Calibration-free Spectroscopic Analysis Of Alloying Elements And THz Spectroscopic Study On Thermal Stability Of Materials

Spectroscopy is a technique to study the composition,structure,and content of matter,employing the characteristic spectrum produced by light's emission and absorption.Spectroscopy has made remarkable contributions to modern science,especially to laser spectroscopy,which not only plays an irreplaceable role in the fields of traditional physics and chemistry but also in the analysis and characterization of materials;it also has critical applications in areas such as environmental monitoring,health and medical care,and space exploration,which is of broad interest today.Both the Laser-induced breakdown spectroscopy technique(LIBS)and terahertz time-domain spectroscopy(THz-TDS)has attracted much attention due to their simple sample processing and fabrication processes and their wide applications.Combined with the work during the master's degree,the research contents of this paper are as follows:1.To address the shortcomings of the LIBS technique for accurate and rapid quantitative analysis of material elements,the present work combines the calibration free algorithm with high repetition rate laser-ablation spark-induced breakdown spectroscopy(HRR LA-SIBS).The silver alloy sample was ablated by an acoustic-optic q-switched fiber laser operating at a30 k Hz pulse repetition rate,and DC high voltage power supply(3k V,200m A)was applied to enhance the signal intensity of the plasma optical radiation.In the analysis of spectral data,firstly,the spectral signal's white background is removed utilizing median filter.The electron number density and the plasma temperature are 1.11×10¹⁷cm^-3 and 7355K,respectively,which obtained by Stark broadening and Saha-Boltzmann diagram Then the plasma is verified to be in a local thermodynamic equilibrium(LTE)state by using the Mc Whirter criterion and the plasma parameters.Finally,the CF method was used to obtain the elemental content of silver alloys with an analytical error within 15%compared to the standard values and an error of<0.5%for the principal elements,demonstrating that the CF HRR LA-SIBS technique can be used for rapid and reliable calibration-free quantitative analysis of alloy samples.2.Thermal stability is a critical problem in the research of materials.The conformational transition temperature(Tc)is a commonly used thermodynamic parameter for the characterization of polymer materials affected by the external environment,such as temperature,the accurate determination of Tc is a hot topic in physical chemistry and analytical chemistry.The polyvinyl alcohol(PVA)crystal pellet samples were treated by the hydrolysis-evaporation-film formation method,and the dynamic spectral data of the PVA samples were recorded in the range of 20-200°C by using a variable temperature cuvette combined with a terahertz time-domain spectroscopy system under the environment of ventilation with dry air.The PVA conformational transition temperature was concluded to be around 70°C based on the calculated absorption coefficient versus temperature.The thermal stability of common terahertz device fabrication materials,polysulfone(PSU),polyetherimide(PEI)and polyetheretherketone(PEEK),was analyzed by the same method in the terahertz band.The results show that all three polymers have good dielectric properties at room temperature with small dielectric loss angle tangent values,PSU has good thermal stability at temperatures below 175°C;PEI can reach 250°C;PEEK can reach 300°C or even higher,and the absorption coefficient and dielectric constant of PEEK fluctuate by only 4.38%and5%compared to room temperature.It shows that the THz-TDS technique is susceptible to the material's internal structural changes and is an excellent technical method for characterizing the material.3.The detection of compounds containing different crystal water is a complex problem in pharmaceutical analysis.In this work,lithium citrate,a commonly used antidepressant,was used as a sample because it is easy to form a strong hydrogen bond with water.It often exists in a tetrahydrate state.THz-TDS technique was used to distinguish anhydrous lithium citrate(anhydrous)from tetrahydrate.The results showed that the lithium citrate tetrahydrate had an obvious absorption peak at 1.66 THz,but the anhydrous lithium citrate(anhydrous)had no absorption peak at room temperature.The results showed that the THz-TDS technique was superior in the detection of compounds containing different crystal water.Secondly,the thermal treatment shows that the absorption peak of lithium citrate(anhydrous)at 1.66 THz decreases continuously until it finally disappears during conversion to anhydrous lithium citrate(anhydrous)water loss temperature is about 65?.Finally,according to the characteristic absorption peak of lithium citrate(anhydrous)at 1.66 THz,the change of water loss rate with the heating temperature at different temperatures was studied,and the relationship between dehydration rate and the heating temperature was fitted by the Arrhenius equation,the apparent activation energy(E_k)of lithium citrate tetrahydrate is 495.1J/g,which is around 3.71%of that obtained by Differential scanning calorimetry test.These results indicate that THz-TDS has advantages in the study of crystalline water compounds and can be used as a supplement to IR,Tunku Abdul Rahman,and other spectroscopic techniques.