Factors influencing development of residual stresses during crystallization firing in a novel lithium silicate glass-ceramic

ObjectiveDevelopment of residual stresses is a potential source of premature fractures in glassy materials, being of special interest in novel lithium silicate glass-ceramics that require a crystallization firing to achieve their final mechanical properties. The aim of this work was to assess the influence of various firing tray systems and the application of different cooling protocols on the development of residual stresses in Suprinity PC crowns. Their effect on the in vitro lifetime of the restorations was also studied.MethodsThirty crowns were milled out of Suprinity PC blocks and crystallized using one of five different commercial firing tray systems (n = 6). Samples in each group were cooled following a fast (FC = 5.5 °C/s), a slow (SC = 0.4 °C/s) or the manufacturer’s reference cooling (REF ). Obtained crowns were sagittally or transversally sectioned and the magnitude and distribution of residual stresses was determined using the light birefringence method. Extra crowns of three of the subgroups (n = 8) were produced and submitted to chewing simulation for 10⁶ cycles or until fracture ensued.ResultsAverage residual stresses ranged between 0 and 1.5 MPa (peaks of 5 MPa). Highest stress magnitudes were observed at the support areas of groups using firing pins, leading to thermal cracks in FC samples and premature failures in the REF subgroup. The use of fibrous pads and firing pastes limited the development of residual stresses, whereas application of SC regimes extended the lifetime of the restorations.SignificanceDevelopment of residual stresses during crystallization firing in lithium silicate glass-ceramics results critical for their mechanical performance and should be therefore avoided by ensuring a homogenous cooling of the structures.     

计算机辅助设计/计算机辅助制作玻璃陶瓷嵌体在Ⅱ类洞修复中的应用研究

目的观察CEREC(瓷睿刻)AC计算机辅助设计/计算机辅助制作(CAD/CAM)系统制作的玻璃陶瓷嵌体修复磨牙Ⅱ类洞的临床效果。方法选取2011年5月至2012年5月沈阳军区总医院口腔门诊收治的牙体缺损需制备Ⅱ类洞患者60例(78颗磨牙),使用CEREC AC CAD/CAM系统制作玻璃陶瓷嵌体,修复后追踪观察3个月、6个月、1年,按照改良美国公共卫生服务标准进行临床评价,并比较修复第1磨牙和第2磨牙缺损的成功率。结果玻璃陶瓷嵌体修复1年成功率达到96.0%,第1磨牙和第2磨牙嵌体修复的成功率比较差异无统计学意义(P>0.05)。结论 CEREC AC CAD/CAM系统制作玻璃陶瓷嵌体修复后磨牙Ⅱ类洞具有较好的临床效果。     

Alkali-free bioactive glasses for bone tissue engineering: a preliminary investigation

An alkali-free series of bioactive glasses has been designed and developed in the glass system CaO-MgO-SiO₂-P₂O₅-CaF₂ along the diopside (CaMgSi₂O₆)-fluorapatite (Ca₅(PO₄)₃F)-tricalcium phosphate (3CaO·P₂O₅) join. The silicate network in all the investigated glasses is predominantly coordinated in Q² (Si) units, while phosphorus tends to remain in an orthophosphate (Q⁰) environment. The in vitro bioactivity analysis of glasses has been made by immersion of glass powders in simulated body fluid (SBF) while chemical degradation has been studied in Tris-HCl in accordance with ISO-10993-14. Some of the investigated glasses exhibit hydroxyapatite formation on their surface within 1-12 h of their immersion in SBF solution. The sintering and crystallization kinetics of glasses has been investigated by differential thermal analysis and hot-stage microscopy, respectively while the crystalline phase evolution in resultant glass-ceramics has been studied in the temperature range of 800-900 °C using powder X-ray diffraction and scanning electron microscopy. The alkaline phosphatase activity and osteogenic differentiation for glasses have been studied in vitro on sintered glass powder compacts using rat bone marrow mesenchymal stem cells. The as-designed glasses are ideal candidates for their potential applications in bone tissue engineering in the form of bioactive glasses as well as glass/glass-ceramic scaffolds.     

Nucleation efficacy and flexural strength of novel leucite glass-ceramics

ObjectivesTo optimize the nucleation mechanism in leucite glass-ceramics to allow more efficacious glass-ceramic manufacture and improvements in microstructure and mechanical reliability.Materials and methodsAn alumino-silicate glass was designed and synthesized using melt quench methods. The glass was crushed and milled using various milling times (48–93 h) and spray drying. Nucleation and growth heat treatment schedules were applied to synthesize glass-ceramics. Glass/glass-ceramic powders and frit specimens were characterized using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) and X-ray diffraction analyses (XRD). Glass-ceramic specimens were tested using the biaxial flexural strength test (BFS).ResultsApplication of defined nucleation heat treatments resulted in the synthesis of Na/Ca titanates. NMR indicated changes to the ²³Na glass spectra in the nucleated glass and TEM/EDX the presence of Na/Ca/Ti domains (<200 nm) within the leucite crystals and associated with the nucleation of the leucite phase. XRD confirmed the presence of a bulk leucite phase in the glass-ceramics. SEM/TEM confirmed the crystallization of the leucite phase (65.5–69.3%) in a thermally matched glass, in conjunction with the nano Na/Ca titanate phase. The leucite glass-ceramics resulted in a high BFS (255–268 MPa), with reduction in powder milling time prior to heat treatments having no significant effect on flexural strength and reliability (p > 0.05).SignificanceNa/Ca titanates were synthesized in leucite glass-ceramics for the first time and associated with its nucleation and efficacious growth. This nucleation optimization provides opportunities for more efficient manufacturing and microstructural/mechanical reliability improvements. Improved synthesis of high strength/reliable leucite glass-ceramics is useful for construction of esthetic minimally invasive restorations.     

电泳沉积多孔钙镁硅生物玻璃陶瓷涂层

目的电泳沉积法制备出钛合金/钙镁硅生物玻璃陶瓷涂层复合材料,优化制备工艺条件。方法用溶胶-凝胶法制备出钙镁硅生物玻璃陶瓷粉体,通过粉体在不同分散介质中的带电特性和沉积结果,筛选合适的溶剂;通过改变悬浮液陈化时间(10分钟、1小时、24小时)、悬浮液体系固含量(0.02kg/L、0.06kg/L、0.12kg/L)、电压(20V、40V、80V)及钛合金基底的预处理方法,观察不同条件对电泳沉积涂层表面形貌和性能的影响,找出较佳的制备条件。结果生物玻璃陶瓷粉体在乙醇中带正电荷,且能在钛合金阴极基底上均匀沉积;在经过机械打磨的钛合金表面沉积的涂层均匀无裂纹,而未经打磨的钛合金表面的涂层开裂严重;悬浮液的固含量为0.02kg/L,陈化时间在10分钟时为最佳分散条件;沉积电压为20V、40V、80V均能制备出均匀无裂纹的涂层,经过煅烧后的涂层为多孔结构,涂层厚度为10~250 m,XRD结果显示涂层是以透辉石,镁黄长石等为主晶相的玻璃陶瓷。结论经过条件优化,可以制备出均匀无开裂的多孔生物玻璃陶瓷涂层,涂层厚度和致密度可控。     

青少年恒磨牙根管治疗后高嵌体修复的疗效观察

选择46颗经根管治疗后的青少年恒磨牙行CAD/CAM玻璃瓷高嵌体修复,运用USPHS标准进行临床评价.经2年复查,在嵌体固位、边缘密合性、邻接关系等方面性能良好,CAD/CAM玻璃瓷高嵌体可作为根管治疗后青少年恒磨牙的有效修复方法.     

Effect of sandblasting,etching and resin bonding on the flexural strength/bonding of novel glass-ceramics

Objectives

To process novel leucite glass-ceramics and test the effects of surface treatment and resin bonding on the biaxial flexural strength (BFS) and shear bond strength (SBS).

多次烧结对二硅酸锂玻璃陶瓷颜色和透光性的影响

目的 探究烧结次数对二硅酸锂玻璃陶瓷的颜色(△E)和透光性(τ)的影响。 方法 采用热压铸工艺制作二硅酸锂玻璃陶瓷(IPS e.max Press)片状试件(直径10mm,厚度1mm)30 个。打磨抛光后按照随机列表法将试件随机分为6 组,每组5 个。在烤瓷炉中分别烧结0(对照组)、1、2、3、4、5 次,用分光测色计和透光率测试仪分别测量各试件的色彩参数(L*a*b*)和透光性(τ), 并计算出色差(△E)。应用SPSS17.0 软件进行统计分析。每组中随机抽取2 个试件用X 线衍射仪(XRD) 分析其晶相组成。结果 试件反复烧结后的色差变化范围为0.88~2.45,明度值明显降低(P<0.05),a*、b*、τ 值明显增大(P<0.05)。 在多次烧结后试件的晶相保持不变,主晶相(Li₂Si₂O₅)的衍射峰强度增大。结论 烧结次数会影响二硅酸锂玻璃陶瓷基底冠的颜色和透光性,这可能与主晶相的晶体含量增加有关。     

Real time neutron diffraction and NMR of the Empress II glass-ceramic system

Objective. This study reports real time neutron diffraction on the Empress II glass-ceramic system.Methods. The commercial glass-ceramics was characterized by real time neutron diffraction, ³¹P and ²⁹Si solid-state MAS-NMR, DSC and XRD.Results. On heating, the as-received glass ceramic contained lithium disilicate (Li₂Si₂O₅), which melted with increasing temperature. This was revealed by neutron diffraction which showed the Bragg peaks for this phase had disappeared by 958 °C in agreement with thermal analysis. On cooling lithium metasilicate (Li₂SiO₃) started to form at around 916 °C and a minor phase of cristobalite at around 852 °C. The unit cell volume of both Li-silicate phases increased linearly with temperature at a rate of +17 × 10⁻³ ų.°C⁻¹. Room temperature powder X-ray diffraction (XRD) of the material after cooling confirms presence of the lithium metasilicate and cristobalite as the main phases and shows, in addition, small amount of lithium disilicate and orthophosphate. ³¹P MAS-NMR reveals presence of the lithiorthophosphate (Li₃PO₄) before and after heat treatment. The melting of lithium disilicate on heating and crystallisation of lithium metasilicate on cooling agree with endothermic and exotermic features respectively observed by DSC. ²⁹Si MAS-NMR shows presence of lithium disilicate phase in the as-received glass-ceramic, though not in the major proportion, and lithium metasilicate in the material after heat treatment. Both phases have significantly long T₁ relaxation time, especially the lithium metasilicate, therefore, a quantitative analysis of the ²⁹Si MAS-NMR spectra was not attempted.Significance. The findings of the present work demonstrate importance of the commercially designed processing parameters in order to preserve desired characteristics of the material. Processing the Empress II at a rate slower than recommended 60 °C min⁻¹ or long isothermal hold at the maximal processing temperature 920 °C can cause crystallization of lithium metasilicate and cristobalite instead of lithium disilicate as major phase.     

Tensile strength of bilayered ceramics and corresponding glass veneers

PURPOSE

To investigate the microtensile bond strength between two all-ceramic systems; lithium disilicate glass ceramic and zirconia core ceramics bonded with their corresponding glass veneers.

MATERIALS AND METHODS

Blocks of core ceramics (IPS e.max® Press and Lava™ Frame) were fabricated and veneered with their corresponding glass veneers. The bilayered blocks were cut into microbars; 8 mm in length and 1 mm² in cross-sectional area (n = 30/group). Additionally, monolithic microbars of these two veneers (IPS e.max® Ceram and Lava™ Ceram; n = 30/group) were also prepared. The obtained microbars were tested in tension until fracture, and the fracture surfaces of the microbars were examined with fluorescent black light and scanning electron microscope (SEM) to identify the mode of failure. One-way ANOVA and the Dunnett''s T3 test were performed to determine significant differences of the mean microtensile bond strength at a significance level of 0.05.

RESULTS

The mean microtensile bond strength of IPS e.max® Press/IPS e.max® Ceram (43.40 ± 5.51 MPa) was significantly greater than that of Lava™ Frame/Lava™ Ceram (31.71 ± 7.03 MPa)(P<.001). Fluorescent black light and SEM analysis showed that most of the tested microbars failed cohesively in the veneer layer. Furthermore, the bond strength of Lava™ Frame/Lava™ Ceram was comparable to the tensile strength of monolithic glass veneer of Lava™ Ceram, while the bond strength of bilayered IPS e.max® Press/IPS e.max® Ceram was significantly greater than tensile strength of monolithic IPS e.max® Ceram.

CONCLUSION

Because fracture site occurred mostly in the glass veneer and most failures were away from the interfacial zone, microtensile bond test may not be a suitable test for bonding integrity. Fracture mechanics approach such as fracture toughness of the interface may be more appropriate to represent the bonding quality between two materials.