| Abstract [eng] |
Nuclear magnetic resonance cross polarization (CP) is a well known technique in solid-state NMR field. This experiment allows to reduce experimental time and to enhance signal-to-noise ratio. In this work, CP kinetics, which are spectral intensity versus time, are recorded and analyzed. These kinetics is a result of the fine and bulk structure of the material. Hence, every kinetic curve contains information about order in nano-scale. In another hand, there are very well acknowledged techniques such as X-ray diffraction (XRD), neutron diffraction (ND), scanning electron microscopy (SEM) etc. And yet these techniques have their own obstacles which is why NMR can be complementary. In this work, two samples were analyzed: ammonium dihydrogen phosphate (ADP) and calcium hydroxyapatite (CaHA). The first one is an interesting material in non-linear optics field and also yields some applicable properties. For this reason, this material is well researched and structural parameters are well determined. Hence, it can be used as a model material to trial new method of analyzing structure. As mentioned before, XRD, ND carries a difficulty in determining a position of the hydrogen atom. In this way, NMR can be complementary crystallographic method because it can manipulate every atom that has a spin. Calcium hydroxyapatite is widely researched material in the medicine. Information about these materials is still needed because they are the main ingredients of the bio-compatible ceramics. Their structure correlates to the bone tissue, because mineral form of CaHA is directly found in the bones. In the field of spectrometry and microscopy, structure and crystallization is analyzed. Using CP, this information can be extracted and can be added to total understanding. Moreover, in previous research an unexpected phenomenon was acquired. It was called cross-polarization beatings, but explanation was not yet presented. For these reasons, the objective of this work is to analyze cross-polarization in ADP and to investigate the reasons for beatings to occur in CaHA. This objective leads to further tasks: 1. Obtain ADP 1H→31P cross-polarization kinetics in the different temperatures; 2. In the same temperature range perform proton longitudinal relaxation measurements and compare acquired results with those found in the literature; 3. Perform optimization experiments with the different MAS rate and excitation width for the CaHA sample; 4. At room temperature obtain CaHA 1H→31P cross-polarization kinetics at which intensity beatings take place; 5. Define conditions for the CP beatings in CaHA to occur. Customized I-I*-S model was employed to obtain structural parameters which are similar to those acquired by ND and XRD techniques. Nevertheless, it was observed how structure of ADP is changing in different temperatures. Moreover, relaxation measurements besides CP analysis had shown the structural change is near the phase transition point. Hence, it was proved that NMR is a good complementary crystallographic method. From the same mathematical analysis it was derived that RF field (or excitation bandwidth) is linked to an actual number of the non-equivalent 1H-31P spin couples. Moreover, appliance of the magic angle spinning at high rates supposedly suppresses long range spin couples that could blur the fine internuclear structure. Hence, high RF field and high MAS rate must be used in order to observe cross-polarization beating effect in calcium hydroxyapatite. In conclusion, one can see, that the applied method is not only good for the structural analysis but also helps to understand the nature of the observed mechanism. |
