n. ehsani

Graphene quantum dots are a new member of the carbon family with various applications rapidly developing due to their unique properties. This research aims to obtain this valuable nanomaterial using a convenient, quick, and easy production method called carburizing. For this purpose, production from the primary source of glutamic acid was done at temperatures of 210, 220, and 230 ºC and times of 60 and 90 s. Next, the Raman test and fourier transform infrared spectroscopy, dynamic light scattering, photoluminescence, and ultraviolet-visible spectroscopy were performed to investigate the properties of graphene quantum dots of the product. The microstructure was examined by field emission escanning electron microscopy images. Also, the morphology of the surface and the distance between the planes were investigated by high resolution transmission electron microscopy. The results showed that the best sample in terms of distribution and particle size was production at 220°C for 60 s with 3 nm in particle size for over 45% of the particles. Moreover, the highest absorption intensity in this sample was appeard at the wavelength of 244 nm. An increase in the production time caused the graphene quantum dots to be coarser with greater dispersion in particle size distribution. Photoluminescence studies in the excitation wavelength range of 340-400 nm revealed the appearance of a strong peak in the sample produced at 220 ºC and calcination time 60 s at the excitation wavelength of 380 nm.

The purpose of this research was to fabricate and investigate the properties of SiC-5TiB2 nano composites reinforced by gaphene quantum dot nanoparticles via pressure less sintering method. In this way, SiC, TiB2, and graphene quantum dot raw materials were used in nanometer dimensions. First, before performing any laboratory operations, experimental samples were designed using Minitab 14 software. The design was done by the Taguchi method according to the L9 array and the parameters of the amount of gaphene quantum dot amplification in three levels were set at 0.2, 0.6, and 1 wt.% and sintering temperatures were defined as 2000, 2100, and 2200°C. The sintering process was carried out at certain temperatures in argon atmosphere for 2 h. XRD, FESEM, FTIR and Raman spectroscopy were performed. Density, micro hardness, and fracture toughness measurements were used for further investigations of physical and mechanical properties. The microstructure of the samples was also observed to determine the fracture toughness mechanisms. The results showed that the parameter of the amount of reinforcement was in the first rank of influence and the sintering temperature was in the second rank, and the best results were obtained in the sample with the amount of 0.6 wt.% of gaphene quantum dot and the sintering temperature of 2100 °C, where hardness and fracture toughness values were obtained to be 27.7 GPa and 3.3 MPa.m1/2, respectively.

In this research, the SiC-matrix composite with different amounts of TiC (0, 2.5, 5, 7.5, and 10 wt%) supplemented with additives including 4.3 wt% Al2O3 and 5.7 wt% Y2O3 were utilized to initiate the required liquid phase. The sintering process was performed using pressureless sintering at      1900 °C for 1.5 hours under argon atmosphere. The composition and microstructure of the obtained composites were analyzed using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), and Energy-Dispersive X-ray Spectroscopy (EDX). The results showed that TiC additives improved the densification of samples and impeded the growth of SiC grains. According to the phase analysis, the SiC was the main phase, while the TiC and YAG were characterized as partial phases. Additionally, due to the reaction of TiC and Al2O3, the composition of the liquid phase contained YAG and YAM. Assessments revealed that the microstructure and the final properties of composites were affected by density, produced phases and their distribution in the matrix, and grain size. According to the results, upon increasing the TiC up to 5 wt%, all the measured properties including density, hardness, elastic modulus, and fracture toughness improved and reached 97.40%, 26.73 GPa, 392 GPa, and 5.80 MPa.m1/2, respectively. However, with increasing the additives to more than 5 wt%, these properties deteriorated. Microscopic evaluations revealed that crack deflection and crack bridging mechanisms contributed to the fracture toughness of SiC ceramics.

Because of their excellent properties such as extremely high hardness, wear resistance, low specific weight, high cross section for neutron absorption and resistance to chemical attacks, boron carbide based ceramics have great potential for industrial applications such as armor sheets, rocket nozzles, cutting tools, high temperature resistant coating, neutron-absorbing materials, etc. In spite of its unique properties, boron carbide has some disadvantages like relatively low strength (200-400MPa), low sinterability and low fracture toughness which have restricted its applications. Due to the presence of strong covalent bond and low diffusion rate, densification of boron carbide above 80% of theoretical density is very difficult. To reduce these problems, a number of secondary phases were added to B4C as sintering aids. It this paper, the effect of SiB6 additive on the sintering behavior and mechanical properties of boron carbide was investigated. It was found that the addition of 0-10% of SiB6 to B4C led to a remarkable improvement in the sinterability and density of boron carbide. The maximum theoretical density of 98% was obtained by 5% SiB6 addition. The mechanical properties such as microhardness, bending strength and fracture toughness were also increased with SiB6 increasing up to 5%. As the amount of SiB6 was increased further, the mechanical properties were reduced.