HT materials

HTS materials, because of their ceramic nature, are quite brittle. This has introduced problems relative to the winding of superconducting magnets. One solution is to first wind the magnet with the powder-in-tube wire before the ceramic powder has been bonded and then heat treat the desired configuration to form the final product. Another solution is to form the superconductor into such fine fila-... 

Since the ionization potential of 238 matches closely the work function of PEDOT (5.1-5.3 eV) [335], the hole injection is dramatically improved. Accordingly, the device ITO/PEDOT/237 238(7 3)/Al has a significantly improved EL efficiency, tjel= 1.5cd/A, two orders of magnitude higher than that of single-layer PLED with 237, six times higher than that of bilayer PLED with triarylamine polymer HTL, and almost twice as high as that of PF blends with low molecular triphenylamine HT materials (in device with Ca electrode) [321]. 

See also Josephson entries HTS materials, 23 819, 839. See also High temperature superconductors (HTS) HTS superconducting current leads, 23 855 HTS tapes, 23 854... 

The additional energy input needed for waste vitrification technologies, compared to conventional incinerators that even recover residual energy, may be considered detrimental to the net energy balance of the complete path resource — waste . However, this additional energy requirement can also be considered as the additional price to pay for the efficient exploitation of natural resources. To be sustainable, HT materials should fulfil the following requirements ... 

The ratio [additional energy needed to produce HT materials] [long-term environmental profit] should be as low as possible. 

The leachability and long-term durability of HT materials should be such that the potential hazard to the environment is significantly reduced in comparison to conventional MSWI products. 

The mechanical characteristics of HT materials should be comparable to those... 

The present chapter deals exclusively with the scientific issues of point (2). It is based on the results of an extensive study under the authority of the Swiss Agency for Environment, Forests and Landscape (SAEFL/BUWAL Perret et al. 2000,2002). The in-depth survey was performed on 23 HT materials originating from 16 different advanced thermal treatment facilities developed by 10 different companies in Switzerland, Germany, France, and Italy, and operated under realistic conditions (e.g., from wastes representative of average MSW). On the basis of the results, two options are tested to assess the long-term behaviour of HT materials ... 

Landfill disposal of HT tnaterials. This scenario supposes that HT materials are stable enough to be considered as inert materials that can be disposed into monofills, hereafter referred to as glassfills , requiring no special attention (e.g., technical barriers or precautionary monitoring of leachates). 

HT materials are re-used as foundation layers for road construction, hereafter referred to as glassroad . 

This section is primarily intended to show that the relevant combination of the intrinsic physico-chemical and microscopic characteristics of HT materials, their behaviour under aggressive conditions of corrosion, and their modelled thermodynamic stability, yields a sound composite picture of these materials. On the basis of the knowledge acquired over decades on high-level nuclear waste glasses (Vernaz Dussosoy 1992 Bates et al. 1994 Thomassin 1995, 1996 Ewing 1996), these are the key parameters that may drastically influence the long-term durability of HT materials. 

For an accurate description of the experimental set-up, the analytical procedure, the HT treatment facilities, and the detailed characteristics of each of the individual 23 HT materials studied, the reader is referred to Ferret et al. (2000, 2002, 2003). 

The 23 HT materials studied were produced in Swiss, German, French, and Italian HT treatment facilities, which were fed by different input materials under realistic conditions of operation (i.e., wastes and residues representative of average MSW, BA, FA, and FC). These technologies can be roughly grouped into two categories (see Fig. 2), in-line processes and post-processes ... 

In-line processes are technologies that directly inertize municipal solid wastes. Five such HT materials were studied. 

Post-processes for bottom ash, which are fed by BA (80-100 wt%, either in rough or fine fractions) and up to 20 wt% FA. Six such HT materials were studied. 

The factor governing the final size and morphology (from millimetre-sized to large irregular blocks) of HT materials is the melt quenching... 

The majority of samples (15 out of 23) were shown to contain no or less than ca. 2 vol% of crystalline inclusions and have a low specific surface area (5spec = 300-600 cm2/g for material ground to 100-125 pm) these samples are considered to be vitreous. The other samples contain higher amounts of crystalline components, either homogeneously or heterogeneously dispersed in a vitreous matrix they are considered to be vitrocrystalline (labelled with an asterisk) and are expected to exhibit a lower resistance to corrosion because of their higher specific surface area. The latter typically varies between 400 and 1000 cm2/g, but may be as high as 8200 cm2/g. None of the HT materials studied was entirely crystalline. 

Fig. 2. Classification of HT samples on the basis of the input material from which they are produced In-line processes directly transform MSW into HT materials. Post-processes transform residues of MSW incineration (BA, FA, FC, or mixtures of them with possible additives) into HT materials. Values are given for Switzerland (year 2002).

Although the role of crystalline phases in the leachability of HT materials is unclear and must be examined from case to case, the identified silicates and oxides are overall more resistant to corrosion than silicate glass and residues of incineration (Scholze 1991). Thus, a clear assessment of the durability of HT materials as a function of crystalline components must take into account the combined effects of their enrichment or depletion in trace metals, their individual leachability, the increase (but sometimes decrease) in overall reactivity due to local heterogeneities and increased Sspec (Jacquet-Francillon et al. 1982 Bickford Jantzen 1984 Jantzen Plodinec 1984 Scholze 1991 Adams 1992 Sproull et al. 1994 Sterpenich 1998). 

Fig. 3. Examples of typical HT materials in their original state. Left column vitreous samples. Right column vitrocrystalline samples. Top rows samples on a 10 cm2 scaling grid. Bottom row scanning electron micrographs (SEM in secondary electrons mode) of samples after grinding to 100-125 p.m.

Fig. 4. Typical X-ray diffractograms of 100-125 pm ground HT materials, showing purely vitreous character (P8) or vitrocrystalline character (P6, PI 6 ). For most vitrocrystalline samples, mineral phases are distributed throughout the glass matrix (e.g., P6", homogeneous vitrocrystalline), but some samples (e.g., PI6, heterogeneous vitrocrystalline) exhibit patches of concentrated crystalline phases visibly separated from the bulk vitreous matrix.

When comparing HT materials to the Swiss Waste Management Ordinance (TVA, Techni-sche Verordnung fiber Abfalle BUWAL 19%), it appears that only 8 samples out of 23 could... 

Fig. 5. Average concentrations of major and minor constituents in the different types of HT materials studied. Left Si02 (network-forming), AI2O3 (network-forming or modifying), and CaO (network-modifying). Right MgO, Na20, K20, and Fe203.

Taking into account all observations on the physico-chemical characteristics of HT materials (macroscopic, microscopic and physical features, chemical composition, metal content), only three HT samples, all of them being vitrocrystalline,... 

Fig. 6. Average concentrations of several potentially toxic and environmentally relevant metals in the different types of HT materials studied.

Figure 8 illustrates the transformations induced on the surface structure of HT materials after 10 days of corrosion. The amorphous gel layer that appears during corrosion is observed on most HT samples and reveals pits, holes, and corrosion paths from case to case. For many samples, crystalline secondary phases in the 1-10 pm range cover the altered surface, and spallation/cxfoliation of the gel layer is observed or at least suggested in several instances, indicating that corrosion is a discontinuous process, even under static conditions. 

After one day, a direct correlation between leachant pH and release of Si in the solution can be seen (Fig. 9a). Later on (3- and 10-day corrosion), the increase in pH becomes much slower, suggesting that the leaching of alkaline elements is not a congruent process, and pH stabilizes rapidly during matrix dissolution. In fact, the proportion of CaO in the matrix of HT materials (11-38 wt%) directly controls the amount of Ca that will be readily leached out (see Fig. 9b). It can thus be expected that Ca-poor/Si-rich HT materials possess a higher durability than Ca-rich/Si-poor ones. The relationship between CaO in the matrix of HT materials, Ca2+ extracted into solution by the leachant, and pH is given by ... 

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