High-pressure High-temperature Annealing Research Articles

Spectroscopic features of electron-irradiated diamond crystals from the Mir kimberlite pipe, Yakutia

The behavior of characteristic centers in diamond crystals from the Mir pipe (Yakutia) was investigated upon electron irradiation. A series of diamond crystals of different types was chosen for experiments based on the nitrogen content and aggregation parameters. In electron-irradiated diamonds of the IaAB type, a new characteristic photoluminescence system was found with a zero-phonon line (ZPL) at 615 nm together with phonon replicas of 41 and 90 meV. The phonons' energies pointed to multiphonon interactions with a quasilocal vibration of a vacancy. According to our data, the nitrogen-related defect responsible for this phenomenon contains a vacancy and may be accompanied by some other impurity. Conversely, in an almost nitrogen-free crystal, a specific system with the ZPL at 558 nm was noted. The center in question is known to be vacancy-related and was formed in type IIa crystals from the Mir pipe not only by electron irradiation but also by high-pressure high-temperature annealing when vacancies were released as a result of motion or annihilation of dislocations. Regardless of the nitrogen impurity, specific systems with the ZPL at 454, 491, and 492 nm were registered in the irradiated diamond crystals from the Mir pipe. To examine the generated defects, the irradiated diamond crystals were subjected to low-temperature annealing at ≤600 °C. Although the 454 and 491 nm systems persisted, the annealing of the 492 nm system along with well-known 523.6, 489.0, and 503.4 nm (3H) centers indicated the interstitial-vacancy nature of the defect.

Formation Process of Long-Period Stacking-Ordered Structures in Mg97Zn1Y2 Alloy Comprising HCP and Cubic Phases Fabricated by High-Pressure High-Temperature Annealing

As-cast Mg97Zn1Y2 alloy consists of α-Mg matrix and 18R-type long-period stacking-ordered (LPSO) structures. We observed that the alloy undergoes a phase transformation to D03 superlattices and α-Mg matrix due to high-pressure high-temperature (HPHT) annealing at 3 GPa and above 773 K. Further, the alloy recovered after HPHT annealing, consisting of the α-Mg matrix and D03 superlattices, transformed into 18R-type LPSO structures during further annealing at ambient pressure. An fcc structure with a lattice parameter of 1.42 nm, which was twice that of D03, emerged in both the collapse process of the 18R-type LPSO structure under high-pressure, and the formation process of the 18R-type LPSO structure at ambient pressure. This fcc phase was an intermediate structure between 18R-type LPSO and D03. From the electron diffraction results, it is considered that 18R-type LPSO is continuously present with 2H including stacking faults, which almost corresponded with previous studies.

Optical properties of CVD single crystal diamonds before and after different post-growth treatments

Single crystal diamonds grown by chemical vapor deposition (CVD) before and after different post-growth treatments were studied using optical spectroscopy. The most pronounced changes in color were observed after irradiation with subsequent annealing at 800 °C whereas a weakening of gray tint took place after the high-pressure high-temperature annealing. Low-pressure high-temperature annealing in microwave plasmas of different composition and by electric arc discharge did not produce a noticeable effect on the diamond properties. Signals from nitrogen-vacancy complexes were detected in the absorption and luminescence spectra of CVD diamonds, but the pink tint in the irradiated and annealed diamonds is due to SiV complexes, not NV. The peaks at 150 and 262 K associated with boron were observed in thermoluminescence (TL) curve of original CVD diamonds. Combined irradiation and annealing allowed us to create deep traps of charge carriers responsible for TL peaks in the 430 to 470 K range, which made CVD diamonds promising for TL dosimetry.

LPHT annealing of brown-to-yellow type Ia diamonds

Low-pressure, high-temperature (LPHT) annealing of yellow-to-brown type Ia natural diamonds was performed to monitor its effects on optical centers within diamond, changes in the observed color, and to assess the process's viability as a commercial gem treatment.With LPHT annealing only, the mostly brown diamonds showed a shift towards yellow coloration; Vis-NIR absorption spectra showed this change was due to a modest increase in H3 intensity. Even at long annealing times (24h at 1800°C) or annealing at high temperatures (2000°C for five minutes), the diamonds did not significantly lose brown coloration. LPHT annealing showed itself as an ineffective means to break apart the vacancy clusters causing the brown color or causing nitrogen disaggregation, which resulted in only a small H3 generation.With LPHT annealing, “amber centers”—a group of several independent bands in the IR between 4200 and 4000cm−1 that disappear with HPHT annealing—were seen to anneal out gradually at various temperatures from 1700 to 2000°C.In contrast, high-pressure, high-temperature (HPHT) annealing effectively removes brown color at similar time/temperature conditions. Without the high stabilizing pressure provided by HPHT annealing techniques, the LPHT annealing showed pronounced damage on inclusions and dramatic surface etching.In subsequent experiments, LPHT annealing was used as a follow-up to laboratory irradiation. The irradiation-related vacancies created greater concentrations of H3 and the vacancy-assisted disaggregation of nitrogen created donors which led to a high concentration of H2 centers. This combination of defects resulted in a pronounced and favorable shift towards saleable yellow colors due to an increase in H3 and a dramatic increase in the H2 center, which led to the suppression of the remaining brownish component.The annealing characteristics for many centers detected by Vis-NIR absorption spectroscopy, FTIR absorption spectroscopy, and photoluminescence spectroscopy were chronicled throughout the study and compared with other LPHT annealing studies and HPHT annealing experiments.

갈색 Type I 다이아몬드의 고압 열처리에 따른 표면 흑연화 생성 연구

We investigated color and graphite layer formation on the surface of Type I tinted brown diamonds exposed for 5 minutes under a high-pressure high-temperature (HPHT) condition in a stable graphite regime. We executed the HPHT processes of Process I, varying the temperature from 1600℃ to 2300℃ under 5.2 GPa pressure for 5 minutes, and Process II, varying the pressure from 4.2 to 5.7 GPa at 2150℃ for 5 minutes. Optical microscopy and micro-Raman spectroscopy were used to check the microstructure and surface layer phase evolution. For Process I, we observed a color change to vivid yellow and greenish yellow and the growth of a graphite layer as the temperature increased. For Process II, the graphite layer thickness increased as the pressure decreased. We also confirmed by 531 nm micro-Raman spectroscopy that all diamonds showed a 1440 cm ?1 characteristic peak, which remained even after HPHT annealing. The results implied that HPHT-treated colored diamonds can be distinguished from natural stones by checking for the existence of the 1440 cm ?1 peak with 531 nm micro-Raman spectroscopy.

High-temperature annealing of optical centers in type-I diamond

Diamonds previously subjected to radiation damage have been annealed at temperatures up to 1750°C at ambient pressure, and at 2300°C using a stabilizing pressure of 5GPa. The results have been compared with those from similar measurements using natural brown diamonds. The investigation has led to an improved understanding of the commercial process for enhancing the color of brown diamonds by high-pressure, high-temperature (HPHT) annealing. The study has confirmed that the H4 center is less stable than the H3 center, and shown that the destruction of the H4 center in irradiated type-IaB diamonds coincides with the formation of a number of optical centers found to occur naturally in brown diamonds. In type-IaA diamonds the annealing out of the H1b absorption coincides with the production of H2 centers. Annealing measurements on brown diamonds indicate that the plastic deformation, associated with the brown color, occurred at a late stage in the diamonds’ history. Optical centers, such as H3 and (NV)−, that are produced in brown diamonds by HPHT annealing, are destroyed by this process in irradiated dislocation-free diamonds. Formation of these centers during the HPHT annealing of brown diamonds occurs as a result of the release of vacancies from the dislocations, and the present results therefore indicate that the production of these centers is a dynamic process with the generation and annealing in competition.

The effect of high-pressure–high-temperature annealing on paramagnetic defects indiamond

The formation and annealing out of paramagnetic defects in high-pressureand high-temperature diamond are investigated through a comprehensiveheat treatment study of as-grown crystals. Diamond crystals are subjected toa sequence of isochronal anneals in the temperature range between 1550 and2000 °C and the concentrations of paramagnetic defects are determined by electron paramagneticresonance measurements after each annealing step. The quantitative data areanalysed in the light of recently published results on the behaviour of nickel and/ornitrogen defects upon annealing. Like the P1 and W8 centres, the NIRIM1nickel-related defects show a strong decrease in their concentration upon annealing at1900 °C,whereas the NIRIM2 centres seem to be rather stable. The evolution of the nickel-relatedcentre concentration through the annealing sequence shows that not all W8 defectstransform into paramagnetic nickel–nitrogen complexes. Indeed, most of the W8 defects aretransformed in other non-paramagnetic centres, with the AB1, AB3, and AB6 defects beingprobably by-products of this process. The relatively low concentration of nickel–nitrogenparamagnetic complexes formed upon annealing shows that their formation does notplay any fundamental role in the kinetics of the nitrogen aggregation during heattreatments.

The influence of pressure on high-pressure, high-temperature annealing of type Ia diamond

The influence of temperature and pressure conditions during high-pressure, high-temperature (HPHT) annealing on the dissociation of nitrogen aggregates is investigated, and special attention is paid to the influence of pressure. When PT conditions are above the diamond–graphite equilibrium line, an activation energy of 5.6±1.4 eV is calculated from the Arrhenius plot of the dissociation rate of A centres by assuming that the dissociation reaction of A centres follows first order kinetics. A second estimate for the binding energy of the A centres, derived from the equilibrium concentrations of A centres and C centres at three different temperatures, gives a value of 7.66±0.6 eV. The dissociation rate of A centres is enhanced by a factor of ∼10 when the PT point during the HPHT annealing is located in the graphite stable region of the carbon phase diagram.

High-pressure high-temperature annealing of ion-implanted GaN films monitored by visible and ultraviolet micro-Raman scattering

Visible and ultraviolet micro-Raman scattering was employed to monitor the high-pressure high-temperature annealing of Mg/P-implanted GaN films. The results illustrate the use of Raman scattering to monitor processing of GaN where fast feedback is required. Temperatures up to 1500 °C with nitrogen overpressures of 1–1.5 GPa were used during the annealing. The crystalline quality, the strain, and the free carrier concentration in the ion-implanted GaN films was monitored, averaged over the layer thickness and in a 40-nm-thin surface layer of the sample. Annealing temperatures of 1400–1500 °C were found to result in the nearly full recovery of the crystalline quality of ion-implanted GaN. No significant surface degradation occurred during the annealing. High nitrogen overpressures proved very effective in preventing the nitrogen out-diffusion from the GaN surface at high temperatures. Strain was introduced during the annealing. Changes in the free carrier concentration were studied.

The Use of Micro-Raman Spectroscopy to Monitor High-Pressure High Temperature Annealing of Ion-Implanted GaN Films

We have investigated the high-pressure high-temperature annealing of Mg/P-implanted GaN films using visible and ultraviolet (UV) micro-Raman spectroscopy. The results illustrate the use of Raman spectroscopy to monitor processing of GaN where fast feedback is required. The structural quality and the stress in ion-implanted GaN films was monitored in a 40nm-thin surface layer of the sample as well as averaged over the sample layer thickness. We find the nearly full recovery of the crystalline quality of ion-implanted GaN films after annealing at 1400-1500°C under nitrogen overpressures of 1.5GPa. No significant degradation effects occurred in the GaN surface layer during the annealing. The high nitrogen overpressures proved very effective in preventing the nitrogen out-diffusion from the GaN surface. Stress introduced during the annealing was monitored. Raman spectra of ion-implanted GaN films were investigated at different temperatures and excitation wavelengths to study the GaN phonon density of states.

The Use of Micro-Raman Spectroscopy to Monitor High-Pressure High-Temperature Annealing of Ion-Implanted GaN Films

AbstractWe have investigated the high-pressure high-temperature annealing of Mg/P-implanted GaN films using visible and ultraviolet (UV) micro-Raman spectroscopy. The results illustrate the use of Raman spectroscopy to monitor processing of GaN where fast feedback is required. The structural quality and the stress in ion-implanted GaN films was monitored in a 40nm-thin surface layer of the sample as well as averaged over the sample layer thickness. We find the nearly full recovery of the crystalline quality of ion-implanted GaN films after annealing at 1400-1500°C under nitrogen overpressures of 1.5GPa. No significant degradation effects occurred in the GaN surface layer during the annealing. The high nitrogen overpressures proved very effective in preventing the nitrogen out-diffusion from the GaN surface. Stress introduced during the annealing was monitored. Raman spectra of ion-implanted GaN films were investigated at different temperatures and excitation wavelengths to study the GaN phonon density of states.

Synthetic diamond crystal strength enhancement through annealing at 50 kbar and 1500 °C

High-pressure, high temperature (HPHT) annealing of synthetic type I diamond crystals at 1200–1700 °C and 50–60 kbar was found to induce aggregate-nitrogen dissociation and metal coalescence as well as heal diamond lattice dislocations. For crystals with low levels of metal inclusions, HPHT annealing was observed to increase the average compressive fracture strength of the crystals by apparently strengthening the strongest crystals of the population. Crystals with high metal-content, or otherwise of low quality, are weakened by anncaling. Strengthening is believed to occur by locally stabilizing the diamond lattice by healing lattice dislocations as well as dispersing nitrogen within the lattice. A general model is presented that ties together these results with those of other researchers.