhombic Sc₂(MoO₄)₃ nanofibers can be fabricated after annealing at different temperatures in 500–800 °C for 2 h. The crystallinity of Sc₂(MoO₄)₃ nanofibers improves and the nanofiber diameter decreases gradually as the annealing temperature increases. However, the nanofiber structure was destroyed at the annealing temperature above 700 °C. Higher precursor concentration results in a slight increase of diameter and decrease in destroying temperature of Sc₂(MoO₄)₃ nanofibers. Spinning distance also affects the diameter of nanofibers, and the nanofiber diameter decreases as the distance increases. One-dimensional orthorhombic Sc₂(MoO₄)₃ nanofibers exhibit anisotropic negative thermal expansion. In 25–700 °C, the coefficients of thermal expansion (CTE) of α_a, α_b and α_c are ?5.81 × 10^?6 °C^?1, 4.80 × 10^?6 °C^?1 and -4.33 × 10^?6 °C^?1, and the α_l of Sc₂(MoO₄)₃ nanofibers is ?1.83 × 10^?6 °C^?1.     

Negative thermal expansion of (Al1/3Fe1/3Cr1/3)2(Mo1/2W1/2)3O12 (AFCMW) and low thermal expansion of AFCMW-(Co1/2Ni1/2)(Mo1/2W1/2)O4 with high entropy

A₂Mo₃O₁₂ (A-Al, Fe, Cr) have large negative thermal expansion (NTE) coefficients and structural stability but high phase-transition temperatures (PTTs). Herein, we prepared (Al_1/3Fe_1/3Cr_1/3)₂(Mo_1/2W_1/2)₃O₁₂ (AFCMW), and found it to have a low NTE coefficient and a low PTT. Furthermore, combination of AFCMW with (Co_1/2Ni_1/2)(Mo_1/2W_1/2)O₄ (CNMW) afforded an AFCMW–CNMW composite with a low thermal expansion (LTE). We determined that the PTT reductions in A₂Mo₃O₁₂ are largely due to the high-entropy effect resulting from the introduction of different ions into its A and M sites. Moreover, we found that the low LTE of the AFCMW–CNMW composite is attributable to the opposite thermal expansion behaviours of AFCMW and CNMW. We suggest that the suppressed thermal expansion during the phase transition process of the AFCMW–CNMW composite could be derived from the high-entropy effect resulting from its increased diversity of polyhedra, the influence of Co²⁺ and Ni²⁺ dopants, and CNMW-induced lattice distortion.     

Synthesis,structure and luminescence of a high-purity and thermal-stable Sr9LiMg(PO4)7: Eu3+ red phosphor

Eu³⁺-activated Sr₉LiMg(PO₄)₇ phosphors, which presented bright red emissions mainly from the ⁵D₀→⁷F₂ transition of Eu³⁺ ions upon the near-ultraviolet excitation, were successfully synthesized in ambient atmosphere. The crystal structure, phase constitution, photoluminescent behaviors, decay time, internal quantum efficiency and thermal stability of the resultant phosphors were investigated in detail. Eu³⁺ ions are found to tend to occupy multiple Sr²⁺ sites, which are 7, 8 and 10-coordinated. The optimal doping concentration is 7 mol% and the electrical multipolar interaction contributed to the non-radiative energy transfer between Eu³⁺ ions in Sr₉LiMg(PO₄)₇ host lattices. Temperature-dependent PL spectra indicated Sr₉LiMg(PO₄)₇: Eu³⁺ possess excellent emission and color stability at elevated temperature. Fabricated single-chromatic LED prototype emit bright red light under 20 mA bias current, which demonstrates that Sr₉LiMg(PO₄)₇: Eu³⁺ phosphor is of great potential as converted phosphor in NUV LED application.     

Expanding negative thermal expansion range of ZrMnMo3O12 to cover room temperature by introducing V5+

Solid solutions of Zr_1+xMn_1-xMo_3-2xV_2xO₁₂ (0 ≤ x ≤ 0.5) are developed with reduced phase transition temperature (from 362 to 160 K) by introducing V⁵⁺ into ZrMnMo₃O₁₂. Zr_1+xMn_1-xMo_3-2xV_2xO₁₂ adopt monoclinic (P2₁/a) and orthorhombic (Pbcn) structure at room temperature (RT) for x ≤ 0.1 and x ≥ 0.2, respectively. The formation of bond V–O induces a larger average effective negative charge on oxygen to enhance the repulsive force between them and then strengthens the bond of Mo–O, which reduces the phase transition temperature due to the reduction in effective electronegativity and expands negative thermal expansion (NTE) range covering RT. NTE property in a wide temperature range (from 160 to 673 K) for Zr_1.5Mn_0.5Mo₂VO₁₂ is realized, implying great potential for applications. The NTE property of the materials is induced by low-frequency phonons.     

Improving room temperature negative thermal expansion performance of Fe2-xScx(MoO4)3

Negative thermal expansion (NTE) performance of Fe₂(MoO₄)₃ is only found in a high-temperature range due to its monoclinic-to-orthorhombic (M-O) phase transformation temperature (PTT) at 503.5°C. To stabilize the orthorhombic phase of Fe₂(MoO₄)₃ at room temperature, a series of Fe_2-xSc_x(MoO₄)₃ (0≤x≤1.5) (abbreviated as F_2-xS_xM) were fabricated via solid-state reaction. Results indicate that the M-O PTT of Fe₂(MoO₄)₃ is successfully reduced from 503.5°C to 34.5°C by A-site cation substitution of Sc³⁺. The regulation mechanism is considered to be the decrease in electronegativity of (Fe_2-xSc_x)⁶⁺ in F_2-xS_xM. Both variable temperature X-ray diffraction (XRD) and thermal mechanical analysis (TMA) analysis results indicate that F_0.5S_1.5 M exhibits anisotropic NTE in 100–700°C. The results indicate that it can effectively improve the densification of Sc-substituted F_0.5S_1.5 M ceramics by two-step calcination process. Furthermore, higher second-step calcination temperature is beneficial for the formation of single-phased orthorhombic F_0.5S_1.5 M. The NTE response temperature range of F_0.5S_1.5 M ceramics second-step sintered at 1000°C is broadened to 30–600°C, and the corresponding coefficient of thermal expansion is -5.74 × 10⁻⁶°C⁻¹. The ease in the proposed design and preparation method makes NTE F_0.5S_1.5 M potential for a wide range of applications in precision mechanical, electronic, optical, and communication instruments.     

Spectral dependency of Eu2+-activated silicate phosphors on the composition for LED application

An Sr₂SiO₄-Ba₂SiO₄ material system doped by Eu²⁺ was studied for light emitting diodes (LEDs) application. The main concern was the precise control of excitation and emission spectra for maximum light yield and color coordinate, which was carried out by changing the composition of the alkaline earth ions in host lattice. The Sr₂SiO₄ : Eu-Ba₂SiO₄ : Eu system was found to be excellent for white LED applications with excitation in the 380–465 nm region. Especially, the yellow light intensity from (Sr,Ba)SiO₄ : Eu phosphors was comparable to YAG : Ce phosphors in case of blue LED excitation.     

SrLaNaTeO6:Eu3+ red-emitting phosphors with high luminescence efficiency and high thermal stability for high CRI white LEDs

A novel single-phase trivalent europium activated red-emitting SrLaNaTeO₆ phosphor was first synthesized in a process of traditional high-temperature solid-state. The phase purity, morphology, and spectroscopy of the prepared phosphor were analyzed. Under 395 nm excitation, the photoluminescence (PL) spectra of the SrLaNaTeO₆:Eu³⁺ products mainly contained five dominant sharp peaks. The intense red emission peak at 615 nm was the typical ⁵D₀→⁷F₂ electric dipole transition of Eu³⁺. The optimum product of high quenching concentration was the SrLaNaTeO₆:0.90Eu³⁺, which reached a high internal quantum efficiency (IQE) of 90.6%. The SrLaNaTeO₆:0.90Eu³⁺ was estimated to have R_c of 6.57 Å and possessed high color purity of 100.0%. The phosphors exhibited excellent thermal stability and high activation energy (E_a = 0.29 eV). The prepared white light-emitting diode (WLED) had a high color rendering index (CRI) R_a of 92 and a low correlated color temperature (CCT) of 5008 K. In conclusion, the phosphors have potential as red components for WLEDs.     

C/SiC composite-Ti6Al4V joints brazed with negative thermal expansion ZrP2WO12 nanoparticle reinforced AgCu alloy

Brazing C/SiC composites to Ti6Al4V alloy is associated with the problem of high residual stress inducing low joining strength. To overcome this problem, negative thermal expansion Zr₂P₂WO₁₂ (ZWP) nanoparticles were introduced into AgCu brazing alloy to obtain robust C/SiC-Ti6Al4V joints. Microstructures and mechanical properties of the joints brazed with different ZWP contents were investigated. Results indicated that 3 wt% ZWP nanoparticles dispersed homogeneously among brazing seam and compatible with brazing alloy. The width of reaction layer at C/SiC side was reduced sharply. Meanwhile, the finite element analysis showed that residual stress was reduced by 52.9 MPa and stress concentration among reaction layer was eliminated. The average shear strength of the joints brazed with AgCu + 3 wt% ZWP increased to 146.2 MPa, which was 70.8% higher than that of joints brazed without ZWP.     

Designing and temperature sensing characteristics of upconversion luminescence core-shell structures with negative and positive thermal expansion

Generally, lanthanum ions doped positive expansion and negative expansion materials exhibit thermal quenching and enhancement of upconversion luminescence (UCL), respectively. Combining the UCL characteristics of positive expansion and negative expansion lattices is of importance for developing efficient temperature sensing systems. Here, positive expansion TiO₂:Yb³⁺, Er³⁺ three dimensionally ordered macroporous film was prepared by the template-assisted approach, and the Yb₂W₃O₁₂: Er³⁺ solution was filled into the TiO₂: Yb³⁺, Er³⁺ three dimensionally ordered macroporous film. After secondary sintering, the shell of negative expansion Yb₂W₃O₁₂: Er³⁺was formed on the surface of TiO₂:Yb³⁺/Er³⁺ core. Under 980 nm excitation, the red and green UCL is predominate for the spectra of TiO₂:Yb³⁺/Er³⁺ core and Yb₂W₃O₁₂: Er³⁺ shell, respectively. With the measurement temperature increasing, the green UCL from negative expansion Yb₂W₃O₁₂: Er³⁺ shell increases, while the red UCL from positive expansion TiO₂:Yb³⁺, Er³⁺ core decreases. The performance of temperature sensing was characterized by the monitoring the UCL intensity ratio between 525 nm and 660 nm. The temperature sensitivity is about 1.12% K^?1, which is larger than that of thermally coupled FIR technology. We believed that the present work is instructive for developing new generation temperature sensor.     

Anomalous thermal expansion behavior in Er1-xCaxMnO3

Perovskite Er_1-xCa_xMnO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) was synthesized using a solid-state method. Thermal expansion behavior was tested using a thermal dilatometer and high-temperature X-ray diffraction (XRD). The experimental results indicated the doping contents of Ca (x) in the Er_1-xCa_xMnO₃ have a dramatic effect on their thermal expansion behavior. The samples of Er_1-xCa_xMnO₃ (x = 0.1,0.2 and 0.25) exhibit positive thermal expansion (PTE) characteristics while Er_0.7Ca_0.3MnO₃ (x = 0.3) exhibits a negative thermal expansion (NTE) property with a thermal expansion coefficient of −3.1 × 10⁻⁶ K⁻¹ in room temperature (RT) −750 K. In addition, Er_0.6Ca_0.4MnO₃ (x = 0.4) exhibits NTE properties only at RT–500 K, and Er_0.5Ca_0.5MnO₃ (x = 0.5) exhibits PTE properties at RT–750 K. The thermal shrinkage mechanism is the Jahn–Teller effect of the Mn³⁺ ions and the double exchange of Mn³⁺–O–Mn⁴⁺ in Er_0.7Ca_0.3MnO₃. This phenomenon causes Mn–O octahedral distortion and oxygen vacancy, causing Er_0.7Ca_0.3MnO₃ to become anisotropic. This feature results in the elastic deformation of Er_0.7Ca_0.3MnO₃ during heating, which consumes the void and displays NTE at macro level.     

Anodic lithium ion battery material with negative thermal expansion

Negative thermal expansion materials will effectively counteract possible severe expansion and contraction due to the insertion and extraction of Li ions in lithium ion batteries. Herein, negative thermal expansion ZrScMo₂VO₁₂ and its carbon-coating composites are prepared as electrode material in lithium ion batteries by a heating treatment route. The galvanostatic charge/discharge process, cyclic voltammetry measurement and electrochemical impedance spectroscopy are tested to relate their thermal expansion and electrochemical properties. The initial specific capacity reaching 1062 mA h g^-1 at the current density of 0.2 A g^-1 is obtained with ideal negative thermal expansion properties. The reversible specific capacity still remains stable at 310 mA h g^-1 for that material coated with carbon after 100 cycles. The corresponding theoretical simulations and in situ XRD patterns propose a Li ion storage mechanism based on Li ion insertion process in open framework structure. As a proof-of-concept research, this work paves a way to the promising application of negative thermal expansion materials in lithium ion batteries and other energy storage systems.     

Giant negative thermal expansion in Zn2-xCuxP2O7 ceramics via microstructure effect

As a novel thermophysical behavior, negative thermal expansion (NTE) has been studied in many materials. However, rare materials have realized giant NTE, and the methods to improve NTE are lacking. Herein, a giant NTE has been achieved in Zn_2-xCu_xP₂O₇ ceramics via microstructure effect. In the Zn_1.96Cu_0.04P₂O₇ ceramic body, the linear contraction measured by dilatometry reaches to 0.9% (3ΔL/L = 2.7%) when heated from ?30 °C to 125 °C, while the intrinsic crystallographic volume contraction derived by X-ray diffraction is only 1.68%. The remarkable NTE enhancement in the ceramic sample is attributed to the microstructure effect. An apparent shrinkage of the voids has been observed by in-situ atomic force microscope (AFM). The voids with large size in the ceramic body is the key factor to enhance NTE. This is the first time to observe direct experimental evidence by AFM for microstructure effect. Microstructure effect is an effective method to produce giant NTE.     

Tunable luminescence and energy transfer properties of MgY4Si3O13: Ce3+, Tb3+, Eu3+ phosphors

The tricolor-emitting MgY₄Si₃O₁₃: Ce³⁺, Tb³⁺, Eu³⁺ phosphors for ultraviolet-LED have been prepared via a high-temperature solid-state method. X-ray diffraction, photoluminescence emission, excitation spectra and fluorescence lifetime were utilized to characterize the structure and the properties of synthesized samples. Two different lattice sites for Ce³⁺ are occupied from the host structure and the normalized PL and PLE spectra. The emissions of single-doped Ce³⁺/Tb³⁺/Eu³⁺ are located in blue, green and red region, respectively. The energy transfer from Ce³⁺ to Tb³⁺ and from Tb³⁺ to Eu³⁺ has been validated by spectra and decay curves and the energy transfer mode from Tb³⁺ to Eu³⁺ was calculated to be electric dipole-dipole interactions. By adjusting the content of Tb³⁺ and Eu³⁺ in MgY₄Si₃O₁₃: Ce³⁺, Tb³⁺, Eu³⁺, the CIE coordinates can be changed from blue to green and eventually generate white light under UV excitation. All the results indicate that the MgY₄Si₃O₁₃: Ce³⁺, Tb³⁺, Eu³⁺ phosphors are potential candidates in the application of UV-WLEDs.     

Fabrication and negative thermal expansion properties of P-substituted ZrV2O7 sintered bodies

Calcined powders of ZrV_1.2P_0.8O₇ (ZVP) and ZrV₂O₇ (ZV) were synthesized by heating gels prepared from NH₄H₂PO₄, NH₄VO₃, and ZrOCl₂ solutions at 500 and 400 °C, respectively. Dense ZV-added ZVP sintered bodies were subsequently fabricated through heating at 850 °C for 20 h, with the addition of ZrV₂O₇ as a sintering additive. The resulting material had a relative density of ca. 92%, while the relative density of pure sintered ZVP was ca. 72%. NH₄H₂PO₄ (NHP) was also added to ZVP along with ZV to obtain a molar ratio of P in NHP for V in ZV = 1. Subsequently, a single phase (NHP, ZV)-added ZVP sintered body was obtained by heating at 850 °C. Thermomechanical analysis showed that the dense ZV-added ZVP sintered body exhibited a positive thermal expansion from 25 to 150 °C and a negative thermal expansion from 150 to 500 °C.     

Preparation of In0.5Sc1.5Mo3O12 nanofibers and its negative thermal expansion property

Orthorhombic In_0.5Sc_1.5Mo₃O₁₂ nanofibers were prepared by electrospinning followed by a heat treatment. The effects of post-annealing temperatures on the phase composition, microstructure and morphology were investigated by XRD, SEM, HRTEM and XPS. Negative thermal expansion (NTE) behaviors of the In_0.5Sc_1.5Mo₃O₁₂ nanofibers were analyzed by high-temperature XRD. Results indicate that the as-prepared In_0.5Sc_1.5Mo₃O₁₂ nanofibers show an amorphous structure with smooth and homogeneous shape. The average diameter of the as-prepared In_0.5Sc_1.5Mo₃O₁₂ nanofibers is around 515 nm. Well crystallized orthorhombic In_0.5Sc_1.5Mo₃O₁₂ nanofibers could be prepared after post-annealing at 550 °C for 2 h with an average diameter of about 192 nm. The crystallinity of In_0.5Sc_1.5Mo₃O₁₂ nanofibers gradually improved with the increase of annealing temperature. However, too high post-annealing temperature leads to a damage of sample's fiber structure. The high-temperature XRD results reveal that In_0.5Sc_1.5Mo₃O₁₂ nanofibers show an anisotropic NTE, and the coefficients of thermal expansion (CTEs) along a-axis and c-axis were −5.95 × 10⁻⁶ °C⁻¹ and -3.54 × 10⁻⁶ °C⁻¹, while the one along b-axis is 5.61 × 10⁻⁶ °C⁻¹. The volumetric CTE of In_0.5Sc_1.5Mo₃O₁₂ nanofibers is −3.90 × 10⁻⁶ °C⁻¹ and the linear one is 1.3 × 10⁻⁶ °C⁻¹ in 25–700 °C.     

Solid state sintering of very low and negative thermal expansion ceramics by Spark Plasma Sintering

Lithium aluminosilicate powder precursors of compositions Li₂O:Al₂O₃:SiO₂ as 1:1:2; and 1:1:3.11 were synthesized and sintered by the Spark Plasma Sintering technique. The sintering conditions were adjusted to obtain dense ceramic materials in an attempt to avoid the presence of a glassy phase. XRD and SEM images were employed for composition and microstructure characterization. The coefficient of thermal expansion of the sintered samples was studied down to cryogenic conditions. Rietveld quantification was performed with the use of an external standard. Pure β-eucryptite of different compositions in dense ceramic bodies was obtained with a negative expansion coefficient.     

Polymer-derived ceramic tapes with small and negative thermal expansion coefficients

A polysiloxane filled with ß-eucryptite and/or SiC was used for the processing of polymer derived ceramic tapes. The combination of both fillers in varying proportions allowed to tailor the overall bulk thermal expansion and the flexural strength of the resulting composite materials simultaneously. Incorporation of SiC increased noticeably the flexural strength of the samples and influenced the phase changes resulting from the interactions between the β-eucryptite filler and the polymer derived ceramic matrix. Changes in the phase composition and changes of the unit cell parameters of β-eucryptite because of the formation of solid-solutions with silica originating from the SiO₂ constituent of the polymer derived ceramic matrix were observed by Rietveld refinement. Tapes resulting from this process possess a sufficient mechanical stability and their coefficient of thermal expansion can be adjusted from slightly positive to moderate negative values.